Dynamics of quantum correlations

The density matrix describing the state of a sybsystem of a physical system whose time dependence is assumed to follow a Schrödinger equation does not itself obey a von Neumann equation. The behavior of the eigenvalues and eigenfunctions of this density matrix is studied. An expression for the rate at which initially separating systems are de-separated is derived in perturbation theory. Indications are given that coherent photon states are more stable in the presence of charged particles than photon number eigenstates. The possible dynamical origin of super selection rules is discussed. A simple model is solved analytically.     

量子关联简介

量子纠缠作为一种非局域的关联,是一种重要的资源而被广泛应用于量子信息处理。然而,最近的研究结果发现,可分态中也可以存在非经典的关联,量子纠缠只是量子关联的一部分;非纠缠的量子关联在一些量子通信和量子计算任务中扮演着重要的角色。文章简要综述了量子关联的基本概念,描述了几种常见的量子关联度量,并介绍了量子关联在量子信息处理中的作用以及一些最新动态。     

量子关联简介

量子纠缠作为一种非局域的关联,是一种重要的资源而被广泛应用于量子信息处理。然而,最近的研究结果发现,可分态中也可以存在非经典的关联,量子纠缠只是量子关联的一部分;非纠缠的量子关联在一些量子通信和量子计算任务中扮演着重要的角色。文章简要综述了量子关联的基本概念,描述了几种常见的量子关联度量,并介绍了量子关联在量子信息处理中的作用以及一些最新动态。     

Spin squeezing and quantum correlations

We discuss the notion of spin squeezing considering two mutually exclusive classes of spin-s states, namely, oriented and non-oriented states. Our analysis shows that the oriented states are not squeezed while non-oriented states exhibit squeezing. We also present a new scheme for construction of spin-s states using 2s spinors oriented along different axes. Taking the case of s=1, we show that the ‘non-oriented’ nature and hence squeezing arise from the intrinsic quantum correlations that exist among the spinors in the coupled state.     

Robustness of quantum correlations against decoherence

We study dynamics of nonclassical correlations by exactly solving a model consisting of two atomic qubits with spontaneous emission. We find that the nonclassical correlations defined by different measures give different qualitative characterizations of those correlations. The relative behaviors of those correlation measures are presented explicitly for various quantum states in the two-qubit atomic system. In particular, we find that the robustness of quantum correlations can be greatly enhanced by performing appropriate local unitary operations on the initial state of the system.     

Observable measure of bipartite quantum correlations

We introduce a measure Q of bipartite quantum correlations for arbitrary two-qubit states, expressed as a state-independent function of the density matrix elements. The amount of quantum correlations can be quantified experimentally by measuring the expectation value of a small set of observables on up to four copies of the state, without the need for a full tomography. We extend the measure to 2×d systems, providing its explicit form in terms of observables and applying it to the relevant class of multiqubit states employed in the deterministic quantum computation with one quantum bit model. The number of required measurements to determine Q in our scheme does not increase with d. Our results provide an experimentally friendly framework to estimate quantitatively the degree of general quantum correlations in composite systems.     

Electron correlations at the fractional quantum Hall edge

This paper reviews tunnel spectroscopy of fractional quantum Hall edges using cleaved-edge overgrown devices. Beginning with an intuitive introduction to various experimental and theoretical aspects, the device structure is reviewed, and the experimental result of a continuum of power-law tunneling exponents is revisited. The unanticipated behavior of the exponent with fractional filling factor is described, and all subsequent theoretical explanations for these results are laid out for comparison. Finally, we propose new directions for experimentally resolving the remaining questions.     

Maximally nonlocal and monogamous quantum correlations

We introduce a version of the chained Bell inequality for an arbitrary number of measurement outcomes and use it to give a simple proof that the maximally entangled state of two d-dimensional quantum systems has no local component. That is, if we write its quantum correlations as a mixture of local correlations and general (not necessarily quantum) correlations, the coefficient of the local correlations must be zero. This suggests an experimental program to obtain as good an upper bound as possible on the fraction of local states and provides a lower bound on the amount of classical communication needed to simulate a maximally entangled state in dxd dimensions. We also prove that the quantum correlations violating the inequality are monogamous among nonsignaling correlations and, hence, can be used for quantum key distribution secure against postquantum (but nonsignaling) eavesdroppers.     

