All Abstracts | Poster Abstracts | Talk Abstracts

1. Homological Stabilizer Codes

Jonas Anderson, University of New Mexico

(Session 13 : Sunday from 11:45am-12:15pm)

Abstract. The discovery of quantum error correction and fault-tolerance were major theoretical breakthroughs on the road towards building a full-fledged quantum computer. Since then thresholds have increased and geometric constraints on the underlying architecture have been added. Homological stabilizer codes provide a method for constructing stabilizer codes constrained to a 2D plane. In this talk I will define and proceed to classify all 2D homological stabilizer codes. I will show that Kitaev's toric code and the topological color codes arise naturally in this classification. I will finally show, up to a set of equivalence relations, that these are the only 2D homological stabilizer codes.

2. Quantum Control and Quantum State Tomography in the Hyperfine Ground Manifold of Atomic Cesium

Brian Anderson, University of Arizona

(Session 1 : Thursday from 4:30pm-5:00pm)

Abstract. Aaron Smith, Brian E. Anderson, Hector Sosa Martinez, Poul Jessen Center for Quantum Information and Control (CQuIC), College of Optical Science and Department of Physics, University of Arizona Carlos Riofrio, Ivan H. Deutsch Center for Quantum Information and Control (CQuIC), Department of Physics and Astronomy, University of New Mexico Quantum systems with Hilbert space dimension greater than two (qudits) provide an alternative to qubits as carriers of quantum information, and may prove advantageous for quantum information tasks if good laboratory tools for qudit manipulation and readout can be developed. We have successfully implemented a protocol for arbitrary quantum state-to-state mapping in the 16 dimensional hyperfine ground manifold of Cesium 133 atoms, using only static, radio frequency (rf) and microwave magnetic fields to drive the atomic evolution. This system is controllable given rf and microwave fields with constant amplitude and frequency, and piecewise constant phase modulation. Control waveforms (rf and microwave phases versus time) are found by numerical optimization, and can be designed to work well in the presence of errors in the driving and background magnetic fields. Experimentally, we achieve an average state mapping fidelity of 99% for a sample of randomly chosen target states. To perform quantum state tomography, we drive an ensemble of identically prepared atoms with phase modulated rf and microwave magnetic fields, and simultaneously probe them by coupling an atomic spin observable to the polarization of a near-resonant optical probe field. A measurement of the probe polarization then yields an informationally complete measurement record that can be inverted to obtain an estimate of the unknown quantum state. We have reconstructed the full density matrix for a set of randomly chosen test states, using computer algorithms based either on least squares fitting or compressed sensing. The latter approach reconstructs our test states with an average fidelity above 90%, limited primarily by errors in applied drive fields.

3. N-Photon Wavepackets Interacting with an Arbitrary Quantum System

Ben Baragiola, University of New Mexico

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. Traveling nonclassical states of light can be important resources for quantum metrology, secure communication, and quantum networking. A theoretical description of systems interacting with nonclassical states becomes more urgent as the generation of more exotic states of light, such as N-photon states and NOON states, become technologically feasible. We approach this problem with the formalism of input-output theory and derive master equations for an arbitrary quantum system (e.g. harmonic oscillator or a multi-level atom) interacting with wavepackets of definite photon number in one and two polarization modes, which we then generalize to wavepackets with arbitrary spectral density functions. We also obtain equations for output field quantities, such as output homodyne current and photon flux. To illustrate our formalism we consider the problem of efficiently transferring field excitations to the excited state population of a two-level atom. For a single-photon wavepacket, this problem has been studied in detail [1]. We show that, for a given photon number, there is a trade off between wavepacket bandwidth and the population of the excited state.

[1] P. Domokos, P. Horak, and H. Ritsch, Phys. Rev. A {\bf 65}, 033832 (2002); M. Stobinska, G. Alber, and G. Leuchs, EPL {\bf 86}, 14007 (2009); Y. Wang, J. J.Minar, L. Sheridan, and V. Scarani, Phys. Rev. A {\bf 83}, 063842 (2011).

4. Integrated Cavity QED in a linear ion trap chip for Enhanced Light Collection

Francisco Benito, Sandia National Laboratories

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract.

Authors: F.M. Benito, M.G. Blain, K. Fortier, D.L. Moehring, J.D. Sterk, D.L. Stick, B.Tabakov.

Title: Enhancement Light Collection of a single trapped ion in a linear trap chip

Realizing a scalable trapped-ion quantum information processor may require integration of tools to manipulate qubits into trapping devices. We present efforts towards integrating a 1 mm optical cavity into a microfabricated surface trap to efficiently connect nodes in a quantum network. The cavity is formed by a concave mirror and a flat coated silicon mirror around a linear trap where ions can be shuttled in and out of the cavity mode. By utilizing the Purcell effect to increase the rate of spontaneous emission into the cavity mode, we expect to collect up to 13% of the emitted photons.

5. Progress on theoretical studies and practical applications of quantum annealing and D-Wave One

Sergio Boixo, Information Sciences Institute at the University of Southern California

(Session 13 : Sunday from 10:45am-11:15am)

Abstract. A D-Wave One quantum optimizer has been installed at the newly created USC-Lockheed Martin Quantum Computing Center. This chip implements quantum annealing at finite temperature as a computational resource, with 90 working qubits. Quantum annealing is a particularly simple branch of adiabatic quantum computation. We report work in progress on exploring practical applications of quantum annealing in general, and this chip in particular. We will also discuss entanglement tests with realistic numerical simulations of the physical devices implemented in the chip. Some of this work is done in collaboration with Aspuru-Guzik's group at Harvard, and D-Wave.

6. Crossing Tsirelsons bound with supersymmetric non-local states

Kamil Bradler, School of Computer Science, McGill University

(Session 11b : Saturday from 5:45pm-6:15pm)

Abstract. We construct a class of supersymmetric entangled states which is used as a nonlocal resource in the CHSH game. If the Grassmann-valued degrees of freedom are accessible to measurement using the proposed measurement model then the entangled state is more nonlocal then a maximally entangled two-qubit state. We show that the winning probability reaches at least pwin=0.8641 which is greater than pwin=cos2(pi/8)=0.8536. This value corresponds to an expected value known as Tsirelsons bound and no ordinary quantum-mechanical entangled state can perform better.

7. Improving robustness of quantum gates to control noise

Constantin Brif, Sandia National Laboratories

(Session 11a : Saturday from 4:15pm-4:45pm)

Abstract. External controls are necessary to enact quantum logic operations, and the inevitable control noise will result in gate errors in a realistic quantum circuit. We investigate the robustness of quantum gates to random noise in an optimal control field, by utilizing properties of the quantum control landscape that relates the physical objective (in the present case, the quantum gate fidelity) to the applied controls. An approximate result obtained for the statistical expectation value of the gate fidelity in the weak noise regime is shown to be in excellent agreement with direct Monte Carlo sampling over noise process realizations for fidelity values relevant for practical quantum information processing. Using this approximate result, we demonstrate that maximizing the robustness to additive/multiplicative white noise is equivalent to minimizing the total control time/fluence. Also, a genetic optimization algorithm is used to identify controls with improved robustness to colored noise.

8. Entanglement of two atoms using the Rydberg Blockade

Antoine Browaeys, Institut Optique, CNRS

(Session 10 : Saturday from 2:00pm-2:45pm)

Abstract. When two quantum systems interact strongly, their simultaneous excitation by the same driving pulse may be forbidden: this is called blockade of excitation. Recently, extensive studies have been devoted to the Rydberg blockade between neutral atoms, which appears due to the interaction induced by their large dipole moments when they are in Rydberg states. This talk will describe our demonstration of the Rydberg blockade between two atoms individually trapped in optical tweezers at a distance of 4 micrometers. The rubidium 87 atoms are prepared in the state |↑〉 = |F =2, M=2〉, and subsequently excited to the Rydberg state 58d3/2, |r〉, by a two-photon transition. A consequence of the blockade mechanism is that the atoms are excited in an entangled state of the form (|r,↑〉 + |↑,r〉) / √2. The signature of the production of this state is the enhanced Rabi frequency of the oscillation of the probability to excite only one of the two atoms, with respect to the Rabi frequency of the excitation of one atom when it is alone. We have then mapped the Rydberg state |r〉 onto a second ground state |↓〉 = |F =1, M=1〉 to generate the Bell state (|↓,↑〉 + |↑,↓〉) / √2. We analyse the amount of entanglement by global Raman rotations on the two atoms. We have measured a fidelity of the two-atom state produced of 0.74. Finally the talk will report our progress on the building of a new setup aiming at entangling a larger number of atoms.

9. Trapped-Ion Physics at GTRI: Towards Large-Scale Integration and Automation

Kenton Brown, Georgia Tech Research Institute

(Session 3 : Friday from 9:00am-9:30am)

Abstract.

From the earliest days of the field of quantum information, trapped atomic ions have had great potential as qubits. Trapped-ion experiments have demonstrated the individual ingredients believed necessary for scalable quantum information processing, and, for small numbers of ions, many of these ingredients have been combined within the same experimental system. Scaling the capabilities of such test-bed systems to larger numbers of qubits will require a higher level of integration between traps, electronics, optics, and control systems than has been achieved to date. Moreover, calibrating and controlling such a complex system with the necessary speed and accuracy will demand a far greater degree of automation than could be achieved through human intervention.

To explore these challenges, the Quantum Information Systems Group at GTRI has microfabricated several ion traps incorporating 40+ control electrodes, including a long linear trap, a trap with a curved mirror microfabricated onto its surface, an X-junction trap, and a trap with integrated microwave lines. In the linear traps we have loaded long chains with more than 20 resolved ions, while the mirror trap enhanced the collection of ion fluorescence by a factor of 1.8. The junction and microwave traps, when fully tested, should allow us to reorder ions into an arbitrary linear configuration and to perform fast qubit rotations, respectively. Successful operation of these traps necessitates accurate and precise modeling of their electromagnetic properties, so we have developed an in-house method-of-moments simulation package, capable of handling millions of elements, which we use to derive an accurate basis set of micromotion compensation potentials. We are planning an experiment to incorporate in-vacuum DAC electronics alongside a trap chip, all mounted on a single compact circuit board within the vacuum chamber. Finally, we have developed a machine language, known as “OPCODEs”, that translates high-level schedules directly into experimental operations for our ion traps. The success of OPCODEs relies on automated calibration and control of trap parameters, accurate modeling of the potentials required for compensation, and automated detection of ion positions. I will present our most recent experimental results in these areas.

10. The achievable values for pairwise concurrences of three qubits

Orest Bucicovschi, University of California San Diego

(Session 11b : Saturday from 4:45pm-5:15pm)

Abstract. We investigate the set of achievable values for the three pairwise concurrences of a state of three qubits. We show that it is the intersection of the convex hull of the Roman Steiner surface with the positive octant in the space of concurrences, first for X-states, then for any pure state of three qubits. We further show that the allowable set is the solution of a linear matrix inequality involving three other entanglement invariants. We further consider the extension of this result to mixed states and n qubits, n>=3.
This is joint work with David A.Meyer and Jon R. Grice

11. Silicon-Based Semiconductor Devices for Quantum Information Science and Technology

Stephen Carr, Sandia National Laboratories

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. M.P. Lilly, L.A. Tracy, N.C. Bishop, K.R. Barkley, J.E. Levy, T.M. Lu, K. Nguyen, J.R. Wendt, J. Stevens, R.K. Grubbs, T. Pluym, J. Dominguez, R.W. Young, H.L. Stalford, R. Muller, E. Nielsen, and M.S. Carroll, Sandia National Laboratories.

The semiconductor quantum information team at Sandia National Laboratory includes microfabrication, cryoelectronics, cryogenic measurement, modeling, and architecture of semiconductor devices for adiabatic and non-adiabatic quantum information processing and the quantum engineering of qubit hardware. We have demonstrated silicon-based electrostatically-defined single and double quantum dots using electron transport measurements and integrated electrometry through quantum-point-contact charge sensors. We present an overview of the device fabrication, broadband cryogenic measurement techniques, and modeling for Metal-Oxide-Semiconductor (MOS) devices and Silicon-Germanium (SiGe) heterostructures.

12. Relationships Between Defect Encodings for Topological Codes

Chris Cesare, Center for Quantum Information and Control, Department of Physics and Astronomy, University of New Mexico

(Session 11c : Saturday from 5:15pm-5:45pm)

Abstract. Several schemes have been proposed in the literature for performing quantum computations using defects in topological codes. In the color codes, there are three schemes for storing qubits using defects: using single defects tethered to boundaries, using two defects tethered to one another, or using three defects tethered to each other. The three defect approach stands apart as the only known way to retain the transversality property of certain color code gates. As such, one might ask whether there is any relationship between this transversality-preserving encoding and the others, and if there exists a way to convert between them. We demonstrate this relationship by presenting such a method of conversion.

13. Reliability of a classical-quantum communication system with noisy feedback

Aman Chawla, University of New Mexico

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. We consider a quantum pure-state forward channel used in conjunction with a feedback quantum pure-state channel for the communication of classical messages. The feedback channel is aided by a classical one-way discrete memoryless channel. We derive a lower bound on the reliability function of this communication system in terms of the overlap between the pure states used for communication over the forward channel.

