All Abstracts | Poster Abstracts | Talk Abstracts

1. Increasing ion trap capabilities: demonstrations of in vacuum control electronics, integrated diffractive optics, and ball grid arrays.*

Jason Amini, Georgia Tech Research Institute

(Session 5 : Thursday from 5:00pm - 7:00pm)

Jason Amini, Curtis Volin, Chris Shappert, Harley Hayden, C.S. Pai, Nicholas Guise, Spencer Fallek, Kenton Brown, True Merrill, and Alexa Harter, Georgia Tech Research Institute; IEMIT collaboration: Lisa Lust, Doug Carlson, Jerry Budach, Kelly Muldoon, and Alan Cornett, Honeywell International; IDM collaboration: Dave Kielpinski, Griffith University; SMIT-BGA collaboration: Daniel Youngner and Matthew Marcus, Honeywell International. We report on three IARPA seedling projects that address issues in scaling of microfabricated ion traps to large numbers of qubits. The first project (IEMIT), in collaboration with Honeywell International, is a successful demonstration of a compact, in-vacuum 80 channel DAC system controlling a microfabricated surface-electrode ion trap. This system reduces the number of through vacuum connections by a factor of ten. Results include ion loading with 40Ca+, 70 m ion transport in the dark at 1 m/s, and a measured ion heating rate that is comparable to external DAC systems. The second project (IDM), in collaboration with Griffith University, takes multiple diffractive optical elements and etches them into the surface of a surface electrode ion trap. We demonstrate optical elements for both collimation and refocusing of light from 171Yb+. For the third project (IDM), Honeywell International is fabricating ion traps with back-side ball-grid-array connections to eliminate wirebonds and to reduce the physical die size. The first run of these traps is nearing completion and we will report on the current state of this project. * This material is based upon work supported by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA) under U.S. Army Research Office (ARO) contracts W911NF1210605 and W911NF1210600 and under Space and Naval Warfare Systems (SPAWAR) contract N6600112C2007.

2. Toward Quantum Communication with Qudits: Measuring Orbital Angular Momentum Entangled Photon Pairs from SPDC

Fangzhao An, Harvey Mudd College

(Session 5 : Thursday from 5:00pm - 7:00pm)

Presenters: Fangzhao A. An and David Spierings van der Wolk Co-authors: David Berryrieser, Julien Devin, and Theresa W. Lynn We describe experimental progress in manipulating orbital angular momentum (OAM) of entangled photon pairs from spontaneous parametric down-conversion (SPDC). OAM provides an infinite-dimensional basis for encoding information in entangled states. Our present work is restricted to the OAM {-1,0,+1} subspace, enabling quantum communication with qutrit and (polarization qubit) x (OAM qutrit) states. OAM measurements are performed with forked-hologram blazed gratings produced photographically in our lab. These diffract into the first order with 37% efficiency while imparting a unit shift in OAM to the diffracted beam. Measurements in an arbitrary superposition of the OAM 0 and +/- 1 states are performed by positioning holograms in the down-converted beams and coupling the first-order diffraction into single-mode fibers. Our OAM-entanglement measurements have in the past been limited by poor mode matching due to the complex spatial mode and spectral profile from SPDC; we present current efforts to measure OAM with higher signal to noise using a multi-mode fiber prefilter in each down-converted beam.

3. Towards an Efficient Decoder for Quantum LDPC Codes

Jonas Anderson, Université de Sherbrooke

(Session 9a : Friday from 5:30pm - 6:00pm)

Quantum low-density parity-checking (LDPC) codes can greatly reduce the overhead associated with fault-tolerant quantum computation (FTQC) by providing a nonzero-rate code family with low-weight stabilizer generators. In principle this means that as the code distance grows so does the number of encoded qubits thus allowing FTQC with constant overhead [1]. Exact decoding of classical LDPC codes is computationally difficult, but approximate decoders such as the belief propagation (BP) decoder are known to work well. For quantum LDPC codes much less is known and BP without modifications is plagued with issues due to degeneracy and short cycles in the Tanner graph. Here we improve upon the work of Poulin and Chung [2] by modifying BP to correct for some of the effects of message passing on a Tanner graph with cycles. Our technique uses nonlinear message weights to offset the additional correlations picked up due to cycles. For physical error rates an order of magnitude below pseudothreshold, arguably the most important regime for FTQC, we improve upon the best-known decoding schemes by an order of magnitude. We will also discuss ideas to further improve upon these schemes. [1] Daniel Gottesman, “What is the Overhead Required for Fault-Tolerant Quantum Computation?” arxiv.org/1310.2984. [2] David Poulin and Yeojin Chung, “On the iterative decoding of sparse quantum codes” Quantum Information and Computation, Vol. 8, No. 10 (2008) 0987–1000.

4. Towards practical quantum simulators for quantum chemistry

Alan Aspuru-Guzik, Harvard University

(Session 8 : Friday from 1:45pm - 2:30pm)

My first talk about quantum computing for chemistry was at SQUINT 2005. Back then, I presented a gate-model approach for the simulation of quantum chemistry. In this talk, almost a decade later, I will present two approaches that are much less demanding on the requirements of the quantum device, yet are able to simulate Fermionic Hamiltonians such as those of molecular quantum chemistry. First, I will talk about the variational quantum eigensolver approach for solving chemistry problems in an arbitrary {\sl hardware ansatz}. I will follow by describing an approach for the simulation of quantum chemistry using adiabatic quantum computers. Both approaches are scalable and good candidates for an early implementation of quantum devices that could carry out a simulation of practical relevance to medical or industrial applications.

5. The role of the global phase in optimal quantum control to implement partial isometries

Charles Baldwin, University of New Mexico

(Session 5 : Thursday from 5:00pm - 7:00pm)

Controlling quantum systems is an important step towards the implementation of quantum information protocols. We consider "geometric control," whereby time-dependent waveforms modulate a set of Hamiltonians that are generators of the Lie algebra su(d) for a d-dimensional Hilbert space. In such a scenario, there is a "quantum speed limit," i.e., the minimum time that it is needed to produce a specified control task for a given set of time dependent Hamiltonians. This speed limit is typically studied for two tasks: state-to-state mappings and the implementation of a full unitary map on the Hilbert space. We study the range of intermediate cases -- partial isometries that map an under-complete set of orthogonal states to another under-complete set of orthogonal states. For full unitary control, it was recently shown that the global phase of the target unitary, restricted to root of unity phases, affects the quantum speed limit. We observe that, in the partial isometry case as well as state-to-state mappings, the idea of global phase is not restricted to root of unity phases but can take any value. This means that each control task has a range of speed limits that must be understood in order to implement the control.

6. Arbitrary 2D-Lattices of Ions

Todd Barrick, Sandia National Laboratories

(Session 5 : Thursday from 5:00pm - 7:00pm)

Arbitrary 2D-Lattices of Ions Todd A. Barrick, Matthew Blain, Peter Maunz, Eric Shaner, Daniel Stick, and Craig R. Clark Sandia National Laboratories Robert Jördens, Dietrich Leibfried, and David Wineland National Institute of Standards and Technology 325 Broadway, Boulder CO 80305 A major aspect of quantum information processing and quantum simulation is the exponential growth in complexity of quantum states as the number of quantum degrees of freedom is scaled up. A cooperative effort at Sandia National Laboratory and the NIST Ion Storage Group has set out to develop ion traps that enable freely configurable quantum interactions over a two-dimensional lattice of trapped ions and have the potential to scale to ion numbers where conventional simulation of the system becomes infeasible. Our near-term goal is to design, fabricate, and test a new set of trap geometries which hold ions in a lattice of individual traps and perform entangling operations mediated by Coulomb interactions between neighboring ions. A configurable interaction between two ions in a double well has been previously demonstrated at NIST. To expand on this work, we plan on fabricating three trap geometries, a 3 and 4 well triangular lattice trap and a 7 well trap with a hexagonal ring around one central well. These designs will be tested at both room temperature and <10K. This poster will present the theoretical motivation for the project along with new trap designs and Sandia National Laboratories’ custom cryogenic chamber design. *Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the US Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000

7. Robust and high-sensitivity nonorthogonal coherent state discrimination

Francisco Elohim Becerra, University of New Mexico

(Session 3 : Thursday from 2:15pm - 2:45pm)

Measurements for accessing the information contained in quantum states are limited by the inherent noise of these states. Strategies for nonorthogonal state discrimination for optimally extracting information from these states have fundamental interest in quantum mechanics and can allow for communications approaching the quantum limits. Conventional measurements for nonorthogonal state discrimination of coherent states with different phases implement a direct phase-sensitive detection, and can ideally reach the standard quantum limit (SQL). However, measurement strategies based on the quantum properties of these states can allow for better measurements which surpass the SQL and approach the ultimate measurement limits allowed by quantum mechanics. We present the demonstration of a receiver based on adaptive measurements and single-photon counting that unconditionally discriminates multiple nonorthogonal coherent states below the SQL. We also discuss the potential of photon-number-resolving detection to provide robustness under realistic conditions for an adaptive coherent receiver with detectors with finite photon-number resolution.

8. Frequency translation with single ions

Francisco Benito, Sandia National Laboratories - University of New Mexico

(Session 5 : Thursday from 5:00pm - 7:00pm)

Frequency translation with single ions Francisco Benito, Hayden McGuinness, Susan Clark, Dan Stick Sandia National Laboratories Here we present an experimental scheme to interact two ion species by creating a photonic link between them. The photons from each ion are frequency converted to an intermediate wavelength by difference frequency generation. These photons can then be interfered on a beam splitter to verify their indistinguishability. In our experiment we use single calcium and ytterbium ions trapped on separate microfabricated ion traps. This technique could have applications in hybrid quantum computing and quantum communication. 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.

