All Abstracts | Poster Abstracts | Talk Abstracts | Tutorial Abstracts

1. Spin-induced non-geodesic motion, Wigner rotation and EPR correlations of massive spin-1/2 particles in a gravitational field

Paul M. Alsing, University of New Mexico

(Session 15 : Sunday from 13:00-13:30)

Abstract. We investigate in a covariant manner, the spin-induced non-geodesic motion of massive spin-1/2 particles in an arbitrary gravitational field for trajectories that are initially geodesic when spin is ignored. Using the WKB approximation for the wave function in an arbitrary curved spacetime, we compute the O(hbar) correction to the Wigner rotation of the spin-1/2 particle, whose O(1) contribution is zero on timelike geodesics. We consider specific examples in the Schwarzschild metric for motions in the equatorial plane for (i) particles falling in from spatial infinity with non-zero angular momentum and (ii) circular geodesic orbits. For the latter case we consider the Bell inequalities for a perfectly anti-correlated EPR entangled pair of spins as the separate qubits traverse the circular geodesic in opposite directions.

2. Scalable Traps and Novel Gates for Quantum Information Processing with Ions

Jason Amini, National Institute of Standards and Technology

(Session 7 : Saturday from 11:30-12:00)

Collaborators: J. M. Amini, R. B. Blakestad, J. J. Bollinger, J. Britton, K. R. Brown, J. Chou, R. J. Epstein [a], J. P. Home, D. B. Hume, W. M. Itano, J. D. Jost, E. Knill, C. R. Langer [b], D. Leibfried, C. Ospelkaus, T. Rosenband, S. Seidelin [c], A. VanDevender, J. H. Wesenberg [d], and D. J. Wineland.

Abstract. Two of the key goals for the ion trap community are scaling ion traps to hold and manipulate the numbers of qubits needed for useful algorithms and improving the quality of all operations. At NIST, we are testing an 18-zone two-layer trap with an "X" intersection and employing microfabrication techniques to simplify the design and construction of future traps [1]. Combined with novel optical and magnetic gates [2], sympathetic cooling [3], and quantum enabled read-out [4] utilizing different ion species, algorithms with large numbers of ions may become tractable. We have also demonstrated cooling of a microcantilever using an RF resonant circuit [5] and are pursuing the coupling of ions to cantilevers for cooling and entanglement.

[1] See the poster by J. Britton, et al.
[2] See the poster by C. Ospelkaus, et al. See also, D. Leibfried, et al., Phys. Rev. A 76, 032324 (2007).
[3] See the poster by J. Jost, et al.
[4] See the poster by D. Hume, et al.
[5] K.R. Brown, et al., Phys. Rev. Lett. 99, 137205 (2007).

Acknowledgements: Work supported by IARPA and NIST.

[a] Current address: Areté Associates, Longmont, CO 80501, USA
[b] Current address: Lockheed Martin, Huntsville, AL, USA
[c] Current address: University of Grenoble, France.
[d] Current address: Oxford University, U.K.

3. Quantum-limited metrology with product states

Sergio Boixo, University of New Mexico

(Session 14 : Sunday from 13:00-13:30)

Abstract. We study the performance of generalized quantum metrology protocols that involve estimating an unknown coupling constant in a nonlinear k-body Hamiltonian. We obtain the theoretical lower bound on the uncertainty in the estimate of the parameter. For arbitrary initial states, the lower bound scales as 1/n^k, and for initial product states, it scales as 1/n^(k-1/2). We show that the latter scaling can be achieved using simple, separable measurements. We analyze in detail the case of a quadratic Hamiltonian (k=2), implementable with Bose-Einstein condensates. We formulate a simple model, based on the evolution of angular-momentum coherent states, which explains the O(n^(-3/2)) scaling for k=2; the model shows that the entanglement generated by the quadratic Hamiltonian does not play a role in the enhanced sensitivity scaling. We show that phase decoherence does not affect the O(n^(-3/2)) sensitivity scaling for initial product states.

4. Efficient feedback controllers for continuous-time quantum error correction

Brad Chase, University of New Mexico

(Session 4 : Friday from 16:00-16:30)

Abstract. We present an efficient approach to continuous-time quantum error correction that extends the low-dimensional quantum filtering methodology developed by van Handel and Mabuchi [quant-ph/0511221 (2005)] to include error recovery operations in the form of real-time quantum feedback. We expect this paradigm to be useful for systems in which error recovery operations cannot be applied instantaneously. While we could not find an exact low-dimensional filter that combined both continuous syndrome measurement and a feedback Hamiltonian appropriate for error recovery, we developed an approximate reduced-dimensional model to do so. Simulations of the five-qubit code subjected to the symmetric depolarizing channel suggests that error correction based on our approximate filter performs essentially identically to correction based on an exact quantum dynamical model.

