All Abstracts | Poster Abstracts | Talk Abstracts | Tutorial Abstracts

1. New Evidence that Quantum Mechanics is Hard to Simulate on Classical Computers

Scott Aaronson, Massachusetts Institute of Technology

(Session 3 : Friday from 1:15-2:00)

Abstract. I'll discuss new types of evidence that quantum mechanics is hard to simulate classically -- evidence that is more complexity-theoretic in character than (say) Shor's factoring algorithm, and that also corresponds to experiments that seem easier than building a universal quantum computer. Specifically: (1) I'll show that linear-optics (that is, systems of non-interacting bosons) produce probability distributions that cannot be efficiently sampled by a classical computer, unless P^#P = BPP^NP and hence the polynomial hierarchy collapses. I'll also discuss an extension of this result to samplers that only approximate the boson distribution. (Based on recent joint work with Alex Arkhipov) (2) Time permitting, I'll also discuss new oracle evidence that BQP has capabilities outside the entire polynomial hierarchy. (arXiv:0910.4698)

2. What is special about quantum entropy?

Howard Barnum, Perimeter Institute for Theoretical Physics

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

Abstract. What is special about quantum entropy? Howard Barnum, Perimeter Institute for Theoretical Physics Pawlowski et. al. recently showed that stronger-than-quantum correlations (ones that violate the Tsirel'son bound on the strength of CHSH/Bell correlations) violate a principle they call Information Causality. The principle states that, using some shared correlations plus classical communication as a resource, the total mutual information Alice can make available to Bob about a set S of classical bits cannot exceed the number of bits of classical communication she uses, even though this total mutual information is the sum of alternative, possibly mutually exclusive strategies Bob can use to get each of the bits of S ("random-access coding", in computer science jargon). They also showed that quantum theory satisfies this principle. We examine the question of what properties of a theory may lead to its correlations satisfying information causality. We define measurement and preparation entropies for states of a general class of theories: the minimum, over finegrained measurements on the state, of the Shannon entropy of the probabilities of the outcomes of the measurement, and the minimum, over pure-state ensembles for the state, of the Shannon entropies of the ensemble probabilities. We find sufficient conditions for information causality in terms of these entropies: If the measurement entropy satisfies a data-processing inequality, and if the conditional measurement entropy is positive when the conditioning is on a classical system, then information causality holds. Besides the principle of strong subadditivity (which is closely related to data processing) this focuses attention on another property of quantum entropy, the positivity of entropy conditional on a classical system. We show that this property follows from another very natural one exhibited by quantum theory, but that does not hold in general: the equality of measurement and preparation entropy. We briefly consider the implications of this principle for the structure and information-processing possibilities of theories. Joint work with Jonathan Barrett, Lisa Orloff Clark, Matthew Leifer, Robert Spekkens, Nicholas Stepanik, Alex Wilce, and Robin Wilke.

3. Spin Squeezing, Large-Scale Entanglement, and Quantum Simulation in Ion Crystals

Michael Biercuk, National Institute of Standards and Technology

(Session 12 : Sunday from 11:15-11:45)

Abstract. M.J. Biercuk, H. Uys, D. Meiser, A. P. VanDevender, C. Ospelkaus, N. Shiga, W. M. Itano, and J. J. Bollinger We describe experimental and theoretical efforts aimed at the realization of nonlinear multipartite interactions using planar ion crystals in a Penning trap. This system benefits from the ability to confine large ion arrays with regular and stable crystalline order, and direct measures of particle number through resonant fluorescence detection. A global entangling interaction is engineered using state-dependent optical dipole forces, resulting in a simple distance-independent Ising interaction similar to single-axis-twisting spin squeezing. We present direct observations of optical-dipole-force excitation of the center-of-mass (COM) mode for a planar crystal using phase-coherent Doppler velocimetry. By combining state-dependent excitation of the COM mode with microwave-mediated global spin control in arrays of up to ~150 ions, we demonstrate a frequency-dependent loss of phase coherence in the spin ensemble due to coherent interaction of spin and motion. Prospects for realizing true deterministic spin squeezing using trapped ions, including the influence of dissipation via elastic Rayleigh scattering are presented, and future experimental directions described.

4. Quantum Information Science with Trapped Ca+ Ions

Rainer Blatt, University of Innsbruck

(Session 2 : Friday from 8:30-9:15)

Abstract. Trapped strings of cold ions provide an ideal system for quantum information processing. The quantum information can be stored in individual ions and these qubits can be individually prepared; the corresponding quantum states can be manipulated and measured with nearly 100% detection efficiency. With a small ion-trap quantum computer based on up to eight trapped Ca+ ions as qubits we have generated genuine quantum states in a pre-programmed way. In particular, we have generated GHZ and W states in a fast and scalable way and we have demonstrated for the first time a Toffoli gate with trapped ions which is analyzed via state and process tomography. High fidelity CNOT-gate operations are investigated towards fault-tolerant quantum computing and using logical qubits encoded in decoherence-free subspaces, a universal set of gate operations was implemented and analyzed. As applications of quantum information processing, an experimental state-independent test of quantum contextuality was performed, a simulation of the Dirac equation was implemented and a quantum walk with a trapped ion was realized.