14. Reflective Parabolic Ion Trap for Efficient Ion Photon Collection

Chen-Kuan Chou, University of Washington

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. Efficiently collecting ion fluorescence is critical for many aspects of trapped ion quantum computation and information, such as qubit state readout and photon-mediated remote entanglement generation. To address this issue, we developed an ion trap combining a reflective parabolic surface with trap electrodes. This parabolic ion trap design covers a solid angle close to 2 Pi, and allows precise ion placement to focal point of the mirror. With the advantage of little photon blocking and collimated ion photon emission, we can couple the ion fluorescence into fiber in a straightforward way. We expect to reach the diffraction limit for single ion imaging, with which >70% fiber coupling efficiency should be achievable. Owing to its simple design, this trap structure can be easily adapted into more complex trap systems.

15. Alternative views for decoherence and discord

Patrick Coles, Carnegie Mellon University

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. The theory of decoherence, e.g. [1]-[3], has made major progress in explaining macroscopic phenomena, yet there have been several distinct approaches. For example, decoherence has been defined as (1) the loss of off-diagonal elements of the density matrix, (2) the loss of interference, and (3) the flow of information to the environment. While Hamiltonian models of decoherence in the literature have suggested a connection, here I explicitly give mathematical connections between these three views of decoherence under very general circumstances, i.e. without invoking any sort of Hamiltonian model [4]. The fact that these three seemingly distinct processes scale with each other in a deterministic quantitative way shows, in a sense, that they are three equivalent views of one underlying phenomenon. A similar situation arises when considering the correlation between two systems. Indeed there are several alternative ways to state the “classicality condition” for correlations, and hence there are several ways to construct measures of the “quantumness” of correlations, often called “discord” measures. In addition to emphasizing the alternative perspectives found in the literature, here I give a novel view of what discord is measuring, as the number of secure classical bits (secure from the purifying system) that can be distilled from the bipartite quantum state. This shows that non-classicality is connected to information security. [1] E. Joos, H. D. Zeh, C. Kiefer, D. Giulini, J. Kupsch, and I.-O. Stamatescu, Decoherence and the appearance of a classical world in quantum theory (Springer, 2003), 2nd ed. [2] W. H. Zurek, Rev. Mod. Phys. 75, 715 (2003). [3] M. Schlosshauer, Decoherence and the quantum-to-classical transition (Springer, 2007). [4] P. J. Coles, Unified view of decoherence with application to quantum discord (2011). Eprint arXiv:1110.1664 [quant-ph].

16. Single shot quantum state estimation via continuous measurement in a strong back-action regime

Robert Cook, Center for Quantum Information and Control (CQuIC) and Department of Physics and Astronomy, University of New Mexico

(Session 11a : Saturday from 5:15pm-5:45pm)

Abstract. Quantum state reconstruction is a fundamental task in quantum information science. The standard approach employs many projective measurements on a series of identically prepared systems in order to collect sufficient statistics of an informationally complete set of observables. An alternative procedure is to reconstruct quantum state by performing weak continuous measurement collectively on an ensemble, while simultaneously applying time varying controls[1,2]. For known dynamics, the measurement history determines the initial state. In current implementations the shot noise of the probe dominates over projection noise so that measurement-induced backaction is negligible. We generalize this to the regime where quantum backaction can play a significant role, even for small numbers of particles. Using the framework of quantum filtering theory, we model the reconstruction of the state of a qubit through collective spin measurement via the Faraday interaction and magnetic field controls, and develop a maximum-likelihood estimate. [1] A. Silberfarb and I. H. Deutsch, Phys. Rev. Lett. 95, 030402 (2005). [2] C.A. Riofrio et. al., J. Phys. B: At. Mol. Opt. Phys. 44, 154007 (2011)

17. Quantum metrology with noisy systems

Luiz Davidovich, Universidade Federal do Rio de Janeiro

(Session 4 : Friday from 10:30am-11:15am)

Abstract. The estimation of parameters characterizing dynamical processes is central for science and technology. The estimation error decreases with the number N of resources employed in the experiment (which could quantify, for instance, the number of probes or the probing energy). For independent probes, it scales as one over the square root of N. Quantum strategies may improve the precision for noiseless processes, so that it scales as 1/N. For noisy processes, it is not known in general if and when this improvement can be achieved. This talk will introduce some basic aspects of quantum metrology, and present a recent proposal [1,2] of a general framework for obtaining attainable and useful lower bounds for the ultimate limit of precision in noisy systems. This method is applied to estimate precision bounds, which are independent of the initial state of the probes, for lossy optical interferometry and atomic spectroscopy in the presence of dephasing. These bounds capture the main features of the transition from the 1/N to the one over square root of N behavior as N increases. References [1] B. M. Escher, R. L. de Matos Filho, and L. Davidovich, General framework for estimating the ultimate precision limit in noisy quantum-enhanced metrology, Nature Physics vol. 7, 406 (2011). [2] B. M. Escher, R. L. de Matos Filho, and L. Davidovich, Quantum metrology for noisy systems, Brazilian Journal of Physics, vol. 41, 229 (2011).

18. niversal Quantum Degeneracy Point and Four-wave Mixing Toolbox of Superconducting Qubits

Xiuhao Deng, University of California, Merced

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. We present a superconducting circuit that can suppress qubit decoherence due to low-frequency noise and to implement well-controlled quantum operations on superconducting resonators. This circuit contains a universal quantum degeneracy point that protects the encoded qubit from arbitrary low-frequency noise. Universal quantum logic gates can be realized on the encoded qubits. This circuit can also generate various quantum operations on superconducting microwave resonators, including the Bogoliubov-linear operations and nonlinear interactions, by exploiting a dispersive four-wave mixing approach. By adjusting the parameters of the qubits, effective quantum operations on the resonators can be realized from virtual transitions.

19. Plug-and-Play Surface Electrode Ion Traps for Scalable Quantum Information Processing

Charlie Doret, Georgia Tech Research Institute

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. At the heart of most ion-based quantum information processing and simulation efforts is an RF-Paul trap to confine the ion qubits. Cutting edge experiments are transitioning from a few qubits to a few tens of qubits with many more qubits envisioned for the future. The underlying ion traps need to both grow with the experiments and provide additional features that can simplify and extend these experiments. The Georgia Tech Research Institute (GTRI) is developing modeling and fabrication processes for these new generations of ion traps in surface electrode architectures using silicon VLSI technology. GTRI has demonstrated traps that approach the plug-and-play ideal, featuring reliable ion loading and transport, long dark lifetimes, stable ion chains, and low heating rates, verified by detailed in-house characterization. Several linear geometries have been demonstrated with novel features such as micromirrors for large NA light collection and shaped RF rails for minimizing deformations to the trapping pseudopotential. Testing of additional features is underway, including integrated microwave current guides for global qubit rotations, a 4-way "X" junction, and a monolithic symmetric trapping architecture with large well depth.

20. Spatial Search by Non-Linear Quantum Walk

Mahdi Ebrahimi Kahou, Institue for Quantum Information Center

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. Authors: Mahdi Ebrahimi Kahou, David L. Feder Affiliation: IQIS, University of Calgary Abstract: One approach to the development of quantum algorithms is the quantum walk. Spatial search can be effected by the continuous-time evolution of a single quantum particle on a lattice or graph containing a marked site. In most conceivable physical applications, however, one would rather expect to have multiple interacting particles. In bosonic systems at low temperatures, the dynamics would be well-described by a discrete non-linear Schr\" o dinger equation. We investigate the role of non-linearity in determining the efficiency of the spatial search algorithm within the quantum walk model, for a variety of graphs including the complete graph, hypercubes, and periodic lattices. The analytical results will be compared with numerical calculations of multiple interacting quantum walkers.

21. Ultimate precision limits for measurement of weak forces on noisy harmonic oscillators

Bruno Escher, Universidade Federal do Rio de Janeiro

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. We obtain the ultimate precision allowed by the quantum mechanics on the estimation of the amplitude of a resonant weak force acting on a noisy harmonic oscillator, using the general method proposed in References [1,2]. In this case, that method leads to an exact analytical expression for the ultimate precision. REFERENCES: [1] B. M. Escher, R. L. de Matos Filho, and L. Davidovich, General Framework for estimating the ultimate precision limit in noisy quantum-enhanced metrology, Nature Physics, vol. 7, 406 (2011). [2] B. M. Escher, R. L. de Matos Filho, and L. Davidovich, Quantum metrology with noisy systems, Brazilian Journal of Physics, vol. 41, 229 (2011).

22. Direct Fidelity Estimation from Few Pauli Measurements

Steven Flammia, University of Washington

(Session 1 : Thursday from 4:00pm-4:30pm)

Abstract. We describe a simple method for certifying that an experimental device prepares a desired quantum state rho. Our method is applicable to any pure state rho, and it provides an estimate of the fidelity between rho and the actual (arbitrary) state in the lab, up to a constant additive error. The method requires measuring only a constant number of Pauli expectation values, selected at random according to an importance-weighting rule. Our method is faster than full tomography by a factor of d, the dimension of the state space, and extends easily and naturally to quantum channels. This is joint work with Yi-Kai Liu.

23. Quantum secret sharing with reduced communication cost

Ben Fortescue, Southern Illinois University

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. Standard techniques for sharing a quantum secret among multiple players (such that certain player subsets can recover the secret while others are denied all knowledge) require a large amount of quantum communication to distribute the secret. Two known methods for reducing this are the use of imperfect secret sharing (in which security is sacrificed for efficiency) and classical encryption. I will demonstrate how one may combine these methods to reduce the required quantum communication below what has been previously achieved (in some cases to a provable minimum) without any loss of security. Joint work with Gilad Gour.

24. Quantum Information Processing using Scalable Techniques

John Gaebler, National Institute of Standards and Technologies

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. Authors: John Gaebler, Ryan Bowler, Yiheng Lin, Ting Rei Tan, David Hanneke, John Jost, Jonathan Home, Adam Meier, Emanual Knill, Dietrich Leibfried, David Wineland We report progress towards improving our previous demonstrations of scalable quantum information processing. In this work we combine all the fundamental building blocks required for scalable quantum information processing using trapped atomic ions. Included elements are long-lived qubits; a laser-induced universal gate set; state initialization and readout; and information transport, including co-trapping a second ion species to reinitialize the quantized motion without qubit decoherence. We are currently studying experimental techniques to efficiently measure the fidelity of quantum sequences involving multiple qubits using randomized benchmarking. To implement the benchmarking we perform quantum information sequences involving as many as 16 two-qubit gates and 50 single-qubit gates. We have also developed an aribtrary waveform generator with an update rate far above the ions' motional frequencies, which is capable of bringing together and then seperating the qubit ions each time a two-qubit gate is peformed.

25. Optimal hybrid quantum secret sharing schemes via stabilizer codes and twirling of symplectic structures

Vlad Gheorghiu, Institute for Quantum Information Sciences and Department of Mathematics and Statistics, Universty of Calgary

(Session 11b : Saturday from 5:15pm-5:45pm)

Abstract. As recently shown in [quant-ph/1108.5541], any quantum error-correcting code can be converted into a perfect "hybrid" quantum secret sharing scheme by allowing the sharing of extra classical bits between the dealer and the players. An advantage of this scheme is that it allows the players' quantum shares to be of smaller dimension than the dimension of the encoded secret, which is impossible for regular perfect quantum secret sharing protocols. Whenever the underlying quantum error correcting code is a stabilizer code (this being the case for the vast majority of known quantum error-correcting codes), I provide a general scheme of reducing the amount of classical communication required, then prove that my scheme is optimal for the stabilizer code being used. The optimality proof is based on the fact that the correlations between the dealer and the players can be fully described by an "information group" [Phys. Rev. A 81, 032326 (2010)]; the symplectic structure of the information group effectively gives the minimum number of classical bits required. Finally I provide an explicit protocol that achieves this minimum by employing the notion of "twirling" (or scrambling) the information group. The results are general and valid for any stabilizer code. I will illustrate the results by simple examples.

26. Decoherence in OAM states due to turbulence

Jose Raul Gonzalez Alonso, University of Southern California

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. Photons have always been the information carriers of choice in quantum information, with many protocols taking advantage of the polarization degrees of freedom to encode quantum information. Exploiting the photon's orbital angular momentum (OAM) can provide distinctive advantages. The main one is an increased alphabet size for information transmission. Since the Hilbert space of OAM states is infinite dimensional, it can be used to encode more than one bit (or qubit) per photon. However, this potential can only be realized if suitable quantum information can be encoded in the OAM photon states, and if it can be protected from the decohering effect of atmospheric turbulence. In this work, we will numerically simulate the errors induced by weak atmospheric turbulence in OAM states.

