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We explore the many body physics of a Bose condensed atom gas at finite temperature through the Raman transition between two hyperfine levels. Unlike the Bragg scattering where the phonon-like nature of the collective excitations has been observed, a different branch of thermal atom excitation is found theoretically in the Raman scattering. This excitation is predicted in the generalized random phase approximation (GRPA) and has a gapped and parabolic dispersion relation. The gap energy results from the exchange interaction and is released during the Raman transition. The scattering rate is determined versus the transition frequency ω and the transferred momentum q and shows the corresponding resonance around this gap. Nevertheless, the Raman scattering process is attenuated by the superfluid part of the gas. The macroscopic wave function of the condensate deforms its shape in order to screen locally the external potential displayed by the Raman light beams. This screening is total for a condensed atom transition in order to prevent the condensate from incoherent scattering. The experimental observation of this result would explain some of the reasons why asuperfluid condensate moves coherentlywithout any friction with its surrounding. [ABSTRACT FROM AUTHOR]

We derive a quantum cloning machine that maximizes the entanglement of formation of the two copies of any maximally entangled input state, while preserving the separability of all unentangled input states. In addition, it is proven to optimally duplicate the entanglement of formation of all isotropic input states. For large d, the cloning machine behaves classically and outperforms a local entanglement cloner, studied for comparison. [ABSTRACT FROM AUTHOR]

We study the properties of a dilute Bose condensed gas at zero temperature in the presence of a strong random potential with arbitrary correlation length. Starting from the underlying Gross–Pitaevskii equation, we use the random phase approximation in order to get a closed integral equation for the averaged density distribution which allows the determination of both the condensate and the superfluid density. The obtained results generalise those of Huang and Meng (HM) to strong disorder. In particular, we find the critical value of the disorder strength, where the superfluid phase disappears by a first-order phase transition. We show how this critical value changes as a function of the correlation length. [ABSTRACT FROM AUTHOR]

We describe a cryptographic protocol consisting of two entangled beams of squeezed light which makes use of statistical tests to deduce the secret key bit. The sender (Alice) encrypts a secret key by modulating the phase of the beam sent in public by the receiver (Bob) who keeps the other beam private. The knowledge of the degree of non classical correlation between the beam quadrature components measured in private and in public allows only Bob to decrypt the secret key. With a view towards absolute security, we formally prove that any external intervention from an eavesdropper (Eve) during the communication process introduces necessarily some modification susceptible to be detected. Statistical confidentiality tests are proposed to detect the presence of Eve. [ABSTRACT FROM AUTHOR]