5. Quantum-circuit guide to optical and atomic interferometry

Carlton Caves, University of New Mexico

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

Abstract. Atomic (qubit) and optical or microwave (modal) phase-estimation protocols are placed on the same footing in terms of quantum-circuit diagrams. Circuit equivalences are used to demonstrate the equivalence of protocols that achieve the Heisenberg limit by employing entangled superpositions of Fock states, such as N00N states. The key equivalences are those that disentangle a circuit so that phase information is written exclusively on a mode or modes or on a qubit. The Fock-state-superposition phase-estimation circuits are converted to use entangled coherent-state superpositions; the resulting protocols are more amenable to realization in the lab, particularly in a qubit/cavity setting at microwave frequencies.

6. Towards wiring up trapped ions

Nikolaos Daniilidis, Unifersity of California, Berkeley

(Session 2 : Friday from 10:45-11:15)

Abstract. We are pursuing experiments aiming at a transmission-line interface to transfer quantum information between distant ions: An oscillating trapped ion induces oscillating image charges in the trap electrodes. If this current is sent to the electrodes of a second trap, it influences the motion of an ion in the second trap. The expected time for a complete exchange of the motional states can be 1 ms for coupling via a floating conductor located above a surface trap. Alternatively resonant-circuit based geometries with increased coupling rates are also considered. We discuss coupling rates and expected heating rates for different approaches. In addition we discuss trap operation in the presence of a floating conductor. The latter will serve as the coupling electrode in experiments aiming at exchange of the motional states of ions in neighboring trapping regions. This “wire-mediated” coupling may be used for scalable quantum information processing, but may also interconnect atomic systems to solid-state systems.

7. Distinguishing the Borel subgroups of PSL(2; q)

Aaron Denney, University of New Mexico

(Session 8 : Saturday from 5:00-5:30)

Abstract. The Hidden Subgroup Problem (HSP) has been a fruitful framework for expressing many quantum algorithms, and the Abelian case is completely understood. Most efforts to attack non-Abelian groups have been based on splitting these groups into simpler factors. The non-Abelian simple group PSL(2; q) has no non-trivial factors, so cannot be attacked in this way. Further, it has no known efficient quantum Fourier transform. As such, the standard method for distinguishing subgroups fails almost from the beginning. However, we can use the 3-transitivity of PSL(2; q) to distinguish among a large set of stabilizer subgroups by working with their intersections, and reducing to a smaller group with an efficient transform. These stabilizer subgroups are conjugates of the Borel subgroup of upper triangular matrices. Although restricted to this one class of subgroups, this is the first efficient algorithm for a case of the HSP in a family of non-Abelian simple groups.

8. Nanomechanical motion measured with an imprecision below the standard quantum limit

Tobias Donner, JILA, National Institute of Standards and Technology and the University of Colorado, Boulder

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

Abstract. Observing quantum behavior of mechanical motion is challenging because it is difficult both to prepare pure quantum states of motion and to detect those states with high enough precision. We present displacement measurements of a nanomechanical oscillator with an imprecision below that at the standard quantum limit [1]. To achieve this, we couple the motion of the oscillator to the microwave field in a high-Q superconducting resonant circuit. The oscillator's displacement imprints a phase modulation on the microwave signal. We attain the low imprecision by reading out the modulation with a Josephson Parametric Amplifier, realizing a microwave interferometer that operates near the shot-noise limit. The apparent motion of the mechanical oscillator due the interferometer's noise is now substantially less than its zero-point motion, making future detection of quantum states feasible. In addition, the phase sensitivity of the demonstrated interferometer is 30 times higher than previous microwave interferometers, providing a critical piece of technology for many experiments investigating quantum information encoded in microwave fields. [1] J. D. Teufel, T. Donner, M. A. Castellanos-Beltran, J. W. Harlow, K. W. Lehnert, Nature Nanotechnology, doi:10.1038/nnano.2009.343, (2009).

9. Quantum secret sharing with qudit graph states

Ben Fortescue, Institute for Quantum Information Science, University of Calgary

(Session 10 : Saturday from 5:00-5:30)

Abstract. We present a formalism for quantum secret sharing using graph states of systems with prime dimension. As we show, such states allow for a unified structure for the sharing of classical and quantum secrets over both classical and quantum channels. We give explicit protocols for three varieties of threshold secret sharing within this formalism. Joint work with Adrian Keet and Barry C. Sanders.