27. Local additivity of the minimum entropy output of a quantum channel

Gilad Gour, Institute for Quantum Information Science

(Session 4 : Friday from 11:45am-12:15pm)

Abstract. In this talk I will show that the minimum von-Neumann entropy output of a quantum channel is locally additive. Hasting's counterexample for the global additivity conjecture, makes this result somewhat surprising. In particular, it indicates that the non-additivity of the minimum entropy output is related to a global effect of quantum channels. I will end with few related open problems.

28. Quantum Circuit Optimization using Symbolic Gate Identities

Brian Granger, California Polytechnic State University

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. Raymond Wong, California Polytechnic State University, San Luis Obispo, CA Addison Cugini, California Polytechnic State University, San Luis Obispo, CA Matt Curry, University of New Mexico, Albuquerque, NM Brian E. Granger, California Polytechnic State University, San Luis Obispo, CA In previous work, we have created an open source software package (SymPy) for simulating quantum computers symbolically. The symbolic manipulation of gates and circuits has many advantages over the traditional numerical approach where gates and qubits are represented as large matrices and vectors. First, extremely large circuits with many gates and qubits can be handled without memory constraints. Second, circuits can be manipulated symbolically using commutation relations, gate decompositions and gate identities. For example, these symbolic manipulations could be used to express a circuit in terms of a different set of universal one and two qubit gates. An important application of this symbolic approach is quantum circuit optimization. For our purposes, quantum circuit optimization consists of taking a known circuit of one and two qubit gates and reducing the overall gate count while taking constraints (for example, CNOT gates are the only allowed two qubit gate) into account. If possible, this type of circuit optimization would be useful for algorithm development as well as for optimizing practical implementations that include quantum error correction. We have started to develop tools for quantum circuit optimization within the context of our open source software. More specifically, we have developed an efficient algorithm for systematically discovering symbolic commutation relations, gate decompositions and gate identities. The algorithm is completely general and works for arbitrary sets of gates and numbers of qubits. We have benchmarked the algorithm by finding known basic gate identities and will report on our ongoing efforts to find additional non-trivial gate identities. We are working to build a library of symbolic gate identities that can be used subsequently for circuit manipulation and optimization. Finally, we will describe our initial work to apply the gate identities to the problem of circuit optimization.

29. Adiabatic Quantum Computing with Neutral Atoms

Aaron Hankin, Sandia National Laboratories

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. We are developing, both theoretically and experimentally, a neutral atom qubit approach to adiabatic quantum computing (AQC). It has been shown in that neutral atoms trapped in optical far off-resonance traps (FORTs) can be used for two-qubit gates using interactions mediated by electric-dipole coupling of a coherently excited Rydberg state. A similar neutral atom system is attractive for this work due to the long-term coherence of the qubit ground states, the potential of multi-dimensional arrays of qubits in FORT traps and the potential for strong, tunable interactions via Rydberg-dressed atoms. If these arrays can be designed to encode a desirable computation into the system Hamiltonian one could use these tunable interactions along with single-qubit rotations to perform an AQC. Taking full advantage of Sandia's microfabricated diffractive optical elements (DOEs), we plan to implement such an array of traps and use Rydberg-dressed atoms to provide a controlled atom-atom interaction in atomic cesium. We forecast that these DOEs can provide the functions of trapping, single-qubit control and state readout resulting in an important engineering stride for quantum computation with neutral atoms. We will develop this experimental capability to generate a two-qubit adiabatic evolution aimed specifically toward demonstrating the two-qubit quadratic unconstrained binary optimization (QUBO) routine. We are studying the two-qubit QUBO problem to test the immunity of AQC to noise processes in the control interactions as well as dissipation mechanisms associated with the trapping. We are developing our theoretical and experimental capabilities through key collaborations with the University of Wisconsin and the University of New Mexico.

30. Microfabricated surface ion trap technology development for localized hyperfine qubit control

Clark Highstrete, Sandia National Laboratories

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. C. Highstrete, S. Scott, C.D. Nordquist, J.E. Stevens, C.P. Tigges, M.G. Blain - Sandia National Laboratories, Albuquerque, NM. Microwave control of hyperfine ion qubits is a promising technology for quantum information processing. However, for free space microwaves, small field gradients on the atomic scale produce negligible coupling to motional modes used for qubit interaction. Additionally, the long wavelength precludes adequate focusing, causing all qubits to be simultaneously addressed. Recently, two-qubit gates were successfully demonstrated with a surface ion trap using sub-wavelength on-chip electrodes to provide the necessary field gradients.[1] At Sandia, we are working to integrate sub-wavelength microwave electrodes into microfabricated surface ion traps using our four-level-metal technology with the further goal of localizing the microwave fields to specific interaction regions. We will present our nascent efforts toward developing this microfabricated surface ion trap technology for localized hyperfine qubit control. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [1] C. Ospelkaus et al., Nature 476, 181 (2011)

31. Finite geometry codes in quantum key expansion

Kung-Chuan Hsu, University of Southern California

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. Quantum key distribution (QKD) generates a common secret key to be securely shared between two distant parties. With the aid of an entanglement-assisted quantum error correcting code (EAQECC), QKD can be made to expand a common key rather than to generate one, in which the process known as quantum key expansion (QKE). Based on good EAQECCs, the performance of QKE is judged by the key rate, which is the rate of key expansion. In our work, we examined closely the families of codes constructed from finite geometry (FG), especially those with low density parity check (LDPC) matrices, and their use in QKE. We simulated one set of FG LDPC codes, where we modeled the noise as a depolarizing channel, and we found that these codes gave good performance only when the noise rate was low. For the set of codes we simulated, only some codes with small block size gave good performance at higher error rates (realistic for QKE), and those had low maximum key rate. Furthermore, it is possible for a classical FG LDPC codes to produce an entanglement-assisted code with negative net rate, further restricting the code selection. However, several other constructions of FG LDPC codes are known, and we continue to work on those constructions to find better codes for QKE, that work better at higher error rates.

32. Conditions imposing physical ancillary states in Stinespring dilations

Zhang Jiang, University of New Mexico

(Session 11c : Saturday from 4:15pm-4:45pm)

Abstract. While unitary transformations are used to describe state evolutions in closed quantum systems, the formalism of quantum operations is the more general approach for open systems. A valid quantum operation has to be completely positive, i.e., the output state for any physical input state, even those entangled with a third party, should also be physical. Often a quantum operation can be described by a Kraus representation. An alternative representation is by a measurement model or ancilla model, which is also called a Stinespring dilation by mathematicians. In an ancilla model, a quantum operation is realized by tracing out the ancilla after a joint unitary is applied on the primary system and the ancilla. Here we answer the following question: given an ancilla model with a particular joint unitary, what are the conditions on the joint unitary so that the ancilla state must be physical, i.e., a density operator, in order that the measurement model gives rise to a valid quantum operation.

33. Adiabatic Quantum Computation via the Rydberg Blockade

Tyler Keating, Univeristy of New Mexico

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. We study an architecture for implementing adiabatic quantum computation with trapped neutral atoms. Ground state atoms are dressed by laser fields in a manner conditional on the Rydberg blockade mechanism, thereby providing the requisite entangling interactions. As a benchmark we study the performance of a Quadratic Unconstrained Binary Optimization (QUBO) problem whose solution is found in the ground state spin configuration of an Ising-like model. Adiabatic evolution of an ensemble of atoms is achieved by ramping down a fictitious magnetic field along the x-direction while ramping up local magnetic fields along the z-direction together with the Rydberg-dressed qubit-qubit coupling. We model a realistic architecture, including details of the atomic implementation, with qubits encoded into the clock states of 133Cs, effective B-fields implemented through microwaves and light shifts, and atom-atom coupling achieved by excitation to the 100P3/2 Rydberg level. Including the fundamental effects of photon scattering we find the fidelity of two-qubit implementation to be on the order of 0.98, with higher fidelities possible with improved laser sources. The system scales favorably, as seen in our models of three and four qubits.

34. Coherent Control of Si-based Qubits

Thaddeus Ladd, HRL Laboratories, LLC

(Session 11a : Saturday from 4:45pm-5:15pm)

Abstract. Electrically defined silicon-based qubits are expected to show improved quantum memory characteristics in comparison to GaAs-based devices due to reduced hyperfine interactions with nuclear spins. Silicon-based qubit devices have proved more challenging to build than their GaAs-based counterparts, but recently several groups have reported substantial progress in single-qubit initialization, measurement, and coherent operation. I will present the recent observation of coherent oscillations in a spin singlet-triplet device built in a Si/SiGe heterostructure, and a measurement confirming that the dephasing time T2* is nearly two orders of magnitude longer than in comparable GaAs devices due to reduced hyperfine effects. Although complete SU(2) control is not yet demonstrated, fully controllable qubits may be enabled using exchange-only operations in Decoherence Free Subsystems (DFS). I will discuss some new control optimizations of the DFS system. Sponsored by the United States Department of Defense. Approved for public release, distribution unlimited.

35. Optimal EAQEC Codes of Small Length

Ching-Yi Lai, University of Southern California

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. The dual of an entanglement-assisted quantum error-correcting (EAQEC) code is defined from the orthogonal group of a simplified stabilizer group. This duality leads to the MacWilliams identity for EAQEC codes by applying the Poisson summation formula from the theory of orthogonal groups, and the linear programming bounds for EAQEC codes follow in a natural way. We establish a table of upper and lower bounds on the minimum distance of any maximal-entanglement EAQEC code with length up to 15 channel qubits. Maximal-entanglement EAQEC codes can be viewed as a class of classical additive quaternary codes whose generators are pairs of symplectic partners. We improve the upper and lower bounds by discussing the existence of certain EAQEC codes.

36. Learning in a variational class of states : efficient tomography method for Matrix Product and Multi Scale Entangled states

Olivier Landon-Cardinal, Université de Sherbrooke

(Session 11a : Saturday from 5:45pm-6:15pm)

Abstract. Quantum state tomography is essential to benchmark quantum devices but standard techniques fundamentally require a number of experiments and a post-processing effort that scales exponentially with the number of particles. However, by taking advantage of efficient representations of quantum states, such as matrix product states (MPS) or multi-scale entanglement renormalisation ansatz (MERA), we can do exponentially better. Indeed, since those variational classes of states are described by a few parameters, identifying the state amounts to learning those parameters. We describe a method for reconstructing a wide range of states from a small number of efficiently-implementable measurements and fast post-processing, namely all states well-approximated by MPS or MERA states. These variational classes are known to faithfully approximate ground states of local Hamiltonians in 1 dimension. Examples of interest include GHZ, W and cluster states. Our method prevents the build-up of errors from both numerical and experimental imperfections and contains a built-in certification procedure. These ideas extend to learning continuous-time quantum dynamics that are described by local Hamiltonians or Lindbladians. This covers work presented in da Silva, Landon-Cardinal and Poulin, PRL 107, 210404 (2011). It complements the Monte Carlo certification scheme presented in the same paper and independently derived by Flammia and Liu in PRL 106, 230501 (2011). Learning complements the work on certification and is of independent interest.

37. Trapped Atoms with an Evanescent Field for Hybrid Quantum Systems

Jongmin Lee, Joint Quantum Institute, University of Maryland

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. We explore uses of atoms trapped in evanescent optical fields for hybrid quantum information. A first system consists of an ensemble of Rb atoms trapped in the evanescent field of a nanofiber [1] coupled to the magnetic field in a lumped-element superconducting resonator [2] operating near the Rb ground state hyperfine frequency (6.8 GHz) as a step towards coupling atoms to a SQUID qubit. A second avenue explores uses of silicon waveguides instead of nanofibers and optical-to-microwave interfaces [3-5] for coherent communication. We will report on the parameter design and possible system performance as well as our experimental. Work supported by the NSF through the PFC at JQI and the ARO Atomtronics MURI. [1] E. Vetsch, D. Reitz, G. Sagué, R. Schmidt, S. T. Dawkins, and A. Rauschenbeutel, Phys. Rev. Lett. 104, 203603 (2010). [2] Z. Kim, C. P. Vlahacos, J. E. Hoffman, J. A. Grover, K. D. Voigt, B. K. Cooper, C. J. Ballard, B. S. Palmer, M. Hafezi, J. M. Taylor, J. R. Anderson, A. J. Dragt, C. J. Lobb, L. A. Orozco, S. L. Rolston, and F. C. Wellstood, AIP Advances 1, 042107 (2011) [3] M. Hafezi, Z. Kim, S. L. Rolston, L. A. Orozco, B. L. Lev., J. M. Taylor, arXiv:1110.3537v1 [quant-ph] (2011). [4] Mankei Tsang, Phys. Rev. A 84, 043845 (2011). [5] C. A. Regal and K. W. Lehnert, J. Phys.: Conf. Ser. 264 012025 (2011).

