April 19, 2014
August 12-16, 2013


Invited Speaker Abstracts

Scott Aaronson, MIT
Quantum Computing as 21st-Century Bell Inequality Violation

I'll advocate a perspective on quantum computing that regards it as almost exactly a 21st-century analogue of Bell inequality violation: that is, an attempt to prove that Nature can't have a classical simulation of a certain kind, with any practical applications coming as a "bonus." I'll argue that taking this perspective leads us to consider quantum computing proposals that could be easier to realize than universal quantum computing (or, say, Shor's factoring algorithm), but for which we still have complexity-theoretic evidence for a quantum advantage. I'll discuss the example of BosonSampling (proposed by Alex Arkhipov and myself in 2011), and survey the progress toward realizing it.

Robert Alicki, University of Gdansk
Thermodynamical cost of accuracy and stability of information processing

A model of a spin-1/2 strongly coupled to a quantum harmonic oscillator and weakly interacting with a heat bath is used to study the minimal thermodynamical cost of a binary quantum measurement. It is shown that this cost is equivalent to a cost of elementary gate performed on the protected single bit. The irreversible dynamics of this system is described by a quantum dynamical semigroup derived from the underlying Hamiltonian model. The formula which gives the relation between stability of encoded bit with respect to combined thermal and quantum noise, accuracy of its readout, and work needed to perform an elementary gate is presented. It differs from the standard Landauer principle and allows to estimate the minimal work needed to perform long computations. Finally, these results illustrate the fundamental conflict between stability and irreversibility of information processing what makes the difference between feasibility of scalable classical and quantum computations.
R. Alicki, arXiv:1305.4910; see also video by Lidia del Rio and
Philipp Kammerlander

Mohammad Amin
, D-Wave Systems Inc.
Entanglement in a quantum annealing processor

Entanglement is believed to be essential for any quantum algorithm that is designed to solve intractable problems. In this talk, I present experimental evidence for existence of entanglement within D-Wave's 512-qubit quantum annealing processor. I provide evidence of 2- and 8-qubit thermal entanglement at three levels: First, using quantum tunneling spectroscopy we directly observe an anticrossing between two ferromagnetically ordered states. The observed ground and first excited states at the center of the anticrossing are close to the maximally entangled GHZ states. The energy spectrum, especially the minimum gap, agrees well with the predictions of the Hamiltonian obtained from independently measured parameters (with no free parameters). Second, we calculate the density matrix using the Hamiltonian and the eigenstate populations that are directly measured. The density matrix allows characterization of the mixed state entanglement via known entanglement measures. Third, we introduce a new susceptibility-based witness for the ground-state entanglement that does not require detailed knowledge of the Hamiltonian. Using independently measured linear cross-susceptibilities we demonstrate ground state entanglement through nonzero values of the witness. All three levels of evidence indicate that the processor has access to the robust thermal entanglement during quantum annealing.

Andrew Cross, IBM T. J. Watson Research Center
Entangling gates for superconducting qubits and robustness of randomized benchmarking

Superconducting qubits show considerable potential for realizing solid state quantum processors. I will describe the architecture that IBM is investigating for a logical qubit based on the surface code. Realizing this architecture requires further investigation of two qubit gates and characterization of residual errors. In this direction, I will present a new experimentally demonstrated two qubit gate that requires only microwave control, and I will discuss numerical results on the performance of randomized benchmarking in the presence of realistic noise, including systematic errors, leakage, and correlated noise.

Joseph Emerson, University of Waterloo
Negative quasi-probability, contextuality and magic are equivalent resources for quantum computation

I will present results identifying necessary resources for universal quantum computation using qudit systems (powers of odd prime). First, I show that negative quasi-probability in a distinguished representation is a necessary resource for universal quantum computation with stabilizer codes via magic-state distillation. This condition defines a natural boundary in the space of quantum states which includes the stabilizer polytope as a strict subset, and hence identifies a large class of "bound magic states". I then show that this negativity boundary coincides with a boundary for contextuality in the graph-theoretic framework recently proposed by Cabello, Severini and Winter. Time-permitting, I will discuss a resource-theory of magic and introduce the concept of "mana", which is a computable magicness monotone that can bound the overhead cost of magic-state distillation.
Joint work with: Victor Veitch, Mark Howard, Dan Gottesman, and Ali Hamed.

