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Frontiers in Optics

Quantum feedback: controlling entangled states in cavity QED and optical quantum computing

Howard M. Wiseman, Jay Gambetta, Joseph E. Reiner, Wade P. Smith, Luis A. Orozco, Timothy C. Ralph

Published Jan 1, 2004• 0 citations
Publisher

Abstract

Quantum optical feedback is emerging as a powerful tool in quantum physics and quantum information. I present new results from the first (2002) “deep quantum” feedback experiment, and a new theoretical analysis showing how entanglement is essential in it. I also present applications of quantum feedback to optical quantum computing.
DOI
10.1364/fio.2004.fwe2

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In this paper, an overview of research in quantum control is presented. Examples of work in the first theme include numerous applications of semi-definite programming and relaxation methods in quantum information theory. Examples of the second include applications of feedback control in the development of new receiver designs for quantum-optical telecom. The third theme is just beginning to emerge as an active research area, linking fundamental issues in quantum field theory with new challenges for applied and computational mathematics in non-commutative stochastic analysis and optimization.
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Quantum feedback in strongly coupled systems can probe a regime where one quantum of excitation is a large fluctuation. We present theoretical and experimental studies of quantum feedback in an optical cavity QED system. The time evolution of the conditional state, following a photodetection, can be modified by changing the drive of the cavity. For the appropriate feedback, the conditional state can be captured in a new steady state and then released. The feedback protocol requires resonance operation, and proper amplitude and delay for the change in the drive. We demonstrate the successful use of feedback in the suppression of the vacuum Rabi oscillations for the length of the feedback pulse and their subsequent return to steady state. The feedback works only because we have an entangled quantum system, rather than an analogous correlated classical system.
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