Control protocols for manipulation of ground-state quantum beats in a cavity QED system

Abstract

Cavity quantum electrodynamics (QED) captures the essential interaction between two quantum systems, an atom (for example) and the quantized electromagnetic field. The cavity reduces the plethora of spatial modes of the field in free-space to one or two. This simplification facilitates the study and control of atom-light interactions. In this thesis we show results where we control both aspects of the interaction.

Our first measurements demonstrate the implementation of a simple feedback mechanism on a two-mode cavity QED system to preserve the Zeeman coherence of a ground state superposition that generates quantum beats. Our investigation shows how to prevent a shift away from the Larmor frequency and associated decoherence caused by Rayleigh scattering. The protocol consists of turning off the drive of the system after the detection of a first photon and letting it evolve in the dark. Turning the drive back on after a pre set time reveals a phase accumulated only from Larmor precession, with the amplitude of the quantum beat more than a factor of two larger than with continuous drive.

We present preliminary conditional measurements in a new cavity QED apparatus that show an environment-assisted speed-up of the evolution of our cavity photon state under weak driving. Changes in the number of atoms (N) that can couple to the field is our way of tailoring the environment. Our results indicate that as N increases, the rate of the re-population of the cavity photon state increases.