10. A neutral atom quantum memory created by diffraction of laser light at an array of pinholes

Katharina Gillen, California Polytechnic State University, San Luis Obispo

(Session 6 : Saturday from 11:15-11:45)

Abstract. We present an idea for a new quantum memory for neutral atom quantum computing: Atom traps formed behind an array of pinholes. The diffraction pattern directly behind a circular aperture exhibits localized intensity maxima and minima that can serve as red-detuned or blue-detuned dipole traps for cold atoms, respectively. Previous calculations [1] suggest that the trap frequencies (kHz to 10s of kHz) achieved for even moderate laser powers (~100 mW) are theoretically sufficient for trapping atoms with low decoherence rates from motional heating and trap light scattering. This approach can be extended to an array of pinholes, thereby creating a 2D array of trapping sites that can be used as a quantum memory. The 2D geometry allows addressing of individual trapping sites with a focused laser beam for performance of single qubit operations. In addition to trapping atoms in the sites of this pattern, the polarization-dependence of atoms in certain atomic substates [2] can be exploited to bring pairs of atoms together and apart to facilitate two-qubit quantum gates. We will discuss our latest computational results on these trap arrays and the ability of bringing pairs of traps together and apart for quantum operations. [1] G. D. Gillen, et al., Phys. Rev. A 73, 013409 (2006), [2] I. H. Deutsch, et al., Phys. Rev. A, 57 (3), 1972-1986 (1998).

11. Putting the pieces together: Recent progress with trapped ions at NIST

David Hanneke, National Institute of Standards and Technology

(Session 2 : Friday from 9:15-9:45)

Abstract. Storing quantum bits in the internal states of trapped atomic ions has proven a successful approach to quantum information processing because of long coherence times and precise interaction with light fields for coherent control and entanglement generation. Here, we present an experiment that combines a complete set of scalable techniques to realize a programmable two-qubit quantum processor. We also highlight other work at NIST that aims at facilitating the realization of large-scale quantum processors using trapped ions. This work includes the development of scalable trap technologies, studies of dynamical-decoupling techniques for memory preservation, and progress towards large scale entanglement generation and quantum simulation. *Work supported by DARPA, NSA, ONR, IARPA, Sandia, and the NIST Quantum Information Program.

12. Proving Hall Conductance Quantization

Matthew Hastings, Microsoft Station Q

(Session 11 : Sunday from 8:30-9:15)

Abstract. Ever since Laughlin's original gauge argument for Hall conductance quantization, mathematicians and physicists have tried to find a general proof. Unfortunately, current approaches either require extra assumptions or are limited to noninteracting electrons. Using quasi-adiabatic continuation, we are able to remove these limitations. I will try to explain how quasi-adiabatic continuation can be used as a general "toolkit": once the machinery is developed, many results follow directly, including this, the higher dimensional Lieb-Schultz-Mattis theorem, Goldstone's theorem, and more. This is joint work with S. Michalakis.

13. Entanglement of Atomic Qubits using an Optical Frequency Comb

David Hayes, Joint Quantum Institute/University of Maryland

(Session 2 : Friday from 10:15-10:45)

Abstract. Our group has demonstrated the use of an optical frequency comb to coherently control and entangle atomic qubits. A train of off-resonant ultrafast laser pulses is used to efficiently and coherently transfer population between electronic and vibrational states of trapped atomic ions and implement entangling quantum logic gates with high fidelity. This technique can be extended to the strong field limit with single ultrafast pulses, and this general approach can be applied to the quantum control of more complex systems, such as large collections of interacting atoms or molecules.

14. Quantum Key Distribution: longer ranges and stronger security with superconducting detectors and decoy states

Richard Hughes, Los Alamos National Laboratory

(Session 1 : Thursday from 8:15-9:00)

Abstract. The past few years have seen dramatic advances in the range, rate and security of quantum key distribution (QKD) over optical fiber. These advances have arisen from the development of decoy-state protocols and new superconducting single-photon detector technologies. The former permit rigorous security without adversely impacting the signal-to-noise, and the later offer lower error rates with higher clock rates than conventional detectors. I will describe the results of QKD experiments using superconducting single photon detectors, and the prospects for incorporating decoy-state QKD into transparent optical fiber networks.

15. Permutational Quantum Computing

Stephen Jordan, California Institute of Technology

(Session 8 : Saturday from 3:30-4:00)

Abstract. In topological quantum computation the geometric details of a particle trajectory are irrelevant; only the topology matters. Taking this one step further, we consider a model of computation that disregards even the topology of the particle trajectory, and computes by permuting particles. Whereas topological quantum computation requires anyons, permutational quantum computation can be performed with ordinary spin-1/2 particles, using a variant of the spin-network scheme of Marzuoli and Rasetti. We do not know whether permutational computation is universal. It may represent a new complexity class within BQP. Nevertheless, permutational quantum computers can in polynomial time approximate matrix elements of certain irreducible representations of the symmetric group and simulate certain processes in the Ponzano-Regge spin foam model of quantum gravity. No polynomial time classical algorithms for these problems are known.