9. Quantum assisted sensing with diamond spins

Ania Bleszynski-Jayich, University of California Santa Barbara

(Session 13 : Saturday from 4:15pm - 5:00pm)

Nitrogen-vacancy (NV) centers in diamond are atomic-scale spin systems with remarkable quantum properties that persist to room temperature. They are highly sensitive to a wide variety of fields (magnetic, electric, thermal) and are easy to initialize, read-out, and manipulate on the individual spin level; thus they make excellent nanoscale sensors. The NV’s sensitivity is a double-edged sword however; environmental fluctuating fields are also a source of decoherence. We use the NV to probe these fluctuating fields, both their frequency spectrum and spatial character, and we mitigate their induced decoherence through engineered CVD diamond growth and quantum control of the NV. I will also present my group’s work on quantum assisted sensing of strain fields on the nanoscale. We demonstrate strain coupling of a single NV spin to a high quality factor mechanical mode of a single-crystal diamond mechanical resonator. This hybrid system has exciting prospects for a phonon-based approach to integrating NVs into quantum networks.

10. Efficient simulation of three-level open quantum systems

Marduk Bolaños, National Autonomous University of Mexico

(Session 5 : Thursday from 5:00pm - 7:00pm)

The Hilbert space of a system of N three-level atoms interacting with classical radiation has dimension 3^N. When spontaneous emission is taken into account, the state of the system is specified by a density matrix obtained as the solution to a master equation. That is, 9^N equations have to be solved. If the evolution of the system, unitary and non-unitary, is symmetric under the exchange of atoms, we show that the state of the system can be described with a basis of symmetric states of polynomial size. This allows for an efficient calculation of the numerical solution to the master equation and also enlarges the class of problems that can be solved analytically. Authors: Marduk Bolaños, Pablo Barberis Institute for Research on Applied Mathematics and Systems, UNAM, Mexico

11. Searching for quantum optimal controls in the presence of control constraints

Constantin Brif, Sandia National Laboratories

(Session 5 : Thursday from 5:00pm - 7:00pm)

Wide success enjoyed by quantum optimal control for a variety of theoretical and experimental objectives has been attributed to the trap-free topology of the corresponding control landscapes. In this work, extensive sets of gradient searches are used to explore how the landscape topology is affected by the inevitable presence of constraints on control fields. We identify several essential control resources, including the number of control variables, control duration, and field strength, and quantify the limits on them. Exceeding these limits produces artificial local traps on the control landscape and can thereby prevent gradient searches from reaching a globally optimal solution. These results suggest that severe field constraints are the primary source of failed searches in both optimal control theory and experiments. We demonstrate how careful choice of relevant control parameters can help to eliminate artificial traps and facilitate successful optimization. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000.

12. Improved Bounds for Eigenpath Traversal

Hao-tien Chiang, University of New Mexico

(Session 5 : Thursday from 5:00pm - 7:00pm)

We present an improved bound on the length of the path defined by the ground states of a continuous family of Hamiltonians in terms of the spectral gap δ. We use this bound to obtain a better cost of recently proposed methods for quantum adiabatic state transformations and eigenpath traversal. In particular, we prove that a method based on evolution randomization, which is a simple extension of adiabatic quantum computation, has an average cost of order 1/δ^2, and a method based on fixed-point search has a maximum cost of order 1/δ^{3/2}. Additionally, if the Hamiltonians satisfy a frustration-free property, such costs can be further improved to order 1/δ^{3/2} and 1/δ, respectively. Our methods offer an important advantage over adiabatic quantum computation when the gap is small, where the cost is of order 1/δ^3.

13. Simulating Hamiltonian evolution on a quantum computer

Richard Cleve, University of Waterloo

(Session 2 : Thursday from 10:45am - 11:30am)

I will explain various quantum algorithms that have been proposed for simulating the evolution of a quantum state under a Hamiltonian, including my recent joint work (with Dominic Berry, Andrew Childs, Robin Kothari, and Rolando Somma) that dramatically improves the running time as a function of the precision of the output data.

14. Probabilistic protocols in quantum information? Probably not.

Joshua Combes, University of New Mexico

(Session 13 : Saturday from 5:00pm - 5:30pm)

Probabilistic protocols in quantum information are an attempt to improve performance by occasionally reporting a better result than could be expected from a deterministic protocol. Here we show that probabilistic protocols can never improve performance beyond the quantum limits on the corresponding deterministic protocol. To illustrate this result we examine three common probabilistic protocols: probabilistic amplification, weak value amplification, and probabilistic metrology. In each of these protocols we show explicitly that the optimal deterministic protocol is better than the corresponding probabilistic protocol when the probabilistic nature of the protocol is correctly accounted for.

15. Studies of electric field noise near metal surfaces using a trapped ion sensor.

Nikos Daniilidis, University of California, Berkeley

(Session 5 : Thursday from 5:00pm - 7:00pm)

Single ions are an extremely sensitive probe for oscillating electric fields in the frequency range between 100 kHz and few MHz. This allows their use to measure electric field noise near conducting surfaces in ultra high vacuum, and several experiments have found the noise to be orders of magnitude higher than expected. Recent work revealed that the noise is related to carbon contamination of the surface, and can be reduced by more than two orders of magnitude by cleaning the surface in vacuum. We report on ongoing progress in using a vacuum system which combines surface cleaning and analysis capabilities with ion trapping. We performed noise measurements, combined with surface cleaning and in-situ analysis of an aluminum-copper alloy surface. Cleaning reduced the noise by between one and two orders of magnitude, but the surface did not need to be carbon or oxide free to show low noise. An analysis of residual gases in our system revealed possible dependence of the noise on the size and type of carbon contaminants on the surface.

16. Shortcuts to adiabaticity in many-body systems

Adolfo del Campo, Los Alamos National Laboratory

(Session 9b : Friday from 5:00pm - 5:30pm)

The evolution of a system induced by counter-diabatic driving mimics the adiabatic dynamics without the requirement of slow driving. Engineering it involves diagonalizing the instantaneous Hamiltonian of the system and results in the need of auxiliary non-local interactions for matter-waves. Here experimentally realizable driving protocols are presented for a large class of single-particle, many-body, and non-linear systems without demanding the spectral properties as an input. The method is applied to the fast decompression of quantum fluids realizing a dynamical quantum microscope, as well as to the fast transport of ion chains.

17. Weak values considered harmful

Chris Ferrie, University of New Mexico

(Session 5 : Thursday from 5:00pm - 7:00pm)

We show using statistically rigorous arguments that the technique of weak value amplification (WVA) does not perform better than standard statistical techniques for the tasks of single parameter estimation and signal detection. Specifically we prove that post-selection, a necessary ingredient for WVA, decreases estimation accuracy and, moreover, arranging for anomalously large weak values is a suboptimal strategy. In doing so, we explicitly provide the optimal estimator, which in turn allows us to identify the optimal experimental arrangement to be the one in which all outcomes have equal weak values (all as small as possible) and the initial state of the meter is the maximal eigenvalue of the square of the system observable. Finally, we give precise quantitative conditions for when weak measurement (measurements without post-selection or anomalously large weak values) can mitigate the effect of uncharacterized technical noise in estimation.

18. Trapped-ion quantum information processing experiments at NIST

John Gaebler, National Institute of Standards and Technology

(Session 1 : Thursday from 9:15am - 9:45am)

We report experiments towards scalable quantum information processing with laser-cooled trapped ions. Quantum information is stored in internal (hyperfine ground) states of ions and gate operations are performed with laser and microwave fields. We describe the current status of quantum information experiments using multi-zone trap arrays to investigate the basic tasks of a quantum information processor including transport of ions between zones and sympathetic cooling. In one recent experiment we created an entangled steady state of two trapped ions using dissipation. The steady state can be maintained for a duration of several times the entanglement generation duration and the entanglement fidelity is currently limited by identified technical issues. We also briefly describe recent progress with other quantum-information-focused experiments in our group including the generation of entanglement between two ions held in distinct but coupled trap zones, efforts to reduce the electric field noise from trap surfaces, superconducting photon detectors and junctions for switching the transport pathway of ions in multi-zone traps structures. *This work is supported by IARPA, ONR, and the NIST Quantum Information Program.

19. Quantum gate set tomography

John Gamble, Sandia National Laboratories

(Session 11 : Saturday from 10:15am - 10:45am)

In this talk, I will discuss a recently-proposed framework called "gate set tomography" (GST) for self-consistently characterizing an entire set of quantum logic gates on a black-box quantum device. Until recently, protocols for quantum tomography relied on a pre-existing and perfectly calibrated reference frame for the measurements used to characterize a device. GST eschews this artificial separation entirely, instead characterizing quantum processes, preparations, and measurements concurrently. I will then describe an explicit closed-form protocol for linear-inversion GST, whose reliability is independent of pathologies such as local maxima of the likelihood function. This initial estimate can then be refined using standard likelihood maximization techniques. Finally, I discuss recent experimental implementations of GST for single qubits in both ion traps and electrostatically defined quantum dot systems. This work was supported in part by the Laboratory Directed Research and Development program at Sandia National Laboratories. 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.

20. Understanding the effects of leakage in superconducting quantum error detection circuits

Joydip Ghosh, University of Calgary

(Session 9a : Friday from 4:30pm - 5:00pm)

The majority of quantum error detection and correction protocols assume that the population in a qubit does not leak outside of its computational subspace. For many existing approaches, however, the physical qubits do possess more than two energy levels and consequently are prone to such leakage events. Analyzing the effects of leakage is therefore essential to devise optimal protocols for quantum gates, measurement, and error correction. In this talk, I discuss the role of leakage in a two-qubit superconducting quantum error detection circuit. We simulate the repeated ancilla-assisted measurement of a single Z operator for a data qubit, record the outcome at the end of each measurement cycle, and explore the signature of leakage events in the obtained readout statistics. An analytic model is also developed that closely approximates the results of our numerical simulations. We find that leakage leads to destructive features in the quantum error detection scheme, making additional hardware and software protocols necessary.

