We present a quantum memory protocol for photons that is based on the direct control of the transition dipole moment. We focus on the case where the light-matter interaction is enhanced by a cavity. We show that the optimal write process (maximizing the storage efficiency) is related to the optimal read process by a reversal of the effective time τ = ∫ dtg(t)/κ, where g(t) is the timedependent coupling and κ is the cavity decay rate. We discuss the implementation of the protocol in a rare-earth ion doped crystal, where an optical transition can be turned on and off by switching a magnetic field.

We present a quantum memory protocol for photons that is based on the direct control of the transition dipole moment. We focus on the case where the light-matter interaction is enhanced by a cavity. We show that the optimal write process (maximizing the storage efficiency) is related to the optimal read process by a reversal of the {\it effective time} $\tau=\int dt g^2(t)/\kappa$, where $g(t)$ is the time-dependent coupling and $\kappa$ is the cavity decay rate. We discuss the implementation of the protocol in certain rare-earth ion doped crystals, where transitions can be turned on and off by switching a magnetic field.

It has recently been shown that light can be stored in Bose-Einstein condensates for over a second. Here we propose a method for realizing a controlled phase gate between two stored photons. The photons are both stored in the ground state of the effective trapping potential inside the condensate. The collision-induced interaction is enhanced by adiabatically increasing the trapping frequency and by using a Feshbach resonance. A controlled phase shift of π can be achieved in 1 s or less.

To build quantum memories for light with atomic ensembles one needs to map single photons into atomic excitations and freeze them until releasing them back as photons on demand. Here we present an idea for realizing this storage-recall procedure by directly controlling the transition dipole moment in a two-level atomic ensemble. An analytical treatment of the problem is performed, and the physical requirements on the proposed scheme are discussed. We propose an implementation employing a magneto-dependent transition dipole moment in a Tm3+:YAG crystal.

The soft-core [image omitted] repulsive interaction together with a Gaussian repulsive interaction are used to reproduce major features of the structure of liquid water, both in direct and reciprocal space, by Monte Carlo and integral equation theories. The study reveals that the structure of liquid water is determined, within the model studied here, by the competition of the two repulsive cores, which results in a two-fold spatial distribution, very reminiscent of the two-state water model proposed by many authors. The fact is that many of the structural features of water could be reproduced without any recourse to direct attractive interactions, such as directional hydrogen bonds, and appear to be the result of long-range competing packing correlations, as witnessed by the particular features of the structure factor. The Hypernetted-Chain integral equation is able to reproduce very accurately the most important features of the experimental structure of room temperature water, whereas the Percus-Yevick approximation fails to reach this state point. A high-temperature study shows that this failure is related to the insufficient diagrammatic structure of this closure.