16. How to build a fault-tolerant logical qubit with quantum dots

Andrew Landahl, Sandia National Laboratories

(Session 1 : Thursday from 7:30-8:15)

Abstract. After a brief overview of Sandia's quantum information science effort, I will focus on our current "Grand Challenge" QIST program, a large part of which is aimed at designing a fault-tolerant logical qubit with quantum dots. Because this technology may not be familiar to everyone, I will spend some time reviewing it with especial focus on why silicon might be a good material for quantum-dot qubits. Many theoretical analyses of fault-tolerant quantum error correction omit engineering-level constraints such as the space needed to route control wires to the qubits. We have found that these kinds of considerations have a HUGE impact on the accuracy threshold, and in fact can even cause the accuracy threshold to disappear altogether. I will discuss how theoretical ideas such as quantum local check codes and dynamical decoupling can ameliorate some of these constraints in the quantum dot setting. We have developed several logical qubit architectures based on these ideas, and using high-performance computing we have generated optimal schedules for processing them. Our Monte-Carlo simulations point to the accuracy threshold disappearing entirely if dynamical decoupling is not used in conjunction with fault-tolerant quantum error correction, and when it is, the threshold lies between roughly 10^{-5} to 10^{-3} depending on which local check code is used. Based on arXiv:0904.0003. This work was supported through the Laboratory Directed Research and Development program at Sandia National Laboratories. 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.

17. Trapping ultracold dysprosium

Benjamin Lev, University of Illinois at Urbana-Champaign

(Session 11 : Sunday from 9:15-9:45)

Abstract. Ultracold dysprosium gases, with a magnetic moment ten times that of alkali atoms and equal only to terbium as the most magnetic atom, are expected to exhibit a multitude of fascinating collisional dynamics and quantum dipolar phases, including quantum liquid crystal physics. We report the first laser cooling and trapping of half a billion Dy atoms using a repumper-free magneto-optical trap (MOT) and continuously loaded magnetic confinement, and we characterize the trap recycling dynamics for bosonic and fermionic isotopes. The first inelastic collision measurements in the few partial wave, 100 uK to 1 mK, regime are made in a system possessing a submerged open electronic f-shell. In addition, we observe unusual stripes of intra-MOT <10 uK sub-Doppler cooled atoms.

18. A photonic cluster state machine gun

Netanel Lindner, Caltech - Institute of Quantum Information

(Session 10 : Saturday from 4:00-4:30)

Abstract. We present a method to convert certain single photon sources into devices capable of emitting large strings of photonic cluster state in a controlled and pulsed ‘‘on-demand’’ manner. Standard spin errors, such as dephasing, are shown to affect only 1 or 2 of the emitted photons at a time. This allows for the use of standard fault tolerance techniques, and shows that the photonic machine gun can be fired for arbitrarily long times. Using realistic parameters for current quantum dot sources, we conclude high entangled photon emission rates are achievable, with Pauli-error rates per photon of less than 0.2%. For quantum dot sources, the method has the added advantage of alleviating the problematic issues of obtaining identical photons from independent, nonidentical quantum dots, and of exciton dephasing.

19. Some new constructions for Local Hamiltonian and universal adiabatic quantum computing

Peter Love, Haverford College

(Session 8 : Saturday from 4:00-4:30)

Abstract. The difficulty of finding the ground state energy of a Hamiltonian is formalized in quantum complexity theory through the problem Local Hamiltonian. Various restrictions of the form of the Hamiltonians in this problem have been studied, including, inter alia, restricted locality and geometry of couplings, coupling strengths, interaction types and stoquasticity of the Hamiltonians. Concomitantly, such results typically tell us which physical Hamiltonians are capable of realizing universal quantum computation adiabatically. In this talk I will describe some new results on the problem Local Hamiltonian that allow universal adiabatic quantum computation in stoquastic Hamiltonians, restrict the form of the Hamiltonian required, and reduce (but do not eliminate) the need for perturbative gadgets. I will discuss the implications of these results for the design of universal adiabatic quantum computers.

20. Tomographic reconstruction of the Wigner function of an itinerant microwave field.

Francois Mallet, Joint Institute for Laboratory Astrophysics

(Session 9 : Saturday from 5:00-5:30)