38. Sub-wavelength Resonance Imaging and Robust Addressing of Atoms in an Optical Lattice

Jae Hoon Lee, University of Arizona

(Session 10 : Saturday from 3:15pm-3:45pm)

Abstract. We demonstrate a resonance imaging protocol for optical lattices that enables robust preparation and single qubit addressing of atoms with sub-wavelength resolution. Our setup consists of a 3D optical lattice, and a superimposed long-period (66 lattice sites) 1D “superlattice” that creates a position dependent shift of the transition frequency between two spin states in the ground manifold. We show that isolated planes of atoms can be prepared by flipping resonant spins with a microwave pulse and removing the remaining non-resonant spins. A second microwave pulses in a translated superlattice subsequently allow us to image these planes with a resolution better than 200 nm. We further show that composite pulse techniques can reduce the sensitivity of the addressing to small variations in the relative position and intensity of the lattices. This robustness is achieved by applying numerically optimized, phase modulated pulses that have a constant atomic response within the region of error. For example, we apply a composite microwave pulse that flips the spin with near unit fidelity for all atoms that are positioned within a target spatial region (e.g., one lattice site), while conserving the spin of the atoms outside of that region (e.g., neighboring lattice sites). Furthermore, with this technique, we show that we are able to implement independent unitaries (single qubit quantum gates) across several adjacent lattice sites with a single composite pulse. Finally, we perform randomized benchmarking, similar to that done by Olmschenk et al., to measure the error per randomized computational gate using composite pulses.

39. Quantum signatures of chaos in quantum tomography

Vaibhav Madhok, University of New Mexico

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. Authors: Vaibhav Madhok, Carlos Riofrio, Ivan H. Deutsch. Abstract: We study the connection between quantum chaos and information gain in the time series of a measurement record used for quantum tomography. The record is obtained as a sequence of expectation values of a Hermitian operator evolving under repeated application of the Floquet operator of the quantum kicked top on a large ensemble of identical systems. We find an increase in information gain and hence higher fidelities in the process when the Floquet maps employed increase in chaoticity. We make predictions for the information gain using random matrix theory in the fully chaotic regime and show a remarkable agreement between the two. Finally we discuss how this approach can be used in general as a benchmark for information gain in an experimental implementation based on nonlinear dynamics of atomic spins measured weakly by the Faraday rotation of a laser probe.

40. Role of quantum discord in quantum communication.

Vaibhav Madhok, University of New Mexico

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. We establish the role of quantum discord in quantum information theory. Firstly we show that discord is a measure of how coherently the mother protocol is performed in the presence of decoherence. Since the mother protocol is a unification of an important class of problems (those which are bipartite, unidirectional and memoryless), we show discord to be a measure of how coherently any of these protocols can be performed. We explicitly demostrate the role quantum discord plays in quantum state merging, noisy super-dense coding, entanglement distillation and noisy teleportation. We also describe a similar role for quantum discord in quantum computation and correlations erasure. Thus quantum discord can be regarded as the advantage of quantum coherence in quantum information theory as a whole.

41. The Photon Shell Game and the Quantum von Neumann Architecture with Superconducting Circuits

Matteo Mariantoni, Department of Physics and California NanoSystems Institute, University of California, Santa Barbara

(Session 13 : Sunday from 12:15pm-12:45pm)

Abstract. Superconducting quantum circuits have made significant advances over the past decade, allowing more complex and integrated circuits that perform with good fidelity. We have recently implemented a machine comprising seven quantum channels, with three superconducting resonators, two phase qubits, and two zeroing registers. I will explain the design and operation of this machine, first showing how a single microwave photon |1> can be prepared in one resonator and coherently transferred between the three resonators. I will also show how more exotic states such as double photon states |2> and superposition states |0>+|1> can be shuffled among the resonators as well [1]. I will then demonstrate how this machine can be used as the quantum-mechanical analog of the von Neumann computer architecture, which for a classical computer comprises a central processing unit and a memory holding both instructions and data. The quantum version comprises a quantum central processing unit (quCPU) that exchanges data with a quantum random-access memory (quRAM) integrated on one chip, with instructions stored on a classical computer. I will also present a proof-of-concept demonstration of a code that involves all seven quantum elements: (1), Preparing an entangled state in the quCPU, (2), writing it to the quRAM, (3), preparing a second state in the quCPU, (4), zeroing it, and, (5), reading out the first state stored in the quRAM [2]. Finally, I will demonstrate that the quantum von Neumann machine provides one unit cell of a two-dimensional qubit-resonator array that can be used for surface code quantum computing. This will allow the realization of a scalable, fault-tolerant quantum processor with the most forgiving error rates to date. [1] M. Mariantoni et al., Nature Physics 7, 287-293 (2011); [2] M. Mariantoni et al., Science 334, 61-65 (2011). Matteo Mariantoni acknowledges support from an Elings Postdoctoral Fellowship. This work was supported by IARPA under ARO award W911NF-08-1-0336 and W911NF-09-1-0375.

42. Information-theoretic approach to the study of symmetric dynamics

Iman Marvian, Perimeter Institute, Institute for quantum computing

(Session 11b : Saturday from 4:15pm-4:45pm)

Abstract. Information theory provides a novel approach to the study of the consequences of symmetric dynamics which goes far beyond the traditional conservation laws that are derived from Noether's theorem. For one, these conservation laws are not applicable to dissipative and open systems. Moreover, even in the case of closed system dynamics, the conservation laws do not capture all the consequences of symmetry if the state of the system is not pure, as we will show. Using the information theoretic approach to this problem, we introduce new quantities called asymmetry monotones, such that if the system is closed they are constant of the motion and otherwise, if the system is open, they are always non-increasing. We also explain how different results in quantum information theory can have non-trivial consequences about the symmetric dynamics of quantum systems.

43. Can the Heisenberg limit be surpassed?

Ruynet Matos Filho, Universidade Federal do Rio de Janeiro

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. The error in the estimation of parameters characterizing dynamical process decreases with the number N of resources employed in the measurement. Quantum strategies may improve the precision for noiseless processes, by an extra factor that depends on the inverse of the square root of N, leading to the so-called Heisenberg limit. Recently, questions have been raised on whether this limit could be surpassed. Here a general framework to tackle this problem is presented. It involves the consideration of corrections to the quantum Cramér-Rao bound that arise when the number of repetitions of the measurement is finite, and it leads to analytical expressions of lower bounds for this number so that these corrections become negligible. These results are used to challenge recent claims that the Heisenberg limit can be beaten.

44. Neutral atom quantum computing with the dipole traps formed behind a circular aperture

Danielle May, California Polytechnic State University, San Luis Obispo

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. The field of neutral atom quantum computing has made encouraging progress towards creating a quantum computer. The neutral atom quantum computing community has experimentally demonstrated the initialization, the readout, and a universal set of quantum gates for neutral atoms trapped in optical dipole traps. Neutral atoms have long decoherence times due to their weak coupling with the environment in their ground state. One remaining problem for neutral atom quantum computing is to create an array of trapped atoms that can be individually addressed for initialization, readout, single, and two qubit gates. Our research team has created a cloud of cold 87Rb atoms in a Magnet-Optical Trap (MOT). The next step is to trap the cold atoms in the dipole traps formed by the bright or dark regions produced in the diffraction pattern formed immediately behind a circular aperture. The atoms can be trapped in the intensity minima for blue laser detuning, or the intensity maxima for red laser detuning. The position of the trapped atoms can be manipulated by tilting the beam incident on the circular aperture. Exploiting the light polarization dependence of the magnetic substates of the atoms allows two trapped atoms to be brought together or apart to facilitate two qubit gates without significant losses due to tunneling or Raman scattering. This system can be scaled up to many dipole traps by creating a 2D array of circular apertures. The center-to-center distance between adjacent circular apertures determines the distance between the trapped atoms. This technique potentially has the ability to produce a scalable system of qubits while allowing each qubit to be individually addressed. We anticipate demonstrating these dipole traps by projecting the diffraction pattern with a lens onto the cloud of cold atoms in the MOT. Once this is accomplished we will measure the trap properties and compare them to our computational results. In this presentation we summarize our computational results for these traps and will report our experimental progress to date. This work was performed in collaboration with Travis Frazer, Sara Monahan, David Roberts, Jennifer Rushing, Jason Schray, Glen D. Gillen, and Katharina Gillen-Christandl (PI). We acknowledge helpful discussions with Thomas D. Gutierrez, Ivan H. Deutsch, and Marianna Safronova. This work was supported by the National Science Foundation Grant No. PHY-0855524.

45. Randomized Benchmarking of Multiple Qubits

Adam Meier, National Institute of Standards and Technology

(Session 9 : Saturday from 11:30am-12:00pm)

Abstract. Randomized benchmarking is a procedure that extracts a "typical" error probability for an experimental quantum computer. This number describes the failure rate of a typical operation in the middle of a long computation and is a worthwhile figure of merit for quantum control demonstrations. I will present a practical, systematic approach to randomized benchmarking of multiple qubits using a recent two-qubit ion trap experiment at NIST as an example. I will also discuss the ways the basic procedure has been extended to reveal information about individual gates.

46. Design and Construction of a Nanofiber-based Quantum Interface

Pascal Mickelson, University of Arizona

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. We describe the design and construction of an experiment that will use atoms trapped in the evanescent-wave field of a tapered optical fiber (nanofiber) as an optical quantum bus to control the atoms' collective spin. When probe laser light interacts with a trapped atomic sample with high optical depth, the probe light undergoes Faraday rotation proportional to the atomic magnetization. If the atom-light coupling is strong enough, polarimetry of the probe light will provide a measurement of the magnetization with resolution better than the spin projection noise, at which point measurement back-action can be used for quantum control of the spin. When atoms are trapped in the evanescent mode of a nanofiber, probe light traveling through the nanofiber is particularly well mode-matched to the atom sample, and high optical depth on the order of 100 is expected. Here, we report experimental progress towards producing cold atom samples and loading them into these nanofiber traps.

47. Quantum Technologies for Light-Matter Interaction

Morgan Mitchell, ICFO - Institute of Photonic Sciences

(Session 12 : Sunday from 9:15am-9:45am)

Abstract. We describe experiments with highly non-classical states (heralded single photons and NooN states) in interaction with atomic ensembles. Our approach uses ultra-bright cavity-enhanced down-conversion and ultra-narrowband ``interaction-free measurement'' filters. With these we demonstrate heralded single photons that are at least 94 % atom-resonant, with multi-photon contamination below 4%. Also 90% fidelity NooN states, and sensitivity beyond the standard quantum limit in a near-resonant Faraday rotation magnetometer. The potential for highly multi-partite, atom-resonant entanglement using these techniques will also be discussed.

48. Progress towards a polarization spectroscopy experiment for quantum control of collective spin

Enrique Montano, University of Arizona

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. We report preliminary results from an experiment that will implement quantum control of the collective spin of an atomic ensemble. In our setup, a weak probe laser interacts with a cold, trapped atomic sample of cesium atoms with high optical depth, leading to Faraday rotation of the probe light proportional to the atomic magnetization. If the atom-light coupling is strong enough, polarimetry of the probe light will provide a measurement of the magnetization with resolution better than the spin projection noise, at which point measurement back-action will become significant enough to be used for quantum control of the spin. Thus far, we have loaded cesium atoms into a ~50 uK deep optical dipole trap, and we observe Faraday rotation of the probe light as it passes through this cloud of atoms. Work is ongoing to increase the optical depth of the atom sample and to optimize the atom-light coupling by mode-matching the probe beam to the atom sample.

49. Surface studies of microfabricated ion traps

Oliver Neitzke, University of California at Berkeley

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. Miniaturization of ion traps may be a suitable approach to scalable quantum information processing if the excessive noise of ion traps is understood and controlled. In recent years, with the increased use of micro fabricated surface traps, the excessive heating of trapped ions has come to the attention and ideas about its possible sources have been proposed. Electric field noise created by surface effects is thought to be one possible cause. Our work will relate the trap electric field noise to its surface properties. With a newly developed design of a ion trap vacuum chamber with integrated surface science tools, we can analytically characterize and modify our trap composition. The apparatus is in the final construction stage. It consists of an annealing lamp, an ion gun, and a UV lamp to alter the structure and composition of our trap surfaces. An Auger spectrometer with LEED capability can analyze these variations of the trap surface. In addition, we have developed micro fabrication methods allowing us to explore a variety of novel trap materials, such as refractory metals and surfaces passivated with graphene.

50. Toward Ion-Photon Entanglement

Thomas Noel, University of Washington

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. We present work toward ion-photon entanglement in a trapped barium ion system. The work is being done in a newly fabricated linear Paul trap, which has an integrated spin-flip electrode for coherent control of the ion's electronic ground state. We excite ¹³⁸Ba⁺ on the 6S_1/2-6P_1/2 dipole transition with frequency doubled light from a Ti:Sapph laser tuned to 986 nm. The polarization state of the subsequent spontaneously emitted photon is entangled with the resulting ionic ground state. This work is an initial step toward long-distance remote entanglement of ions. Barium is a good candidate for such work due the relatively long wavelengths of the transitions involved.