Andrew N. Jordan, University of Rochester
Action Principle for Continuous Quantum Measurement

The process of continuous quantum measurement will be formulated in terms of a stochastic path integral encoding every possible quantum trajectory, the probability density of those trajectories, the continuous measurement results, and the state disturbance. This approach gives a new way to calculate any expectation value or correlation function of the measurement result or state. As an application of this approach, we provide an answer to the question of what is the most likely path the quantum state takes in its state space between a preselected and a postselected state, separated by a fixed time. We show how this answer may be found from a least action principle and illustrate how it is important for the theory of quantum jumping of a qubit in the Zeno measurement limit.
Action principle for continuous quantum measurement.
A. Chantasri, J. Dressel, A. N. Jordan

Paul Kwiat, University of Illinois at Urbana-Champaign
The Death of Nonlocality

Ever since John Bell showed that quantum mechanics could give different predictions from a local realistic model in an appropriate experiment, there has been wide interest in carrying out such a test. Unfortunately, to date no truly unambiguous test has ever been completed, due to the existence of two experimental loopholes: the “locality loophole” and the “detection loophole”. Each of these have been closed individually in different experimental systems –photons, atoms, ions, and superconductors – but no system has simultaneously closed each of the loopholes. In fact, here we argue that until now, no system has closed both of the loopholes, even independently. Here we present a photon-based experiment that violates the Bell inequality, free of the detection loophole for the first time. This experiment has enough “efficiency overhead" to eventually perform a fully loophole-free test of local realism, and the high entanglement quality of the source allows us to perform precise tests of the upper limit of quantum correlations. Finally, we have used this source to generate secure quantum random numbers at rates several orders of magnitude beyond previous experiments.

Shunlong Luo, Chinese Academy of Sciences
Discord versus Entanglement

Correlations in quantum world are basic, subtle, and complex, and play important roles in quantum foundations and quantum information processing. In this talk, we present a historic review of some aspects of quantum correlations from the perspectives of quantum measurements and information transferring, with emphasis on quantum discord and its various features. We further highlight the fundamental and intrinsic relationships between quantum discord and entanglement.

Marco Merkli, Memorial University
Dynamics of open quantum systems via resonances

We consider the dynamics of a quantum system consisting of a `system of interest' coupled to an `environment'. We explain how the evolution of the full system plus environment can be expressed in terms of exponentials of complex energies (resonances). When reduced to the open system, they give its irreversible dynamics. This approach is mathematically rigorous and valid for all times. We present some applications of this method. In particular, we compare the two regimes of isolated and of overlapping resonances, in which the typical system energy gaps are much larger and much smaller than the system-reservoir coupling, respectively.

Christopher Monroe, JQI and University of Maryland
Quantum Magnetism from the Bottom Up

Crystals of laser-cooled atomic ions are standards for quantum information science, with psuedospins within each atom representing qubits that have unsurpassed levels of quantum coherence and can be measured with near-perfect efficiency. When spin-dependent optical dipole forces are applied to a collection of atomic ions, their Coulomb interaction is modulated in a way that allows the tailoring of spin-spin interactions that are found in theories of quantum magnetism. Recent experiments have implemented variable-range Ising interactions with up to 16 trapped ion spins, the largest system of interacting qubits assembled to date. Direct measurements of spin-spin correlations has shown the emergence of antiferromagnetic order in this highly frustrated system as well as coherent nonequilibrium dynamics following a quench. Soon the number of spins will be high enough where no classical computer can predict the behavior of such a fully-connected quantum magnet, allowing a direct quantum simulation of the murky behavior of quantum spin liquids and spin glasses, the measurement of entanglement near a quantum phase transition, and investigations in the thermalization of a closed quantum system.