Abstract. Francois Mallet, Manuel Castellanos-Beltran, Hsiang-Sheng Ku, Kent Irwin, Leila Vale, Gene Hilton, Konrad Lehnert. In an increasing number of experiments, the desired information (for example the state of nanomechanical resonators or of superconducting qubits) is successfully encoded into the state of a coherent microwave field. However these experiments suffer from the lack of high efficiency detectors at microwave frequencies: the best commercially available amplifiers add twenty times more noise than the intrinsic quantum fluctuations of the field. Our group has made a crucial step to overcome this important limitation by developing quantum limited Josephson Parametric Amplifiers (JPAs) [1]. In this talk I will show how we dramatically increase the performance of the Quantum State Tomography of a squeezed state of the microwave field by using our JPAs. The achieved degree of squeezing and the quantum efficiency of the state tomography will be presented from the point of view of using these squeezed states as building blocks of a more global strategy to perform Quantum Information experiments. Indeed it has been shown in the field of Continuous Variables Quantum Information that theses squeezed states, can be combined to create EPR-like entangled states. Conveniently, the non-classical squeezed states are themselves created by the JPAs. [1] Amplification and squeezing of quantum noise with a tunable Josephson metamaterial, M. Castellanos-Beltran et al., Nat. Phys. 4, 929-931 (2008).

21. Frequency Translation of Single-Photon States by Four-Wave Mixing in a Photonic Crystal Fiber

Hayden McGuinness, University of Oregon

(Session 10 : Saturday from 4:30-5:00)

Abstract. We study the effect of frequency translation of single-photon states in optical fiber through use of the Bragg scattering four-wave mixing process. Preliminary evidence shows that we have successfully translated single-photon wave-packets from wavelength 696 nm to 680 nm, while maintaining photon statistics in the nonclassical regime.

22. "Quantum Interference Experiments with One and More Neutral Atoms"

Dieter Meschede, Universitaet Bonn

(Session 6 : Saturday from 8:30-9:15)

Abstract. The wave properties of material particles are one of the most widely known features of quantum physics. Wave properties become apparent in diffraction and perhaps most strikingly in interference phenomena. In this talk I will present experiments where we trap and control up to a dozen neutral atoms by means of optical dipole forces. I will show how to selectively address individual atoms, how to transport and sort them, and how to store and retrieve information from the atomic qubits. Recently, we have have taken the atoms to the full quantum regime, i.e. to the observation of matter wave interferences at the single trapped atom level. We have demonstrated the quantum analogue of Brownian motion, the quantum walk, a concept of relevance in quantum information science. We have furthermore obtained excellent control of atomic motion using microwaves, including cooling to the vibrational ground levels and the creation of single particle entangled states. In a separate line of experiments we have been able to read out the spin quantum states in dispersive manner. I will discuss the options to create correlated many atom quantum states based on the available protocols.

23. Multi-query quantum algorithms for summation

David Meyer, University of California/San Diego

(Session 7 : Saturday from 2:00-2:30)

Abstract. PARITY is the problem of determining the parity of a string f of n bits given access to an oracle that responds to a query x ∈ {0, ..., n-1} with the x^th bit of the string, f(x). Classically n queries are required to succeed with probability greater than 1/2, but only the least integer greater than n/2 number of quantum queries suffice to determine the parity with probability 1. We consider a generalization to strings f of n elements of Z_k and the problem of determining ∑f(x). By constructing an explicit algorithm, we show that with n - r quantum queries the optimal success probability is at least the greatest integer less than n/r, divided by k. This quantum algorithm utilizes the n - r queries sequentially and adaptively, like Grover's algorithm, but in a different way that is not amplitude amplification.

24. Quantum control and computation in circuit quantum electrodynamics.

Gerard Milburn, The University of Queensland

(Session 1 : Thursday from 4:30-5:15)

Abstract. The new field of circuit quantum electrodynamics (circuit QED for short) has developed in less than a decade driven by technological improvements in the ability to fabricate small circuits from superconducting metals. Much of this development has been motivated by the possibility of implementing quantum computing in such systems, but they are of much wider interest. In this talk I will discuss the feasibility of a number of schemes for quantum feedback control enabled by the new technology. Lehnert has recently demonstrated quantum limited interferometry with high readout efficiency, equivalent to that of a photo-detector reading out an ideal interferometor with efficiency=0.27. This opens up the possibility of doing some important quantum feedback control experiments that are very difficult to do in an atomic or quantum optical setting but very much more feasible in circuit QED. In a quantum optical setting, quantum limited feedback requires that we use all the light leaving the cavity in the measurement process. This is difficult to do in an optical setting but in principle easier in a circuit QED setting. Unlike in an optical setting, all the measured fields are voltages and currents at GHz frequencies on a superconducting wire and thus there is no need to convert from an optical frequency down to a fast electronic signal. Finally the time scales are slower in a circuit to what they are in an all-optical setting and thus fast feedback is more feasible, even with some in-line signal processing. On the other hand, circuit QED presents a difficulty that is not found in optics: we need to make quantum limited homodyne measurements on the cavity output. Lehnert's scheme uses a Josephson parametric amplifier (JPA) which is a phase sensitive amplifier. JPAs have long been used in superconducting electronics, but a key difference in the new devices is the presence of a significant Kerr nonlinearity. I will discuss the quantum noise performance of such devices in circuit QED.

