In this paper we consider the decay of quantum entanglement, quantified by the concurrence, of a pair of two-level systems each of which is interacting with a reservoir at finite temperature T. For a broad class of initially entangled states, we demonstrate that the system always becomes disentangled in a finite time i.e. "entanglement sudden death" (ESD) occurs. This class includes all states which previously had been found to have long-lived entanglement in zero temperature reservoirs. Our general result is illustrated by an example.In the past few years there has been considerable interest in the properties of entangled quantum systems. Spurred on by the emergence of compelling applications in quantum information processing, useful methods by which the entanglement of quantum systems can be established and characterized have emerged. Perhaps the most impactful to date has been the simple procedure derived by Wootters [1] for quantifying entanglement for an arbitrary mixed state of a pair of two-level systems. This has provided a very useful tool for measurement of experimental quantum states [2] and is today commonly used in assessing the capabilities of emerging quantum technologies. Building on Wootter's work, recently Yu and Eberly [3] investigated the time evolution of entanglement (quantified using the concurrence) of a bipartite qubit system undergoing various modes of decoherence. Remarkably, they found that, even when there is no interaction, (either directly or through a correlated environment), there are certain states whose entanglement decays exponentially with time, while for other closely related states, the entanglement vanishes completly in a finite time. This "entanglement sudden death" (ESD) is an intriguing and potentially very important discovery. Since the first theoretical demonstration of ESD, further investigations of different systems have been made by various groups [4,5,6,7,8,9,10,11,12,13]. Extending Yu and Eberly's model by considering correlated reservoirs and interactions [5,8,10,12,13], it was shown that entanglement may be created by spontaneous emission (something which has been known for some time [17] in a different context). The ESD has also been predicted for more complicated systems using other entanglement measures [15,16], and an attempt to give a geometric interpretation for the phenomena of ESD has also been made [18]. Very recently, experimental studies have also been carried out to demostrate ESD, using carefully engineered interactions between systems and environments: Sudden death has been observed both in photons [14] and in atomic ensembles [19].From the technological point of view, states which exhibit exponential decay of entanglement, and therefore retain some vestige of this all-important correlation for long periods, are of great interest. Thus it is important to identify precisely in what circumstances ESD will occur. In this paper, we consider qubits in finite temperature reservoirs: instead of the energy of the qubits being lost via spontaneous decay to...
X states are a broad class of two-qubit density matrices that generalize many states of interest in the literature. In this work, we give a comprehensive account of various quantum properties of these states, such as entanglement, negativity, quantum discord and other related quantities. Moreover, we discuss the transformations that preserve their structure both in terms of continuous time evolution and discrete quantum processes. * nquesada@physics.utoronto.ca arXiv:1207.3689v1 [quant-ph]
A necessary and sufficient condition is derived for certain ad hoc expressions that are frequently used in the literature to represent correctly the degree of polarization of a light beam.
When two Bose-Einstein condensates are suddenly coupled by a tunneling junction, the Gross-Pitaevskii mean-field theory predicts that caustics will form in the number-difference probability distribution. The caustics are singular but are regularized by going to the many-body theory where atom number is quantized. However, if the system is subject to a weak continuous measurement the quantum state decoheres and classicality is restored. We investigate the emergence of singularities during the quantum-to-classical transition paying attention to the interplay between particle number N and the quantum noise introduced by the measurement.
We study the dynamics of entanglement in continuous variable quantum systems. Specifically, we study the phenomena of entanglement sudden death (ESD) in general two-mode-N-photon states undergoing pure dephasing. We show that for these circumstances, ESD never occurs. These states are generalizations of the so-called high NOON states (i.e., a superposition of N photons in the first mode, O in the second, with O photons in the first, N in the second), shown to decrease the Rayleigh limit of lambda to lambda/N, which promises great improvement in resolution of interference patterns if states with large N are physically realized [Phys. Rev. Lett.85, 2733 (2000)]. However, we show that in dephasing NOON states, the time to reach some critical visibility Vcrit, scales inversely with N2. On the practical level, this shows that as N increases, the visibility degrades much faster, which is likely to be a considerable drawback for any practical application of these states.
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