William J. Munro, NTT Basic Research Labs
Hybrid Quantum Systems: a route forward for distributed information processing

The development of moderate scale quantum devices requires stable quantum bits in which we can process, store and transport quantum information. A single physical system is unlikely to achieve all of these tasks efficiently and so a natural solution is to examine hybrid system approaches, where we can utilize the best aspects of the individual parts. We demonstrate a hybrid system composed of a superconducting flux qubit (a processor) directly but selectively coupled to an ensemble of nitrogen-vacancy centers (a memory) and show how information can be transferred from the flux qubit to the memory, stored and subsequently retrieved. With NV centers possessing a natural optical transition one can envisage the transform of information between the optical and microwave regimes (an optical interface). With information transport between superconducting qubits already realized, a simple scalable integrated quantum hybrid device seems achievable.

Joshua Nunn, Oxford University
Generating entanglement in solids, and detecting entanglement for QKD.

The talk is in two parts. First we describe a recent demonstration of entanglement between the vibrational modes of a pair of remote diamond crystals, by means of Raman scattering. In the second part we introduce a new protocol for quantum key distribution with spectrally entangled photon pairs, based on time-to-frequency conversion, and discuss the security of the protocol.

Kenji Ohmori, Institute for Molecular Science
Ultrafast Coherent Control of an Ultracold Rydberg Gas

We have developed spatiotemporal coherent-control in which the ultrafast wave-packet interference in a molecule is visualized and controlled with precisions on the picometer spatial and attosecond temporal scales [1-5]. Here we apply this high-precision coherent control to quantum many-body systems of ultracold Rydberg atoms.
A picosecond laser pulse produces Rydberg electronic wave-packets in laser-cooled Rb atoms in an optical dipole trap. We have measured the temporal evolution of those Rydberg wave-packets and their Ramsey interference as a function of the atom density in the Rydberg gas. We have observed that the temporal evolution changes, and the interferogram is phase-shifted when we change the atom density. These results indicate that the interatomic interactions have successfully been triggered by a Rydberg wave-packet generated in each Rb atom. This approach could lead to the development of a novel simulator of quantum many-body dynamics.
[1] H. Katsuki and K. Ohmori et. al., Science 311, 1589 (2006).
[2] H. Katsuki and K. Ohmori et. al., Phys. Rev. Lett. 102, 103602 (2009).
[3] K. Hosaka and K. Ohmori et al., Phys. Rev. Lett. 104, 180501 (2010)
(Highlighted by Nature 465, 138 (2010); Physics 3, 38 (2010)).
[4] H. Goto and K. Ohmori et al., Nature Physics 7, 383 (2011).
(Highlighted by Nature Physics 7, 373 (2011); Nature Photonics 5, 382 (2011)).
[5] H. Katsuki, Y. Kayanuma, and K. Ohmori, Phys. Rev. B 88, 014507 (2013).

Martin B. Plenio, Ulm University
Diamond Quantum Devices and Biology

The detection of smallest magnetic fields emanating for example from small number of nuclear and electronic spins holds the promise for a wide variety of applications. The presence of environmental noise presents a key obstacle on the path towards this goal. Considering a single colour center in diamond I will explain how resonance conditions can be exploited to achieve the dual of goal of sensing minute fields while protecting against environmental noise. I will proceed to explain how such a sensor may lead to the design of a novel room temperature quantum simulator on the basis of nuclear or electron spin arrays in diamond. Finally, I will present a novel approach towards nanoscale quantum devices in which regular arrays of nanodiamonds created by exploiting the self-assembly capabilities of biological systems.
[1] J.M. Cai, F. Jelezko, N. Katz, A. Retzker and M.B. Plenio. New J. Phys. 14, 093030 (2012)
[2] J. Cai, B. Naydenov, R. Pfeiffer, L. McGuiness, K.D. Jahnke, F. Jelezko, M.B. Plenio and A. Retzker. New J. Phys. 14, 113023 (2012)
[3] J.M. Cai, F. Jelezko, M.B. Plenio, A. Retzker. New J. Phys. 15, 013020
[4] J.M. Cai, A. Retzker, F. Jelezko and M.B. Plenio. Nature Physics 9, 168 -173 (2013)
[5] A. Ermakova, G. Pramanik, J.-M. Cai, G. Algara-Siller, U. Kaiser, T. Weil, H.C. Chang, L.P. McGuinness, M.B. Plenio, B. Naydenov and F. Jelezko. Nano Letters (2013)
[6] A. Albrecht, A. Retzker, G. Koplovitz, F. Jelezko, S. Yochelis, Y. Nevo, O. Shoseyov, Y. Paltiel and M.B. Plenio. E-print arXiv:1301.1871 [quant-ph]
[7] P. London, J. Scheuer, J.M. Cai, I. Schwarz, A. Retzker, M.B. Plenio, M. Katagiri, T. Teraji, S. Koizumi, J. Isoya, R. Fischer, L.P. McGuinness, B. Naydenov and F. Jelezko. To appear in Phys. Rev. Lett. 2013 and E-print arXiv:1304.4709

