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Program Manager
Prof Howard Wiseman - Griffith University

Participating GU Researchers
Prof David Pegg
Dr John Vaccaro
Dr He-Bi Sun
Dr Kurt Jacobs

Participating Centre Researchers
Prof Gerard Milburn
A/Prof Tim Ralph
Dr Geoff Pryde
Dr Jeremy O'Brien
Dr Andrew White

Collaborating Researchers
Dr Andrew Doherty - University of Queensland
Dr Steve Bartlett - University of Sydney
Dr Rob Spekkens - Perimeter Institute, Canada
A/Prof Hideo Mabuchi - California Institute of Technology, USA
Dr Keith Schwab
- University of Maryland, USA

Program Description
The aim of this program is to understand the measurement and control of quantum devices and to apply this understanding to quantum information processing. The program has four sub-programs:

1. Quantum Measurement Theory for Read-Out Devices
A crucial part of quantum computing is read-out. Schemes for reading out the state of a solid-state QC include a single-electron transistor, and a quantum point contact. The theory of quantum trajectories, as developed in quantum optics, has proven very useful for understanding continuous measurement in the solid state context. However, as traditionally formulated it is a highly idealized theory, assuming that individual quantum events (such as electrons tunnelling through a barrier) can be observed. In practice, the electronic noise and bandwidth of the external amplifier will mask this. A generalization of the quantum trajectory approach to model the effect of such read-out imperfections on the conditioned system state was developed for photon detectors [1]. We are now applying this technique in the solid-state context (see figure below).



2. Quantum Feed Back Control
Applications for quantum feedback in quantum computers include quantum error correction for detected errors [2], testing read-out devices, and state engineering. We are investigating all of these aspects, as well as seeking a general understanding of quantum control.

3. Quantum Computing - Measurement Interface
Limitations on the measurements that parties can undertake on their quantum systems places limitations on what quantum information processing tasks they can achieve. We are developing the general theory of such limitations [3], as well as studying other questions relating to quantum information and measurement.

4. Quantum Computing - Measurement Interface
We are seeking new schemes for quantum measurement that may be useful for quantum computing, in particular adaptive phase measurements in linear optics quantum computing. On the flip-side, quantum computing also suggest new measurement schemes.

[1] P. Warszawski, H.M. Wiseman and H. Mabuchi, Phys. Rev. A 65, 023802 (2002).
[2] C. Ahn, H.M. Wiseman, and G.J. Milburn, Phys. Rev. A 67, 052310 (2003).
[3] H.M. Wiseman and J.A. Vaccaro, Phys. Rev. Lett. 91, 097902 (2003); S.D. Bartlett and H.M. Wiseman, ibid., 097903 (2003).




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