Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session G35: Quantum Communication, Decoherence, & Cryptography |
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Sponsoring Units: GQI Room: 702 |
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G35.00001: An Exactly Solvable Model of Quantum Communications Graeme Smith, John Smolin Information theory establishes the ultimate limits on performance for noisy communication systems. Accurate models of physical communication devices must include quantum effects, but these typically make the theory intractable. As a result, communication capacities are not known, even for transmission between two users connected by an electromagnetic waveguide with gaussian noise[6]. We present an exactly solvable model of communications with a fully quantum electromagnetic field. This gives explicit expressions for all point-to-point capacities of noisy quantum channels, with implications for quantum key distribution and fiber optical communications. We also develop a theory of quantum communication networks by solving some rudimentary networks for broadcasting and multiple access. We compare the predictions of our model with the orthodox gaussian model and in all cases find agreement to within a few bits. At high signal to noise ratios (SNRs) our simple model captures the relevant physics while remaining amenable to exact solution. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G35.00002: Strong converse rates for classical communication over thermal bosonic channels Bhaskar Roy Bardhan, Mark Wilde We prove that several known upper bounds on the classical capacity of thermal bosonic channels are actually strong converse rates. Our results strengthen the interpretation of these upper bounds, in the sense that we now know that the probability of correctly decoding a classical message rapidly converges to zero in the limit of many channel uses if the communication rate exceeds these upper bounds. In order for these theorems to hold, we need to impose a maximum photon number constraint on the states input to the channel (the strong converse property need not hold if there is only a mean photon number constraint). Our first theorem demonstrates that a capacity upper bound due to Koenig and Smith is a strong converse rate, and we prove this result by utilizing their structural decomposition of a thermal channel into a pure-loss channel followed by an amplifier channel. Our second theorem demonstrates that an upper bound due to Giovannetti {\it et al.} corresponds to a strong converse rate, and we prove this result by relating success probability to coding rate and bosonic entropies. Both bounds are within 1.45 bits of the known lower bound on capacity that arises from a coherent-state coding scheme. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G35.00003: Entropic error-disturbance relations Patrick Coles, Fabian Furrer We derive an entropic error-disturbance relation for a sequential measurement scenario as originally considered by Heisenberg, and we discuss how our relation could be tested using existing experimental setups. Our relation is valid for discrete observables, such as spin, as well as continuous observables, such as position and momentum. The novel aspect of our relation compared to earlier versions is its clear operational interpretation and the quantification of error and disturbance using entropic quantities. This directly relates the measurement uncertainty, a fundamental property of quantum mechanics, to information theoretical limitations and offers potential applications in for instance quantum cryptography. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G35.00004: Mismatched quantum filtering and entropic information Mankei Tsang Quantum filtering is a signal processing technique that estimates the posterior state of a quantum system under continuous measurements and has become a standard tool in quantum information processing, with applications in quantum state preparation, quantum metrology, and quantum control. If the filter assumes a wrong model due to assumptions or approximations, however, the estimation accuracy is bound to suffer. In this talk I shall present formulas that relate the error penalty caused by quantum filter mismatch to the relative entropy between the true model and the nominal model, with one formula for Gaussian measurements, such as homodyne detection, and another for Poissonian measurements, such as photon counting. These formulas generalize recent seminal results in classical information theory and provide new operational meanings to relative entropy, mutual information, and channel capacity in the context of quantum experiments. See http://arxiv.org/abs/1310.0291 for details. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G35.00005: Action principle for continuous quantum measurement and quantum trajectories with pre and post-selection Areeya Chantasri, Justin Dressel, Steven Weber, Kater Murch, Irfan Siddiqi, Andrew Jordan We apply an action principle to continuous quantum measurement by introducing a joint probability density function of measurement outcomes and quantum state trajectories in a path integral form. Using a modified principle of least action, we find the paths of maximum likelihood connecting boundary states between any two points in time, at which we call the most-likely paths. We present, as an example, the most-likely paths for a continuous qubit measurement with pre and post-selected states, along with a preliminary comparison to data from a superconducting qubit coupled to a microwave cavity. We, furthermore, introduce other interesting statistical characterizations of the quantum trajectories such as mean paths, variances and most-likely times, that can be derived from our path integral formalism. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G35.00006: Long-Distance Continuous-Variable Quantum Key Distribution with Scalar Reconciliation and Gaussian Adaptive Multicarrier Quadrature Division Laszlo Gyongyosi, Sandor Imre The two-way Continuous-Variable Quantum Key Distribution (CVQKD) systems allow higher key rates and improved transmission distances over standard telecommunication networks in comparison to the one-way CVQKD protocols. To exploit the real potential of two-way CVQKD systems a robust reconciliation technique is needed. It is currently unavailable, which makes it impossible to reach the real performance of a two-way CVQKD system. We propose an efficient logical layer-based reconciliation method for two-way CVQKD to extract binary information from correlated Gaussian variables. We demonstrate that by operating on the raw-data level, the noise of the quantum channel can be corrected in the scalar space and the reconciliation can be extended to arbitrary high dimensions. The results allow to significantly improve the currently available key rates and transmission distances of two-way CVQKD. We show that by exploiting the proposed adaptive multicarrier modulation scheme, two-way CVQKD can be extended to a range of 160 km over optical fiber with improved tolerable loss and excess noise. The proposed scalar reconciliation can also be applied in one-way systems as well, and can be extended for multiuser communication. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G35.00007: Dynamics of the spin in slowly rotating magnetic field Amrit Poudel, Canran Xu, Maxim G. Vavilov We study the dynamics of a spin coupled to its environment in a slowly rotating magnetic field. We show that once rotation starts abruptly, the spin exhibits precession around rotating magnetic field. This precession is suppressed due to the decoherence of the spin induced by the environment. At longer times, the spin rotates with the magnetic field and has a component perpendicular to the plane of rotation of the field, which is proportional to the product of the Berry curvature and the angular velocity of the rotation. Finite temperature environment causes thermalization of the spin and, in particular, effectively reduces the magnitude of the spin in the direction perpendicular to the plane of rotation. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G35.00008: Semiclassical Decoherence in He-Surface Scattering Matthew Schram The field of Helium-surface scattering has been recently reexamined with great fervor thanks to recent technological advances and a more focused interest in studying issues surrounding decoherence and the quantum to classical transition. Recent work (Schuller et al 2007, Bundaleski et al 2008) has unexpectedly observed diffraction peaks in He-LiF and He-Ag interactions. This raises fundamental questions about the degree of elasticity of these high-energy collisions, and moreover what the scattering particle is coherent ``with.'' We present results using semiclassical gaussian wavepackets to simulate surface scattering and report on the relative contributions on coherence from different types of inelastic interactions. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G35.00009: Exact non-Markovian two-time correlation functions and current noise spectrum of electron transport through a quantum dot Chung-Chin Jian, Hsi-Sheng Goan Two-time correlation functions (CF's) of the electric currents through nanostructure devices are important in the study of the transport properties of current fluctuations and noise spectra. In the Markovian case, an extremely useful procedure to calculate the two-time (multiple-time) CF's is the so-called quantum regression theorem (QRT). For transport problems, a widely used method to calculate the steady-state current noise spectrum (i.e., Fourier transform of the steady-state current-current two-time CF's) is the MacDonald's formula which can be shown to be equivalent to QRT. However, similar to the QRT where only the evolution equations of the single-time expectation values are required to evaluate two-time CF's, the MacDonald's formula involves also only the single-time expectation values. Thus the MacDonald's formula, in our opinion, may not be applicable to calculate the current noise spectrum for transport problems that involves processes with non-Markovian (memory) effects. Here we develop a correct method to calculate the non-Markovian two-time CF's and finite-frequency noise power spectra based on the approach of the non-Markovian quantum state diffusion (NMQSD) or diffusive stochastic Schrodinger equation. This powerful NMQSD method allows us to calculate the exact current-current two-time CF and thus the exact current noise power spectrum for electron transport through a quantum dot. Our exact results reduce to those obtained by QRT or the MacDonald's formula in the Markovian limit. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G35.00010: Uncertainty Principle Consequences on Thermal Equilibrium Thermodynamics Johan Triana Galvis, Leonardo Pachon Contreras, David Zueco, Paul Brumer In the framework of classical mechanics, it is shown that the thermal equilibrium distribution of a system interacting via central forces with a non self-interacting environment, irrespectively of the interaction strength, is exactly characterized by the canonical Boltzmann distribution. In the framework of quantum mechanics, we show that the fundamental constraints on the contraction of the phase-space volume, imposed by the uncertainty principle, not only inhibits the system thermal-equilibrium-state to be described by the canonical Boltzmann distribution but also it is the responsible of the failure of the Onsager's regression hypothesis and a violation of the KMS condition. Furthermore, as a consequence of this analysis, we discuss the emergence of an \emph{effective coupling} to the environment that depends on all the energy scales involved in the system and reservoir interaction. This effective coupling defines a new quantum limit and has immediately consequences: (i) For the case of strong effective coupling, the system thermal equilibrium state does not match into the canonical distribution and (ii) For the case of weak effective coupling, quantum fluctuations are able to maintain, e.g., stationary entanglement at higher temperatures. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G35.00011: Quantum-limited amplification via reservoir engineering A. Metelmann, A.A. Clerk We describe a new kind of phase-preserving quantum amplifier which utilizes dissipative interactions in a parametrically-coupled three-mode bosonic system [1]. The use of dissipative interactions provides a fundamental advantage over standard cavity-based parametric amplifiers: large photon number gains are possible with quantum-limited added noise, with no limitation on the gain-bandwidth product. Our approach is related to reservoir engineering, where one constructs a non-trivial dissipative reservoir that relaxes the system to a desired target state. We instead realize a dissipative amplification process mediated via an engineered reservoir. The proposed scheme is simple enough to be implemented both in optomechanical systems and in superconducting microwave circuits.\\ \noindent \begin{footnotesize} \hfill [1] A.Metelmann and A.A. Clerk, ArXiv e-prints (2013), 1311.0273. \end{footnotesize} [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G35.00012: Autonomous Fock state stabilization by reservoir engineering E. Holland, B. Vlastakis, R. Heeres, U. Vool, Z. Leghtas, L. Frunzio, G. Kirchmair, M. Mirrahimi, R.J. Schoelkopf Quantum computing requires the ability to create and maintain quantum states. However, due to persistent coupling to the environment a quantum state suffers from decoherence. In order to fight decoherence physicists have come up with different approaches such as circuit based quantum error correction and reservoir engineering. Here we present a reservoir engineering scheme which autonomously stabilizes Fock states in a superconducting waveguide cavity. We will discuss how a vertical transmon qubit is used as a nonlinear coupler between two superconducting waveguide cavities. This nonlinear coupling creates a direct, strong-dispersive interaction between the two cavities. We utilize this interaction to autonomously stabilize Fock states by applying classical continuous wave drives. We present preliminary experimental results. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G35.00013: Decay of the Loschmidt echo in an open, out of equilibrium quantum system Silvia Viola Kusminskiy, Mark Thomas, Torsten Karzig, Felix von Oppen The dynamics of a classical heavy particle moving in a quantum environment is determined by a Langevin equation which encapsulates the effect of the environment-induced reaction forces on the particle. For an open quantum system these include a Born-Oppenheimer force, a dissipative force and a stochastic force due to shot and thermal noise. Recently it was shown that these forces can be expressed in terms of the scattering matrix of the system by considering the classical heavy particle as a time-dependent scattering center. At the same time, it is well known that small changes in a scattering potential can have a profound impact on a fermionic system due to the Anderson orthogonality catastrophe. A useful tool to study this effect on the dynamics of the quantum system is the Loschmidt echo. In this work we study the decay of the Loschmidt echo due to a small change in a scattering potential, for an open quantum system which is out of equilibrium due to an applied bias potential. With methods of scattering theory, and relying on the expressions obtained previously for the environment-induced forces on a heavy particle, we determine the decay of the Loschmidt echo in terms of the fluctuations and dissipation of the system. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G35.00014: Landau-Zener Transition for a qubit coupled to an Ohmic environment Maxim G. Vavilov, Canran Xu, Amrit Poudel We study dynamics of a qubit coupled to an Ohmic environment using the Bloch-Redfield approach. We first discuss how Bloch-Redfield equations can be modified to describe a quantum system with a slowly varying Hamiltonian. We apply this method to the Landau--Zener problem in the presence of environment at zero and finite temperatures. We show that the environment causes relaxation and excitation processes with time--dependent transition rates and the transition probability is greatly affected by these processes. In particular, the transition probability is reduced for environment at zero temperature, when only relaxation is present. At finite temperatures, the competition between relaxation and excitation give rise to non--monotonic dependence of the transition probability on the coupling strength. We also discuss the applicability of the Lindblad formalism to this problem. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G35.00015: Dynamical and thermodynamical control of Open Quantum Walks Francesco Petruccione, Ilya Sinayskiy Over the last few years dynamical properties and limit distributions of Open Quantum Walks (OQWs), quantum walks driven by dissipation, have been intensely studied [S. Attal et. al. J. Stat. Phys. 147, Issue 4, 832 (2012)]. For some particular cases of OQWs central limit theorems have been proven [S. Attal, N. Guillotin, C. Sabot, ``Central Limit Theorems for Open Quantum Random Walks,'' to appear in Annales Henri Poincar\'e]. However, only recently the connection between the rich dynamical behavior of OQWs and the corresponding microscopic system-environment models has been established. The microscopic derivation of an OQW as a reduced system dynamics on a 2-nodes graph [I. Sinayskiy, F. Petruccione, Open Syst. Inf. Dyn. 20, 1340007 (2013)] and its generalization to arbitrary graphs allow to explain the dependance of the dynamical behavior of the OQW on the temperature and coupling to the environment. For thermal environments we observe Gaussian behaviour, whereas at zero temperature population trapping and ``soliton''-like behaviour are possible. Physical realizations of OQWs in quantum optical setups will be also presented. [Preview Abstract] |
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