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Communications in Theoretical Physics

Concurrence of Quantum States: Algebraic Dynamical Method Study XXX Models in a Time-Depending Random External Field

Chuan‐Ji Fu, Qinsheng Zhu, Wu Shao-Yi

Published Jun 1, 2010• 2 citations
Publisher

Abstract

Based on algebraic dynamics and the concept of the concurrence of the entanglement, we investigate the evolutive properties of the two-qubit entanglement that formed by Heisenberg XXX models under a time-depending external held. For this system, the property of the concurrence that is only dependent on the coupling constant J and total values of the external field is proved. Furthermore, we found that the thermal concurrence of the system under a static random external field is a function of the coupling constant J , temperature T , and the magnitude of external held.

DOI
10.1088/0253-6102/53/6/16

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Based on algebraic dynamics and the concept of the concurrence of the entanglement,we investigate the evolutive properties of the TwoQubit entanglement that formed by Heisenberg XXX models under a time-dependent arbitrary external field.For this system,the property of the concurrence that is only dependent on the coupling constant J and total values of the external field is proved.Furthermore,we found that the thermal concurrence of the system under a static arbitrary external field is a function of the coupling constant J,temperature T and the magnitude of external field.
We study the dynamics evolution of a two-qubit Heisenberg XXX spin chain under a time-dependent rotating magnetic field.Based on the algebraic structure of the non-autonomouse system,the exact solution of the Schrdinger equation is obtained by using the method of algebraic dynamics.Based on the time-dependent analytical solution,we further study the entanglement evolution between the two coupled spins for different initial states,and find that the entanglement is determined by the coefficients of the initial state and the coupling constant J of the system.
We analyze the time evolution of a spin chain of three qubits with nearest-neighbour Heisenberg XX interaction, coupled to two thermal reservoirs. We use the ‘reduced density matrix approach’ to calculate the populations and correlations (concurrence) among the spins. The spin-reservoir interaction is of the energy conserving type. This allows us to analyze the evolution analytically without approximations – yielding explicit, yet complicated formulas for the spin dynamics. To analyze the latter and discuss it as a function of the various physical parameters and time (intial conditions, termperatures, coupling strenghts), we resort to numerical methods. Our main finding is that the external noise (coupling to reservoirs) causes the appearence of an asymptotic, stationary (time-periodic) regime for the dynamics of concurrence. We discuss the dependence of the relaxation time and the characteristics of the regime itself, on the various parameters.
Abstract. We have investigated the analytical and numerical dynamics of entanglement for two qubits that interact with each other via Heisenberg XXX-type interaction and subject to local time-specific external kick and Gaussian pulse-type magnetic fields in x-y plane. The qubits have been assumed to be initially prepared in different pure separable and maximally entangled states and the effect of the strength and the direction of external fast pulses on concurrence has been investigated. The carefully designed kick or pulse sequences are found to enable one to obtain constant long-lasting entanglement with desired magnitude. Moreover, the time ordering effects are found to be important in the creation and manipulation of entanglement by external fields.
We study the dynamics of a two-qubit system coupled through time dependent anisotropic XYZXYZXYZ Heisenberg interaction in presence of a time varying non-uniform external magnetic field. Exact results are presented for the time evolution of the system under certain integrability conditions. Furthermore, the corresponding entanglement of the system is studied for different values of the involved parameters. We found that the time evolution and different properties of entanglement such as amplitude, frequency and profile can be finely tuned by the interplay of the characteristics of the time-dependent magnetic field and exchange coupling. Also we show how the discrete symmetries of the system Hamiltonian, which splits its Hilbert space into two distinct subspaces, can be utilized to deduce the dynamics in one of its two distinct subspaces from the other one. Moreover, approximate results for the time evolution are provided utilizing the rotating wave as well as the perturbation approximations for the special case of either static magnetic field or exchange coupling. We compare the range of validity of the two approximation methods and their effectiveness in treating the considered system and determine their critical parameters.