25. Quantum Control of Neutral Atoms Qudits and Transport

Brian Mischuck, University of New Mexico

(Session 6 : Saturday from 10:15-10:45)

Abstract. Quantum control offers a variety of techniques to manipulate quantum systems in order to perform a desired evolution. We describe the application of these ideas to two different problems in the control of neutral atoms. In the first problem, we consider control of the hyperfine spin manifold a cloud of cold atoms driven by microwave and radio-frequency fields. The large number of spin states available in individual atoms makes them candidates for a qudit based quantum computer. Because the Hamiltonians that drive the system may vary spatially and/or temporally, collections of atoms form ensembles of distinguishable qudits. Borrowing from ideas originally developed for NMR, we show how to drive the ensembles through a given desired evolution. This allows for robust control and spatial selectivity of ensembles of atoms. In the second problem we show how atoms’ transport in an optical lattice can be controlled through polarization control of the optical lattice and global microwave pulses. This control is a necessary first step in many of the neutral atom based schemes for quantum computation and simulation, as well as a realization of a quantum walk. We show that with the available global control, any unitary or state synthesis consistent with translational invariance may be performed.

26. Single-Atom Single-Photon Quantum Interface

David Moehring, Sandia National Laboratories

(Session 6 : Saturday from 9:15-9:45)

Abstract. We report on the implementation of a deterministic protocol where a single rubidium atom trapped within a high-finesse optical cavity is entangled with a single emitted photon. After a chosen time, the atomic state is mapped onto a second photon, thus generating an entangled photon pair. Compared to previous experiments, the long trapping times of exactly one atom in the mode of the cavity allow for 10^5 times more entangled photons per atom. The entanglement is verified by a Bell inequality measurement and via quantum state tomography, both showing a clear violation of classical physics. The combination of two independent atom-cavity systems may further allow for the efficient generation of remote-atom entanglement in the near future. *The presented work was completed at the Max Planck Institute of Quantum Optics in the group of Gerhard Rempe.

27. Simulated Electric and Magnetic Fields for Quantum Degenerate Neutral Atoms

William Phillips, Joint Quantum Institute

(Session 1 : Thursday from 6:45-7:30)

Abstract. William D. Phillips, Robert L. Compton, Karina Jiménez-García, Yu-Ju Lin, James V. Porto, and Ian B. Spielman Joint Quantum Institute, National Institute of Standards and Technology, and University of Maryland, Gaithersburg, Maryland, 20899, USA We create an effective vector potential for ultra-cold neutral 87Rb atoms by applying laser beams that Raman-couple different magnetic sublevels having different linear momenta. The resulting distorted energy-momentum dispersion relationship is analogous to the Hamiltonian for a charged particle in a magnetic vector potential. A time-varying effective vector potential creates a synthetic electric field, and a spatially varying vector potential creates a synthetic magnetic field. Measuring the momentum imparted to the atoms allows a direct measurement of the impulse imparted by the synthetic electric field, and observation of vortices in the atom cloud reveals the action of the synthetic magnetic field. Such synthetic fields should address some of the difficulties in other approaches to using neutral atoms as quantum simulators of the integer and fractional quantum Hall effects. This work was supported by DARPA/ARO, the NSF, and the ONR.

28. Geometrization of quantum adiabatic computation

Ali Rezakhani, University of Southern California

(Session 8 : Saturday from 4:30-5:00)

Abstract. A time-optimal approach to adiabatic quantum computation (AQC) will be formulated. The corresponding natural Riemannian metric is also derived, through which AQC can be understood as the problem of finding a geodesic on the manifold of control parameters. This geometrization of AQC and its relation with quantum phase transitions are demonstrated through some examples, where we show that the geometric formulation leads to improved performance of AQC, and sheds light on the roles of entanglement and curvature of the control manifold in algorithmic performance.

29. Quantum mechanical aspects of photosynthesis

Mohan Sarovar, University of California, Berkeley

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

Abstract. Identification of non-trivial quantum mechanical effects in the functioning of biological systems has been a long-standing and elusive goal in the fields of physics, chemistry and biology. Recent progress in control and measurement technologies, especially in the optical spectroscopy domain, have made possible the identification of such effects. In particular, electronic coherence was recently shown to survive for relatively long times in photosynthetic light harvesting complexes despite the effects of noisy bio-molecular environments. Combining techniques from quantum information, quantum dynamical theory and chemical physics, we performed several detailed studies to characterize the extent and nature of quantum dynamics in light harvesting structures. I will present results that demonstrate (i) the presence of long-lived quantum entanglement in these biologically relevant structures, (ii) the lack of sustained quantum speedup in light harvesting complex dynamics, and (iii) the effect of environmental fluctuations on coherence and transport properties in these systems. Our results scrutinize the fine details of light harvesting complex dynamics and reveal the complex interplay between coherent and decoherent dynamics present in these systems.