21. Practical and Fast Gaussian State Estimation

Scott Glancy, National Institute of Standards and Technology

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

Many experiments on quantum systems involve the preparation and measurement of Gaussian states of a multi-system continuous variable Hilbert space. Examples include optical and microwave systems involving squeezing and linear interactions and nanomechanical resonators described with second order Hamiltonians. The state space that these systems access is much smaller than the full Hilbert space and can be fully characterized with a 2Nx2N covariance matrix and 2N means vector, where N is the number of individual modes or resonators. We describe here a very simple and fast method for estimating the covariance matrix and means vector from homodyne (or quadrature) measurement data collected at arbitrary phases. The method computes observed means of simple functions of the homodyne (phase, quadrature) pairs, which are easily related to the covariance matrix and means vector. We characterize uncertainty through a parametric bootstrap strategy. Our method is particularly useful for the analysis of large data sets.

22. Building quantum hybrids from wires and single ions

Dylan Gorman, University of California at Berkeley

(Session 5 : Thursday from 5:00pm - 7:00pm)

We report experimental and theoretical progress towards constructing hybrid quantum systems from single trapped ions and solid state devices. An instructive proof-of-principle experiment is to use a wire to create entanglement between distant (d ≈ 500μm) ions. Two distant ions trapped near (≈ 100μm) a conducting wire will experience an interaction potential mediated by their Coulomb interaction with the wire. With reasonable parameters, such an experiment would generate entanglement between the motional states of two ions in about 50 ms, suggesting that coupling experiments can be constructed from the wire with heating rates already achieved in surface-electrode ion traps. This experiment is important for developing an experimental toolbox to study the interactions of ions with quantum circuits. At present, we have mounted a wire on a moveable stage above a surface-electrode ion trap. We have moved the wire to within 100μm of a single ion, and measured heating rates as the ion-wire distance is varied. The heating rates appear to remain acceptably low as the wire approaches, suggesting that a coupling experiment is immediately feasible. Current work focuses on moving to a new trap design where the wire is integrated directly into the trap for performing the first coupling experiments.

23. Simulation of Stochastic Quantum Systems Using Polynomial Chaos Expansions

Matthew Grace, Sandia National Labs

(Session 5 : Thursday from 5:00pm - 7:00pm)

We present an approach to the simulation of quantum systems driven by classical stochastic processes that is based on the polynomial chaos expansion, a well-known technique in the field of uncertainty quantification. The polynomial chaos technique represents the density matrix as an expansion in orthogonal polynomials over the principle components of the stochastic process and yields a sparsely coupled hierarchy of linear differential equations. We provide practical heuristics for truncating this expansion based on results from time-dependent perturbation theory and demonstrate, via an experimentally relevant one-qubit numerical example, that our technique can be significantly more computationally efficient than Monte Carlo simulation.

24. Quantum Hamiltonian Learning

Christopher Granade, Institute for Quantum Computing

(Session 5 : Thursday from 5:00pm - 7:00pm)

A long-standing problem in the development of practical quantum simulators is how to certify that a given quantum device implements a desired Hamiltonian. For devices on the 100-qubit scale, as are currently being proposed, classical simulation cannot certify the dynamics of a quantum device. Here, we address this problem by providing an algorithm that exploits trusted quantum simulation resources in order to characterize and certify the Hamiltonian dynamics of an untrusted quantum system. Moreover, our algorithm provides a powerful resource for the characterization of quantum information processing devices, thus allowing for processors to be used as resources in the development of further processors. We show that our algorithm, in some analytically-tractable cases, admits near-optimal performance. Moreover, we demonstrate analytic and numeric evidence that our algorithm is robust to sampling errors, decoherence and excluded terms. By using quantum simulation resources together with classical statistical inference techniques, our algorithm provides a powerful tool for certifying quantum simulators and for developing new quantum information processing devices.

25. Stochastic Master Equations in the Circuit Model

Jonathan Gross, University of New Mexico

(Session 5 : Thursday from 5:00pm - 7:00pm)

We present a derivation of several stochastic master equations that model the trajectory of a system interacting with a continuously monitored Gaussian field. Our approach differs from previous work in using a continuum of finite-dimensional ancillary systems to model the field, allowing the derivation to make use of techniques common in quantum information and measurement theory instead of the traditional quantum-optics-based approach. In particular, we can, before taking the continuous limit, draw circuit diagrams that portray the interaction of the system with the finite-dimensional ancillas.

26. The effect of realistic noise models on quantum error correction thresholds

Mauricio Gutierrez, Georgia Institute of Technology

(Session 9a : Friday from 5:00pm - 5:30pm)

Classical simulations of noisy stabilizer circuits are often used to estimate the threshold of a quantum error-correcting code (QECC). In this context, it is common to model the noise as a depolarizing channel by inserting Pauli gates randomly throughout the circuit [1]. However, it is not clear how sensitive a code's threshold is to the noise model, and whether or not a depolarizing channel is a good approximation for realistic non-stabilizer errors. Within the stabilizer formalism, we have shown that for a single qubit more accurate approximations can be obtained by including in the noise model Clifford operators and Pauli operators conditional on measurement [2]. Independent work by Magesan et al. has also shown the utility of adding Clifford operators to error models [3]. We now examine the feasibility of employing these error approximations at the single-qubit level to obtain better estimates of a QECC's threshold. For several codes and various noise models, we simulate an error-correction step and compute the pseudo-threshold by determining the noise strength above which encoding reduces the qubit fidelity. We compare the pseudo-threshold values for the real noise with its Pauli and expanded Pauli approximations. In most cases, the expanded Pauli channel provides a significantly better approximation to the real pseudo-threshold suggesting that our expanded error models will lead to more accurate stabilizer-based threshold estimations for realistic noise models. [1] A.M. Steane, Phys. Rev. A 68, 042322 (2003) [2] M. Gutiérrez, L. Svec, A. Vargo, and K. R. Brown, Phys. Rev. A. 87, 030302(R) (2013) [3] E. Magesan, D. Puzzuoli, C. E. Granade, D. G. Cory, Phys. Rev. A 87, 012324 (2013)

27. Controlled-Phase Gate using Rydberg-Dressed States in Cesium

Aaron Hankin, Sandia National Laboratories and University of New Mexico

(Session 5 : Thursday from 5:00pm - 7:00pm)

We are implementing a controlled-phase gate based on trapped neutral atoms whose coupling is mediated by the dipole-dipole interaction of Rydberg states. Ground state cesium atoms are dressed by an off-resonant laser field in a manner conditional on the Rydberg blockade mechanism [1,2,3], providing the required entangling interaction. We will present the calculated controlled-phase gate fidelity for realistic experimental parameters as well as preliminary measurements of the Rydberg-dressed state interaction. Sandia National Laboratories is a multi-program laboratory managed and operated b 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] S. Rolston, et al. Phys. Rev. A, 82, 033412 (2010) [2] T. Keating, et al. Phys. Rev. A, 87, 052314 (2013) [3] A. Hankin, et al. to be published

28. Donor-quantum-dot qubit in silicon

Patrick Harvey-Collard, Sandia National Laboratories

(Session 5 : Thursday from 5:00pm - 7:00pm)

We propose and show experimental progress towards a donor-quantum-dot silicon qubit. Electron spins in silicon are of increasing interest because of recent successes in demonstrating single quantum bits (qubits) including demonstrations of electron spin resonance (ESR) of a single electron spin bound to a phosphorus donor and formation of a two spin logical qubit (i.e. the singlet-triplet qubit in the m=0 subspace) using a double quantum dot in the SiGe/Si system. Silicon is of particular interest because very long decoherence times can be achieved in isotopically enriched 28Si, however, this spin depleted material also lacks any built-in magnetic field gradient or spin-orbit coupling necessary for rotating the qubit states as is utilized in GaAs and other III-V materials. Therefore, alternative methods of control are being pursued, like local inductors (e.g. for ESR), micro-magnets or more complex multi-electron logical qubit encodings (e.g. triple dots). We examine, instead, coupling a single quantum dot to a donor for which the donor nucleus provides a built-in magnetic field gradient, therefore eliminating the need for magnets, inductors or spin-orbit coupling in a very compact and natural way. In addition, it provides a qubit platform to test many ideas relevant to both Si quantum dot and donor qubit systems.

29. Spacetime, quantum cloning and black holes

Patrick Hayden, Stanford University

(Session 7 : Friday from 10:30am - 11:15am)

Reconciling black hole evaporation with the unitarity of quantum mechanics is an endeavour frought with conceptual difficulties. Not least among them is the apparent need for quantum cloning or, equivalently, violations of the monogamy of entanglement. The most recent and confusing incarnation of this problem is the so-called firewall paradox, which interprets monogamy violations as an indication that black holes may not have interiors. This talk will begin with a more pedestrian question: understanding all the ways in which quantum information can be replicated in Minkowski spacetime. It turns out that there is an amazing variety, perhaps an indication that we should not be so worried about apparent violations of no-cloning in situations in which the causal structure of spacetime is itself in doubt. Towards the end, I will return to the black hole firewall problem and sketch some of the quantum information theoretic ideas that have been proposed as possible resolutions.

30. Period Finding with Adiabatic Quantum Computation

Itay Hen, University of Southern California

(Session 9a : Friday from 4:00pm - 4:30pm)

We outline an efficient quantum-adiabatic algorithm that solves Simon's problem, in which one has to determine the `period', or xor-mask, of a given black-box function. We show that the proposed algorithm is exponentially faster than its classical counterpart and has the same complexity as the corresponding circuit-based algorithm. Together with other related studies, this result supports a conjecture that the complexity of adiabatic quantum computation is equivalent to the circuit-based computational model in a stronger sense than the well-known, proven polynomial equivalence between the two paradigms. We also discuss the importance of the algorithm and its implications for the existence of an optimal-complexity adiabatic version of Shor's integer factorization algorithm and the experimental implementation of the latter.

