Motion, Measurement and Control in an Open Quantum System
Abstract (Summary)Restricted Item. Print thesis available in the University of Auckland Library or available through Inter-Library Loan. The interaction of a single atom with the light field inside a Fabry-Pérot microcavity in the strong-coupling regime is now an experimentally realizable example of a continuously measured open quantum system. This thesis investigates the motion of an atom due to the interaction with the cavity light field, the possibility of determining the state of the such a system given a series of measurement results and schemes for feedback control of the dynamics which make use of the resulting state estimates. A semiclassical model of atomic motion in an optical cavity is derived and the dependence of the forces on the different parameters is described. A major obstacle to trapping atoms inside the cavity field is that the fluctuations of these forces are typically very much larger than in free space. A quantum mechanical treatment of the atomic motion is also given and it is shown that the motional state of the atom can be highly non-classical. Detection of the phase shift of the light caused by the atom constitutes a high precision measurement of the atomic position and this motivates an investigation of the quantum limits to state estimation under continuous position measurement that accounts for uncertainty about the initial state and the presence of environmental noise or detection inefficiency. Quantum trajectory theory is used to show that after a sufficient period of observation the state of the system is uniquely determined by the measurement results. It is proposed that this state estimate be used to manipulate the system as in techniques of classical control such as dynamic programming. This significantly broadens the range of available quantum mechanical feedback strategies. Finally, a major reason for interest in strong-coupling cavity quantum electrodynamics is the possible application to the developing field of quantum information processing. The limitations imposed by atomic motion on the realization of several quantum computing and communication schemes are investigated. It is shown that schemes which are not critically dependent on the length of the interaction, for example those which employ adiabatic passage, are more robust when atomic motion is considered.
School Location:New Zealand
Source Type:Master's Thesis
Date of Publication:01/01/2000