Eugene Polzik, University of Copenhagen
Deterministic teleportation of spin dynamics and other non-local protocols with atomic ensembles

Continuous in phase space and in time interaction of spin ensembles with light combined with continuous measurement has allowed for realization of a broad variety of novel quantum information protocols. Generation of steady state entanglement between distant atomic objects [1], measurement of magnetic fields beyond the standard quantum limit [2] and stroboscopic teleportation of atomic dynamics [3] are the most recent examples. In the experiments quantum variables of interest are encoded in the collective spin of a macroscopic atomic ensemble. Entanglement required for the protocols is distributed by light propagating from one ensemble to the other. Homodyne measurements on light ensure the deterministic and continuous character of the protocols which succeed in every attempt. In a recent proposal [4] this platform has been shown to be capable of teleportation of continuous atomic dynamics and simulation of non-local atomic interactions. Hybrid protocols demonstrating continuous variable approach to a non-Gaussian atomic excitation are in progress. The protocols demonstrated for atomic ensembles are applicable to other important systems, such as mechanical oscillators coupled to light or spin ensembles coupled to microwaves.
1.Entanglement generated by dissipation and steady state entanglement of two macroscopic objects. H. Krauter, C. Muschik, K. Jensen, W. Wasilewski, J. M. Petersen, J. I. Cirac, and E. S. Polzik. Phys. Rev. Lett. 107, 080503 (2011).
2.Quantum noise limited and entanglement-assisted magnetometry. W. Wasilewski, K. Jensen, H. Krauter, J. Renema, M. Balabas, and E.S. Polzik. Phys. Rev. Lett., 104, 133601 (2010).
3.Deterministic quantum teleportation between distant atomic objects. H. Krauter, D. Salart, C. A. Muschik, J. M. Petersen, Heng Shen, T. Fernholz, and E. S. Polzik. Nature Phys., (July 2013).
4.Quantum teleportation of dynamics and effective interactions between remote systems. C. A. Muschik, K. Hammerer, E. S. Polzik, and I. J. Cirac. Phys. Rev. Lett. 111, 020501 (2013).

Michael G. Raymer, University of Oregon
Quantum Frequency Conversion of States of Light

Four-wave mixing in third-order nonlinear optical media can exchange (or swap) the quantum states between two narrow spectral bands of the optical spectrum. [1] When one spectral band is occupied by a single-photon wave-packet state, and the other band is occupied by vacuum, this process can achieve quantum frequency conversion (QFC) - changing the carrier frequency of the photon, as demonstrated experimentally in [2]. When both spectral bands are occupied by single-photon wave-packet states, two-photon (Hong-Ou-Mandel) interference is predicted to create the state |20> + |02>, that is, the two photons end up with the same color, but that color is indeterminate. [3] By tailoring the phase-matching conditions, one can achieve selectivity for different temporally orthogonal wave packets, creating add/drop functionality for a quantum internet. These operations can also be accomplished using three-wave mixing in second-order nonlinear optical media, with constraints on the smallness of the frequency shifts achievable. [4, 5]
1. "Translation of quantum states by four-wave mixing in fibers," C. J. McKinstrie, J. D. Harvey, S. Radic and M. G. Raymer, Opt. Express 13, 9131 (2005).
2. "Quantum frequency translation of single-photon states in photonic crystal fiber,"
H.J. McGuinness, M.G. Raymer, C.J. McKinstrie, and S. Radic, Phys. Rev. Lett. 105, 093604 (2010).
3. "Interference of two photons of different color," M. G. Raymer, S. J. van Enk, C. J. McKinstrie, and H. J. McGuinness, Opt. Commun. 238, 747 (2010).
4. "A quantum pulse gate based on spectrally engineered sum frequency generation," A. Eckstein, B. Brecht, and C. Silberhorn, Opt. Express 19, 13770 (2011).
5. "Temporal mode selectivity by frequency conversion in second-order nonlinear optical waveguides," D. V. Reddy, M. G. Raymer, C. J. McKinstrie, L. Mejling, and K. Rottwitt (Opt. Express, 2013).

