Coherent control, quantum memories and quantum repeaters

Abstract

In the domain of quantum communication, one of the main challenges is the distribution of entanglement over long distances. A scheme, the so-called quantum repeater, has been proposed in order to overcome the problem of channel losses. In this thesis we adresse different problems related to quantum repeaters, both theoretically and experimentally. We present an alternative scheme of quantum repeater based on atomic ensembles and linear optics. It is based on the use of a qubit amplifier and two photon interference. The main features of the protocol are that the distributed entangled state does not need post selection and the entanglement distribution rates approach the optimal rates one might expect from quantum repeaters using probabilistic swapping. An important concern of the whole class of quantum repeaters based on one photon interference is the phase stability in the communication channels. We performed an experiment evaluating the realistic phase noise in installed optical fibers in the Geneva urban area. We found fluctuations of order of 0.1 rad on the time scale of order of 100 $mu$s. The feasibility of the phase stabilization is discussed. One of the essential building blocks of quantum repeater is a quantum memory. We focus our attention on quantum memories based on solid state atomic ensembles and photon echo like techniques. In particular, we show, that the atomic coherence in eryso can be controlled to a very high degree using an external electric field. This allowed us to implement the CRIB protocol in this material and realize the first storage of light at the single photon level at telecommunication wavelength. A necessary step for the implementation of CRIB and AFC storage protocols, in order to achieve long storage times and unit efficiencies, is the use of control light pulses. We developped a numerical Maxwell-Bloch simulator allowing us to study a coherent control of atomic excitations. In particular, we investigate the use of adiabatic chirped pulses in the context of AFC protocol. We study the relationship between the pulse characteristics and the echo properties. The comparison with the conventional $pi$ pulses and the impacts on the multimode capacity of the AFC protocol are also discussed.