51. Enhanced Spin Squeezing Through Quantum Control of Qudits

Leigh Norris, University of New Mexico

(Session 12 : Sunday from 9:45am-10:15am)

Abstract. Spin squeezed states have applications in metrology and quantum information processing. While there has been significant progress in producing spin squeezed states and understanding their properties, most spin squeezing research to date has focused on ensembles of qubit spins. We explore squeezed state production in an ensemble of spin f>1/2 alkali atoms (qudits). Collective interactions are achieved through coherent quantum feedback of a laser probe, interacting with the ensemble through the Faraday interaction. This process can be enhanced through further control of the atomic qudits. We control the internal atomic state both before and after the collective interaction. Initial preparation increases the collective squeezing parameter through enhancement of resolvable quantum fluctuations. Qudit control can then be used to map entanglement created by the collective interaction to different pseudo-spin subspaces where they are metrologically useful, e.g., the clock transition or the stretched state for magnetometry. In the latter case, additional internal control can be used to squeeze the individual atoms, further enhancing the total squeezing in a multiplicative manner. The actual squeezing will depend on a balance between the enhanced coupling and decoherence. These considerations highlight the unique capabilities of our platform: we are able to transfer coherences and correlations between subspaces and integrate control tools to explore a wider variety of nonclassical states, with ultimate application in sensors or other quantum information processors.

52. Integrated quantum photonics

Jeremy OBrien, University of Bristol

(Session 0 : from 1:45pm-2:30pm)

Abstract. Integrated quantum photonics K Aungskunsiri, D Bonneau, J Carolan, E Engin, D Fry, J Hadden, P Kalasuwan, J Kennard, S Knauer, T Lawson, L Marseglia, E Martin-Lopez, J Meinecke, G Mendoza, A Peruzzo, K Poulios, N Russell, A Santamato, P Shadbolt, J Silverstone, A Stanley-Clark, M Halder, J Harrison, D Ho, P Jiang, A Laing, M Lobino, J Matthews, B Patton, A Politi, M Rodas Verde, P Zhang, X-Q Zhou, M Cryan, J Rarity, M Thompson, S Yu, J O’Brien & non-Bristol collaborators Centre for Quantum Photonics, H.H. Wills Physics Laboratory & Department of Electrical and Electronic Engineering, University of Bristol www.phy.bris.ac.uk/groups/cqp Quantum information science aims to harness uniquely quantum mechanical properties to enhance measurement and information technologies, and to explore fundamental aspects of quantum physics. Of the various approaches to quantum computing [1], photons are particularly appealing for their low-noise properties and ease of manipulation at the single qubit level [2,3]. Encoding quantum information in photons is also an appealing approach to quantum communication, metrology (eg. [4]), measurement (eg. [5]) and other quantum technologies [6]. However, the implementation of optical quantum circuits with bulk optics has reached practical limits. We have developed an integrated waveguide approach to photonic quantum circuits for high performance, miniaturization and scalability [7]. Here we report high-fidelity silica-on-silicon integrated optical realizations of key quantum photonic circuits, including two-photon quantum interference and a controlled-NOT logic gate [8]. We have demonstrated controlled manipulation of up to four photons on-chip, including high-fidelity single qubit operations, using a lithographically patterned resistive phase shifter [9]. We have used this architecture to implement a small-scale compiled version of Shor’s quantum factoring algorithm [10], demonstrated heralded generation of tunable four photon entangled states from a six photon input [11], a reconfigurable two-qubit circuit [12], and combined waveguide photonic circuits with superconducting single photon detectors [13]. We describe complex quantum interference behavior in multi-mode interference devices with up to eight inputs and outputs [14], and quantum walks of correlated particles in arrays of coupled waveguides [15]. Finally, we give an overview of our recent work on fundamental aspects of quantum measurement [16,17], diamond [18,19] and nonlinear [20,21] photon sources, fast manipulation of single photons [22]. [1] T. D. Ladd, F. Jelezko, R. Laflamme, Y. Nakamura, C. Monroe, and J. L. OBrien, Nature 464, 45 (2010). [2] J. L. O’Brien, Science 318, 1567 (2007). [3] R. Okamoto, J. L. O’Brien, H. F. Hofmann and S. Takeuchi Proc. Natl. Acad. Sci. 108, 10067 (2011) [4] T. Nagata, R. Okamoto, J. L. O’Brien, K. Sasaki, and S. Takeuchi, Science 316, 726 (2007). [5] R. Okamoto, J. L. O’Brien, H. F. Hofmann, T. Nagata, K. Sasaki, S. Takeuchi, Science 323, 483 (2009). [6] J. L. O’Brien, A. Furusawa, and J.Vuckovic, Nature Photon. 3, 687 (2009). [7] A. Politi, M. J. Cryan, J. G. Rarity, S. Yu, and J. L. O’Brien, Science 320, 646 (2008). [8] A. Laing, A. Peruzzo, A. Politi, M. R. Verde, M. Halder, T. C. Ralph, M. G. Thompson, and J. L. O’Brien, Appl. Phys. Lett. 97, 211109 (2010) [9] J. C. F. Matthews, A. Politi, A. Stefanov, and J. L. O’Brien, Nature Photon. 3, 346 (2009). [10] A. Politi, J. C. F. Matthews, and J. L. O’Brien, Science 325, 1221 (2009). [11] J. C. F. Matthews, A. Peruzzo, D. Bonneau, and J. L. O’Brien, Phys. Rev. Lett. 107, 163602 (2011) [12] P. J. Shadbolt, M. R. Verde, A. Peruzzo, A. Politi, A. Laing, M. Lobino, J. C. F. Matthews, J. L. O'Brien, Nature Photon. doi:10.1038/nphoton.2011.283 [13] C. M. Natarajan, A. Peruzzo, S. Miki, M. Sasaki, Z. Wang, B. Baek, S. Nam, R. H. Hadfield, and J. L. O’Brien, Appl. Phys. Lett. 96, 211101 (2010). [14] A. Peruzzo, A. Laing, A. Politi, T. Rudolph, and J. L. O’Brien, Nature Comm. 2, 224 (2011) [15] A. Peruzzo, M. Lobino, J. C. F. Matthews, N. Matsuda, A. Politi, K. Poulios, X.-Q. Zhou, Y. Lahini, N. Ismail, K. Worhoff, Y. Bromberg, Y. Silberberg, M. G. Thompson, and J. L. O’Brien, Science 329, 1500 (2010) [16] A. Laing, T. Rudolph, and J. L. O’Brien, Phys. Rev. Lett. 102, 160502 (2009). [17] X-Q Zhou, TC Ralph, P Kalasuwan, M Zhang, A Peruzzo, BP Lanyon, and JL O’Brien, Nature Comm. 2 413 2011 [18] J. P. Hadden, J. P. Harrison, A. C. Stanley-Clarke, L. Marseglia, Y.-L. D. Ho, B. R. Patton, J. L. OBrien, and J. G. Rarity, Appl. Phys. Lett. 97, 241901 (2010) [19] L. Marseglia, J. P. Hadden, A. C. Stanley-Clarke, J. P. Harrison, B. Patton, Y.-L. D. Ho, B. Naydenov, F. Jelezko, J. Meijer, P. R. Dolan, J. M. Smith, J. G. Rarity, J. L. O'Brien, Appl. Phys. Lett. 98, 133107 (2011) [20] C. Xiong, et al. Appl. Phys. Lett. 98, 051101 (2011) [21] M. Lobino, et al, Appl. Phys. Lett. 99, 081110 (2011) [22] D Bonneau, M Lobino, P Jiang, CM Natarajan, MG Tanner, RH Hadfield, SN Dorenbos, V Zwiller, MG Thompson, JL O'Brien Phys. Rev. Lett.; arXiv:1107.3476

53. Complete characterization of linear amplifiers including the quantum limits for nongaussian noise

Shashank Pandey, Center for Quantum Information and Control, University of New Mexico

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. We characterize the quantum limitations on the entire probability distribution of added noise in a phase-preserving linear amplifier. Previously the quantum limits on amplifiers have been given entirely in terms of second moments. As Josephson parametric amplifiers approach fundamental quantum limits on noise temperature, it becomes important to investigate the limits on higher moments of the amplifier noise. We prove that all phase-preserving linear amplifiers with arbitrary noise are formally equivalent to a parametric amplifier ,i.e. a two-mode squeeze operator with a physical state of an ancillary mode whose quantum noise determines the noise properties of the amplifier. We also discuss generalizations to the nondeterministic linear amplifiers proposed by Ralph and Lund.

54. Adiabatic Quantum Computing with Neutral Atoms

L. Paul Parazzoli, Sandia National Laboratories

(Session 10 : Saturday from 2:45pm-3:15pm)

Abstract. We are developing, both theoretically and experimentally, a neutral atom qubit approach to adiabatic quantum computing (AQC). It has been shown in that neutral atoms trapped in optical far off-resonance traps (FORTs) can be used for two-qubit gates using interactions mediated by electric-dipole coupling of a coherently excited Rydberg state. A similar neutral atom system is attractive for this work due to the long-term coherence of the qubit ground states, the potential of multi-dimensional arrays of qubits in FORT traps and the potential for strong, tunable interactions via Rydberg-dressed atoms. If these arrays can be designed to encode a desirable computation into the system Hamiltonian one could use these tunable interactions along with single-qubit rotations to perform an AQC. Taking full advantage of Sandia’s microfabricated diffractive optical elements (DOEs), we plan to implement such an array of traps and use Rydberg-dressed atoms to provide a controlled atom-atom interaction in atomic cesium. We forecast that these DOEs can provide the functions of trapping, single-qubit control and state readout resulting in an important engineering stride for quantum computation with neutral atoms. We will develop this experimental capability to generate a two-qubit adiabatic evolution aimed specifically toward demonstrating the twoqubit quadratic unconstrained binary optimization (QUBO) routine. We are studying the two-qubit QUBO problem to test the immunity of AQC to noise processes in the control interactions as well as dissipation mechanisms associated with the trapping. We are developing our theoretical and experimental capabilities through key collaborations with The University of Wisconsin, NIST and The University of New Mexico.

55. Magnetic field gradient effects on a trapped ion frequency standard

Heather Partner, Sandia National Labs / University of New Mexico

(Session 3 : Friday from 9:30am-10:00am)

Abstract. We are developing a low-power, high-stability miniature atomic frequency standard based on trapped 171Yb+ ions.  The ions are buffer-gas cooled and held in a linear quadrupole trap that is integrated into a sealed 10 cm^3, getter-pumped vacuum package, and interrogated on the 12.6 GHz hyperfine transition. We hope to achieve a long-term fractional frequency stability of 10^-14 with this miniature clock. To achieve this exceptional long-term stability, the sensitivity of the clock frequency to magnetic fields must be minimized. Because the “clock” transition frequency of 171Yb+ depends quadratically on the magnetic field, it is advantageous to operate at a bias field of ~100 mG or below. However, in small-sized ion traps, magnetic field gradients can prevent operation at low fields because of broadening of the clock resonance. This broadening occurs due to the secular motion of the ions in the trap, which in interaction with a spatially varying magnetic field can induce transitions among the Zeeman sublevels of the upper ground state when the Zeeman frequency is close to the secular frequency of the trap, creating a dephasing effect.  Understanding this mechanism for a particular trap geometry as well as taking steps to eliminate background gradients allows us to operate the clock with a much lower bias field in a region below this secular frequency resonance, where we can both minimize broadening and reduce our clock’s sensitivity to magnetic field fluctuations while reducing the overall power required to run the clock.  We have studied these effects in several traps and will discuss these results as well as clock performance. Peter Schwindt, Yuan-Yu Jau, Michael Descour, Lu Fang, Adrian Casias, Ken Wojciechowski, Roy Olsson, Darwin Serkland, Ron Manginell, Matthew Moorman, Robert Boye, John Prestage, Nan Yu, Robert Lutwak, Sheng Chang

56. Open-loop methods for protection of encoded information

Gerardo Paz-Silva, University of Southern California

(Session 11c : Saturday from 5:45pm-6:15pm)

Abstract. We study the interplay of two well-known open-loop decoherence suppression methods, the Quantum Zeno effect and Dynamical decoupling, with quantum error correction codes. For the first part, the quantum Zeno effect case, we analyze the decoherence suppression induced only by the weak (non-selective) syndrome measurements in every error correction round, i.e. multiple rounds of error correction without the recovery step. We show that there is indeed a suppression effect despite the absence of recovery operations. For the second part, dynamical decoupling, we discuss the use of elements of the code as pulses and show that they provide an advantage over brute force multiqubit dynamical decoupling: they not only generate shorter sequences but impose no additional locality constraints on the noise besides the ones demanded by fault-tolerant models.

