A photon-photon interface with an atomic ensemble in an optical cavity

Abstract

Since the emergence of quantum computing as a field, interactions between two-level systems that can carry quantum information have been conceptualized in terms of quantum devices. Here, I demonstrate four such devices for optical photons traveling in two different modes: an all-optical switch and transistor, a nondestructive photon detector, a phase shifter, and a quantum state generator. These devices rely on a nonlinear optical interface comprised of a few thousand atoms inside of a high-finesse optical cavity. Signal light is sent through a mode transverse to the cavity and is stored or travels through the atoms as a collective excitation. The corresponding atomic population couples to the cavity mode's optical field. The strength of the resulting photon-photon interaction is governed by the optical depth of the atoms for the signal light, and the strong atom-photon coupling in the optical cavity. I first demonstrate an all-optical switch and transistor using blocking interactions that reduce the cavity's transmission by a factor of 11 +/- 1 in the presence of a stored signal photon. This interaction creates anticorrelations between the output light in the signal and cavity paths. I show that these anticorrelations persist in time continuous operation when the photon signal and cavity arrive at the same time. Then, I turn to photon detection. I reconfigure the system and reconceptualize it as a nondestructive, cavity-based detector for signal light. I demonstrate strong correlations between this nondestructive detection and a subsequent destructive detection with non-destructive detection efficiency of 0.5%. Next, I show that a single cavity photon can shift the phase of stored signal light by up to 1.0 +/- 0.4 rad and demonstrate entanglement between output cavity and signal photons. Finally, I present recent experiments where this entanglement is used to modify the phase and amplitude of the signal light by making a projective measurement on the cavity light.