Asymptotics of quantum spin networks at a fixed root of unity

A classical spin network consists of a ribbon graph (i.e., an abstract graph with a cyclic ordering of the vertices around each edge) and an admissible coloring of its edges by natural numbers. The standard evaluation of a spin network is an integer number. In a previous paper, we proved an existence theorem for the asymptotics of the standard evaluation of an arbitrary classical spin network when the coloring of its edges are scaled by a large natural number. In the present paper, we extend the results to the case of an evaluation of quantum spin networks of arbitrary valency at a fixed root of unity. As in the classical case, our proofs use the theory of G-functions of André, together with some new results concerning holonomic and q-holonomic sequences of Wilf-Zeilberger.     

Thermal Quantum Discord and Super Quantum Discord Teleportation Via a Two-Qubit Spin-Squeezing Model

We study thermal quantum correlations (quantum discord and super quantum discord) in a two-spin model in an external magnetic field and obtain relations between them and entanglement. We study their dependence on the magnetic field, the strength of the spin squeezing, and the temperature in detail. One interesting result is that when the entanglement suddenly disappears, quantum correlations still survive. We study thermal quantum teleportation in the framework of this model. The main goal is investigating the possibility of increasing the thermal quantum correlations of a teleported state in the presence of a magnetic field, strength of the spin squeezing, and temperature. We note that teleportation of quantum discord and super quantum discord can be realized over a larger temperature range than teleportation of entanglement. Our results show that quantum discord and super quantum discord can be a suitable measure for controlling quantum teleportation with fidelity. Moreover, the presence of entangled states is unnecessary for the exchange of quantum information.     

View of bunching and antibunching from the standpoint of classical signals

The similarity between classical wave mechanics and quantum mechanics was noted in the works of De Broglie, Schr?dinger, ??late?? Einstein, Lamb, Lande, Mandel, Marshall, Santos, Boyer, and many others. We present a new wave model of quantum mechanics, the so-called prequantum classical statistical field theory, in which an analogy between some quantum phenomena and the classical theory of random fields is investigated. Quantum systems are interpreted as symbolic representations of such fields (not only for photons, cf. Lande and Lamb, but even for massive particles). All quantum averages and correlations (including composite systems in entangled states) can be represented as averages and correlations for classical random fields. We use the prequantum classical statistical field theory to obtain bunching and antibunching in the framework of classical signal theory. We note that antibunching at least is typically considered an essentially quantum (nonclassical) phenomenon.     

Linear conic formulations for two-party correlations and values of nonlocal games

In this work we study the sets of two-party correlations generated from a Bell scenario involving two spatially separated systems with respect to various physical models. We show that the sets of classical, quantum, no-signaling and unrestricted correlations can be expressed as projections of affine sections of appropriate convex cones. As a by-product, we identify a spectrahedral outer approximation to the set of quantum correlations which is contained in the first level of the Navascués, Pironio and Acín (NPA) hierarchy and also a sufficient condition for the set of quantum correlations to be closed. Furthermore, by our conic formulations, the value of a nonlocal game over the sets of classical, quantum, no-signaling and unrestricted correlations can be cast as a linear conic program. This allows us to show that a semidefinite programming upper bound to the classical value of a nonlocal game introduced by Feige and Lovász is in fact an upper bound to the quantum value of the game and moreover, it is at least as strong as optimizing over the first level of the NPA hierarchy. Lastly, we show that deciding the existence of a perfect quantum (resp. classical) strategy is equivalent to deciding the feasibility of a linear conic program over the cone of completely positive semidefinite matrices (resp. completely positive matrices). By specializing the results to synchronous nonlocal games, we recover the conic formulations for various quantum and classical graph parameters that were recently derived in the literature.     