31. Single charged impurities inside a Bose-Einstein condensate

Sebastian Hofferberth, Universität Stuttgart

(Session 1 : Thursday from 8:30am - 9:15am)

We investigate the interaction of a single electron as well as a single ion with a Bose-Einstein condensate (BEC). The charge impurities are produced by exciting exactly one atom from the BEC to a Rydberg state. Since the ionic core and the Rydberg electron have vastly different mass and interaction range with the surrounding ground state, we can consider both parts separately. Firstly, for low-L Rydberg states, the electron wavefunction is fully immersed in the BEC, and we observe electron-phonon coupling. We observe that single electron excite collective modes of the whole condensate. Alternatively, for high-L states the electron can be moved completely outside of the BEC, enabling us to study the interaction of the ionic core with the BEC. We are currently studying ion-ground state Feshbach resonances and investige the possibility of trapping the ion inside the BEC without any external electric fields.

32. Dressed-state master equation for an optomechanical system in the ultra-strong coupling regime

Dan Hu, School of Natural Sciences, University of California, Merced

(Session 5 : Thursday from 5:00pm - 7:00pm)

We study the open system dynamics of an optomechanical system in the ultra-strong coupling regime. In our system, the mechanical oscillator couples to a cavity mode via radiation pressure force, and the coupling strength is comparable to the mechanical frequency. The environmental degrees of freedom of both the mechanical mode and the cavity mode are modeled as bosonic baths coupling linearly with the system modes. In contrast to the standard approach to describing the effects of the environment, we derive the Lindblad master equation in the normal-mode basis of the optomechanical system (dressed states). We find that the mechanical damping in our approach depends sensitively on the state of the cavity mode. We illustrate this result using numerical results of the correlations of the cavity field, the optomechanical entanglement, and the Wigner functions.

33. Quantum Fisher information for states in exponential form

Zhang Jiang, University of New Mexico

(Session 5 : Thursday from 5:00pm - 7:00pm)

We derive explicit expressions for the quantum Fisher information and the symmetric logarithmic derivative (SLD) of a quantum state in the exponential form; the SLD is expressed in terms of the generator. Applications include quantum metrology problems with Gaussian states and general thermal states. Specifically, we give the SLD for a Gaussian state in two forms, in terms of its generator and its moments; its Fisher information is also calculated with the latter form. Special cases are discussed, including pure and very noisy Gaussian states.

34. Quantum process tomography of near-unitary maps

Amir Kalev, University of New Mexico

(Session 11 : Saturday from 11:15am - 11:45am)

We study the problem of quantum process tomography given the prior information that the implemented map is near to a unitary map on a d-dimensional Hilbert space. In particular, we show that a perfect unitary map is completely characterized by a minimum of d^2 + d measurement outcomes. This contrasts with the d^4 measurement outcomes required in general. To achieve this lower bound, one must probe the system with a particular set of d states in a particular order. This order exploits unitarity but does not assume any other structure of the map. We further numerically study the behaviors of two of compressed sensing estimators based on correct or faulty prior information caused by noise. The results show two important features: (1) When we have accurate prior information, one can drastically reduce the required data needed; (2) Different estimators applied to the same data are sensitive to different types of noise. The estimators could, therefore, be used as indicators of particular error models and to validate the use of prior assumptions for compressed sensing quantum process tomography. Finally, we consider the more general case of noisy quantum maps, with a low level of noise. Our study indicates that transforming to the interaction picture, where the noiseless map is represented by a diagonal operator, can provide a useful tool to identify the noise structure. This, in turn, can lead to a substantial reduction in the numerical resources needed to estimate the noisy map.

35. Mutually unbiased measurements in finite dimensions

Amir Kalev, University of New Mexico

(Session 5 : Thursday from 5:00pm - 7:00pm)

We generalize the concept of mutually unbiased bases (MUB) to measurements which are not necessarily described by rank one projectors. As such, these measurements can be a useful tool to study the long standing problem of the existence of MUB. We derive their general form, and show that in a finite, d-dimensional Hilbert space, one can construct a complete set of d+1 mutually unbiased measurements. Beside of their intrinsic link to MUB, we show, that these measurements' statistics provide complete information about the state of the system. Moreover, they capture the physical essence of unbiasedness, and in particular, they satisfy non-trivial entropic uncertainty relation similar to d+1 MUB.

36. Off-resonant CPHASE Gate in Neutral Atoms

Tyler Keating, University of New Mexico

(Session 5 : Thursday from 5:00pm - 7:00pm)

The dipole blockade effect between Rydberg atoms is a promising tool for quantum information processing in neutral atoms. There have been numerous proposals to exploit this effect in order to perform a controlled-phase quantum logic gate between two neutral atom qubits, but most use near- or on-resonance pulses to excite the Rydberg state. By instead using significantly off-resonant lasers to adiabatically dress the atomic ground states, one can make a gate that is more robust against atomic motion at finite temperature. We analyze the benefits of such a scheme as compared to near-resonance approaches and show how we can attain fidelities greater than 0.99, limited primarily by the finite lifetime of the Rydberg state. We also describe how the off-resonant dressing mechanism can be generalized to produce multi-qubit gates, such as the Toffoli gate.

37. Certifying violations of local realism

Emanuel Knill, University of Colorado at Boulder

(Session 12 : Saturday from 1:45am - 2:30pm)

Many applications of quantum systems require measurements that verify the presence of sufficiently strong quantum correlations. The probability of the following unwanted event must be extremely small: The event where the correlations are not sufficiently strong but one is nevertheless convinced that they are strong enough. Important examples of quantum correlation occur in experiments showing violations of Bell's inequalities, which are thought to invalidate local realism. This is a review of how such violations are quantified and robustly certified, with or without predetermined Bell's inequalities.

38. True Quantum Precision and Unique Optimal Probes in presence of Decoherence.

Sergey Knysh, NASA Ames Research Center

(Session 13 : Saturday from 5:30pm - 6:00pm)

Quantum instruments derived from composite systems allow greater measurement precision than their classical counterparts due to coherences maintained between the N component elements; spins, atoms or photons. Typical decoherence that plagues real-world devices can be dephasing, particle loss, thermal excitation and relaxation. All these adversely affect precision (mean squared error), whether one is measuring time or phase, or even the noise amplitude itself. We develop a novel technique that uncovers the uniquely optimal probe states of the N `qubits' alongside new tight bounds on precision under local and collective mechanisms of these noise types above.   For large quantum ensembles (where numerical techniques fail), the problem reduces by analogy  to finding the ground state of a 1-D particle in a potential well, with the shape of the well dictated by the type and strength of decoherence. Under collective dephasing alone we find that optimal estimation of phase and noise parameter can be effected simultaneously, utilizing the same optimal probe and measurement scheme. The formalism is applied to real-world devices such as the Mach-Zehnder interferometer and atom clocks.

39. Towards a rigorous link between anyonic excitations and 2D topological codes

Olivier Landon-Cardinal, Caltech

(Session 5 : Thursday from 5:00pm - 7:00pm)

Topological codes are the best candidates for a self-correcting quantum memory. However, they are thermally unstable in 2D. The intuitive argument is that low-energy excitations are anyons whose free diffusion changes the ground state and require only finite energy. However, no formal link has been proven between generic 2D topological codes and anyons, even if it holds for all known model Hamiltonians. The thermal instability of 2D codes was thus proven using a stochastic construction. In this work, we improve it to build unitary operators which move the excitations. Our construction sheds light unto the emergence of anyons from the Hamiltonian. Joint work with David Poulin.

40. Optimal quantum-enhanced interferometry using a laser power source

Matthias Lang, University of New Mexico

(Session 9c : Friday from 5:00pm - 5:30pm)

We consider an interferometer powered by laser light (a coherent state) into one input port and ask the following question: what is the best state to inject into the second input port, given a constraint on the mean number of photons this state can carry, in order to optimize the interferometer’s phase sensitivity? This question is the practical question for high-sensitivity interferometry. We answer the question by considering the quantum Cramer-Rao bound for such a setup. The answer is squeezed vacuum, if there are no photon losses in the interferometer. For a lossy interferometer, the squeezed vacuum is the best choice for the practical case where the laser power is much bigger than the power put into the squeezing.

41. Progress in Quantum Information Processing with Trapped Ions at NIST

Dietrich Leibfried, National Institute of Standards and Technology

(Session 5 : Thursday from 5:00pm - 7:00pm)

This poster will provide an overview of the progress in quantum information processing and quantum simulation with trapped ions at NIST. In particular, improvements of ion transport and cooling within a scalable architecture for quantum information processing and experiments entangling the internal states of ions held in separate trapping wells, a basic building block for quantum simulation, will be discussed.

42. Scalable Source of Multipartite Continuous Variable Entangled Beams of Light

Alberto Marino, University of Oklahoma

(Session 5 : Thursday from 5:00pm - 7:00pm)

The development of efficient and scalable sources of multipartite entanglement is required for the further development of quantum information. We propose a scalable configuration based on cascaded four-wave mixing (FWM) processes for the generation of multipartite continuous variable (CV) entanglement. The FWM process is based on a double-lambda configuration in rubidium vapor and has been previously used to generate highly entangled twin beams. In the proposed configuration, one of the twin beams is used to seed another FWM process. This leads to the amplification of the beam used as the seed and the generation of an additional entangled beam of light, thus increasing the number of entangled parties by one. One of the advantages of the proposed source is that is phase insensitive, which makes it easily scalable to a large number of parties by cascading multiple FWM processes. We have experimentally verified that a configuration of two cascaded FWM processes leads to the generation of three beams that contain quantum correlations in the form of intensity-difference squeezing and show that the level of squeezing produced by the first FWM process is increased by the second one. We also derive a sufficient and necessary criterion for the presence of multipartite entanglement for the proposed configuration that shows that one should expect the beams generated by the cascaded FWM processes to be entangled.

43. Symmetry-Protected Topological Entanglement

Iman Marvian, University of Southern California

(Session 9b : Friday from 5:30pm - 6:00pm)

We propose an order parameter for the Symmetry-Protected Topological (SPT) phases which are protected by an Abelian on-site symmetry. This order parameter, called the SPT entanglement, is defined as the entanglement between A and B, two distant regions of the system, given that the total charge (associated with the symmetry) in a third region C is measured and known, where C is a connected region surrounded by A and B and the boundaries of the system. In the case of 1-dimensional systems we prove that at the limit where A and B are large and far from each other compared to the correlation length, the SPT entanglement remains constant throughout a SPT phase, and furthermore, it is zero for the trivial phase while it is nonzero for all the non-trivial phases. Moreover, we show that the SPT entanglement is invariant under the low-depth local quantum circuits which respect the symmetry, and hence it remains constant throughout a SPT phase in the higher dimensions as well. Finally, we show that the concept of SPT entanglement leads us to a new interpretation of the string order parameters and based on this interpretation we propose an algorithm for extracting the relevant information about the SPT phase of the system from the string order parameters.

