It is shown that radiation pressure can be profitably used to entangle macroscopic oscillators like movable mirrors, using present technology. We prove a new sufficient criterion for entanglement and show that the achievable entanglement is robust against thermal noise. Its signature can be revealed using common optomechanical readout apparatus.

We investigate the possibility of realising effective quantum gates between two atoms in distant cavities coupled by an optical fibre. We show that highly reliable swap and entangling gates are achievable. We exactly study the stability of these gates in presence of imperfections in coupling strengths and interaction times and prove them to be robust. Moreover, we analyse the effect of spontaneous emission and losses and show that such gates are very promising in view of the high level of coherent control currently achievable in optical cavities.

By using a generalization of the optical tomography technique we describe the dynamics of a quantum system in terms of equations for a purely classical probability distribution which contains complete information about the system.Comment: 12 pages, LATEX,preprint of Camerino University, to appear in Phys.Lett.A (1996

We propose a simple optomechanical model in which a mechanical oscillator quadrature could be "cooled" well below its equilibrium temperature by applying a suitable feedback to drive the orthogonal quadrature by means of the homodyne current of the radiation field used to probe its position. PACS numbers(s): 03.65. Bz, 42.50.Dv, 42.50.Vk The problem of considering a macroscopic oscillator in terms of Quantum Mechanics is usually avoided because one can obtain the right results without using any quantum mechanical hypothesys. When, however, one whishes to use it as a device to detect extremely small displacements due to very weak forces, as in the gravitational wave detectors, one has to be careful in considering it as a mere macroscopic object. Should one consider a macroscopic oscillator as a quantum oscillator, once all other possible noise sources were eliminated by using filters, screens, insulators etc., the ultimate criterion one has to satisfy is the one associated with the thermal noise [1,2]. For the harmonic oscillator it means k B T

It is shown that because of the radiation pressure a Schrödinger cat state can be generated in a resonator with oscillating wall. The optomechanical control of quantum macroscopic coherence and its detection is taken into account introducing new cat states. The effects due to the environmental couplings with this nonlinear system are considered developing an operator perturbation procedure to solve the master equation for the field mode density operator. PACS number

We propose a protocol for deterministic communication that does not make use of entanglement. It exploits nonorthogonal states in a two-way quantum channel to attain unconditional security and high efficiency of the transmission. We explicitly show the scheme is secure against a class of individual attacks regardless of the noise on the channel. Its experimental realization is feasible with current technology.

We show that a linear Fabry-Perot cavity with an oscillating end mirror can be used for quantum noise reduction. For a high-quality factor of the mechanical oscillator the output quantum fluctuations of the monochromatic light beam can be significantly squeezed at a frequency very close to that of the impinging light. The analysis is performed by taking into account the coupling of the system with the external world.PACS number(s): 42.50.Lc, 42.65.Vh

Quantum cryptography has been recently extended to continuous variable systems, e.g., the bosonic modes of the electromagnetic field. In particular, several cryptographic protocols have been proposed and experimentally implemented using bosonic modes with Gaussian statistics. Such protocols have shown the possibility of reaching very high secret key rates, even in the presence of strong losses in the quantum communication channel. Despite this robustness to loss, their security can be affected by more general attacks where extra Gaussian noise is introduced by the eavesdropper. In this general scenario we show a "hardware solution" for enhancing the security thresholds of these protocols. This is possible by extending them to a two-way quantum communication where subsequent uses of the quantum channel are suitably combined. In the resulting two-way schemes, one of the honest parties assists the secret encoding of the other with the chance of a non-trivial superadditive enhancement of the security thresholds. Such results enable the extension of quantum cryptography to more complex quantum communications.In recent years, quantum information has entered the domain of continuous variable (CV) systems, i.e., quantum systems described by an infinite dimensional Hilbert space [1,2]. So far, the most studied CV systems are the bosonic modes, such as the optical modes of the electromagnetic field. In particular, the most important bosonic states are the ones with Gaussian statistics, thanks to their experimental accessibility and the relative simplicity of their mathematical description [3,4]. Accordingly, quantum key distribution (QKD) has been extended to this new framework [5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21] and Gaussian cryptographic protocols using coherent states have been shown to exploit fully the potentialities of quantum optics [12,16]. These coherentstate protocols are robust with respect to the noise of the quantum channel, as long as such noise can be ascribed to pure losses [12,16]. By contrast, their security is strongly affected when channel losses are used to introduce a thermal environment, which is assumed to be controlled by a malicious eavesdropper [12,22]. In this Gaussian eavesdropping scenario, we present a method to enhance the security thresholds of the basic coherent-state protocols. This is achieved by extending them to two-way quantum communication protocols, where one of the honest parties (Bob) uses its quantum resources to assist the secret encoding of the other party (Alice). In particular, the enhancement of security is proven to be effective since the security thresholds are superadditive with respect to the double use of the quantum channel. Such a result is achieved when the Gaussian attack corresponds to a memoryless Gaussian channel. More generally, we also consider Gaussian channels with memory, therefore creating classical and/or quantum correlations between the paths of the two-way quantum communication. In order to overcome this kind of eavesdropping strategy, the tw...