30. Mathematical model for real-world entanglement swapping and applications to practical long-distance quantum key distribution

Artur Scherer, Institute for Quantum Information Science at the University of Calgary

(Session 10 : Saturday from 3:30-4:00)

Abstract. Entanglement swapping between photon pairs is a key building block in entanglement-based quantum communication schemes using quantum relays or quantum repeaters to overcome the range limits of long-distance quantum key distribution (QKD). We present a nonperturbative mathematical model for practical entanglement swapping, which accounts for real-world imperfections due to detector inefficiencies, detector dark counts and the unavoidable multipair events of current realistic sources of entangled photon pairs. Our closed-form solution for the actual quantum states prepared by realistic entanglement swapping is useful for planning long-distance QKD experiments. In particular, our analysis provides the optimal photon-pair production rate (brightness) of the sources that maximizes the secret key rate for a given distance between a sender (Alice) and a receiver (Bob).

31. An efficient algorithm to find mean field and Matrix Product State solutions for one-dimensional systems

Norbert Schuch, California Institute of Technology

(Session 12 : Sunday from 10:45-11:15)

Abstract. We prove that the best approximation to ground states of one-dimensional quantum systems within the two most common variational ansatzes, namely the mean field ansatz and Matrix Product States, can be found efficiently. This shows that the corresponding variational methods, in particular the Density Matrix Renormalization Group method, can be realized in a provably efficient way, placing their success on a rigorous footing. Moreover, our findings imply that ground states of commuting Hamiltonians in one dimension can be found efficiently.

32. Preparation and Detection of an RF Mechanical Resonator Near the Ground State of Motion

Keith Schwab, Caltech

(Session 4 : Friday from 3:30-4:15)

Abstract. The tools and techniques to prepare mechanical structures in fundamental quantum states are being rapidly developed, using both optical and electrical techniques. To prepare the quantum ground state, we are preforming experiments with a mechanical resonator parametrically coupled to a electrical resonator. The mechanical resonator is a very low dissipation (Q>1M), 6 MHz, nanomechanical structure; the electrical resonator is a lithographic, low dissipation (Q=20,000), superconducting niobium, 7.5 GHz resonator. We pump this structure with carefully prepared microwave photons and demonstrate cooling of the mechanical structure of quantum occupation =3.8. The deep quantum limit, <<1, appears within reach with a modified device.

33. Novel Ion Trap for Efficient Fluorescence Collection from Trapped Ion Qubits

Gang Shu, University of Washington

(Session 2 : Friday from 11:15-11:45)

Abstract. Efficient ion fluorescence collection is critical for fast reliable qubit state detection and higher photon collection rates from single trapped ions or atoms would lead to more efficient single-photon sources and ion-photon entanglement. By integrating a high N.A. spherical mirror into a linear Paul trap, we achieved 10% photon collection efficiency from a single Barium ion qubit. Based on the current successful trap, we designed and built a novel trap in which the reflective optical surface serves as the RF electrode. The new trap geometry is very open and almost 30% of the photons emitted by the ion will be intercepted. Additionally, the axial symmetry of the trap provides means for self-alignment of the ion trapping position and the optical axis of the spherical mirror. Its smaller size will proportionally reduce the spherical aberration so that we can achieve diffraction-limited ion image, and attempt to couple ion fluorescence into a single mode optical fiber for remote ion entanglement. The design can be easily miniaturized and fabricated with standard MEMS technology. Compared to refractive optics systems, our solution has the advantage of simplicity, low cost, flexibility and scale-up potential.

34. Quantum State Mapping in the Cs 133 Full Hyperfine Ground Manifold

Aaron Smith, University of Arizona

(Session 6 : Saturday from 10:45-11:15)

Abstract. Aaron Smith, Brian E. Anderson, Poul Jessen: Center for Quantum Information and Control, College of Optical Sciences, University of Arizona Quantum systems with Hilbert space dimension greater than two (qudits) are often thought of as carriers of quantum information, usually by isolating a convenient pair of states (qubit) and working entirely within this two dimensional embedded subspace. Quantum control of the entire qudit system could prove to be very useful for information processing tasks allowing for the implementation of novel protocols for robust qubit manipulation and error correction. Quantum control of systems with large Hilbert space dimension, especially collective spins, also has near-term applications in quantum metrology. We will describe a method in which to achieve universal quantum control of the entire 16 dimensional hyperfine ground manifold of Cesium using a nearly decoherence free protocol involving the application of static, RF, and microwave magnetic fields. A simple numerical optimization routine can be used to design time dependent control fields that map any initial state onto any target state. We have implemented this control protocol in our experiment and have successfully mapped our initial state, |F=4, m=4>, onto all 16 magnetic eigenstates. We measure the fidelity of the state mapping using Stern-Gerlach analysis and we have achieved fidelities in the range ~ 94% - 98% which is limited almost entirely by errors in the control fields. Our next step is to implement a weak measurement in combination with dynamical control, and to perform quantum state reconstruction based on the resulting measurement record.