Multimode Relativistic String: Classical Solutions and Quantization

We find all independent classical solutions for the three-mode Nambu–Goto string. We investigate the geometry of the closed curves obtained. Canonical quantization is performed for a part of the solutions obtained. We consider the spectrum of the quantum states obtained. It is consistent with the experimental masses and quantum numbers of mesons corresponding to glueballs. The leading Regge trajectory of the obtained states is consistent with the available data on the Pomeron trajectory.Deceased     

Remote control of quantum correlations in a two-qubit receiver by a three-qubit sender

We study remote control of quantum correlations (discord) in a subsystem of two qubits (receiver) via parameters of the initial state of another three-qubit subsystem (sender) connected to the receiver by an inhomogeneous spin s = 1/2 chain. We propose two parameters characterizing the creatable correlations. The first parameter is the discord between the receiver and the remainder of the spin s = 1/2 chain, and it concerns the mutual correlations between these two subsystems. The second parameter is the discord between the two nodes of the receiver and describes the inner correlations of the receiver. We study the dependence of these two discords on the inhomogeneity parameter of the spin chain.     

Separability and entanglement in tripartite states

While classical correlations can be freely distributed among many systems, this is not true for entanglement and quantum correlations. If a quantum system S^a is entangled with another quantum system S^b, then its entanglement with any third quantum system S^c cannot be arbitrary. This is the celebrated monogamy of entanglement. Implicit in this general statement is the plausible belief that only entanglement between the systems S^a and S^b constrains the entanglement between S^a and the third system S^c. We demonstrate that even classical correlations between S^a and S^b may impose surprisingly stringent restrictions on the possible entanglement between S^a and S^c. In particular, perfect bipartite classical correlations and full entanglement cannot coexist in any tripartite state. An intuitive explanation of this monogamy of hybrid classical and quantum correlations might be that the system S^a has a correlating capability, which cannot be used to establish any entanglement with a third system (but can still be used to establish classical correlations) if it is exhausted when correlated with S^b (in either a classical or quantum fashion). This may be interpreted as an alternate version of monogamy.     

Action of Clifford Algebra on the Space of Sequences of Transfer Operators

We deduce from a determinant identity on quantum transfer matrices of generalized quantum integrable spin chain model their generating functions. We construct the isomorphism of Clifford algebra modules of sequences of transfer matrices and the boson space of symmetric functions. As an application, tau-functions of transfer matrices immediately arise from classical tau-functions of symmetric functions.     

Kinetic Model for Spin Correlation Swapping

We further develop the previously proposed kinetic model of the recombination of radical pairs that demonstrates the phenomenon of the swapping of quantum spin correlations for an arbitrary source of the correlated pairs. We discuss how information about the potential recombination events that did not occur can be taken into account and how a consistent model that includes the spin correlation swapping can be constructed.     

Correlation and Entanglement

In quantum mechanics, it is long recognized that there exist correlations between observables which are much stronger than the classical ones. These correlations are usually called entanglement, and cannot be accounted for by classical theory. In this paper, we will study correlations between observables in terms of covariance and the Wigner-Yanase correlation, and compare their merits in characterizing entanglement. We will show that the Wigner-Yanase correlation has some advantages over the conventional covariance.     

Tomography of Spin States, the Entanglement Criterion, and Bell's Inequalities

We review the method of spin tomography of quantum states in which we use the standard probability distribution functions to describe spin projections on selected directions, which provides the same information about states as is obtained by the density matrix method. In this approach, we show that satisfaction or violation of Bell's inequalities can be understood as properties of tomographic functions for joint probability distributions for two spins. We compare results obtained using the methods of classical probability theory with those obtained in the framework of traditional quantum mechanics. __________ Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 146, No. 1, pp. 172–185, January, 2006.     

Spin parity effect resulting from time-inversion symmetry

The spin parity effect resulting from time-inversion symmetry is derived by generalizing the pure quantum theory of the effect resulting from finite-fold axial-rotation symmetry. The quantum system concerned may consist of either one spin or many spins with arbitrary spin quantum numbers and the states involved may be any pair of eigenstates of spin operator projected on arbitrary axis with their eigenvalues having opposite signs. The two kinds of spin parity effect, resulting from time-inversion symmetry and from axial-rotation symmetry, are in general not equivalent but complementary to each other. Project supported by the National Natural Science Foundation of China (Grant No. 19677101).     

Quantum correlation with entanglement and mutual entropy

We discuss the correlations on classical and quantum systems from the information theoretical points of view. There exists an essential difference between such two types of correlation. How can we understand such difference? This report is a review of our recent works on the quantum information theory with entanglement.     