44. A generalization of Schur-Weyl duality with applications in quantum estimation

Iman Marvian, University of Southern California

(Session 5 : Thursday from 5:00pm - 7:00pm)

Schur-Weyl duality is a powerful tool in representation theory which has many applications to quantum information theory. We provide a generalization of this duality and demonstrate some of its applications. In particular, we use it to develop a general framework for the study of a family of quantum estimation problems wherein one is given n copies of an unknown quantum state according to some prior and the goal is to estimate certain parameters of the given state. In particular, we are interested to know whether collective measurements are useful and if so to find an upper bound on the amount of entanglement which is required to achieve the optimal estimation. In the case of pure states, we show that commutativity of the set of observables that define the estimation problem implies the sufficiency of unentangled measurements.

45. CNOT Decompositions for Clifford Operators

Adam Meier, Georgia Tech Research Institute

(Session 5 : Thursday from 5:00pm - 7:00pm)

The Clifford group of unitary operators shows up in quantum error correction, randomized benchmarking, and many fault-tolerant techniques for quantum computing. Experimentally, any operator in this group can be implemented by multiple applications of the Hadamard, “phase” or CNOT operators on subsets of qubits. Of these generating operators, the CNOT is almost certain to have the lowest experimental fidelity, so it is worthwhile to optimize such operator decompositions to reduce the number of CNOT applications. After a brief introduction to the Clifford group, I will describe attempts to understand the structure of the Clifford group with respect to the minimal number of CNOTs needed to generate its elements. These attempts include both exhaustive characterizations of optimal decompositions for each group element and efficient algorithms for nearly optimal decompositions. This work was performed in collaboration with Emanuel Knill.

46. An on-chip toolset for surface-electrode trap based quantum processors

True Merrill, Georgia Tech Research Institute

(Session 1 : Thursday from 9:45am - 10:15am)

Increasing the size and complexity of ion-trap quantum computing experiments requires improvements in automation, hardware, and control.  We report on several technologies which incorporate control electronics, diffractive ion imaging optics, and quantum control techniques for microwave gates in microfabricated surface-traps.  We demonstrate a compact, in-vacuum 80 channel digital-to-analog converter (DAC) system controlling a surface-electrode trap.  The DAC system transports  40Ca+ ions for over 70 m at 1 m/s without cooling, and the measured 0.8 quanta/ms  ion-heating rate is comparable to external DAC systems.  A second project incorporates diffractive Fresnel mirrors onto a trap surface for enhanced ion imaging and state detection.  Optics for light collimation and refocusing are demonstrated, achieving a ~8.3 x enhancement in the total fluorescence signal.  We comment on possible limits to asynchronous ion-qubit readout and strategies to mitigate decoherence from stray photons during measurement processes. Finally, we discuss composite pulse techniques for gates on 171Yb+ qubits that yield accurate quantum control despite classical control errors.

47. Symmetry-protected topological ordered phases and their use for quantum computation

Akimasa Miyake, University of New Mexico

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

Collective phenomena, like superconductivity and magnetism, are usually robust features of quantum many-body systems. They only depend on a few key parameters of a system Hamiltonian, and often symmetries are sufficient enough to characterize different (quantum) phases associated with different collective behaviors. Through remarkable progress at the crossover between quantum information and quantum many-body physics, it gets more and more clear that certain strongly-correlated ground states could be harnessed for quantum information processing, based on their underlying entanglement structure and thus inherent complexity. For example, some concrete models of topological orders are most known by the application to quantum error correction. Here we address a question: "to what extent computational usefulness of quantum many-body ground states would be determined ubiquitously by symmetries, without the system Hamiltonian specified in detail?" Our approach may be also applicable to how quantum state preparation and verification can be made without detailed knowledge about the system, in the context of quantum simulation. This is a joint work with Jacob Miller.

48. Progress towards quantum control and squeezing of collective spins

Enrique Montano, University of Arizona

(Session 5 : Thursday from 5:00pm - 7:00pm)

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 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 QND measurement of the atomic 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 prepared an ensemble of atoms with high optical depth in a crossed optical dipole trap and have generated squeezing of the collective spin of the ensemble. To achieve metrologically relevant spin squeezing, we have implemented a two color probe scheme to suppresses the detrimental effects of tensor light shifts. We are now working to increase atom-light coupling in our experiment, by optimizing the 3D geometry and by using individual-atom control to prepare initial states that exhibit greater spin projection noise.

49. Quantum limits on Probabilistic Amplifiers

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

(Session 5 : Thursday from 5:00pm - 7:00pm)

An ideal phase-preserving linear amplifier is a deterministic device that adds to an input signal the minimal amount of noise consistent with the constraints imposed by quantum mechanics. A noiseless linear amplifier takes an input coherent state to an amplified coherent state, but only works part of the time. Such a device is actually better than noiseless, since the output has less noise than the amplified noise of the input coherent state; for this reason we refer to such devices as immaculate. We bound the working probabilities of probabilistic and approximate immaculate amplifiers and construct theoretical models that achieve some of these bounds. Our chief conclusions are the following: (i) the working probability of any phase-insensitive immaculate amplifier is very small in the phase-plane region where the device works with high fidelity; (ii) phase-sensitive immaculate amplifiers that work only on coherent states sparsely distributed on a phase-plane circle centered at the origin can have a reasonably high working probability.

50. Topological defect formation and dynamics in ion Coulomb crystals

Heather Partner, Physikalisch-Technische Bundesanstalt

(Session 8 : Friday from 3:00pm - 3:30pm)

Topological defects (kinks) in laboratory systems have attracted recent interest because of their universal nature. In our system, kinks form during nonlinear quenches from the linear to zigzag phase in Coulomb crystals of about 30 172Yb+ ions in a segmented linear trap. I will present our experimental study of probabilistic kink creation in the context of the Kibble-Zurek mechanism, which predicts a scaling for defect creation as a function of quench rate, and discuss the effects of inhomogeneity and finite size in such systems. In addition, I will describe the dynamics of such defects and how they can be controllably modified. These methods provide a toolbox for using kinks to study phase transitions and soliton physics, and as a potential carrier of quantum information.

51. Optimal dissipative encoding and state preparation for topological order

Fernando Pastawski, California Institute of Technology

(Session 9b : Friday from 4:30pm - 5:00pm)

We study the suitability of dissipative (non-unitary) processes for (a) encoding logical information into a topologically ordered ground space and (b) preparing an (arbitrary) topologically ordered state. We give a construction achieving (a) in time O(L) for the LxL-toric code by evolution under a geometrically local, time-independent Liouvillian. We show that this scaling is optimal: even the easier problem (b) takes at least O(L) time when allowing arbitrary (possibly time-dependent) dissipative evolution. For more general topological codes, we obtain similar lower bounds on the required time for (a) and (b). These bounds involve the code distance and the dimensionality of the lattice. The proof involves Lieb-Robinson bounds, recent cleaning-lemma-type arguments for topological codes, as well as uncertainty relations between complementary observables. By allowing general locality-preserving evolutions (including, e.g., circuits of CPTPMs), our results extend earlier work characterizing unitary state preparation.

52. Autoresonance control protocols in an open quantum system

Arjendu Pattanayak, Carleton College

(Session 5 : Thursday from 5:00pm - 7:00pm)

A classical nonlinear oscillator can be driven to increasingly higher energy by chirping the driving frequency with a chirp rate chosen by various protocols, including one that analyzes the Teager-Kaiser energy operator. We report on the effect of applying this protocol to an open quantum system, particularly as the system size is changed so that the effective Planck's constant increases in size and the behavior becomes more quantum-mechanical. We comment on the connection with the Quantum Ladder Climbing protocol applicable in the extreme quantum limit. (Henry Luo, Ali Ehlen, Zhilu Zhang, and Arjendu Pattanayak)

53. An atomic superfluid Bose-Einstein condensate in a ring

William Phillips, Joint Quantum Institute/National Institute of Standards and Technology

(Session 6 : Friday from 8:30am - 9:15am)

A Bose-Einstein condensate extending around a ring-shaped trap that is interrupted by a repulsive barrier can exhibit behavior similar to that of a superconducting loop interrupted by a weak link or Josephson junction. [ See, for example, K. Wright, R. Blakestad, C. Lobb, W. Phillips, and G. Campbell, Phys. Rev. Lett 110, 025302 (2013). DOI: 10.1103/PhysRevLett.110.025302]. We observe controllable, discrete phase slips (jumps in the phase winding number around the ring) and hysteretic behavior. Analogies to the superconducting circuit can describe much of the behavior of this atomtronic circuit. This work was partially supported by the ONR, the NSF, and the ARO.