5. A Quantum Kicked Top with Cold Atomic Spins

Souma Chaudhury, University of Arizona

(Session 4 : Friday from 17:30-18:00)

Abstract. Complexity in classical as well as quantum physics arises through the coupling of multiple degrees of freedom. Recent theoretical studies have shown a connection between the dynamical rate of entanglement generation in a bipartite quantum system and the presence of chaos in the corresponding classical dynamics. In order to explore this and similar questions that lie at the boundary between quantum information science and quantum chaos we have developed a version of the quantum kicked top based on laser cooled atomic spins driven by a pulsed magnetic field and a rank 2 tensor light shift. Among the advantages offered by our system are the ability to prepare arbitrary initial spin states, the ability to precisely implement the desired non-linear dynamics, and the ability to accurately measure the entire spin density matrix and thus obtain accurate snapshots of the evolving quantum state.

We will present results from an experiment that implemented a quantum kicked top for the F=3 hyperfine ground state of Cs. Initial spin states were chosen to overlap with regular or chaotic areas of the classical phase space map, and the resulting spin Husimi distribution measured after each step in a series of 50 kicks. The spin dynamics seen in the experiment agrees closely with the predictions of theory, including dynamical tunneling between regular islands, rapid spreading of states throughout the chaotic sea, and surprisingly robust signatures of classical phase space structures even after many kicks and significant decoherence. As expected, the entanglement generated between electronic and nuclear spin is larger when the corresponding classical dynamics is chaotic, though the difference "while clear" is modest due to the small size of the total spin. Future versions of the experiment may circumvent this limitation by driving the electronic and nuclear spins independently, or by working with the collective spin of an ensemble of atoms.

6. Using the cutting edge of matrix product state techniques to slice infinitely large entangled systems down to size

Gregory Crosswhite, University of Washington, Department of Physics

(Session 3 : Friday from 15:00-15:30)

Abstract. The numerical simulation of quantum systems is inherently very difficult because the presence of entanglement means that one is faced with a state space exponentially large with respect to the number of particles. The only hope one has to get around this is to employ a clever form of representation that approximates quantum states of interest adequately while remaining small enough to be tractable. Matrix product states have garnered much interest over the past decade because they have these properties. In particular, matrix product states make an excellent ansatz for using the variational method to determine properties of the ground state. In my talk, I shall present an algorithm which uses a local direct variational optimization algorithm to obtain a translationally invariant representation of ground states for infinitely large one-dimensional systems.

7. Polarons in Bose-Einstein condensates

Fernando Cucchietti, Los Alamos National Laboratory

(Session 13 : Sunday from 13:30-14:00)

Abstract. I will describe the behavior of impurities in a Bose-Einstein condensate using analogies with the problem of electrons in ionic crystals -- i.e. the "quantum simulation" of condensed-matter polarons using ultra-cold atoms. In the strong coupling regime, the impurities take on a self-localized state that is smaller than the healing length of the condensate. For intermediate to weak coupling, a different variational approach allows us to calculate analytic expressions for the effective mass of the BEC-polarons and its dispersion relation. I will discuss applications of this quantum simulation as well as its experimental viability.

8. Quantum Circuits Architecture

Giacomo Mauro D'Ariano, Università di Pavia

(Session 8 : Saturday from 13:30-14:15)

Abstract. A method method for optimizing quantum circuits architecture is presented. The method is based on the notion of "quantum comb", which describes a circuit board in which one can insert variable subcircuits, and mathematically corresponds to a generalization of the notions of quantum operation and POVM. The method allows to address novel kinds of quantum processing tasks, such as optimal storing-retrieving and cloning of channels, and optimal quantum circuit board testers.

