The author presents a quantum Langevin equation in the Schrodinger picture. The friction term is motivated by a natural generalisation of Wigner's theorem or, equivalently, of Dirac's superposition principle. The author applies the model to spin relaxation and to the Brownian motion of the harmonic oscillator. In both cases the state vectors evolve asymptotically to a distribution which has the Gibbs state as corresponding density matrix. It is shown that for any initial state, the harmonic oscillator tends to a coherent state.
We demonstrate an implementation of quantum key distribution with continuous variables based on a go-&-return configuration over distances up to 14km. This configuration leads to self-compensation of polarisation and phase fluctuations. We observe a high degree of stability of our set-up over many hours.
Quantum cryptography is reviewed, first using entanglement both for the intuition and for the experimental realizations. Next, the implementation is simplified in several steps until it becomes practical. At this point entanglement has disappeared. This method can be seen as a lesson of Applied Physics. Finally, security issues, e.g. photon number splitting attacks, and counter-measures are discussed.
An alternative to the electrical microphone for photoacoustical detection of trace gases is presented. This microphone is made of a monomode optical fiber wound and glued on a thin plate. This fiber is one arm of a Michelson interferometer. A Sagnac interferometer has also been used and has given better results.
One of the last open problems concerning two qubits in a pure state is to find the exact local content of their correlation, in the sense of Elitzur, Popescu, and Rohrlich (EPR2) [A. C. Elitzur, S. Popescu, and D. Rohrlich, Phys. Lett. A162, 25 (1992)]. We propose an EPR2 decomposition that allows us to prove, for a wide range of states $|\ensuremath{\psi}(\ensuremath{\theta})\ensuremath{\rangle}=\mathrm{cos}\ensuremath{\theta}|00\ensuremath{\rangle}+\mathrm{sin}\ensuremath{\theta}|11\ensuremath{\rangle}$, that their local content is $\overline{{p}_{L}}(\ensuremath{\theta})=\mathrm{cos}2\ensuremath{\theta}$. We also share reflections on how to possibly extend this result to all two-qubit pure states.
We use a recent stochastical diffusion model of quantum evolution to represent the evolution of a three-level quantum system undergoing quantum jumps. This is possible because the continuous change in the quantum state in this diffusion model is so rapid that it appears to be instantaneous in comparison with the time between transitions. Experimental data from a study of the intermittent fluorescence of a single trapped 24Mg+ ion and equivalent theoretical data are shown to be strikingly similar. Statistical comparisons of the data are also made.
The Differential-Phase-Shift (DPS) and the Coherent-One-Way (COW) are among the most practical protocols for quantum cryptography, and are therefore the object of fast-paced experimental developments. The assessment of their security is also a challenge for theorists: the existing tools, that allow to prove security against the most general attacks, do not apply to these two protocols in any straightforward way. We present new upper bounds for their security in the limit of large distances ($d \gtrsim 50$km with typical values in optical fibers) by considering a large class of collective attacks, namely those in which the adversary attaches ancillary quantum systems to each pulse or to each pair of pulses. We introduce also two modified versions of the COW protocol, which may prove more robust than the original one.
In systems containing several elements with polarization dependent losses (PDL) connected by standard optical fibers, the global attenuation and the global PDL may fluctuate. After discussing these phenomena, the corresponding statistics are investigated. Basic definitions are proposed and formulae for the mean values and the standard deviations are derived.
A definition of a {\it Realistic} Physics Theory is proposed based on the idea that, at all time, the set of physical properties possessed (at that time) by a system should unequivocally determine the probabilities of outcomes of all possible measurements.
