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Communications in Theoretical Physics

Disentanglement of Two Qubits Coupled to an XY Spin Chain at Finite Temperature

Nie Jing, Lincheng Wang, Yi Xue-Xi

Published May 1, 2009• 6 citations
Publisher

Abstract

The disentanglement evolution of bipartite spin- 1 / 2 system coupled to a common surrounding XY chain in transverse fields at nonzero temperature is studied in this letter. The dynamical process of the entanglement is numerically and analytically investigated. We find that thermal effects can enhance disentanglement if the entangled initial state of the central spins does not in the decoherence free space. The critical phenomenon of quantum phase transitions reflected in the disentanglement can be washed out by the thermal effect eventually.

DOI
10.1088/0253-6102/51/5/11

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The disentanglement evolution of bipartite spin- 1 / 2 system coupled to a common surrounding XY chain in transverse fields at nonzero temperature is studied in this letter. The dynamical process of the entanglement is numerically and analytically investigated. We find that thermal effects can enhance disentanglement if the entangled initial state of the central spins does not in the decoherence free space. The critical phenomenon of quantum phase transitions reflected in the disentanglement can be washed out by the thermal effect eventually.
Quantum correlations (including entanglement and quantum discord) of two qubits symmetrically coupled to the surrounding XY spin chain in the presence of a transverse magnetic field are studied. We investigate the dynamic evolution of the quantum correlations of the two qubits when the surrounding spin chain is in the thermal equilibrium state. Numerical results show that the thermal effects can accelerate the decay process of the quantum correlations if the initial state of the composite system is not in the decoherence free space. When the temperature is high enough, the quantum phase transition driven by the external magnetic field completely disappear.
We study the disentanglement evolution of two spin qubits which interact with a general XYXYXY spin-chain environment. The dynamical process of the disentanglement is numerically and analytically investigated in the vicinity of a quantum phase transition (QPT) of the spin chain in both weak and strong coupling cases. We find that the disentanglement of the two spin qubits may be greatly enhanced by the quantum critical behavior of the environmental spin chain. We give a detailed analysis to facilitate the understanding of the QPT-enhanced decaying behavior of the coherence factor. Furthermore, the scaling behavior in the disentanglement dynamics is also revealed and analyzed.
Recently, along with the development of quantum information, quantum entanglemant became a hot topic of people. Quantum entanglemant is one of the most amazing phenomenon in quantum mechanics that is totally different from classical physics. However, system would interact with environment in the practical quantum information process. The entanglement would be broken. In this paper, we study the disentanglement evolution of three spin qubits in an XY spin-chain environment. The dynamical process of the disentanglement is investigated. We found the exact expression of the coherence factor. We discuss the coherence factor and the parameters, and then we illustrate that the disentanglement of central spins is best enhanced by the quantum critical behavior of the environmental spin chain. Furthermore, a scaling rule is obtained.
By using the concept of concurrence, we numerically investigate the thermal entanglement between any two nearest-neighbour spins in uniform and periodic anisotropic XY chains in a transverse field at finite temperature T . It is found that the entanglement has more than one critical temperatures on some parameter regions for uniform and periodic chains. We also discuss the behaviour of the thermal entanglement at the vicinity of quantum phase transition of periodic anisotropic XY chains and find that all the derivatives ∂ λ C have similar behaviour as that of the uniform chain.
We study the entanglement of thermal and ground states in the Heisenberg XY spin chain with an inhomogeneous magnetic field.A general result is found that for two-qubit case,pairwise entanglement is independent of the sign of coupling constant J,magnetic field B,anisotropic parameter γ and inhomogeneous parameter b of the magnetic field.At zero absolute temperature,an analytical expression of concurrence C is given through which we show that the critical magnetic field Bc increases with the increase of inhomogeneity.At finite temperature,we show analytically for the isotropic case and numerically for the anisotropic case that the thermal entanglement can be apparently enhanced by introducing the inhomogeneity of the magnetic field.We also show that for isotropic case the critical temperature Tc is alone an increasing function of the inhomogeneous parameter b,while for anisotropic case,it is determined by both B and b.
We study the dynamics of entanglement in the infinite asymmetric XYXYXY spin chain, in an applied transverse field. The system is prepared in a thermal equilibrium state (ground state at zero temperature) at the initial instant, and it starts evolving after the transverse field is completely turned off. We investigate the evolved state of the chain at a given fixed time, and show that the nearest-neighbor entanglement in the chain exhibits a critical behavior (which we call a dynamical phase transition), controlled by the initial value of the transverse field. The character of the dynamical phase transition is qualitatively different for short and long evolution times. We also find a nonmonotonic behavior of entanglement with respect to the temperature of the initial equilibrium state. Interestingly, the region of the initial field for which we obtain a nonmonotonicity of entanglement (with respect to temperature) is directly related to the position and character of the dynamical phase transition in the model.