Entangled Markov chains

Motivated by the problem of finding a satisfactory quantum generalization of the classical random walks, we construct a new class of quantum Markov chains which are at the same time purely generated and uniquely determined by a corresponding classical Markov chain. We argue that this construction yields as a corollary, a solution to the problem of constructing quantum analogues of classical random walks which are “entangled” in a sense specified in the paper.The formula giving the joint correlations of these quantum chains is obtained from the corresponding classical formula by replacing the usual matrix multiplication by Schur multiplication.The connection between Schur multiplication and entanglement is clarified by showing that these quantum chains are the limits of vector states whose amplitudes, in a given basis (e.g. the computational basis of quantum information), are complex square roots of the joint probabilities of the corresponding classical chains. In particular, when restricted to the projectors on this basis, the quantum chain reduces to the classical one. In this sense we speak of entangled lifting, to the quantum case, of a classical Markov chain. Since random walks are particular Markov chains, our general construction also gives a solution to the problem that motivated our study.In view of possible applications to quantum statistical mechanics too, we prove that the ergodic type of an entangled Markov chain with finite state space (thus excluding random walks) is completely determined by the corresponding ergodic type of the underlying classical chain. Mathematics Subject Classification (2000) Primary 46L53, 60J99; Secondary 46L60, 60G50, 62B10     

Two-time correlation functions in an exactly solvable spin-boson model

We consider a spin-boson model describing the dephasing process in an open quantum system and obtain exact expressions for the two-time spin correlation function and the decoherence function applicable for any values of the coupling constants. We show that the initial statistical correlations between the dynamical system and the heat bath considerably affect the time dependence of the decoherence function.     

Quantum Cohomology of Minuscule Homogeneous Spaces

We study the quantum cohomology of (co)minuscule homogeneous varieties under a unified perspective. We show that three points Gromov–Witten invariants can always be interpreted as classical intersection numbers on auxiliary varieties. Our main combinatorial tools are certain quivers, in terms of which we obtain a quantum Chevalley formula and a higher quantum Poincaré duality. In particular, we compute the quantum cohomology of the two exceptional minuscule homogeneous varieties. DOI: .     

Nonequilibrium Green’s Functions in the Atomic Representation and the Problem of Quantum Transport of Electrons Through Systems With Internal Degrees of Freedom

We develop the theory of quantum transport of electrons through systems with strong correlations between fermionic and internal spin degrees of freedom. The atomic representation for the Hamiltonian of a device and nonequilibrium Green’s functions constructed using the Hubbard operators allow overcoming difficulties in the perturbation theory encountered in the traditional approach because of a larger number of bare scattering amplitudes. Representing the matrix elements of effective interactions as a superposition of terms each of which is split in matrix indices, we obtain a simple method for solving systems of very many equations for nonequilibrium Green’s functions in the atomic representation. As a result, we obtain an expression describing the electron currents in a device one of whose sites is in tunnel coupling with the left contact and the other, with the right contact. We derive closed kinetic equations for the occupation numbers under conditions where the electron flow leads to significant renormalization of them.     

The classical limit of a quantum kinetic equation for quarks

The classical limit of the exact quantum kinetic equation for quarks with spin is obtained on the basis of a spinor decomposition. A calculation scheme for the Lenard-Balescu-type collision term is presented. The quantum correction to the classical matter equation is calculated for Abelian plasma.Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 108, No. 1, pp. 159–175, July, 1996.     

The master T-operator for vertex models with trigonometric R-matrices as a classical τ-function

We apply the recently proposed construction of the master T-operator to integrable vertex models and the associated quantum spin chains with trigonometric R-matrices. The master T-operator is a generating function for commuting transfer matrices of integrable vertex models depending on infinitely many parameters. It also turns out to be the τ-function of an integrable hierarchy of classical soliton equations in the sense that it satisfies the same bilinear Hirota equations. We characterize the class of solutions of the Hirota equations that correspond to eigenvalues of the master T-operator and discuss its relation to the classical Ruijsenaars-Schneider system of particles.