Themes in Howard Wiseman's Research in Physics

This is an area which has gained a great deal of attention in recent years. My interest is primarily in quantum optical systems which obey (approximately) a master equation. One of the fascinating aspects of open quantum systems is how the system dynamics can be qualitatively altered by altering the way in which the environment is monitored, without changing the master equation. This sets up a tension with the popular idea that somehow the environment of open quantum systems forces them to behave in a unique, pseudo-classical way. Current work is proceeding in investigating this tension.

Quantum Feedback

One sort of open quantum system which is, as yet, relatively unexplored, is that in which the results of monitoring the environment are used to alter the system dynamics. This quantum feedback can lead to interesting changes in the global system dynamics and more subtle changes in the quantum fluctuations. It has also contributed to ideas in quantum error correction for quantum computing and also for protection of Schroedinger cat states. The use of non-Markovian feedback, in particular State-based feedback, is an active topics of investigation.

Adaptive Quantum Measurements

In text books of quantum mechanics measurements are usually treated quite formally, and one finds statements such as "all Hermitian operators represent an observable". Little space is devoted to how general physical quantities could be observed in a laboratory. In the quantum optics context, it is obvious how to measure some observables. For example, photon number can be measured using a photodetector. Other observables, such as phase, cannot be measured in any simple way. To attempt to measure phase one needs complicated techniques. One such technique is that of adaptive single-shot measurements. By single-shot I mean that this is a single measurement on a single copy of the system. But even single-shot measurements take a finite amount of time. This allows adaptive algorithms to be used. The results from the early part of the measurement can be used to alter the apparatus at later times of the same measurement. So far results have been obtained only for phase measurements, and show that an adaptive phase measurement is far superior to standard phase measurement techniques. Future projects may involve adaptive measurements of other quantities in quantum and atom optics. I am also collaborating with experimental groups to test predictions regarding adaptive phase measurements.

Atom Lasers and BEC

The recent experimental achievement of Bose-Einstein condensation of dilute atomic gases has opened up a great many lines of theoretical speculation. One of the main ones has been the idea of an atom laser. As the author of the first published atom laser model, I have thought about this idea for a long time. As a result of this time for thinking, I believe that it is important to attempt to define exactly what is meant by the term "atom laser" in order for experimentalists to know what to aim for (See my 1997 paper "Defining the (atom) laser"). I am not so active in this area any more, but work on a small scale is continuing on investigating various atom laser models, and other topics in BEC theory.

Fundamental Issues in Quantum Mechanics

I have interests in the "quantum measurement problem" and the philosophy of quantum mechanics. One specific issue is whether the uncertainty relation has anything to say about the destruction of interference in which-path measurements. A number of ways of answering this question have been investigated, including using Bohmian mechanics. I am interested in this, and other interpretations especially in how they treat probabilities and time. I am agnostic as to the best interpretation of quantum mechanics. More recently, I have become interested in weak values, how they can be applied to which-way experiments, and what they may have to say about the interpretation of quantum mechanics.

Quantum Information

This is a major research area for me, in the Centre for Quantum Computer Technology. As well as the research discussed there,  I have investigated a new model of quantum computation (ROM-based), and its application to question of space-efficiency of quantum computation over classical computation. This included collaborating with NMR experimentalists to realise some of the algorithms. Recently, I have become interested in entanglement issues, in particular entanglement and super-selection rules, and also the EPR paradox.

Non-Markovian Open Quantum Systems

For quantum systems which obey (approximately) a Markovian master equation, the relationship between these irreversible dynamics and quantum meaurements (discussed above) is now well-understood. Much less well understood is this relationship in the case of systems having strongly non-Markovian evolution. This work touches on fundamental issues like the meaning of the state vector of a system.