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We show that under the influence of pure vacuum noise two entangled qubits become completely disentangled in a finite time, and in a specific example we find the time to be given by lntimes the usual spontaneous lifetime.PACS numbers: 03.65. Yz, 03.65.Ta, 42.50.Lc Superposition and entanglement are two basic features that distinguish the quantum world from the classical world. While quantum coherence is recognized as a major resource, decoherence due to the interaction with an environment is a crucial issue that is of fundamental interest [1,2,3,4]. When coherence exists among several distinct quantum subsystems the issue becomes more complicated because, along with the local coherence of each constituent particle, their entanglement brings a special kind of distributed or nonlocal coherence. It is this distributed coherence that really matters in many important applications of quantum information [5,6]. Consequently, the fragility of nonlocal quantum coherence is recognized as a main obstacle to realizing quantum computing and quantum information processing (QIP) [7,8]. Apart from the important link to QIP realizations, a deeper understanding of entanglement decoherence is also expected to lead to new insights into quantum fundamentals, particularly quantum measurement and the quantum-classical transition [9,10,11]. Although quantum decoherence has been extensively studied in recent years, it remains unclear how a local decoherence rate is related to a nonlocal disentanglement rate when a multiparticle quantum state is in contact with one or more noisy environments.Therefore, a deep understanding of the decoherence in any viable realization of qubits is desirable and it is surprising that few if any fundamental treatments exist The two atoms are initially entangled but have no direct interaction afterwards. * Electronic address: ting@pas.rochester.edu † Electronic address: eberly@pas.rochester.edu of decoherence that include the dynamics of disentanglement on better than an empirical or phenomenological basis.Here we consider two initially entangled qubits and examine the dynamics of their disentanglement due to spontaneous emission without phenomenological approximation. There is perhaps no simpler realistic bipartite model in which all of the effects of quantum noise can be considered fully analytically. We show that decoherence caused by vacuum fluctuations can affect localized and distributed coherences in very different ways. As one surprising consequence, we show that spontaneous disentanglement may take only a finite time to be completed, while local decoherence (the normal single-atom transverse and longitudinal decay) takes an infinite time.To make our model and results concrete, we restrict our attention to two two-level atoms A and B coupled individually to two cavities which are initially in their vacuum states (see Fig. 1). In the general framework of system-plus-environment, the two two-level atoms are identified as the system of interest, whereas the two cavities serve as the environments. The...

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Quantum entanglement associated with transverse wave vectors of down conversion photons is investigated based on the Schmidt decomposition method. We show that transverse entanglement involves two variables: orbital angular momentum and transverse frequency. We show that in the monochromatic limit high values of entanglement are closely controlled by a single parameter resulting from the competition between (transverse) momentum conservation and longitudinal phase matching. We examine the features of the Schmidt eigenmodes, and indicate how entanglement can be enhanced by suitable mode selection methods.

We examine the quantum structure of continuum entanglement and in the context of short-pulse down-conversion we answer the open question of how many of the uncountably many frequency modes contribute effectively to the entanglement. We derive a set of two-photon mode functions that provide an exact, discrete, and effectively finite basis for characterizing pairwise entanglement. Our analysis provides a basis for entropy control in two-photon pulses generated from down-conversion.