9. Entanglement is an important resource ??!!

Animesh Datta, University of New Mexico

(Session 15 : Sunday from 13:30-14:00)

Abstract. We attempt at characterizing the correlations present in the quantum computational model DQC1, introduced by Knill and Laflamme [Phys. Rev. Lett. 81, 5672 (1998)]. The model involves a collection of qubits in the completely mixed state coupled to a single control qubit that has nonzero purity. Although there is little or no entanglement between two parts of this system, it provides an exponential speedup in certain problems. On the contrary, we find that the quantum discord across the most natural split is nonzero for typical instances of the DQC1 ciruit. Nonzero values of discord indicate the presence of nonclassical correlations. We propose quantum discord as figure of merit for characterizing the resources present in this computational model. This might be a complementary measure for counting resources in quantum information science.

10. Ultra-Low Noise Photon Pair Source in Dispersion Shifted Optical Fiber

Shellee Dyer, National Institute of Standards and Technology

(Session 11 : Sunday from 09:45-10:15)

Collaborators: Shellee D. Dyer, Lenson Pellouchoud, and Sae Woo Nam

Abstract. Single photon and photon pair sources are important resources for optical quantum information processing. We demonstrate a fiber-based photon pair source in which the photon pairs are generated through four-wave mixing in dispersion shifted fiber (DSF). Previous demonstrations of photon pair generation in DSF were limited by the strong Raman scattering background in the fiber. By cooling the fiber to 4 K, we demonstrate that we can achieve almost complete suppression of the Raman photons, yielding a coincidence-to-accidental ratio larger than 300, exceeding previous best-case results by a factor of 4.

11. A graphical description of stabilizer states

Matthew Elliott, University of New Mexico

(Session 9 : Saturday from 16:45-17:15)

Abstract. Stabilizer states are ubiquitous elements of quantum information theory, as a consequence of both their power and of their relative simplicity. The purpose of this talk is to augment the stabilizer formalism by introducing a graphical representation of stabilizer states. We furthermore demonstrate how Clifford operations, Pauli measurements, and stabilizer codes can be interpreted graphically using this approach.

12. A quantum computer can determine who wins a game faster than a classical computer

Edward Farhi, Massachusetts Institute of Technology

(Session 6 : Saturday from 08:30-09:15)

Abstract. Imagine a game where two players go back and forth making moves and at the end of a fixed number of moves the position is either a win or a loss for the first player. In this case, if both players play best possible, it is determined at the first move who wins or loses. To figure out who will be the winner you need not look at all of the final positions but only at N^.753 where N is the number of final positions. I will show that with a quantum computer the exponent can be reduced to 1/2. The technique involves quantum scattering theory.

13. Ultracompact Generation of Continuous-Variable Cluster States

Steve Flammia, Perimeter Institute

(Session 1 : Friday from 10:30-11:00)

Abstract. We propose an experimental scheme that has the potential for large-scale realization of continuous-variable (CV) cluster states for universal quantum computation. We do this by mapping CV cluster-state graphs onto two-mode squeezing graphs, which can be engineered into a single optical parametric oscillator (OPO). The desired CV cluster state is produced directly from a joint squeezing operation on the vacuum using a multi-frequency pump beam. This method has potential for ultracompact experimental implementation. As an illustration, we detail an experimental proposal for creating a four-mode square CV cluster state with a single OPO. (PRA 76, 010302 (2007) and arXiv:0710.4980)

14. Generation of optical Cat States by squeezed photon subtraction

Thomas Gerrits, National Institute of Standards and Technology

(Session 1 : Friday from 09:30-10:00)

Collaborators: Thomas Gerrits, Tracy Clement, Scott Glancy, Sae Woo Nam, Richard Mirin, Manny Knill (National Institute of Standards and Technology, Boulder, CO, 80303)

Abstract. Optical Cat States are superpositions of coherent states with opposite phases. Those states may be useful for optical phase measurements, as an interferometer's sensitivity is enhanced compared to a classical interferometer, when the light in both interferometers contains equal mean number of photons and wavelength. Also, in quantum computing they are a fundamental resource of fault-tolerant algorithms. Cat States are very sensitive to decoherence, and as a result their preparation is challenging and can serve as a demonstration of good quantum control. We will present our recent effort in generating and detecting these Cat States. Using a femtosecond laser and a KNbO3 downconversion source we are able to generate non-Gaussian states, which are similar to a Schroedinger Cat State.

