Quantum information storage in atomic media

The answer to the atom-light interface problem could be given by the recently developed technique of storage of light by means of electromagnetically-induced transparency. EIT is a quantum interference effect that permits light to propagate through an otherwise opaque medium. Observed in gaseous atomic media, EIT is associated with high linear dispersion which leads to a tremendous reduction of the group velocity of light. Reducing this velocity to zero will stop and store the light pulse in the medium. The process can be reversed and the light pulse regenerated in its original quantum state, thus implementing a quantum memory cell for light. 2 Theoretically, this method is good for storing quantum and classical information alike; the regenerated pulse should possess exactly the same quantum properties as the one initially stored. Existing experimental tests are however limited to the classical domain. The next step extending the EIT storage technique to nonclassical states of light is the goal of our project. Our non-classical states will be single photons the actual carriers of quantum bits in the hypothetical optical quantum computer. The idea is to generate a photon pair by means of parametric down-conversion; one photon in a pair will be detected immediately to serve as a reference while the other one will be stored in an EIT medium. The second photon will be detected upon its release; nonclassical correlations between the two photons will verify whether the EIT storage preserves the single-photon state. The experiment is rather complicated due to some special requirements a photon must meet in order to be stored. Namely, its line width must be on the order of hundreds of kHz. Such narrowband single photons have not yet been produced. The idea is to use an optical parametric amplifier a nonlinear optical cavity pumped by a continuous-wave laser source below its threshold. This cavity must be stabilized so that the signal photon wavelength is exactly the same as that of the desired atomic transition. We report in our present experimental results together with a global overview of future experiments.