Single atom detection using optical cavities
Creators: name: Poldy, Rachel
Publisher: name: The Australian National University
Year: 2012
Subjects: subject: QC446.2.P65 2012
subject: Quantum optics
subject: Atoms

description: In the same way that we have the ability to detect single particles of light - photons - in optical experiments and devices, the role of a single atom counter in analogous set-ups using atoms is of notable interest. It is also a role that has yet to be properly fulfilled, particularly in the case of cold, neutral atoms. In this thesis, we investigate the use of an optical cavity of modest finesse to detect single rubidium atoms. We explore the system with theoretical modelling and experimental testing. This is the first study of optical cavities that thoroughly considers the optimisation of design for high-quantum efficiency, low-noise single-atom counting, with an investigation of such a comprehensive range of parameters. We model the atom detection with a quantum description of a two-level atom and single cavity mode. Specifically, we numerically solve a set of ODEs that define the field of a Fabry-Perot cavity mode in a truncated Fock basis. Their solution gives the steady-state density matrix for the cavity-atom system that is subsequently used to determine the expectation values for a range of operators. Our modelling presents a set of variables that define a large parameter space. We consider the full parameter space in order to find the best theoretical signal-to-noise ratio for a maximally coupled atom. We find good signal-to-noise ratios are achieved using both resonant and detuned frequencies for the probe laser and cavity resonance, provided the intra-cavity intensity corresponds approximately to the atomic saturation intensity. This requires increasing driving power in the probe beam as the detuning is increased. A corollary is that the probe powers resulting in the best signal-to-noise ratios - particularly for detuned detection - are higher than the saturation intensity for a typical avalanche photodiode. Consequently heterodyne detection is determined to be the best photon detection system for the cavity set-up. We proceed to refine the model, with simulations of atomic trajectories that account for various forces within the cavity mode. The dipole-force results in a strong channelling of atoms into high-coupling regions of the mode, meaning that detuned detection results in better quantum efficiencies than resonant detection. High quantum efficiencies accompanied by low false-count rates suggests the ability to detect the statistics of an atomic source with high sensitivity. A clearance of more than 20dB above the atomic noise-floor is found for detuned detection, and approximately 8dB for resonant detection. The experimental investigation of cavity detection is realised with an under-coupled Fabry-Perot cavity with a finesse of approximately the same value as our modelled cavity: 10000. We study resonant detection. The set-up differs somewhat from the simulated process in two respects; our measurements are not shot-noise limited as in the model, and our atoms transit the cavity much faster and with lower density resulting in a substantially weaker signal. We have begun the investigation of a new atomic source that will produce slower atoms, and hopefully allow a more complete experimental study of the cavity detector. - provided by Candidate