Thermodynamical approach to quantifying quantum correlations

We consider the amount of work which can be extracted from a heat bath using a bipartite state rho shared by two parties. In general it is less then the amount of work extractable when one party is in possession of the entire state. We derive bounds for this "work deficit" and calculate it explicitly for a number of different cases. In particuar, for pure states the work deficit is exactly equal to the distillable entanglement of the state. A form of complementarity exists between physical work which can be extracted and distillable entanglement. The work deficit is a good measure of the quantum correlations in a state and provides a new paradigm for understanding quantum nonlocality.     

Dynamical decoupling pulses on the quantum correlations for the system of superconducting quantum circuit

We investigate the influence of the dynamical decoupling pulses on the quantum correlations in a superconducting system consisting of two noninteracting qubits interacting with their own data buses. It is found that the geometric discord and entanglement between the two superconducting qubits can be increased by applying a train of π-phase pulses. We then proceed to explore how the decoupling pulses affect the quantum transfer of information between the two superconducting qubits by making use of the change of trace distance.     

Two-pulse correlations in noisy quantum channels

The modification of initially entangled light pulses passing through noisy quantum channels, modeled by dispersive and absorbing four-port devices, is studied with the help of recently obtained results on quantum state transformations. The fidelity and indices of quantum correlations based on the von Neumann entropy are calculated. Their dependence on both the pulse shape and the four-port device parameters is studied. It is shown that due to dispersion and absorption, the quantum correlations are reduced, and they decrease with increasing initial entanglement.     

Locking classical correlations in quantum States

We show that there exist bipartite quantum states which contain a large locked classical correlation that is unlocked by a disproportionately small amount of classical communication. In particular, there are (2n+1)-qubit states for which a one-bit message doubles the optimal classical mutual information between measurement results on the subsystems, from n/2 bits to n bits. This phenomenon is impossible classically. However, states exhibiting this behavior need not be entangled. We study the range of states exhibiting this phenomenon and bound its magnitude.     

Quantum correlations in dual quantum measurements

 We analyze the quantum measurement properties of dual non-degenerate parametric amplifers in the twin-beam configuration, in the cascaded back-action-evasion configuration, and in Kerr-type photon-number quantum non-demolition measurements. It is found that Einstein-Podolsky-Rosen correlations can be obtained between the quadrature components of an idler mode and the sum of the readout of two signal modes. Furthermore, we discuss dual-mode quantum non-demolition measurements on the combination of two light modes, and the generation of number-state entanglement. Received: 12 April 1996/Revised version: 2 July 1996     

Stronger quantum correlations with loophole-free postselection

One of the most striking nonclassical features of quantum mechanics is in the correlations it predicts between spatially separated measurements. In local hidden variable theories, correlations are constrained by Bell inequalities, but quantum correlations violate these. However, experimental imperfections lead to loopholes whereby LHV correlations are no longer constrained by Bell inequalities, and violations can be described by LHV theories. For example, loopholes can emerge through selective detection of events. In this Letter, we introduce a clean, operational picture of multiparty Bell tests, and show that there exists a nontrivial form of loophole-free postselection. Surprisingly, the same postselection can enhance quantum correlations, and unlock a connection between nonclassical correlations and nonclassical computation.     

Remanent quantum correlations in dissipative qubits

Starting from the exact evolution of a Markovian dissipative quantum walk, a non-Markovian decoherence of two qubits interacting with a phonon thermal bath has been investigated analytically using quantum information tools. Concurrence and quantum discord are affected in a complex way, showing that entanglement decreases with dissipation. At the limit where dissipation dominates, quantum correlations survive in time as ∝t^−1/2$\propto t^{-1/2}$. Thus, even under the influence of dissipation, two qubits retain their quantumness for a long time. Quantum correlations could be therefore observed for a long time in related photonic experiments.