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We study the Loschmidt echo (LE) of a coupled system consisting of a central spin and its surrounding environment described by a general XY spin-chain model. The quantum dynamics of the LE is shown to be remarkably influenced by the quantum criticality of the spin chain. In particular, the decaying behavior of the LE is found to be controlled by the anisotropy parameter of the spin chain. Furthermore, we show that due to the coupling to the spin chain, the groundstate Berry phase for the central spin becomes nonanalytical and its derivative with respect to the magnetic parameter λ in spin chain diverges along the critical line λ = 1, which suggests an alternative measurement of the quantum criticality of the spin chain.

We study the disentanglement evolution of two spin qubits which interact with a general XY spin-chain environment. The dynamical process of the disentanglement is numerically and analytically investigated in the vicinity of a quantum phase transition ͑QPT͒ 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.The coupling between an entangled quantum system and its environment leads to disentanglement of the system, the process through which quantum information is degraded. Disentanglement is a crucial issue that is of fundamental interest due to the fact that the distributed nonlocal coherence among multiparticles by the entanglement really matters in many important applications of quantum information ͓1,2͔. Consequently, the fragility of nonlocal entanglement is recognized as a main obstacle to realizing quantum computing and quantum information processing ͑QIP͒ ͓3,4͔. Apart from the important link to QIP realizations, a deeper understanding of disentanglement is also expected to lead to new insights into quantum fundamentals, particularly quantum measurement and quantum-classical transitions ͓5͔. Recently, Yu and Eberly ͓6͔ have showed that two entangled qubits become completely disentangled in a finite time under the influence of pure vacuum noise. Zubairy et al. ͓7͔ have demonstrated how high quality cavities can be used to realize the new class of quantum erasers referred to as quantum disentanglement erasers. Dodd and Halliwell ͓5͔ have studied the competing effects of environmental noise and interparticle coupling on disentanglement by solving the dynamics of two harmonically coupled oscillators. Cucchietti et al. ͓8͔ have considered the decoherence effect of a noninteracting spin chain on a single qubit.In this paper, we study the disentanglement dynamics of a two-qubit quantum system. Here, the key point is that we choose a special correlated XY spin chain to model the surrounding environment. This choice of the correlated environment is directly motivated by the recent recognition that the single-qubit decoherence induced by a spin-chain environment displays highly interesting properties ͓9-12͔ due to the unique occurrence of a quantum phase transition ͑QPT͒ in the spin-chain environmental subsystem. Quan et al. ͓9͔ have studied the transition dynamics of a quantum two-level system from a pure state to a mixed one induced by a QPT of the surrounding many-body system. They have shown that the decaying behavior of the Loschmidt echo ͑LE͒ is best enhanced by the QPT of the surrounding system. Cucchietti et al. ͓10͔ have found that the QPT of the spin-chain environment will drive the decay of the quantum coherence in the central quantum system ...