57. Manipulation of small atom clouds in a microscopic dipole trap

Joseph Pellegirno, Laboratoire Charles Fabry

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. Ronan Bourgain, Joseph Pellegrino, Yvan R.P. Sortais and Antoine Browaeys Laboratoire Charles Fabry, Groupe Optique Quantique - Atomes, Institut d’Optique, Avenue de la Vauve 91127 Palaiseau Cedex, France Recent years have seen a growing interest in the study of small, but dense cold atomic ensembles. Here we present our progress on the manipulation of cold atomic clouds in a regime where they contain only a few tens of atoms. In our case we use 87Rb atoms, trapped in a microscopic optical dipole trap, to study this mesoscopic regime. We use a single atom to measure the resolution of our imaging system [1]. This provides a calibration of our detection scheme which is useful to understand the regime where a few atoms are trapped [2]. In particular it was applied to the measurement of dipole trap losses due to the presence of near resonant light [3]. These results indicate that the loading, in presence of two body losses leads to a subpoissonian atomic number distribution.We also perform a lossless state preparation, and detection on single atom [4]. All these tools should be useful for the realisation of a BEC with a few atoms only. [1] A. Fuhrmanek, A. Lance, C. Tuchendler, P. Grangier, Y.P.R. Sortais and A. Browaeys, "Imaging a single atom in a time-of-flight measurement.”, NJP 12, 053028 (2010). [2] A. Fuhrmanek, Y. R. P. Sortais, P. Grangier, and A. Browaeys, "Measurement of the atom number distribution in an optical tweezer using single-photon counting”, PRA 82, 023623 (2010). [3] A. Fuhrmanek, R. Bourgain, Y.R.P. Sortais, A. Browaeys, "Large light-assisted collision rates between cold atoms in amicroscopic dipole trap", ArXiv 1107.5781 (2011). [4] A. Fuhrmanek, R. Bourgain, Y. R. P. Sortais, and A. Browaeys, "Free-Space Lossless State Detection of a Single Trapped Atom", PRL 106, 133003 (2011).

58. Backaction of Microwave Photon Detection by a Strongly Coupled Josephson Junction

Emiy Pritchett, Saarland University

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. We analyze the functionality of on-chip Josephson junctions as single microwave photon detectors, as has been demonstrated recently in Chen, et al., arXiv:1011.4329. The Josephson junction device, which we refer to as a Josephson Photomultiplier (JPM), acts as a nearly perfect binary detectors of microwave photons by undergoing an observable switching event when there are one or more photons in an incident cavity. We analyze the backaction of this switching event on the state of incident light, including the energy dissipation and dephasing affecting an imperfect JPM. This analysis improves the efficiency and fidelity with which a JPM reconstructs the state of light in an incident transmission line `cavity', which are commonly used to store and transfer quantum states in implementations of circuit-QED.

59. Quantum Nonlocal Boxes Exhibit Stronger Distillability

Jibran Rashid, Institute for Quantum Information Science at the University of Calgary

(Session 2 : Thursday from 8:15pm-8:45pm)

Abstract.

Introduction

We approach stronger that quantum correlations with a new perspective. The nonlocal box model (NLB) allows two spatially separated parties, Alice and Bob, to exhibit stronger than quantum correlations. Rather than considering a hypothetical box resource, we allow the spatially separated parties Alice and Bob, access to a trusted third party Charlie. Charlie is allowed to communicate with Alice and Bob without allowing communication between Alice and Bob. A natural generalization is to consider the case when Alice and Bob utilize quantum states for communicating with Charlie and he communicates back using quantum states as well.

We model Charlie as a quantum nonlocal box, abbreviated qNLB, which takes as input a joint quantum state and outputs a joint quantum state. A priori, such a model may not obey our non-signalling requirement since any unitary U_AB not of the form U_A⊗U_B allows for signalling. It thus may appear that a quantum generalization of the NLB model would always allow for signalling, but this only holds true if we restrict the maps to be unitary. Quantum nonlocal boxes that satisfy the non-signalling requirement and allow for quantum states as output are possible when we drop the requirement of the box being unitary. Such boxes have previously been studied under the notion of causal maps, completely positive trace-preserving maps, and non-signalling operations. Here we initiate a systematic study of such boxes in terms of nonlocality.

Nonlocality distillation occurs if it is possible for Alice and Bob to concentrate the nonlocality in n copies of an imperfect NLB/qNLB to form a stronger NLB/qNLB. Given the apparent limited distillability of NLBs, we propose a new non-adaptive protocol for nonlocality distillation of qNLBs. As our main result, we show that qNLBs exhibit strictly stronger nonlocality distillation than NLBs, for non-adaptive distillation protocols. We prove our main result by setting up a semi-definite programming framework for analyzing non-adaptive protocols for qNLB distillation. We then use this framework to define and give a protocol for qNLB distillation and show that it asymptotically distills the class of correlated quantum nonlocal boxes to the value 3.098, whereas in contrast, the optimal non-adaptive parity protocol for classical correlated nonlocal boxes asymptotically distills to the value 3.0 (Figure 1). The protocol is also proven to be an optimal non-adaptive protocol for 1, 2 and 3 copies of qNLBs by constructing a matching dual solution for the semi-definite program.

Figure 1: (a) Distillation achievable for correlated NLBs by our protocol for qNLBs and the parity protocol for NLBs. (a) Value attained by a single copy of qNLB (solid line) and NLB (dotted line). (b) Distilled value attained for n copies of qNLBs (solid lines) and NLBs (dotted lines), for n = 2 and n = 100, respectively

Motivation

A major component of the research on nonlocality can be linked to identifying a set of restrictions that produce physical theories of varying strength in terms of their correlations. From quantum strategies that violate Bell inequalities to the no-signalling principle, information causality, local quantum measurements and macroscopic locality, one of the goals is to obtain a useful understanding of the conditions that imply quantum correlations. These conditions serve as fundamental physical principles that guide the development of physical theories. One attempt to develop our understanding of the limits on nonlocality is via nonlocality distillation protocols.

The class of correlated NLBs are already known to be asymptotically distillable to a perfect NLB. This is only achieved by an adaptive protocol. In our current work we have shown that even if we restrict out attention to non-adaptive protocols, qNLBs offer improved distillation over NLBs. A generalization of our SDP approach that allows for adaptive protocols may reveal a similar improvement for adaptive protocols. This may imply distillability for correlations that are currently not known to be distillable and at the same time an increased understanding of correlations that violate principles such as information causality.

As a consequence of the work nonlocality distillation we propose a two-pronged approach for classifying correlations which are not known to satisfy the principle of macroscopic locality. We provide numerical evidence that correlations with non-trivial marginals which are not known to satisfy the macroscopic locality principle may be distillable even when corresponding correlations with trivial marginals are not. On the flip side, we argue that if reality does allow correlations that violate the principle, then nonlocality distillation could still be utilized to improve the possibility of detecting such a violation in the lab.

60. Trapping atoms around nanofibers

Sylvain Ravets, University of Maryland

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. Coupling atomic and superconducting qubits in a hybrid quantum platform is a promising option for quantum information. To build such a system, drastic experimental conditions have to be met: in our project, classical atom-light techniques used to control atoms must fit in the millikelvin temperature environment necessary to work with superconducting qubits. To limit the heating induced by the trapping light, we take advantage of nanofiber traps [1]: by superposing the repulsive and attractive potentials induced by the evanescent blue-detuned and red-detuned light fields propagating around a tapered nanofiber, it is possible to create a potential well to trap atoms. In this poster, we present recent results on long tapered fibers from our fiber puller. We can reliably produce tapered optical fibers with a waist up to 10 cm in length and down to 500 nm in diameter. By controlling the geometry of the tapered region, we can reach the adiabatic regime and minimize losses given our size constraints. Work supported by the NSF through the PFC at JQI and SR thanks the Fulbright Fellowship. [1] E. Vetsch, D. Reitz, G. Sagué, R. Schmidt, S. T. Dawkins, and A. Rauschenbeutel, Phys. Rev. Lett. 104, 203603 (2010).

61. Quantum Optical Pulse Shaping, Routing, and Frequency Translation by Four-Wave Mixing in Optical Fiber

Michael Raymer, University of Oregon

(Session 5 : Friday from 2:30pm-3:00pm)

Abstract. Our collaboration of U Oregon, Bell Labs, and UC San Diego is developing quantum frequency translation (background-free frequency conversion) of quantum states of light by using four-wave mixing in optical fiber. The involvement of two pump pulses at distinct frequencies leads to useful capabilities not present when using single-pump three-wave mixing. It allows pulse reshaping and routing of quantum optical wave packets, including single-photon states. It also allows translating between nearby frequency channels, opening the possibility of quantum-level wavelength-division multiplexing.

62. Continuous measurement quantum state tomography of atomic spins

Carlos Riofrio, University of New Mexico

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. C. Riofrío and I. H. Deutsch, Department of Physics and Astronomy, Center for Quantum Information and Control , University of New Mexico. A. Smith, B. E. Anderson, H. Sosa, and P. Jessen, College of Optical Sciences, Center for Quantum Information and Control, University of Arizona. Quantum state tomography is a fundamental tool in quantum information science and technology. It requires estimates of the expectation values of an "informationally complete" set of observables. This is, in general, a very time-consuming process that requires a large number of measurements to gather sufficient statistics. There are, however, systems in which the data acquisition can be done more efficiently. An ensemble of quantum systems can be prepared and controlled by external fields while being continuously and collectively probed, producing enough information in the time-evolving measurement record to estimate the initial state [1]. Such protocol has the advantage of being fast and robust. In this poster, we present a study of a continuous-measurement quantum-state tomography protocol and its application to controlling large spin ensembles. We perform reconstruction of quantum states prepared in the 16 dimensional ground-electronic hyperfine manifolds of an ensemble of 133Cs atoms, driven by microwaves and radio-frequency magnetic fields and probed via polarization spectroscopy [2]. We present theoretical and experimental results of its implementation and discuss two estimation methods: constrained maximum likelihood and compressed sensing. We show the exquisite level of control achieved in the lab and the excellent agreement between theory and experiment. Moreover, we are able to achieve fidelities >95% for low complexity quantum states, and >92% for arbitrary random states. [1] A. Silberfarb and I. H. Deutsch, "Quantum-state reconstruction via continuous measurement", Phys. Rev. Lett. 95, 030402 (2005). [2] C. A. Riofrío, P. S. Jessen, and I. H. Deutsch, "Quantum tomography of the full hyperfine manifold of atomic spins via continuous measurement on an ensemble", J. Phys. B: At. Mol. Opt. Phys. 44 (2011) 154007.

63. Decoherence Leads to Non-monotonicity in the "Quantumness" of Fock States

Peter Rose, Carleton College

(Session 11c : Saturday from 4:45pm-5:15pm)

Abstract. We consider the evolution of Fock states |n> of a harmonic oscillator coupled to a Markovian bath. The master equation in the number basis is an infinite number of coupled, first order differential equations which can be solved analytically at any temperature. Using the negative volume of the Wigner function as a metric of "quantumness", we show that in the absence of environmental coupling, quantumness increases with n, but the presence of any environmental interaction causes high-n states to lose their quantum features more rapidly leading to a time-dependent quantumness peak across the eigenstates. Our results are consistent with recent experiments.

64. Entanglement and Quantum Algorithms with Superconducting Circuits

Robert Schoelkopf, Yale University

(Session 8 : Saturday from 8:30am-9:15am)

Abstract. By using the unique properties of quantum physics, such as entanglement and superposition, quantum computers are predicted to be vastly more powerful than their classical counterparts for certain tasks. While some technologies, such as NMR and trapped ions, have succeeded in making and manipulating a handful of quantum bits (qubits), they look quite different from a conventional computer, and there are many obstacles to building large-scale processors. At Yale, we use superconducting circuits to make macroscopic, solid-state qubits which are controlled and measured entirely by a sequence of electronic pulses on wires. These devices have advanced to the point where we can generate and detect highly-entangled states, and perform universal quantum gates. I will describe recent experiments showing two and three qubit entanglement, the operation of Grover’s search algorithm, and the successful realization of simple quantum error correction.

65. Matrix Product States, Projected Entangled Pair States, and the entanglement spectrum of two-dimensional quantum systems

Norbert Schuch, California Institute of Technology

(Session 6 : Friday from 4:00pm-4:45pm)

Abstract. Matrix Product States (MPS) and Projected Entangled Pair States (PEPS) provide a description of correlated quantum many-body states from a local perspective. They faithfully approximate ground states of local Hamitonians which makes them powerful numerical tools, while at the same time their ability to explain the global behavior of quantum many-body systems from local properties makes them useful for analytical studies. In my talk, I will give an introduction to Matrix Product States and PEPS as analytical and numerical tools, and illustrate their usefulness by showing how PEPS can be used to establish a full bulk-boundary duality for two-dimensional quantum systems. In particular, PEPS provide an explicit construction relating the entanglement spectrum of a two-dimensional region to the spectrum of a one-dimensional model associated to its boundary, and thereby provide new tools for the analytical and numerical study of boundary models.

66. Entanglement quantification with finite data

Lucia Schwarz, Oregon Center for Optics, University of Oregon

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. In recent experiments, more and more qubits have been entangled in a GHZ state. We simulated a measurement on a noisy GHZ state with fixed measurement settings. Thanks to information criteria, we can simulate states with up to 52 qubits. The question is, how often do we have to repeat those measurements in order to get a reliable estimate of entanglement? And how does this depend on the number of qubits?