54. Control of Quantum Chaos

Bibek Pokharel, Carleton College

(Session 5 : Thursday from 5:00pm - 7:00pm)

We have recently computed Lyapunov exponents describing the chaotic behavior of the quantum trajectories of an open quantum nonlinear oscillator using the Quantum State Diffusion formalism. We have seen several interesting features as a function of changing system parameters. We report on progress towards controlling the observed quantum chaotic behavior using the classical Ott‐Grebogi‐Yorke protocol. [With Arjendu K. Pattanayak, Carleton College]

55. Progress towards experimentally realizing movable atom traps behind an array of pinholes for quantum computing

Ian Powell, California Polytechnic State University, San Luis Obispo

(Session 5 : Thursday from 5:00pm - 7:00pm)

The neutral atom quantum computing community has successfully demonstrated all criteria for the implementation of a quantum computer except for scalability. We propose to use atoms trapped in the diffraction pattern behind a two-dimensional array of pinholes as a scalable, addressable array of quantum bits (qubits). Changing the angle of incidence of the laser beams illuminating the pinhole array will facilitate two-qubit gates by bringing pairs of atoms together and apart controllably. The current areas of focus of our work are to directly measure the pinhole diffraction pattern for laser beams at large incident angles and to experimentally achieve the transfer of rubidium atoms from a magneto-optical trap (MOT) to the pinhole traps.We have designed and built a circuit for quickly switching off the MOT magnetic field in order to transfer the cold atoms to the pinhole traps. We are building optical setups for projecting the diffraction pattern into the MOT and for characterizing the MOT cloud and pinhole traps using a high-speed camera and photodiode. We are in the process of developing a LabVIEW program for controlling the atom transfer sequence and recording images and photodiode signals of the trapped atoms.We will present the latest progress and results of our work. This work was performed in collaboration with Sanjay Khatri, Jason Schray, Taylor Shannon,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.

56. Local Detection of Quantum Correlations with a Single Trapped Ion

Thaned Pruttivarasin, University of California, Berkeley

(Session 5 : Thursday from 5:00pm - 7:00pm)

Entanglement is one of the most important feature of quantum mechanics. In small systems, full state tomography can reveal such quantum correlations between subsystems and has been implemented in modern experiments routinely. For larger system, full state tomography is time consuming and tedious. We show a realization of quantum correlations detection scheme between two subsystems, represented by two degree-of-freedoms of a single trapped ion, namely, the electronics state and motional states, by accessing only one of the subsystem. Using this protocol, we can infer a lower bound of quantum correlations between them without having to do full state tomography.

57. Towards space-time crystals with trapped ions

Anthony Ransford, University of California, Berkeley

(Session 5 : Thursday from 5:00pm - 7:00pm)

Recent work has shown that spontaneous symmetry breaking can lead to a crystal not only in space but also in time [F. Wilczek, (2012)]. For instance, there exist static situations with time dependent (quasi) ground states. A proposal for constructing such a phase of matter has been presented for ions in a cylindrically symmetric RF trap with a constant magnetic field [T. Li et al, (2012)]. We present some experimental challenges towards implementing such time crystals with trapped ions. For 100 ions trapped in a ring structure with diameter of 100 micrometers, we expect a level spacing of 1 kHz between the quasi ground states and the next excited state. Thus, exceptionally low heating is required to maintain the ion ring in the quasi ground state and to study the time crystal. We discuss plans to adiabatically cool the ion ring to its motional ground state with a variable pinning potential. Zig-zag phase transitions exhibiting Kibble-Zurek topological defects might also be studied in a similar set up.

58. Spectrally Entangled Photon Pairs for Ultrafast Probing of Molecules

Michael Raymer, University of Oregon

(Session 3 : Thursday from 2:45pm - 3:15pm)

We introduce a new method, called entangled photon-pair two-dimensional fluorescence spectroscopy (EPP-2DFS), to sensitively probe the nonlinear electronic response of complex molecular systems. The method incorporates a separated two-photon (Franson) interferometer, which generates time-frequency-entangled photon pairs, into the framework of a fluorescence-detected 2D optical spectroscopic experiment. The EPR-like correlations in time and frequency allow circumventing the usual restrictions imposed by the time-frequency uncertainty principle.

59. Classical command of quantum systems

Ben Reichardt, University of Southern California

(Session 12 : Saturday from 2:30pm - 3:15pm)

Can a classical experimentalist command an untrusted quantum system to realize arbitrary quantum dynamics, aborting if it misbehaves?  We give a way for a classical system to certify the joint, entangled state of a bipartite quantum system, as well as command the application of specific operators on each subsystem. This is accomplished by showing a strong converse to Tsirelson's optimality result for the CHSH game: the only way to win many games is if the bipartite state is close to the tensor product of EPR states, and the measurements are the optimal CHSH measurements on successive qubits. This leads directly to a scheme for device-independent quantum key distribution. Control over the state and operators can also be leveraged to create more elaborate protocols for reliably realizing general quantum circuits. Joint work with Falk Unger and Umesh Vazirani.

60. Quantum optics experiments at Earth-orbital scales and beyond

David Rideout, University of California, San Diego

(Session 9c : Friday from 4:30pm - 5:00pm)

A number of national and international entities are racing to set up satellite-based quantum communication infrastructure, which would allow the construction of a global network for quantum key distribution. The construction of such a network poses numerous technological challenges, considering that quantum entanglement has not yet been demonstrated at such scales. I will provide some details of a Canadian Space Agency funded mission to demonstrate quantum entanglement between the Earth's surface and Low Earth Orbit, and sketch some tests of fundamental physics which could be enabled by such satellites, ranging from verification of quantum mechanics to tests of spacetime discreteness from quantum gravity.

61. What constitutes a resource state for measurement-based quantum computation?

Eleanor Rieffel, NASA Ames Research Center

(Session 5 : Thursday from 5:00pm - 7:00pm)

We consider the issue of what should count as a resource for measurement-based quantum computation (MBQC), and propose some minimal criteria. The term “resource” is used most frequently when discussing which states support universal MBQC and which do not [1-6]. Universality is a sufficient condition, but seems too strong as a necessary condition given known classes of MBQCs that likely give an advantage over classical computing but which are not universal [7-9]. One could try to characterize which resource states support (or do not support) computationally interesting MBQC. The problem is that, almost certainly, not all the interesting MBQC tasks are known. Instead, we concentrate on a weaker property, namely what makes families of MBQCs worthy to be called “measurement-based.” We introduce the notion of inherently measurement-based computations, and give a series of necessary conditions for families of MBQCs to be considered inherently measurement-based. We propose that for a state to be considered a resource for MBQC it must support a family of MBQCs that is inherently measurement-based. Using these criteria, we explain why discord-free states cannot be resources for MBQC, in spite of claims to the contrary [9]. We do not answer the question as to whether entanglement is required. Joint work with Howard M. Wiseman. [1] M. Van den Nest, A. Miyake, W. Du ̈r, and H. J. Briegel, Physical Review Letters 97, 150504 (2006). [2] D. Gross, J. Eisert, N. Schuch, and D. Perez-Garcia, Physical Review A 76, 052315 (2007). [3] H. Briegel, D. Browne, W. Du ̈r, R. Raussendorf, and M. Van den Nest, Nature Physics 5, 19 (2009). [4] D. Gross, S. T. Flammia, and J. Eisert, Physical Review Letters 102, 190501 (2009). [5] M. J. Bremner, C. Mora, and A. Winter, Physical Review Letters 102, 190502 (2009). [6] R. A. Low, Large deviation bounds for k-designs, arXiv:0903.5236 (2009). [7] J. Anders and D. E. Browne, Physical Review Letters 102, 050502 (2009). [8] M. J. Bremner, R. Jozsa, and D. J. Shepherd, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Science 467, 459 (2011). [9] M. J. Hoban, J. J. Wallman, H. Anwar, N. Usher, R. Raussendorf, and D. E. Browne, Exact sampling and entanglement-free resources for measurement-based quantum computation, arXiv:1304.2667v1 (2013).

62. Tomography of Quantum Fields

Carlos Riofrio, Freie Universität Berlin

(Session 11 : Saturday from 10:45am - 11:15am)

Understanding the fundamental interactions in many-body physical systems is of great interest in current theoretical and experimental efforts. In particular, continuous many-body systems or fields are exciting because they offer the tools for performing quantum simulations of processes of non-equilibrium, equilibration and thermalization. In this context, the problem of developing tools for identifying and reconstructing the state or some aspect of such systems is needed on a practical level. In this talk, I will present a first approach in that direction and possible applications for reconstructing quantum fields from low order correlation functions readily measurable in current experiments. We concentrate on one dimensional systems with spatially limited entanglement which are well described by the continuous matrix product state (cMPS) formalism.

63. Mapping the topological phase diagram of superconducting qubit systems

Pedram Roushan, UCSB

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

Building a practical quantum simulator requires a scalable architecture suitable for large numbers of qubits. By combining the high coherence Xmon qubits with an adjustable inductance, we have developed a new qubit architecture called g-mon, which has a tunable qubit-qubit interaction. To demonstrate this tunability, we have performed high fidelity single and two-qubit gates. Turning on the qubit-qubit interaction allows for fast multi-qubit operations implemented in less than 30 ns, achieving multi-qubit gate times approaching that of single qubit gates. Furthermore, we show the versatility of this system by mapping the topological phase diagram of interacting Hamiltonians. So far, experimental studies of topological invariants in condensed matter systems have been limited to transport measurements in non-interacting systems. Recently, it was proposed [1] that the topological properties of Hamiltonians can be inferred from quantum dynamics. Using superconducting g-mon qubits, we experimentally measure the Berry curvature, a quantity that reflects the geometrical properties of the eigenstates, for various eigenstates of the Hamiltonian of the system. We will discuss the phase diagram of various topological phases and the robustness of the measured Chern numbers. [1] Gritsev and Polkovnikov, PNAS, 109, 6457 (2012).

64. Strong converse rates for classical communication over thermal bosonic channels

Bhaskar Roy Bardhan, Louisiana State University

(Session 5 : Thursday from 5:00pm - 7:00pm)

We prove that several known upper bounds on the classical capacity of thermal bosonic channels are actually strong converse rates. Our results strengthen the interpretation of these upper bounds, in the sense that we now know that the probability of correctly decoding a classical message rapidly converges to zero in the limit of many channel uses if the communication rate exceeds these upper bounds. In order for these theorems to hold, we need to impose a maximum photon number constraint on the states input to the channel (the strong converse property need not hold if there is only a mean photon number constraint). Our first theorem demonstrates that a capacity upper bound due to Koenig and Smith is a strong converse rate, and we prove this result by utilizing their structural decomposition of a thermal channel into a pure-loss channel followed by an amplifier channel. Our second theorem demonstrates that an upper bound due to Giovannetti et al. corresponds to a strong converse rate, and we prove this result by relating success probability to rate, the effective dimension of the output space, and the purity of the channel as measured by the Renyi collision entropy.