15. Polygamy of entanglement of assistance: duality for monogamy of entanglement

Gilad Gour, Institute of Quantum Information Science

(Session 15 : Sunday from 12:30-13:00)

Abstract. In contrast to classical multi-partite systems, which can enjoy arbitrary correlations between components, shared entanglement is restricted in a multipartite system. In this talk I will introduce a duality for monogamy of entanglement: whereas monogamy of entanglement inequalities provide an upper bound for bipartite sharability of entanglement in a multipartite system, I will show that the same quantity provides a lower bound for distribution of bipartite entanglement in a multipartite system. I will then show that our results for monogamy of entanglement can be used to establish relations between bipartite entanglement that separate one qubit from the rest vs separating two qubits from the rest.

16. Quantum Non-Demolition counting of photons in Cavity QED

Serge Haroche, Ecole Normale Supérieure

(Session 1 : Friday from 08:45-09:30)

Abstract. Rydberg atoms crossing one by one a high-Q cavity extract information from the field stored in it, without absorbing the photons. The procedure realizes an ideal quantum-non demolition (QND) measurement of light. Initially prepared in a coherent state, the field quickly collapses into a Fock state of well-defined photon number, then undergoes successive jumps towards vacuum due to cavity relaxation. We have checked Planck's law and the predictions of quantum field theory by performing a statistical analysis of thousands of individual quantum trajectories recorded in this way. As the photon number is pinned down to a single value by the QND procedure, the field's phase is blurred. The first stage of this blurring process, induced by a single atom, prepares a photonic Schrödinger cat in the cavity, i.e. a coherent superposition of two field states with different phases. By displacing this cat state in phase space and performing a QND measurement on the translated field, we have reconstructed its Wigner function. It exhibits two classical components and, between them, an interference feature presenting negative parts. which is a signature of the cat state quantum coherence. This interference component vanishes much faster than the decay of the field intensity. This tomographic procedure opens the way to a direct investigation of the decoherence process on cat states containing up to a few tens of photons.

17. Practical long distance quantum key distribution

Jim Harrington, Los Alamos National Laboratory

(Session 11 : Sunday from 09:15-09:45)

Abstract. We implemented a quantum key distribution protocol of phase-encoded BB84 with decoy states in optical fiber, and we achieved secret bits over more than 140 km with high confidence of security against any eavesdropping attack. The protocol included finite statistics effects for decoy state analysis, reconciliation, deskewing, information estimate, and privacy amplification.

18. Black Holes as Mirrors

Patrick Hayden, McGill University

(Session 3 : Friday from 13:45-14:30)

Abstract. I'll discuss information retrieval from evaporating black holes, assuming that the internal dynamics of a black hole is unitary and rapidly mixing, and also assuming that the retriever has unlimited control over the emitted Hawking radiation. If the evaporation of the black hole has already proceeded past the "half-way" point, where half of the initial entropy has been radiated away, then additional quantum information deposited in the black hole is revealed in the Hawking radiation very rapidly. Information deposited prior to the half-way point remains concealed until the half-way point, and then emerges quickly. These conclusions hold because typical local quantum circuits are efficient encoders for quantum error-correcting codes that nearly achieve the capacity of the quantum erasure channel. Our estimate of a black hole's information retention time, based on speculative dynamical assumptions, is just barely compatible with the black hole complementarity hypothesis.

19. Decoherence-free subspaces and incoherently generated coherences

Raisa Karasik, University of California, Berkeley

(Session 14 : Sunday from 12:30-13:00)

Abstract. A decoherence-free subspace (DFS) is a collection of states that is immune to the dominant noise effects created by the environment. DFS is usually studied for states involving two or more particles and is considered a prominent candidate for quantum memory and quantum information processing.

We present rigorous criteria for the existence of DFS in finite-dimensional systems coupled to the Markovian reservoirs. This allows us to identify a new special class of decoherence free states that relies on rather counterintuitive phenomenon, which we call an “incoherent generation of coherences.” We provide examples of physical systems that support such states.

20. A Nuclear Clock

Alex Kuzmich, Georgia Institute of Technology

(Session 2 : Friday from 13:00-13:45)

Abstract. Th-229 nucleus has an exceptionally low-lying first excited state, 7.5 eV relative to the ground state. As the nuclei are affected less by background electromagnetic fields than atoms, laser excitation of the nuclear transition has been proposed as a basis for an ultrastable clock. In this talk, I will report our progress towards trapping triply ionized Th-229.