Moshe Shapiro, The University of British Columbia
The quantum dynamics experienced by a single molecular eigenstate excited by incoherent light

Contrary to conventional wisdom that all dynamics is a result of interference (or ``dephasing'') between many (at least 2) energy eigenstates, we show that when a continuum of states is present, even a single molecular eigenstate undergoes ``steady-state'' quantum dynamics. Moreover, this type of dynamics can be initiated by incoherent (e.g., solar) light sources. Continua are invariably involved in molecular systems due to a variety of sources such as the ever present bath modes; spontaneously emitted photons; the detachment of electrons; or the dissociation of chemical bonds. Contrary to a single bound energy-eigenfunction which is a real (``standing-waves'') function that carries no flux, hence has no dynamics, a single (complex) continuum energy-eigenfunction carries ``steady-state'' flux given by the group velocity of the energetically narrow wave packet it represents. When this energy eigenfunction is a multi-mode resonance embedded in a continuum via a chain of intramolecular couplings, this dynamics may be initiated by any (light) source, and is controlled, contrary to coherent wave packet dynamics, by the position of the resonance rather than its width.

Yaron Silberberg, The Weizmann Institute of Science
Quantum Walks in Photonic Lattices

Photonic circuits are a natural choice for experimental studies of quantum walks because of their excellent transmission properties and robustness to various decoherence processes. We have shown that the propagation of photons in waveguide lattices are essentially an implementation of continuous-time quantum walks. These systems enable implementation of large scale, decoherence-free quantum walks exhibiting, in periodic lattices, linear expansion vs time. In this talk I shall review our studies on the emergence of unique quantum correlations between two indistinguishable quantum walkers in these systems. We show that two such non-interacting walkers develop unusual correlations depending on their initial state. We discuss experimental results with photons, but also extend these ideas to fermions and entangled particles. We also discuss quantum walks in random lattices and the quantum correlations of walkers undergoing Anderson localization.

Kartik Srinivasan, National Institute of Standards and Technology
Frequency conversion interfaces for photonic quantum systems

Manipulating the wavelength of quantum states of light is an important resource in the development of photonic quantum information technology, where it can be used to interface disparate physical systems, overcome fabrication-induced inhomogeneity, and allow for more optimal detection. In this talk, I will outline our laboratory’s efforts at generating and manipulating the color of single photon states. I will begin by briefly reviewing how we generate single photons from single semiconductor quantum dots embedded in guided wave nanophotonic structures. I’ll then describe experiments in which we use such a single photon source in conjunction with three-wave-mixing in a nonlinear crystal to demonstrate telecom-to-visible conversion and produce identical photons from initially spectrally distinct sources. Finally, I will discuss efforts to develop quantum frequency converters in a scalable, chip-based platform, using both material nonlinearities (four-wave-mixing) and engineered nonlinearities based on radiation pressure coupling between photons and phonons (cavity optomechanics

Gregor Weihs, University of Innsbruck
Semiconductor sources of entanglement

For fundamental tests of quantum physics as well as for quantum communications non-classical states of light are an important tool. I will present two approaches towards semiconductor-based and integrated sources of single photons and entangled photon pairs.
In the first approach we demonstrate entangled photon pair generation in an AlGaAs Bragg-reflection waveguide. Spontaneous parametric down-conversion creates photon pairs at telecommunication wavelengths. This approach can to lead to a fully integrated photon pair source with the pump laser, active and passive optical devices all on a single semiconductor chip.
In our second approach we use resonant two-photon excitation of a single InAs/GaAs quantum dot to deterministically trigger a biexciton-exciton cascade. We demonstrate Rabi oscillations, Ramsey interference and all-optical coherent control of the quantum dot resulting in single and paired photons with a high degree of indistinguishability. This indistinguishability results in time-bin entanglement, which is a useful variant for long distance communication.

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