65. Quantum Behavior of Electro-mechanical Structures

Keith Schwab, Caltech

(Session 6 : Friday from 9:15am - 10:00am)

Electro-mechanical structures composed of a radio-frequency mechanical resonator parametrically coupled to a superconducting microwave frequency electrical resonator, offer an opportunity to study quantum behavior of both the electronic and mechanical degrees of freedom. We have recently demonstrated detection of a single motional quadrature with imprecision less than x_zp, and avoidance of the backaction due to the shot noise of the microwave field. We have also demonstrated the measurement of the imbalance between up and down conversion and will discuss the interpretation of these measurements. Finally, we have also recently demonstrated quantum squeezing of the motion using a reservoir engineering technique described by Aash Clerk et al and will present this new data.

66. Optimal phase estimation in the presence of dephasing noise using photon number and parity measurements

Kaushik Seshadreesan, Louisiana State University

(Session 5 : Thursday from 5:00pm - 7:00pm)

We study interferometric phase estimation in the presence of dephasing noise using photon number, and photon number parity, measurements. We show that both the above measurements can achieve phase sensitivities at the quantum Cramer-Rao bound of the optimal probe state preparation. Furthermore, we show that when operated using a Bayesian update protocol, photon number measurement can be made optimally sensitive to phase fluctuations independently of the actual value of the unknown phase.

67. Autonomously stabilized entanglement between two superconducting qubits

Shyam Shankar, Yale University

(Session 10 : Saturday from 8:30am - 9:15am)

Quantum error-correction codes are designed to protect an arbitrary state of a multi-qubit register against decoherence-induced errors, but their implementation is an outstanding challenge for the development of large-scale quantum computers. A first step is to stabilize a non-equilibrium state of a simple quantum system such as a qubit or a cavity mode, in the presence of decoherence. Several groups have recently accomplished this goal using measurement-based feedback schemes. A next step is to prepare and stabilize a state of a composite system. Here we demonstrate the stabilization of an entangled Bell state of a quantum register of two superconducting qubits for an arbitrary time. Our result[1] is achieved by an autonomous feedback scheme which combines continuous drives along with a specifically engineered coupling between the two-qubit register and a dissipative bath. Similar bath engineering techniques have recently been used for qubit reset, single qubit state stabilization, as well as for the creation and stabilization of states of multipartite quantum systems. Unlike conventional, measurement-based schemes, an autonomous approach which uses engineered dissipation to counteract decoherence, obviates the need for a complicated external feedback loop to correct errors. Instead the feedback loop is built into the Hamiltonian such that the steady state of the system in the presence of drives and dissipation is a Bell state, an essential building-block for quantum information processing. Such autonomous schemes, which are broadly applicable to a variety of physical systems, will be an essential tool for the implementation of quantum-error correction. [1] http://dx.doi.org/10.1038/nature12802

68. Tensor networks and Symmetries

Sukhbinder Singh, Macquarie University, Sydney

(Session 9b : Friday from 4:00pm - 4:30pm)

Tensors networks methods, which are based on ideas of entanglement and renormalization group, have significantly progressed our understanding of strongly correlated quantum lattice systems in recent years. Examples of popular tensor network states include the matrix product state (MPS)[1], which results naturally from both Wilson’s numerical renormalization group[2] and White’s density matrix renormalization group(DMRG)[3], and the multi-scale entanglement renormalization ansatz (MERA) [4] which is based a specific RG scheme known as entanglement renormalization [5]. These tensor networks have been applied to the exploration of frustrated antiferromagnets, interacting fermions, quantum criticality, topological order and symmetry protected order, and more recently, the MERA has been used to explore[6] the holographic correspondence[7] of string theory. The careful incorporation[8] of lattice symmetries (both spacetime and internal symmetries) in tensor networks is playing an increasingly important role in these applications. In this talk I will outline some aspects of this role in the context of the MPS and the MERA where symmetries have been exploited to (i) target specific quantum number sectors of the Hilbert space, which subsequently also allows for the efficient simulation of lattice systems of anyons[9], (ii) identification of the quantum order of a ground state from its tensor network description[10], and (iii) to realize certain symmetry features of the holographic correspondence in the MERA. References [1] S. Ostlund and S. Rommer, Phys. Rev. Lett. 75, 3537 (1995). [2] K.G. Wilson, Rev. Mod. Phys. 47, 4, 773 (1975). [3] S.R. White, Phys. Rev. Lett. 69, 2863 (1992). [4] G. Vidal, Phys. Rev. Lett. 101, 110501 (2008). [5] G. Vidal, Phys. Rev. Lett. 99, 220405 (2007). [6] B. Swingle, Phys. Rev. D 86, 065007 (2012). [7] J. Maldacena, Adv. Theor. Math. Phys. 2, 231 (1998). [8] S. Singh, R. N. C. Pfeifer, and G. Vidal, Phys. Rev. A 82, 050301 (2010). [9] S. Singh, R. N. C. Pfeifer, G. Vidal, and G. K. Brennen. arXiv:1311.0967 (2013). [10] S. Singh and G. Vidal. Phys. Rev. B 88, 121108(R) (2013).

69. A quantum fractional Fourier transform

Rolando Somma, Los Alamos National Laboratory

(Session 2 : Thursday from 11:30am - 12:00pm)

The Fourier transform (FT) is ubiquitous in signal processing, as it can be used to filter noise. The digital version, often named the discrete Fourier transform, when formulated on a basis of quantum states, is the quantum Fourier transform (QFT). The efficiency in the implementation of the QFT is the main reason for several quantum speedups, including the one for factoring and the one in phase estimation at the Heisenberg limit. The fractional FT (frFT) is a generalization of the FT. The frFT has recently gained attention in signal analysis as it can filter noise in scenarios where the FT is not useful. Quantum frFTs (QfrFTs), however, have never been analyzed or applied; We propose a QfrFT and show that a good approximation of this transformation can be implemented on a quantum computer with exponentially less resources than those required for its conventional implementation. We then analyze some problems in signal analysis (parameter estimation) for which our defined QfrFT is useful. Applications of the QfrFT for the simulation of continuous-variable quantum mechanics will also be considered.

70. New Tools for Unitary Control of Cold Atom Qudits

Hector Sosa Martinez, University of Arizona

(Session 9c : Friday from 4:00pm - 4:30pm)

Accurate and robust quantum control of single or coupled qubit systems is a key element of quantum information science. In practice, the actual physical building blocks (atoms, ions, superconducting devices) are often qudits with state space dimension d>2, and the available auxiliary levels have proven useful for information processing tasks such as implementing Toffoli gates with two-body interactions. More generally, large internal state spaces may prove a useful resource if good laboratory tools for qudit manipulation can be developed. As a laboratory test bed for such development, we have implemented a protocol to perform arbitrary unitary transformations in the 16 dimensional ground hyperfine manifold of individual 133Cs atoms, by driving this system with phase modulated rf and microwave magnetic fields and using the tools of optimal control to find appropriate control waveforms. Similar to what can be achieved for qubits, we show that accurate unitary control can be achieved in the presence of simultaneous static and dynamical perturbations and imperfections in the control fields, simply by optimizing with respect to the appropriate cost function when designing control waveforms. We anticipate this approach to prove helpful for control in less than ideal environments, such as atoms moving around in the light shift potential of a dipole trap. We are currently exploring the prospects for inhomogeneous quantum control, with the goal of performing different unitary transformations on qudits that see different light shifts from an optical addressing field. Ultimately this may lead to addressable unitary control similar to what has been demonstrated for atomic qubits in optical lattices.

71. Towards a Quantum Memory with Telecom-wavelength Conversion

Daniel Stack, United States Army Research Laboratory

(Session 5 : Thursday from 5:00pm - 7:00pm)

Fiber-based transmission of quantum information over long distances may be achieved using quantum memory elements and quantum repeater protocols. However, atom-based quantum memories typically involve interactions with light fields outside the telecom window needed to minimize absorption in transmission by optical fibers. We report on progress towards a quantum memory based on the generation of 780 nm spontaneously emitted single photons by a write-laser beam interacting with a cold 87Rb ensemble. The single photons are then frequency-converted into (out of) the telecomm band via difference (sum) frequency generation in a PPLN crystal. Finally, the atomic state is read out via the interaction of a read-pulse with the quantum memory. With such a system, it will be possible to realize a long-lived quantum memory that will allow transmission of quantum information over many kilometers with high fidelity, essential for a scalable, long-distance quantum network.

72. Repeat-Until-Success: Non-deterministic decomposition of single-qubit unitaries

Krysta Svore, Microsoft Research

(Session 8 : Friday from 2:30pm - 3:00pm)

We present a non-deterministic circuit decomposition technique for approximating an arbitrary single-qubit unitary to within distance epsilon that requires significantly fewer non-Clifford gates than deterministic decomposition techniques. We develop ``Repeat-Until-Success" (RUS) circuits and characterize unitaries that can be exactly represented as an RUS circuit. Our RUS circuits operate by conditioning on a given measurement outcome and using only a small number of non-Clifford gates and ancilla qubits. We construct an algorithm based on RUS circuits that approximates an arbitrary single-qubit Z-axis rotation to within distance epsilon, where the number of T gates scales as 1.26*log_2(1/\epsilon) - 3.53, an improvement of roughly three-fold over state-of-the-art techniques. We then extend our algorithm and show that a scaling of 2.4 * log_2(1/\epsilon) - 3.28 can be achieved for arbitrary unitaries and a small range of epsilon, which is roughly twice as good as optimal deterministic decomposition methods.