21. Single-Photon Spin-Orbit Coupling for Cluster State Quantum Computation

Cody Leary, Oregon Center for Optics, University of Oregon

(Session 13 : Sunday from 12:30-13:00)

Abstract. When a quasi-paraxial photon propagates through a cylindrically symmetric inhomogeneous transparent medium such that the inhomogeneity is slowly varying over the spatial extent of the photon’s transverse electric field, its spin angular momentum s and its orbital angular momentum l are coupled. That is, photons in eigenmodes with the formerly degenerate propagation constant k but different values of s and l undergo splitting in k according to k + k(A + B s l) in the presence of the inhomogeneity. The constants A and B are both small compared to unity and are determined by the properties of the medium. This is photon spin-orbit coupling (SOC). In the case of a multimode step-index optical fiber, this k splitting gives rise to a rotational effect in the transverse spatial field distributions of the higher order fiber modes, in which left (right) circularly polarized modes resembling free-space Hermite-Gauss (H-G) modes rotate clockwise (counterclockwise) as they propagate through the fiber. Due to these rotations, single-photon SOC can be used to exploit the transverse spatial photonic degrees of freedom in order to create cluster states for use in fiber-based linear optical quantum computation. We propose fiber-based spin-orbit fusion gate elements towards the creation of cluster states entangled in H-G mode.

22. Ultracold atoms in a radiofrequency-dressed optical lattice

Nathan Lundblad, National Institute of Standards and Technology

(Session 2 : Friday from 11:00-11:30)

Abstract. We load cold atoms into an optical lattice dramatically reshaped by the rf dressing of a strongly state-dependent bare lattice. This rf dressing changes the unit cell of the lattice at a subwavelength scale, such that its curvature and topology departs strongly from that of a simple sinusoidal lattice, and in certain limits is ringlike. Such a lattice is generally interesting from a band-structure engineering perspective, and more specifically from a need for lattices that will realize more complicated solid-state analogues. Radiofrequency dressing has previously been performed at length scales from millimeters to tens of microns, but not at the single-optical-wavelength scale. At this length scale significant coupling between adiabatic potentials leads to nonadiabatic transitions, which we characterize. We also investigate the dressing itself by measuring the momentum distribution of the dressed states.

23. Optimal Control of Large Spin-Atomic Systems with Coherent Electromagnetic Fields

Seth Merkel, University of New Mexico

(Session 4 : Friday from 17:00-17:30)

Abstract. Cold atomic systems provide an excellent testing ground for quantum control protocols due to the isolation of these systems from their environment and the availability of high precision fields from the “quantum optics toolbox”. In this talk, we look at a variety of way to control large spins confined to the ground state hyperfine manifold of 133Cs. In particular, we present a scheme for controlling spins coherently using microwaves and rf-magnetic fields and compare this some previous experiments that utilized quasi-static magnetic fields and a nonlinear AC-Stark shift. We look at the requirements for controllability and find state preparations protocols, fields that map a fiducial state to an arbitrary target state, through a simple stochastic search algorithm. Additionally, we show that in this system the ability to easily find state preparation protocols translates into the ability to easily find arbitrary unitary maps.

24. When a quantum query is no better than a classical one

David Meyer, University of California at San Diego

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

Abstract. We consider a simple generalization of Deutsch's problem in which a single quantum query, rather than solving the problem, provides no more information than a single classical query. This result can be explained by properties of quantum interference, and also follows from results in the early quantum hypothesis testing literature.

25. The role of state preparation in quantum process tomography

Kavan Modi, University of Texas

(Session 13 : Sunday from 12:00-12:30)

Abstract. The immense computational power of a quantum computer comes with a cost - the fragility of entangled quantum states from coherence loss. Although decoherence is present in all physical systems, the effect of these logic errors can be eliminated by using error correcting codes provided gate errors fall below a fault tolerance threshold. This threshold depends on system architecture and specific forms of decoherence, but is likely to be in the 10^-4 range. The measurement of gate fidelity is thus a critical step for implementing fault tolerant quantum computing.

Most experiments determine coherence through T1 and T2 measurements, which gives only a simple description of error process in qubits. A more full and precise measurement is based on density matrix measurements of qubit states, which leads to a description of coherence in terms of state and process tomography. We study the effects of preparation of input states in quantum process tomography experiments. We study two preparation procedures, stochastic preparations and preparations by measurements. We show that for stochastic preparation procedure, linear process maps adequately describe the process. But when linear process maps are obtained from systems initially prepared using von Neumann measurements, they cannot describe the process adequately. We introduce a quadratic process map that can describe the processes initialized by preparation by measurements. I will discuss the consequences of the quadratic map and its properties.

