

Professor of Quantum Information, Griffith University
Howard M. Wiseman is Director of the Centre for Quantum Dynamics, Griffith University, Australia. He did his Ph.D. with Milburn at the University of Queensland and a postdoc with Walls at the University of Auckland. He is best known for his work on quantum feedback control and adaptive measurements (he recently coauthored the book Quantum Measurement and Control with Milburn, published by Cambridge in 2010), atom laser fundamentals, and entanglement in the presence of superselection rules. His current research areas include entanglement and quantum cryptography, quantum metrology, and quantum foundations. He has won a number of Australian honours: the Bragg Medal of the AIP, the Pawsey Medal of the AAS and the Malcolm Macintosh Medal of the Federal Government. He was elected as a Fellow of the AAS in 2008.
Abstract
In general if one obtains information about an open quantum system by measuring its environment, that measurement will alter the future evolution of the system. However in the Markovian case this backaction is negligible and one can “track” the system i.e. assign it a (stochastically evolving) pure state at all times without disturbing its (deterministic) average evolution. In general this stochastic evolution creates a trajectory passing through infinitely many different pure states, even for a finite dimensional quantum system. Hence an infinite classical memory would be required to track such evolution. Here we show that, for any ergodic master equation, there should exist a monitoring scheme on the environment that can confine the system state to jumping between finitely many states, so that only a finitely large classical memory is required. We prove explicitly that one bit is always sufficient to track a qubit.