67. Towards building hybrid quantum systems out of wires, ions, and solid-state devices.

Sankaranarayanan Selvarajan, University of California, Berkeley

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. Oscillating trapped ions induce currents in nearby electrodes. These currents can be send either to electrical devices such as tank circuits or to another ion. Here we discuss progress towards both goals. We developed a high-quality tank circuit with a resonance frequency of approximately 1 MHz. Winding the coil from niobium wires, we find for the fundamental and higher order modes quality factors between 27,000 and 160,000 at temperatures around 5 K. For the fundamental mode (Q=27,000 at 1.1 MHz) we find a damping time of 3.5 ms. Connecting the resonator to trap electrodes and assuming an ion 50 μm above a surface trap, we expect an ion-resonator coupling of 2π*450 Hz, entering the strong coupling regime. Initial experiments will use multiple ions to increase the coupling strength. We also discuss progress in coupling ions through a macroscopic wire. With two ions trapped close to either end of a gold wire about 1mm in length, we estimate that the ions can coherently exchange a motional quantum in about 1ms, significantly faster than the estimated T1 times at cryogenic temperatures. By determining the capacitance between the wire and the trap electrodes, we have modified our electrostatic trap simulations to include the effect of the electrically floated wire on the trapping potentials. Both the ion-resonator coupling and the wire mediated coupling between two ions can be used to construct hybrid quantum systems out of ions and solid state devices.

68. Quantum control of spin correlations in an atomic ensemble

Robert Sewell, Institute of Photonic Sciences

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. We report on an an experimental project designed to measure and control spin correlations in an ensemble of laser cooled rubidium atoms. Through a combination of spin-squeezing via quantum-non-demolition (QND) measurement, and real-time incoherent feedback (optical pumping), we aim to generate a macroscopic singlet state in an ensemble of up to 1 millions atoms. The singlet state is characterised by a generalized spin-squeezing inequality, ξ=Σi(ΔFi)^2/Nf ≤ 0, where ξ<0 implies non-classical spin correlations and entanglement amongst the atoms. It has potential application in quantum memories for storing information in decoherence-free subspaces, and in quantum metrology for magnetic field gradiometry, and is an example of real-time engineering of quantum correlations in a macroscopic atomic ensemble. We conditionally squeeze the quantum noise in each angular momentum component below the standard quantum limit via projection-noise limited QND measurement. Real-time optical pumping feedback based on the measurement outcome restores the average angular momentum to zero. In this way, by working with an unpolarised atomic ensemble, we avoid back-action and are able to simultaneously squeeze all three angular momentum components. In the experimental apparatus, micro-second pulses of polarised light interact with an elongated atomic cloud and are detected by a shot-noise-limited polarimeter. The experiment achieves projection-noise limited sensitivity, as calibrated against a thermal spin state, and we have recently demonstrated spin squeezing of atoms in the F=1 hyperfine ground state. We have developed an FPGA-based detection and feedback device for real-time control of quantum correlations. Here we report preliminary results towards generating a macroscopic singlet state.

69. Integrated quantum photonics

Pete Shadbolt, University of Bristol

(Session 5 : Friday from 1:45pm-2:30pm)

Abstract.

70. Spectral Gap Amplification

Rolando Somma, Los Alamos National Laboratory

(Session 6 : Friday from 4:45pm-5:15pm)

Abstract. Several problems in science can be solved by preparing a specific eigenstate of some Hamiltonian H. The generic cost of quantum algorithms for these problems is determined by the inverse spectral gap of H for that eigenstate and the cost of evolving with H for some fixed time. The goal of spectral gap amplification is to construct a Hamiltonian H' with the same eigenstate as H but a bigger spectral gap, requiring that constant-time evolutions with H' and H are implemented with nearly the same cost. I will show that a quadratic spectral gap amplification is possible when H satisfies a frustration-free property and construct H' for these cases. This results in quantum speedups for optimization problems. It also yields improved constructions for adiabatic simulations of quantum circuits and for the preparation of projected entangled pair states (PEPS), which play an important role in quantum many-body physics. Defining a suitable black-box model, I will establish that the quadratic amplification is optimal for frustration-free Hamiltonians and that no spectral gap amplification is possible, in general, if the frustration-free property is removed. Interestingly, this results in some limits on the power of some classical methods that simulate quantum adiabatic evolutions.

71. Quantum Control and Quantum State Tomography in the Hyperfine Ground Manifold of Atomic Cesium

Hector Sosa-Martinez, University of Arizona

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. Aaron Smith, Brian E. Anderson, Hector Sosa Martinez, Poul Jessen Center for Quantum Information and Control (CQuIC), College of Optical Science and Department of Physics, University of Arizona Carlos Riofrio, Ivan H. Deutsch Center for Quantum Information and Control (CQuIC), Department of Physics and Astronomy, University of New Mexico. Quantum systems with Hilbert space dimension greater than two (qudits) provide an alternative to qubits as carriers of quantum information, and may prove advantageous for quantum information tasks if good laboratory tools for qudit manipulation and readout can be developed. We have successfully implemented a protocol for arbitrary quantum state-to-state mapping in the 16 dimensional hyperfine ground manifold of Cesium 133 atoms, using only static, radio frequency (rf) and microwave magnetic fields to drive the atomic evolution. This system is controllable given rf and microwave fields with constant amplitude and frequency, and piecewise constant phase modulation. Control waveforms (rf and microwave phases versus time) are found by numerical optimization, and can be designed to work well in the presence of errors in the driving and background magnetic fields. Experimentally, we achieve an average state mapping fidelity of 99% for a sample of randomly chosen target states. To perform quantum state tomography, we drive an ensemble of identically prepared atoms with phase modulated rf and microwave magnetic fields, and simultaneously probe them by coupling an atomic spin observable to the polarization of a near-resonant optical probe field. A measurement of the probe polarization then yields an informationally complete measurement record that can be inverted to obtain an estimate of the unknown quantum state. We have reconstructed the full density matrix for a set of randomly chosen test states, using computer algorithms based either on least squares fitting or compressed sensing. The latter approach reconstructs our test states with an average fidelity above 90%, limited primarily by errors in applied drive fields.

72. Formulating Quantum Theory as a Causally Neutral Theory of Bayesian Inference

Robert Spekkens, Perimeter Institute for Theoretical Physics

(Session 2 : Thursday from 7:00pm-7:45pm)

Abstract. Quantum theory can be thought of as a noncommutative generalization of Bayesian probability theory, but for the analogy to be convincing, it should be possible to describe inferences among quantum systems in a manner that is independent of the causal relationship between those systems. In particular, it should be possible to unify the treatment of two kinds of inference: (i) from beliefs about one system to beliefs about another, for instance, in the Einstein-Podolsky-Rosen or ``quantum steering” phenomenon, and (ii) from beliefs about a system at one time to beliefs about that same system at another time, for instance, in predictions or retrodictions about a system undergoing dynamical evolution or undergoing a measurement. I will present a formalism that achieves such a unification by making use of “conditional quantum states”, which are noncommutative generalizations of conditional probabilities. I argue for causal neutrality by drawing a comparison with a classical statistical theory with an epistemic restriction. (Joint work with Matthew Leifer)

73. Entangled State Synthesis for Superconducting Resonator Qudits

Frederick Strauch, Williams College

(Session 8 : Saturday from 9:15am-9:45am)

Abstract. I will present a theoretical analysis of methods to synthesize entangled states of two superconducting resonators, and their extension to general unitary operations on resonators as qudits. These methods use experimentally demonstrated interactions of resonators with artificial atoms, and offer efficient routes to generate nonclassical states and processes for high-dimensional quantum systems.

74. Trapping ⁴⁰Ca⁺ in a segmented annular (ring) trap

Boyan Tabakov, University of New Mexico, Sandia National Laboratories

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract.

Authors: B. Tabakov, F. Benito, M. G. Blain, M. Descour, L. Fang, K. M. Fortier, R. A. Haltli, C. Highstrete, T. L. Lindgren, D. L. Moehring, M. E. Smith, J. D. Sterk, D. L. Stick, C. P. Tigges

Abstract: We report on operating and characterizing a surface ion trap with segmented annular (ring) geometry under the Multi-Qubit Coherent Operations MUSIQC collaboration. We describe a surface trap technology that utilizes four metal level technology allowing for leadless electrodes. Other key advances implemented in the ring trap fabrication are precision backside loading holes and low profile wire bonds. Robust loading and shuttling of single ⁴⁰Ca⁺ has been demonstrated. Efforts towards loading multiple ions, micromotion compensation, and heating rate measurements are underway.

75. Entanglement-based perturbation theory for highly anisotropic Bose-Einstein condensates

Alexandre Tacla, University of New Mexico

(Session 9 : Saturday from 12:00pm-12:30pm)

Abstract. We investigate the emergence of three-dimensional behavior in a reduced-dimension Bose-Einstein condensate trapped by a highly anisotropic potential. We handle the problem analytically by performing a perturbative Schmidt decomposition of the condensate wave function between the tightly confined direction(s) and the loosely confined direction(s). The perturbation theory is valid when the nonlinear scattering energy is small compared to the transverse energy scales. Our approach provides a straightforward way, first, to derive corrections to the transverse and longitudinal wave functions of the reduced-dimension approximation and, second, to calculate the amount of entanglement that arises between the transverse and longitudinal spatial directions. Numerical integration of the three-dimensional Gross-Pitaevskii equation for different cigar-shaped potentials and experimentally accessible parameters reveals good agreement with our analytical model even for relatively high nonlinearities. In particular, we show that even for such stronger nonlinearities the entanglement remains remarkably small, which allows the condensate to be well described by a product wave function that corresponds to a single Schmidt term.

76. Towards understanding thermodynamics and energy transport in strings of trapped ions.

Ishan Talukdar, University of California, Berkeley

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. We report experiments on laser induced heating of ions confined in a linear Paul trap. Specifically, we investigate the mechanism of melting of a crystallized ion chain due to heating by light detuned blue from an atomic resonance. In these experiments, we observe the decay of ion fluorescence as we shine laser light on either the entire ion string or a small subset. From these measurements we hope to extract information on the thermodynamic properties of such Coulomb crystals. Understanding these properties, together with the ability to address individual ions will facilitate the study of excitation transfer dynamics along the chain.

77. Effect of the basis-dependent flaw on the security of quantum key distribution system

Kiyoshi Tamaki, NTT Basic Research Laboratories

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. In this presentation, we study the effect of the basis-dependent flaw on the security of quantum key distribution system, especially in the context of measurement device independent quantum key distribution (MDIQKD). We propose two schemes for the phase encoding scheme for MDIQKD, the first one employs a phase locking technique with the use of non-phase-randomized coherent pulses, and the second one uses conversion of standard BB84 phase encoding pulses into polarization modes. We prove the unconditional security of these schemes and we also simulate the key generation rate based on simple device models that accommodate imperfections. Our simulation results show the feasibility of these schemes with current technologies and highlights the importance of the state preparation with good fidelity between the density matrices in the two bases. Since the basis-dependent flaw is a problem not only for MDIQKD but also for standard QKD, our work highlights the importance of an accurate signal source in practical QKD systems.

78. Ensemble Cavity QED & Precision Metrology

James Thompson, JILA

(Session 12 : Sunday from 8:30am-9:15am)

Abstract. I will discuss quantum metrology experiments using large ensembles of cold, trapped atoms and cavity QED. The first portion of the talk will describe conditional spin squeezing of the clock transition of a million Rb atoms, achieved by utilizing the vacuum Rabi splitting as a collective QND meter. The second portion of the talk will describe a Raman laser that operates deep into the superradiant or bad-cavity regime. The system is demonstrated to operate with <1 intracavity photon and with an effective excited state decay linewidth <1 Hz. This model system demonstrates key physics for future active optical clocks that may achieve frequency linewidths approaching 1 mHz due to reduced sensitivity to thermal mirror noise.

79. Transient and Adiabatic Quantum State Transfer in Optomechanical Systems

Lin Tian, University of California, Merced

(Session 8 : Saturday from 9:45am-10:15am)

Abstract. Light-matter interaction in optomechanical systems can be explored for optical quantum information processing. Here, we present transient and adiabatic schemes for quantum state transfer between optical modes with distinct frequencies via the optomechanical forces. In the transient scheme, red-detuned laser pulses generate state-swappings between the optical and the mechanical modes to achieve the state transfer. In the adiabatic scheme, the cavity dark mode that is immune to the mechanical noise is explored to transfer quantum states. The transfer fidelity for gaussian states can be derived by solving the Langevin equation in the adiabatic limit.

80. Mutually unbiased bases for quantum states defined over p-adic numbers

Wim van Dam, University of California, Santa Barbara

(Session 4 : Friday from 11:15am-11:45am)

Abstract. We describe sets of mutually unbiased bases (MUBs) for quantum states defined over the p-adic numbers Q_p, i.e. the states that can be described as elements of the (rigged) Hilbert space L2(Q_p). We find that for every prime >2 there are at least p+1 MUBs, which is in contrast with the situation for quantum states defined over the real line R for which only 3 MUBs are known. We comment on the possible reason for the difference regarding MUBs between these two infinite dimensional Hilbert spaces. This is joint work with Alexander Russell. http://arxiv.org/abs/1109.0060

81. Information criteria for quantum state estimation and everything else

Steven van Enk, University of Oregon

(Session 1 : Thursday from 5:00pm-5:30pm)

Abstract. The thesis of this talk is that every good experiment (in which the experimentalist knows more or less what she is doing) can be analyzed efficiently, by using so-called information criteria developed for model selection. This holds true even for tomographically complete measurements on many-qubit systems.