26. Optimal control of light storage and retrieval

Irina Novikova, The College of William & Mary

(Session 11 : Sunday from 08:30-09:15)

Abstract. Mapping of quantum states between light and matter (light storage) using a dynamic form of electromagnetically induced transparency is a topic of great current interest. We demonstrate experimentally a general approach to obtain the maximum efficiency for the storage and retrieval of light pulses in atomic media by finding optimal temporal profile for a strong control field or a signal wavepacket. The procedure uses time reversal to obtain optimal input signal pulse-shapes. Experimental results in warm Rb vapor are in good agreement with theoretical predictions and demonstrate a substantial improvement of efficiency. These optimization procedures are applicable to a wide range of systems.

27. Fault-tolerant holonomic computation on quantum error-correcting codes

Ognyan Oreshkov, University of Southern California

(Session 9 : Saturday from 16:15-16:45)

Collaborators: Ognyan Oreshkov, Todd Brun, Daniel Lidar, and Paolo Zanardi

Abstract. Holonomic quantum computation is a method of computation that uses non-abelian generalizations of the Berry phase. Due to its geometric nature, this approach is robust against various types of errors in the control parameters driving the evolution. In this study, we propose a scheme for fault-tolerant holonomic computation on stabilizer codes, which combines the virtues of error correction with those of the geometric approach. The scheme implements single-qubit operations on different qubits in the code by adiabatically varying Hamiltonians that are elements of the stabilizer, or in the case of subsystem codes---operators that act on the noisy subsystem. Two-qubit operations between qubits from different blocks require Hamiltonians whose weights are higher by one. Thus for certain codes, like the 9-qubit Shor code or its subsystem versions, it is possible to realize universal fault-tolerant computation using Hamiltonians of weight two and three, which is the optimal Hamiltonian weight for holonomic computation on a system of qubits. We also study the regime in which the adiabaticity condition becomes compatible with the fault-tolerance condition for fast gates on the time scale of the noise. Both conditions can be satisfied for a sufficiently large Hamiltonian strength, or equivalently, for a sufficiently low noise rate. This requires only a constant overhead of resources compared to those needed for fault-tolerant dynamical computation.

28. Preserved information in quantum processes

David Poulin, California Institute of Technology

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

Abstract. I will derive a general structure theorem characterizing the information that can be preserved by a quantum process (CPTP map). This characterization builds on a very simple yet powerful operational definition of the notion of being preserved: a set of quantum states is preserved by a process if the states are as distinguishable before and after the process. This definition encompasses noiseless subsystems, decoherence-free subspaces, pointer bases, and error-correcting codes. More generally, I will demonstrate that all such information-preserving structure (IPS) is isomorphic to a matrix algebra. This provides a simple and efficient algorithm for finding all noiseless and unitarily noiseless IPS.

29. quantum thermodynamic cycles and quantum heat engines

Haitao Quan, Los Alamos National Laboratory

(Session 15 : Sunday from 12:00-12:30)

Abstract. In this work, We are trying to make quantum mechanical generation of thermodynamics. our discussion will focus on the so-called quantum heat engines, which use quantum mechanical systems as the working substance. Quantum heat engines have some different properties from their classical counterpart. In order to describe quantum heat engines, we systematically studyisothermal and isochoric processes for quantum thermodynamic cycles. Based on these results the quantum versions of both the Carnot heat engine and the Otto heat engine are defined without ambiguities. We also study the properties of quantum Carnot and Otto heat engines in comparison with their classical counterparts. In addition, we discuss the role of Maxwell\'s demon in quantum thermodynamic cycles. We find that there is no violation of the second law, even in the existence of such a demon, when the demon is included correctly as part of the working substance of the heat engine.

30. On measurement-based quantum computation with the toric code states

Robert Raussendorf, University of British Columbia

(Session 8 : Saturday from 14:15-14:45)

Abstract. We study measurement-based quantum computation (MQC) using as quantum resource the planar code state on a two-dimensional square lattice (planar analogue of the toric code). It is shown that MQC with the planar code state can be efficiently simulated on a classical computer by mapping to non-interacting fermions via the planar Ising model.

