A thermal field, which frequently appears in problems of decoherence, provides us with minimal information about the field. We study the interaction of the thermal field and a quantum system composed of two qubits and find that such a chaotic field with minimal information can nevertheless entangle the qubits which are prepared initially in a separable state. This simple model of a quantum register interacting with a noisy environment allows us to understand how memory of the environment affects the state of a quantum register

An entangled two-mode coherent state is studied within the framework of 2 × 2 dimensional Hilbert space. An entanglement concentration scheme based on joint Bell-state measurements is worked out. When the entangled coherent state is embedded in vacuum environment, its entanglement is degraded but not totally lost. It is found that the larger the initial coherent amplitude, the faster entanglement decreases. We investigate a scheme to teleport a coherent superposition state while considering a mixed quantum channel. We find that the decohered entangled coherent state may be useless for quantum teleportation as it gives the optimal fidelity of teleportation less than the classical limit 2/3. PACS number(s); 03.67.-a, 89.70.+c

Transfer of entanglement and information is studied for quantum teleportation of an unknown entangled state through noisy quantum channels. We find that the quantum entanglement of the unknown state can be lost during the teleportation even when the channel is quantum correlated. We introduce a fundamental parameter of correlation information which dissipates linearly during the teleportation through the noisy channel. Analyzing the transfer of correlation information, we show that the purity of the initial state is important in determining the entanglement of the replica state. PACS number(s); 03.65. Bz, 89.70.+c The nonlocal property of quantum mechanics enables a striking phenomenon called quantum teleportation. By quantum teleportation an unknown quantum state is destroyed at a sending place while its perfect replica state appears at a remote place via dual quantum and classical channels [1,2]. For the perfect quantum teleportation, a maximally entangled state, e.g. a singlet state, is required for the quantum channel. However, the decoherence effects due to the environment make the pure entangled state into a statistical mixture and degrade quantum entanglement in the real world. Popescu [3] studied the quantum teleportation with the mixed quantum channel and found that even when the channel is not maximally entangled, it has the fidelity better than any classical communication procedure. For a practical purpose, a purification scheme may be applied to the noisy channel state before teleportation. [4][5][6].Earlier studies have been confined to the teleportation of single-body quantum states: Quantum teleportation of two-level states [1], N -dimensional states [7], and continuous variables [8,9]. In this Letter, we are interested in teleportation of two-body entangled quantum states, especially regarding the effects of the noisy environment. Direct transmission of an entangled state was considered in a noisy environment [10]. A possibility to copy pure entangled states was studied [11]. Extending the argument of the single-body teleportation we can easily show that an entangled N -body state can be perfectly teleported using the N maximally-entangled pairs for the quantum channel. However, for the noisy channel, it is important and nontrivial to know how much the entanglement is transferred to the replica state and how close the replica state is to the original unknown state, depending on the entanglement of the unknown state and channel state.Bennett et al.[1] argued that teleportation is a linear operation for the perfect quantum channel so that it would also work with mixed states and could be extended to what is now called entanglement swapping [12]. We rigorously found that teleportation is linear even for the mixed channel, considering the maximization of the average fidelity [13]. With the property of the linearity, one may conjecture that quantum teleportation preserves the nature of quantum correlation in the unknown entangled state if the channel is quantum-mechanically correlated. We investiga...

There have been theoretical and experimental studies on quantum nonlocality for continuous variables, based on dichotomic observables. In particular, we are interested in two cases of dichotomic observables for the light field of continuous variables: One case is even and odd numbers of photons and the other case is no photon and the presence of photons. We analyze various observables to give the maximum violation of Bell's inequalities for continuous-variable states. We discuss an observable which gives the violation of Bell's inequality for any entangled pure continuous variable state. However, it does not have to be a maximally entangled state to give the maximal violation of the Bell's inequality. This is attributed to a generic problem of testing the quantum nonlocality of an infinite-dimensional state using a dichotomic observable.

We present generic Bell inequalities for multipartite arbitrary dimensional systems. The inequalities that any local realistic theories must obey are violated by quantum mechanics for even dimensional systems. A large set of variants are shown to naturally emerge from the generic Bell inequalities. We discuss particular variants of Bell inequalities, that are violated for all the systems including odd dimensional systems.PACS numbers: 03.65. Ud, 03.65.Ta, Quantum nonlocality is the most significant evidence of physical observations that cannot be explained by theories based upon local realism. Local realism is rooted in the classical view of measurement, namely that an observation on one of a pair of subsystems cannot affect the other system faster than the speed of light. In fact, since the advent of quantum mechanics, the nonclassical implications have given rise to fundamental questions on the nature of the act of measurement. In quantum mechanics, a measurement does not provide a preexisting value of the system, rather, it is a manifestation of the state of the probed system and the probing apparatus, as advocated by Mermin [1].As early as 1964, Bell [2] proved that local realism implies constraints on a correlation of measurements between two separate systems. These constraints are incompatible with the quantitative predictions by quantum theory in the case of two coupled spin-1/2 particles. These constraints, expressed as so-called Bell inequalities, are of paramount importance in the conceptual foundations of quantum mechanics. But these are idealised experiments with archetypal non-classical system. A key question remains whether a complex system of high-dimensional quantum subsystems could eventually simulate a pseudo-classical system that does not contradict local realism.Since this startling discovery [2], investigating Bell theorem for a general system has been regarded as one of the most important challenges in quantum mechanics and quantum information science (QIS) [3,4,5,6,7,8,9,10,11,12,13,14,15]. The motivation is obvious from a scientific and technological viewpoint. Firstly, proving Bell theorem for a general system, would show that quantum physics would apply to macroscopic complex systems. Secondly, for QIS to outperform the acquisition, manipulation and transmission of information over its classical counterpart, the property of nonlocality is closely related to its extraordinary power. In fact, the manipulation of a complex quantum system rather than a simple one has practical advantages. Gaining access to the states is easier and more efficient (for example, efficient cluster state QIS [16] and increased security in high-dimensional quantum cryptography [17]). It follows that a nonlocality test for such a complex system is highly desirable. Thirdly, the controllability of quantum operations depends on the nature of the system. Certain logical operations are relatively easy for one system but impossible or difficult for another. It is then essential to understand the nonlocal properties of ...