Conditional measurements, quantum feedback, and cold atoms in cavity QED.

摘要

Two-time correlation functions are equivalent to conditional measurements in the sense that given a fluctuation at time t, they give the evolution of the system at time t + τ. The theoretical description of conditional measurements is well described with the formalism of quantum trajectories, which provide a “measurement friendly” means for understanding the evolution of a quantum system.; The quantum system studied in this thesis is the strongly-coupled; atom-cavity QED system which consists of N-atoms coupled to a single electro-magnetic field mode of a Fabry-Perot cavity. When the cavity emits a single photon the intra-cavity field undergoes large fluctuations. The coherent evolution of the intra-cavity field; following a photoemission, reduces the cavity field noise below the shot-noise limit. A connection exists between this reduction, known as squeezing, and the conditioned field evolution. The cosine-Fourier transform of the conditioned field evolution and the spectrum of squeezing are proportional. In the first part of my thesis I use this connection, along with quantum trajectory theory, to study the dynamic origins of the spectrum of squeezing. This led to a better understanding of previous experimental results in our cavity QED system.; In the second and third parts of my thesis I used quantum trajectories to formulate two different quantum feedback schemes for a strongly-coupled cavity QED system. In both feedback proposals it is the experimenter's knowledge of the system, and the detection of a single photon, that is used to control the evolution of the cavity QED system. We have implemented the first of these feedback proposals which conditions feedback upon single photon detections from our low-intensity cavity QED system.; Previous experimental realizations have used a thermal beam to place the atoms inside the cavity. This degrades the effectiveness of the feedback proposals and the detection of quantum fluctuations. The final portion of my thesis is dedicated to overcoming this degradation with the construction of a continuous cold beam of rubidium atoms. These atoms have been extracted from a two-dimensional magneto-optical trap with a moving molasses.