J-Ref: S. Bravyi and R. Raussendorf, Phys. Rev. A 76, 022304 (2007)

Acknowledgements: Joint work with Sergey Bravyi

31. Superoperator Dynamics Approach for Identification and Control of Hamiltonian Systems

Ali Rezakhani, University of Southern California Center for Quantum Information Science and Technology

(Session 14 : Sunday from 13:30-14:00)

Abstract. Characterization and control of open quantum systems are among the fundamental tasks/challenges in quantum physics and quantum information science. In particular, there is much interest in the identification of quantum systems which have unknown interactions with their embedding environment. Quantum process tomography is known to be a general method for characterization of quantum dynamical processes, through an inversion of experimental data obtained from a complete set of state tomographies. In an earlier work we demonstrated that the utilization of quantum error detection techniques leads to the direct estimation of all independent parameters of a superoperator. Motivated by that approach, we now introduce new dynamical equations for superoperators – leading to novel ways for Hamiltonian identification and control of open quantum systems. As an application, we show that this method could lead to efficient identification of certain properties of some sparse Hamiltonians. We also briefly discuss some possible applications to open-loop/learning control of Hamiltonian systems.

32. Quantum walk on a circle in phase space via superconducting circuit

Barry Sanders, University of Calgary

(Session 8 : Saturday from 14:45-15:15)

Abstract. We show how a quantum walk, with a single walker and controllable decoherence, can be implemented for the first time in a quantum quincunx created via superconducting circuit quantum electrodynamics (QED). Two resonators are employed to provide simultaneously fast readout and controllable decoherence over a wide range of parameters. The Hadamard coin flip is achieved by directly driving the cavity, with the result that the walker jumps between circles in phase space but still exhibits quantum walk behavior over 15 steps.

33. Resources and decoherence in qubit metrology

Anil Shaji, University of New Mexico

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

Abstract. In quantum parameter estimation, accuracies that beat the standard quantum limit can be obtained by using the quantum properties of the probes and by modulating the nature of the interaction between the probe and the measured system. When qubits are used to construct a quantum probe, it is known that initializing n qubits in an entangled state, rather than in a separable state, can improve the measurement uncertainty by a factor of $1/\\sqrt{n}$. It is also known that if the interaction between the probe and the measured system involves $k$-qubit couplings then the best possible scaling of the measurement uncertainty is $1/n^k$ for a probe initialized in an entangled state and $1/n^{k-1/2}$ for a probe initialized in a product state. We investigate how the measurement uncertainty is affected when the individual qubits in a probe are subjected to decoherence in measurement schemes involving both linear and nonlinear couplings. In the face of such decoherence, we regard the rate $R$ at which qubits can be generated and the total duration $\\tau$ of a measurement as fixed resources, and we determine the optimal use of entanglement among the qubits and the resulting optimal measurement uncertainty as functions of $R$ and $\\tau$.

34. Engineering coherent quantum states in superconducting systems

Raymond Simmonds, National Institute of Standards and Technology

(Session 7 : Saturday from 10:45-11:30)

Abstract. Wouldn't it be great to custom design your own individual quantum systems, then connect them up in interesting arrangements and play around with quantum mechanics? Recently, we have taken the first step towards creating and controlling quantum information using superconducting circuits. We have observed for the first time a coherent interaction between two superconducting “atoms” (quantum bits or qubits) and an LC cavity formed by a ~7 mm long coplanar waveguide resonant at ~9 GHz. When either qubit is resonant with the cavity, we observe the vacuum Rabi splitting of the qubit's spectral line. In a time-domain measurement, we observe coherent vacuum Rabi oscillations between either qubit and the oscillator. Using controllable shift pulses, we have shown coherent transfer of a arbitrary quantum state. We first prepare the first qubit in a superposition state, then this state is transferred to the resonant cavity and then after a short time, we transfer this state into the final qubit. These experiments show that developing custom designed quantum systems on chip is possible, opening up new possibilities for studying quantum mechancis and information science.

35. Quantum Simulated Annealing

Rolando Somma, Perimeter Institute

(Session 13 : Sunday from 13:00-13:30)

Abstract. During the last years it has been shown that if a large quantum computer existed today, certain problems could be solved with them much more efficiently than their classical counterparts. Some of these problems include the quantum simulations of physical systems. In this talk I will show how quantum computers can be used to simulate and compute properties of classical systems in equilibrium. In particular, I will present a quantum algorithm that simulates annealing processes, where the (quantum) annealing rate greatly outperforms other classical methods like Markov chain Monte-Carlo based algorithms.