35. Fast Quantum Algorithms for Traversing Paths of Eigenstates

Rolando Somma, Los Alamos National Laboratory

(Session 7 : Saturday from 2:30-3:00)

Abstract. We present optimal quantum algorithms to traverse the path of eigenstates of a discrete or continuous family of Hamiltonians. The implementation cost of the algorithms is the total evolution time with the Hamiltonians. Under some assumptions, the cost of the method is proportional to the ratio of the length of the eigenstate path to the minimum eigenvalue gap of the Hamiltonians. When no assumptions are made, the worst-case cost scales with the inverse of the gap squared. Our algorithms advance by preparing eigenstates from previous ones through a version of fixed point search that is approximated using the phase estimation algorithm. The cost of our methods is optimal and significantly improves upon the cost of known general methods for quantum adiabatic state preparation. In some cases, our methods yield a quantum speed-up over well-known classical annealing methods. (Unitary versions of the method can also be considered.)

36. Practical quantum metrology with Bose-Einstein condensates

Alexandre Tacla, University of New Mexico

(Session 9 : Saturday from 3:30-4:00)

Abstract. We analyze in detail the recently proposed experiment [Boixo et al., Phys. Rev. Lett. 101 , 040403 (2008)] for achieving better than 1/N scaling in a quantum metrology protocol using a two mode Bose-Einstein condensate of N atoms. There were several simplifying assumptions in the original proposal that made it easy to see how a scaling approaching 1/N^(3/2) may be obtained. We look at these assumptions in detail to see when they may be justified. We present numerical results that confirm our theoretical predictions for the effect of the spreading of the BEC wave function with increasing N on the scaling. Numerical integration of the coupled Gross-Pitaevskii equations for the two mode BEC also shows that the assumption that the two modes share the same spatial wave function is justified for a length of time that is sufficient to run the metrology scheme.

37. Quantum Eraser and Phase-Matching for Exponential Spin-Squeezing via Coherent Optical Feedback

Collin Trail, University of New Mexico

(Session 9 : Saturday from 4:00-4:30)

Abstract. A scheme for squeezing collective atomic spin states via coherent optical feedback was proposed by M. Takeuchi et. al., Phys. Rev. Lett. 94, 023003, 2005. In the first pass, the Faraday effect acts to entangle the light with the atoms. In a coherent second pass, this information is imprinted back onto the atoms, creating an effective nonlinear interaction and entanglement between atoms. However, the light is still entangled to the atoms when it escapes, leading to substantial decoherence, and moreover, the interaction slowly rotates the system out of sync with the squeezing axis, both of which result in suboptimal squeezing. We show how the addition of a quantum eraser and phase matching can lead to radically improved exponential scaling. We analyze this system in the presence of realistic imperfections such as photon scattering, optical pumping, losses in transmission and reflection, finite detector efficiency, and nonprojective measurements, and show that spin squeezing near 10 dB should be possible.

38. Time-Symmetric Quantum Smoothing: A General Theory of Optimal Quantum Sensing

Mankei Tsang, University of New Mexico

(Session 9 : Saturday from 4:30-5:00)

Abstract. In real-world sensing applications, the signal to be estimated, such as the position of an aircraft, a gravitational wave, or a magnetic field, is seldom a parameter constant in time but a fluctuating random process. Drawing insights from Bayesian estimation theory, I shall demonstrate how the optimal estimation of a random process coupled to a quantum sensor can be done using the recently proposed quantum smoothing theory. The theory calls for the use of not one but two density operators, one to be solved forward in time and one backward in time, and can out-perform conventional quantum filtering methods if delay is permitted in the estimation. Potential applications include gravitational wave sensing and atomic magnetometry. The accuracy improvement of quantum optical phase estimation due to smoothing has recently been experimentally demonstrated by an Australian-Japanese collaboration [Wheatley et al., arXiv:0912.1162].

39. QIP = PSPACE

John Watrous, University of Waterloo

(Session 7 : Saturday from 1:15-2:00)

Abstract. The interactive proof system model of computation is a cornerstone of complexity theory, and its quantum computational variant has been a topic of study in quantum complexity theory for the past decade. In this talk I will present an exact characterization of the expressive power of quantum interactive proof systems that I recently proved in collaboration with Rahul Jain, Zhengfeng Ji, and Sarvagya Upadhyay. The characterization states that QIP = PSPACE, or in other words that the collection of computational problems having quantum interactive proof systems consists precisely of those problems solvable in polynomial space with an ordinary classical Turing machine. This characterization implies the striking fact that quantum computing does not provide an increase in computational power over classical computing in the context of interactive proof systems.

40. An information-theoretic interpretation of topological entanglement entropy

Jon Yard, Los Alamos National Laboratory

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

Abstract. TBA