82. Magically, negativity of the Wigner function is useful

Victor Veitch, Institute for Quantum Computing

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. It is possible to represent d-dimensional quantum states as probability distributions over a phase space of d^2 points. However, to encompass the full quantum formalism we must allow negative representations. The well known magic state model of quantum computation gives a recipe for universal quantum computation using perfect Clifford operations and repeated preparations of a noisy ancilla state. It is an open problem to determine which ancilla states enable universal quantum computation in this model. In this paper we show that for systems of odd dimension a necessary condition for a state to enable universal quantum computation is that it have negative representation in a particular quasi-probability representation which is a discrete analogue to the Wigner function. This condition implies the existence of a large class of bound states for magic distillation: states which cannot be prepared using Clifford operations but which are not useful for quantum computation. This settles in the negative the conjecture that all states not representable as a convex combination of stabilizer states enable universal quantum computation.

83. The approach to typicality in many-body quantum systems

Sai Vinjanampathy, University of Massachusetts, Boston

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. The recent discovery that for large Hilbert spaces, almost all (that is, typical) Hamiltonians have eigenstates that place small subsystems in thermal equilibrium, has shed much light on the origins of irreversibility and thermalization. Here we give numerical evidence that many-body lattice systems generically approach typicality as the number of subsystems is increased, and thus provide further support for the eigenstate thermalization hypothesis. Our results indicate that the deviation of many-body systems from typicality scales as an inverse power of the number of systems, and we compare this with the equivalent scaling for random Hamiltonians.

84. Coupling Neutral Atoms to Superconducting Circuits

Kristen Voigt, Joint Quantum Institute, Department of Physics, University of Maryland and National Institute of Standards and Technology, College Park, MD 20742, United States

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. We have developed a lumped-element thin-film superconducting resonator [1] for coupling to the hyperfine transition of ⁸⁷Rb at 6.834683 GHz. The resonator operates on the cold stage of a dilution refrigerator. It is made by photolithographic patterning of Al that has been deposited on a sapphire substrate. By moving a carefully machined Al pin towards the inductor of the resonator using a piezoelectric stage, we can to tune the resonance frequency, in situ at 15mK, over a range of 35 MHz and within a few kHz of the Rb resonance while maintaining a quality factor greater than 60,000. We will discuss our tuning results and an initial design for coupling the resonator to ⁸⁷Rb atoms trapped on a tapered optical fiber.

Work supported by NSF through the Physics Frontier Center at the Joint Quantum Institute.

85. TIQC-SPICE: a simulator for trapped-ion quantum computing

Shannon Wang, Massachusetts Institute of Technology

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. Quantum computing experiments are growing in capability and complexity; to realize algorithms, the effects of noise on long computations need to be predicted, understood and minimized. TIQC-SPICE is a modelling system for trapped ion quantum computing experiments. It simulates practical pulse sequences for realizing quantum algorithms by numerically evolving the system Hamiltonian in the presence of various noise sources corresponding to physical errors and technical imperfections in the experiment. These noise sources are modelled by making close correspondence between physically relevant parameters and theoretical noise models, and are simulated via Monte-Carlo methods. Simulated and experimentally measured gate fidelities for a pulse sequence that realizes the Quantum Fourier Transform on 3 ions show good agreement. Finally, we propose and evaluate the in-circuit fidelity of a number of single- and two-qubit gates; this fidelity measure bounds individual gates given a limited set of measurements on a complete pulse sequence and provides a practical alternative to full process tomography.

86. Trapped-ion quantum information processing experiments at NIST*

Ulrich Warring, NIST Ion Storage Group; National Institute of Standards and Technology

(Session 3 : Friday from 8:30am-9:00am)

Abstract. In our experiments we employ internal states of trapped and laser-cooled ions as qubits. Typically, laser light is utilized to introduce the coherent coupling between qubits for entangling gates. In recent work we demonstrated microwave near-field control of the qubit states, i.e. single-qubit and entangling two-qubit gates. In this experiment laser light is used only for Doppler cooling, state preparation and state detection, significantly reducing laser power and laser control requirements. Recent experiments on this technique will be reported. In addition we will summarize efforts and progress on benchmarking the fidelity of one- and two-qubit gates, ion transport in multi-zone traps, engineering of Ising-spin interaction with a few hundred ion qubits in a Penning trap, investigations of anomalous heating, and quantum limited metrology. *work supported by IARPA, NSA, ONR, DARPA and the NIST Quantum Information Program.

87. Quantum to Classical Randomness Extractors

Stephanie Wehner, Centre for Quantum Technologies, National University of Singapore

(Session 8 : Saturday from 10:45am-11:30am)

Abstract. Even though randomness is an essential resource for many information processing tasks, it is not easily found in nature. The goal of randomness extraction is to distill (almost) perfect randomness from a weak source of randomness. When the source yields a classical string X, many extractor constructions are known. Yet, when considering a physical randomness source, X is itself ultimately the result of a measurement on an underlying quantum system. When characterizing the power of a source to supply randomness it is hence a natural question to ask, how much classical randomness we can extract from a quantum state. To tackle this question we here take on the study of quantum-to-classical randomness extractors (QC-extractors). We provide constructions of QC-extractors based on measurements in a full set of mutually unbiased bases (MUBs), and certain single qubit measurements. As the first application, we show that any QC-extractor gives rise to entropic uncertainty relations with respect to quantum side information. Such relations were previously only known for two measurements. As the second application, we resolve the central open question in the noisy-storage model [Wehner et al., PRL 100, 220502 (2008)] by linking security to the quantum capacity of the adversary's storage device.

88. Improved Error-Scaling for Adiabatic Quantum Evolutions

Nathan Wiebe, Institute for Quantum Computing

(Session 13 : Sunday from 11:15am-11:45am)

Abstract. We present a new technique that improves the scaling of the error in the adiabatic approximation with respect to the evolution duration, thereby enabling the design of more efficient adiabatic quantum algorithms and adiabatic quantum gates. Our method is conceptually different from previously proposed techniques: it exploits a commonly overlooked phase interference effect that occurs predictably at specific evolution times, suppressing transitions away from the adiabatically transferred eigenstate. Our method can be used in concert with existing adiabatic optimization techniques, such as local adiabatic evolutions or boundary cancellation methods. We perform a full error analysis of our phase interference method along with existing boundary cancellation techniques and show a tradeoff between error-scaling and experimental precision. We illustrate these findings using two examples, showing improved error-scaling for an adiabatic search algorithm and a tunable two-qubit quantum logic gate.

89. Heating studies with an in-situ-cleaned surface-electrode ion trap

Andrew Wilson, National Institute of Standards & Technology

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. Anomalous heating is a major obstacle to the development of scalable quantum information processors based on trapped atomic ions. Miniaturized traps can achieve high trap frequencies for fast gate operations and quick transport, but the ions are confined in close proximity to surfaces where motional decoherence is rapid. Suppression of motional heating has been demonstrated at cryogenic temperatures, but fabrication issues appear to cause variable results. Moreover, motional decoherence remains substantially more rapid than expected from Johnson electric field noise. Despite investigations by many groups, the physical origin of the electric field noise that causes this motional heating has not yet been identified. However, a number of studies suggest that it might be caused by electrode surface effects. We describe a micro-fabricated, surface-electrode, 9Be+ ion trapping apparatus with an integrated ion-bombardment cleaning capability that allows us to remove contaminant layers from the surfaces of the trap's gold electrodes. We report characterization of the surface properties of our traps (including Auger spectroscopy analysis), results of our cleaning procedures, as well as heating measurements performed before and after cleaning. Work in the laboratory is on-going, but to date the observed heating rate has been unaffected by the surface cleaning. The observed electric field noise is amongst the lowest measured in a room-temperature ion trap with an ion-electrode distance comparable to ours, but still more than three orders of magnitude higher than the Johnson electric field noise calculated for this apparatus. We characterized the heating under a variety of trapping conditions, and describe our efforts to test for and eliminate sources of externally injected noise. * Supported by IARPA, NSA, DARPA, ONR, and the NIST Quantum Information Program

90. Towards a quantum-limited, broadband microwave parametric amplifier

Emma Wollman, California Institute of Technology

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. It is well known that a phase-preserving linear amplifier must add, at minimum, a half-quantum of noise. For microwave signals, however, commonly-used cryogenic HEMT amplifiers operate more than 50 times above this limit. Recently demonstrated Josephson parametric amplifiers are now operating near the quantum limit, but are fundamentally narrowband due to their standing-wave design. We have developed parametric amplifiers that use the non-linear kinetic inductance of a superconducting NbTiN transmission line to amplify microwave signals with a wide bandwidth. Due to the low-loss properties of NbTiN, these amplifiers should be able to operate near the quantum limit for a large range of input powers. In addition, by utilizing a traveling-wave rather than a standing-wave configuration, these devices can have bandwidths of several octaves and can be designed to operate at frequencies from the microwave to the submillimeter-wave band. These amplifiers are expected to have many applications in low-temperature fields such as sub-millimeter astronomy, precision measurement, quantum information, and tests of fundamental quantum physics. We present initial results that demonstrate wide-bandwidth gain and low added noise values.

91. PCB-based Ion Chip Trap Mounting

John Wright, University of Washington

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. Planar ion chip traps are good quantum computer candidates because DC control electrodes placed within hundreds of microns of the trapping region allow fine control over multiple trapped ions simultaneously. Recently fabricated chips include mounted optical cavities or fabricated mirrors for photon collection and printed microwave coils for driving hyperfine transitions. In order to facilitate easily mounting and frequently upgrading these traps, we have designed a UHV-compatible printed circuit board that connects CPGA-100 socket chip trap carriers to four DB-25 vacuum feedthroughs. The board incorporates low-pass filters on the DC control pins approximately 1 cm from the chip to quickly route capacitive RF pickup to ground. The PCB should provide more stable DC control voltages than the more common method of routing individual wires for each control pin to filters outside the vacuum chamber. We have reached UHV with the assembled system without significant problems, and are currently attempting to trap Barium ions.

92. Strong Photon-photon Interaction in Cavity-Quantum Dot System

Jian Yang, University of Illinois at Urbana-Champaign

(Session 5 : Friday from 3:00pm-3:30pm)

Abstract. The photon plays a critical role in quantum communication and quantum computation. Photon-photon interaction is essential to construct efficient quantum logic gates, but it is typically extremely weak in nonlinear media. Exploiting cavity-quantum dot (C-QD) interactions in the strong coupling regime, we found that photons with ˇ°time-reversedˇ± line-shapes of the C-QD emissions can excite the system with near-unity efficiency. In this way, photons can acquire strong interactions with each other, which may be useful in a variety of quantum information applications, including quantum non-demolition detectors and constructing high-efficiency quantum logic gates.

93. Fast controlled unitary protocols using group or quasigroup structures

Li Yu, Carnegie Mellon University

(Session 7 : Friday from 5:15pm-7:15pm)

Abstract. A nonlocal bipartite unitary gate can sometimes be implemented using prior entanglement and only one round of classical communication in which the two parties send messages to each other simultaneously. This cuts the classical communication time by a half compared to the usual protocols, which require back-and-forth classical communication. We introduce such a fast protocol that can implement a class of controlled unitaries exactly, where the controlled operators form a subset of a projective representation of a finite group, which may be Abelian or non-Abelian. We also introduce a modified version of the protocol for the approximate implementation of controlled unitaries. This protocol makes use of quasigroups, which are closely related to Latin squares. We then show that by using enough entanglement, this fast protocol can implement all controlled unitaries approximately. The entanglement cost of our protocols is compared with other fast unitary protocols in the literature. The cost is quite small when the form of the unitary is relatively simple.

94. Asymptotically optimal data analysis for rejecting local realism

Yanbao Zhang, National Institute of Standards and Technology, Boulder

(Session 2 : Thursday from 7:45pm-8:15pm)

Abstract. Reliable experimental demonstrations of violations of local realism are highly desirable for fundamental tests of quantum mechanics. One can quantify the violation witnessed by an experiment in terms of statistical p-values, where high violation corresponds to small p-values. We propose a prediction-based ratio (PBR) analysis protocol whose p-values are valid even if the prepared quantum state varies arbitrarily and local realistic models can depend on previous measurement settings and outcomes. It is therefore not subject to the memory loophole [J. Barrett et al., Phys. Rev. A 66, 042111 (2002)]. If the prepared state does not vary in time, the p-values are asymptotically optimal. For comparison, we consider protocols derived from the number of standard deviations of violation of a Bell inequality and from martingale theory [R. Gill, arXiv:quant-ph/0110137]. We find that the p-values of the former can be too small and are therefore not statistically valid, while those derived from the latter are sub-optimal. PBR $p$-values do not require a predetermined Bell inequality and can be used to compare results from different tests of local realism independent of experimental details. This talk is based on the paper [Y. Zhang, S. Glancy, and E. Knill, arXiv:1108.2468, to be published in Phys. Rev. A].