36. Generic local distinguishability and completely entangled subspaces

Jon Walgate, Perimeter Institute for Theoretical Physics

(Session 3 : Friday from 15:30-16:00)

Abstract. The geometry of Hilbert space entails many necessary and generic properties of quantum systems. In fact, expressing quantum information theoretic questions in geometric terms can transform apparently complex problems into exceedingly simple results. We present an example - a theorem concerning subspaces of projective Hilbert space with immediate and surprising consequences for entanglement and local state distinguishability.

A subspace of a multipartite Hilbert space is completely entangled if it contains no product states. Such subspaces can be large with a known maximum size, S, approaching the full dimension of the system, D. We show that almost all subspaces with dimension less than or equal to S are completely entangled, and then use this fact to prove that n random pure quantum states are unambiguously locally distinguishable if and only if n does not exceed D-S. This condition holds for almost all sets of states of all multipartite systems, and reveals something unexpected. The criterion is identical for separable and for nonseparable states: entanglement makes no difference.

Acknowledgements: Joint work with Andrew Scott, see arXiv:0709.4238

37. Quantum Convolutional Coding with Entanglement Assistance

Mark Wilde, University of Southern California

(Session 9 : Saturday from 15:45-16:15)

Abstract. We have recently developed quantum convolutional coding techniques for both entanglement distillation and quantum error correction. These techniques assume that the two parties participating in the communication protocols possess prior shared entanglement. Using these methods, we can import arbitrary classical binary or quaternary convolutional codes for use in quantum coding, with no requirement that these codes be self-orthogonal. Moreover, high-performance classical convolutional codes lead to high-performance quantum convolutional codes. We explicitly show how a convolutional entanglement distillation protocol operates, and how to encode and decode a stream of quantum information in an entanglement-assisted quantum convolutional code.

38. A general quantum algorithm for knot and link polynomials

Jon Yard, Los Alamos National Laboratory

(Session 6 : Saturday from 09:15-9:45)

Abstract. In this talk, I will present a quantum algorithm for approximating topological invariants of knots and links coming from Markov traces on centralizer algebras of quantum groups. The method is based on a general formalism for efficiently implementing, on a quantum computer, representations of braid groups associated with path algebras. The general framework presented accommodates known quantum algorithms for approximately evaluating the Jones and HOMFLYPT polynomials - which arise from Markov traces on Temperley-Lieb and Hecke algebras associated to deformations of unitary groups. The framework also allows one to approximately evaluate the Kauffman polynomial invariants which arise from Markov traces on Birman-Wenzl-Murakami algebras associated to deformations of the orthogonal and symplectic groups. Time permitting, I will also comment on the cases in which approximating the Kauffman polynomial is a universal quantum algorithm which solves a Promise-BQP-complete problem.

Acknowledgements: This is joint work with Cris Moore (University of New Mexico, Santa Fe Institute).

39. Codeword Stabilized Quantum Codes

Bei Zeng, Massachusetts Institute of Technology

(Session 14 : Sunday from 12:00-12:30)

Abstract. Quantum error correction codes play a central role in quantum computation and quantum information. While considerable understanding has now been obtained for a broad class of quantum codes, almost all of this has focused on stabilizer codes, the quantum analogues of classical additive codes. However, such codes are strictly suboptimal in some settings---there exist nonadditive codes which encode a larger logical space than possible with a stabilizer code of the same length and capable of tolerating the same number of errors. There are only a handful of such examples, and their constructions have proceeded in an ad hoc fashion, each code working for seemingly different reasons.

We present a unifying approach to quantum error correcting code design, namely, the codeword stabilized quantum codes, that encompasses additive (stabilizer) codes, as well as all known examples of nonadditive codes with good parameters. In addition to elucidating nonadditive codes, this unified perspective promises to shed new light on additive codes as well. Our codes are described by two objects: First, the codeword stabilizer that can be taken to describe a graph state, and which transforms the quantum errors to be corrected into effectively classical errors. And second, a classical code capable of correcting the induced classical error model. With a fixed stabilizer state, finding a quantum code is reduced to finding a classical code that corrects the (perhaps rather exotic) induced error model.

We use this framework to generate new codes with superior parameters ((n,K,d)) to any previously known, the number of physical qubits being n, the dimension of the encoded space K, and the code distance d. In particular, we find ((10,18,3)) and ((10,20,3)) codes. We also show how to construct encoding circuits for all codes within our framework.