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...

Helpful elements: A facile bottom-up method using citric acid and L-cysteine as a precursor has been developed to prepare graphene quantum dots (GQDs) co-doped with nitrogen and sulfur. A new type and high density of surface state of GQDs arises, leading to high yields (more than 70 %) and excitation-independent emission. FLQY = fluorescence quantum yield.

Graphene has many unique properties that make it an ideal material for fundamental studies as well as for potential applications. Here we review recent results on the Raman spectroscopy and imaging of graphene. We show that Raman spectroscopy and imaging can be used as a quick and unambiguous method to determine the number of graphene layers. The strong Raman signal of single layer graphene compared to graphite is explained by an interference enhancement model. We have also studied the effect of substrates, the top layer deposition, the annealing process, as well as folding (stacking order) on the physical and electronic properties of graphene. Finally, Raman spectroscopy of epitaxial graphene grown on a SiC substrate is presented and strong compressive strain on epitaxial graphene is observed. The results presented here are highly relevant to the application of graphene in nano-electronic devices and help in developing a better understanding of the physical and electronic properties of graphene.

A new development in the dynamical behavior of elementary quantum systems is the surprising discovery that correlation between two quantum units of information called qubits can be degraded by environmental noise in a way not seen previously in studies of dissipation. This new route for dissipation attacks quantum entanglement, the essential resource for quantum information as well as the central feature in the Einstein-Podolsky-Rosen so-called paradox and in discussions of the fate of Schrödinger's cat. The effect has been labeled ESD, which stands for early-stage disentanglement or, more frequently, entanglement sudden death. We review recent progress in studies focused on this phenomenon.