73. Progress towards attaining an equidistant ion chain in an annular segmented surface ion trap

Boyan Tabakov, Sandia National Laboratories and University of New Mexico

(Session 5 : Thursday from 5:00pm - 7:00pm)

Over almost a decade, a primary drive for developing microfabricated segmented surface electrode ion traps has been the application of trapped ions as a quantum information processing platform. At Sandia National Laboratories we design, fabricate, and test such traps, the utility of which extends beyond the realm of quantum computation. One highly symmetric design that has the potential to provide periodic boundary conditions allowing studies of quantum phase transitions, or could be a testbed for observing Hawking radiation from acoustic black holes, is the ring trap. We demonstrate forming a ring of hundreds of Ca⁺ ions in that trap, and report on the challenges and our progress towards attaining equal spacing in the ion chain. In the absence of undesired stray fields, we envision the ions separated by their mutual Coulomb repulsion and moving on the ring.

74. Trapped Atoms and Polarimetry in a Nanofiber-based Quantum Interface

Kyle Taylor, University of Arizona

(Session 5 : Thursday from 5:00pm - 7:00pm)

D. Melchior, K. Taylor, P. G. Mickelson, and P. S. Jessen We describe an experiment that will use the evanescent-wave field of a tapered optical fiber (nanofiber) to trap cold atoms and control their 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 10^2 is expected. Here, we report experimental progress towards loading cold atoms samples into these nanofiber traps.

75. Quantum simulation of chemical systems based on the sparsity of the CI-matrix

Borzu Toloui, Haverford College/ visting Harvard University

(Session 5 : Thursday from 5:00pm - 7:00pm)

Quantum chemistry is an area where quantum simulation algorithms can make considerable contributions in science and technology. The majority of algorithms for simulating electronic structures to date have used a second-quantized representation of the respective Hamiltonian. The qubit requirements for such algorithms that scale linearly with the maximum number of orbitals that are included in the problem. However, storing the full Fock space of the orbitals is unnecessary because the number of electrons is a fixed and known parameter of the problem. Representing the wave function in a basis of slater determinants for fixed electron number suffices. We show how to apply techniques developed for the simulation of sparse Hamiltonians to the CI-matrix that is expressed in such basis. We show that it is possible to use the minimal number of qubits to represent the wave function. We also show that these methods can offer improved scaling in the number of gates required by cleverly exploiting the structure of the CI-matrix.

76. On generating macroscopic superpositions via nonlinear dynamics of stopped light in a two-component Bose-Einstein condensate

Collin Trail, University of Calgary

(Session 9c : Friday from 5:30pm - 6:00pm)

We investigate a method for generating nonlinear phase shifts on superpositions of photon number states. The light is stored in a Bose-Einstein condensate via electromagnetically-induced transparency memory techniques. The atomic collisions are exploited to generate a nonlinear evolution for the stored state. The stored light is then revived with the nonlinear phase shift imprinted upon it. For the special case of a coherent state input we find that this method can be used to generate an optical cat state. We investigate the validity of using the Thomas-Fermi and mean-field approximations.

77. Mismatched quantum filtering and entropic information

Mankei Tsang, National University of Singapore

(Session 7 : Friday from 11:15am - 11:45am)

Quantum filtering is a signal processing technique that estimates the posterior state of a quantum system under continuous measurements and has become a standard tool in quantum information processing, with applications in quantum state preparation, quantum metrology, and quantum control. If the filter assumes a wrong model due to assumptions or approximations, however, the estimation accuracy is bound to suffer. I shall present formulas that relate the error penalty caused by quantum filter mismatch to the relative entropy between the true model and the nominal model, with one formula for Gaussian measurements, such as homodyne detection, and another for Poissonian measurements, such as photon counting. These formulas generalize recent seminal results in classical information theory and provide new operational meanings to relative entropy, mutual information, and channel capacity in the context of quantum experiments. See http://arxiv.org/abs/1310.0291 for details.

78. Distinguishability of Qudit Hyperentangled States with Linear Evolution and Local Measurement

Andrew Turner, Harvey Mudd College

(Session 5 : Thursday from 5:00pm - 7:00pm)

Measurement of an entangled state in the Bell state basis is an integral part of many protocols in quantum communication. Of particular interest is measurement that uses only linear evolution and local measurement (LELM). Previous work has shown that for two identical particles entangled in n qubits, 2^(n+1)-1 classes of the 4^n hyperentangled Bell states can be distinguished using an LELM device. I will present recent progress on the Bell state distinguishability problem for general hyperentangled states using LELM. This includes limits on distinguishability for the qutrit and (qubit) x (qutrit) entangled states of two particles.

79. All-Optical Switching and Router via the Direct Quantum Control of Coupling between Cavity Modes

Jason Twamley, Macquarie University

(Session 12 : Saturday from 3:15pm - 3:45pm)

We describe a scheme to execute all-optical routing of photonic information by optically controlling the internal quantum state of a individual scatterer coupled to two independent cavity modes. We show that through this quantum control one can dynamically and rapidly modulate the cavity coupling. This allows all-optical modulation of intercavity couplings via ac Stark or shuffle (stimulated Raman adiabatic passage) control of the scatterer’s internal states, and from this modulation, we show that we can perform all-optical switching and all-optical routing with near-unit switching contrast and with high bandwidth. [1] K. Xia and J. Twamley, Phys Rev X 3, 031013 (2013)

80. General relativistic quantum information

James van Meter, National Institute of Standards and Technology

(Session 5 : Thursday from 5:00pm - 7:00pm)

Relativistic quantum information theory is an emerging field concerned with new phenomena and methods that may emerge from a fully relativistic treatment of quantum information theory. Of particular interest are the effects of curved spacetime, which may for example have measurable effects on quantum communication with satellites. Here we consider the sensitivity of quantum devices to the gravitational field, and the potential for relativistic quantum metrology.

81. A polynomial-time algorithm for the ground state of 1D gapped local Hamiltonians

Thomas Vidick, UC Berkeley

(Session 4 : Thursday from 3:45pm - 4:30pm)

Computing ground states of local Hamiltonians is a fundamental problem in condensed matter physics. We give the first randomized polynomial-time algorithm for finding ground states of gapped one-dimensional Hamiltonians: it outputs an (inverse-polynomial) approximation, expressed as a matrix product state (MPS) of polynomial bond dimension. The algorithm combines many ingredients, including recently discovered structural features of gapped 1D systems, convex programming, insights from classical algorithms for 1D satisfiability, and new techniques for manipulating and bounding the complexity of MPS. Our result provides one of the first major classes of Hamiltonians for which computing ground states is provably tractable despite the exponential nature of the objects involved.

82. Strongly interacting photons

Vladan Vuletic, MIT

(Session 3 : Thursday from 1:30pm - 2:15pm)

I discuss two experimental systems where individual photons interact strongly with one another. One is a cavity QED system with an atomic ensemble inside an optical resonator where one photon stored in the ensemble in the form of a collective excitation can switch the transmission of more than one photon through the cavity. The other is a free-space system where photons traveling slowly in an atomic ensemble interact with one another via the coupling to strongly mutually interacting Rydberg states. We observe attractive forces between individual photons leading to the formation of a two-photon bound state, and measure a conditional two-photon phase shift exceeding pi/4.

83. Microwave shot noise and quantum motional sideband asymmetry in an electro-mechanical device

Aaron Weinstein, California Institute of Technology

(Session 5 : Thursday from 5:00pm - 7:00pm)

A quantum harmonic oscillator has an asymmetric position noise spectral density between the positive and negative frequencies, which is the direct outcome of the Heisenberg uncertainty principle. Here, we report a measurement of the up and down-converted sidebands of a radio-frequency mechanical resonator parametrically coupled to a super-conducting microwave transducer. By accounting for the classical microwave noise in the device, we measure a sideband imbalance of 1.2+-0.2 quanta at mechanical occupations near the ground state. Finally, we show that the interpretation of this imbalance must incorporate the type of detection scheme used in the measurement. For amplitude detection of the sidebands presented here, the asymmetry arises solely from the quantum fluctuations of the microwave field, not of the mechanics, and shows good agreement with the imbalance observed in this measurement.

84. Nonlinear Analog Quantum Computation

Thomas Wong, University of California, San Diego

(Session 5 : Thursday from 5:00pm - 7:00pm)

Extensive experimental work has shown that the effect of any fundamental nonlinear generalization of quantum mechanics must be tiny. Nevertheless, there are quantum mechanical systems with multiple interacting particles whose effective evolution is governed by a nonlinear Schrödinger equation with a term proportional to f(|ψ|²)ψ. This includes the Gross-Pitaevskii equation with a cubic nonlinearity that describes Bose-Einstein condensates, the cubic-quintic nonlinear Schrödinger equation that describes light propagation in nonlinear Kerr media with defocusing corrections, and the logarithmic nonlinear Schrödinger equation that describes Bose liquids under certain conditions. We quantify the computational speedup that this general nonlinearity has in solving the unstructured search problem. In doing so, we identify a host of physically realistic nonlinear quantum systems that can be used to perform continuous-time computation faster than (linear) quantum computation, up to a bound on the size of the problem such that the nonlinear equation is a good approximation of the linear dynamics of the system, unless the nonlinearity is fundamental.

85. Violation of the Arrhenius law for memory time below magnetic and topological transition temperature

Beni Yoshida, California Institute of Technology

(Session 7 : Friday from 11:45am - 12:15pm)

When interacting spin systems possess non-zero magnetization or topological entanglement entropy below the transition temperature, they often serve as classical or quantum self-correcting memory whose memory time grows exponentially in the system size due to polynomially growing energy barrier. Here, we argue that this is not always the case; we demonstrate that memory time of classical clock model (a generalization of ferromagnet to q-state spins) or Zq Toric code may be only polynomially long even when the system possesses finite magnetization or topological entanglement entropy. This violation of the Arrhenius law occurs above the percolation temperature (but below the transition temperature) where excitation droplets percolate the entire lattice while the system as a whole still remains ordered. We present numerical evidences for polynomial scaling as well as analytical argument showing that energy barrier is effectively suppressed and is only logarithmically divergent. The models we study are physically natural as they converge to 2d XY model and U(1) gauge theory as q goes to infinity where excitations are vortex-like with logarithmically divergent excitation energy. We also derive an asymptotic formula of mutual information and topological entanglement entropy at finite temperature for 2d clock model and 3d toric code as a function of q, which is consistent with large q behaviors.