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Adaptive Quantum Measurements of a Continuously Varying Phase | | D. W. Berry
; H. M. Wiseman
; | | Date: |
2 Nov 2001 | | Journal: | Phys. Rev. A 65, 043803 (2002) DOI: 10.1103/PhysRevA.65.043803 | | Subject: | quant-ph | | Abstract: | We analyze the problem of quantum-limited estimation of a stochastically varying phase of a continuous beam (rather than a pulse) of the electromagnetic field. We consider both non-adaptive and adaptive measurements, and both dyne detection (using a local oscillator) and interferometric detection. We take the phase variation to be dotphi = sqrt{kappa}xi(t), where xi(t) is delta-correlated Gaussian noise. For a beam of power P, the important dimensionless parameter is N=P/hbaromegakappa, the number of photons per coherence time. For the case of dyne detection, both continuous-wave (cw) coherent beams and cw (broadband) squeezed beams are considered. For a coherent beam a simple feedback scheme gives good results, with a phase variance simeq N^{-1/2}/2. This is sqrt{2} times smaller than that achievable by nonadaptive (heterodyne) detection. For a squeezed beam a more accurate feedback scheme gives a variance scaling as N^{-2/3}, compared to N^{-1/2} for heterodyne detection. For the case of interferometry only a coherent input into one port is considered. The locally optimal feedback scheme is identified, and it is shown to give a variance scaling as N^{-1/2}. It offers a significant improvement over nonadaptive interferometry only for N of order unity. | | Source: | arXiv, quant-ph/0111013 | | Services: | Forum | Review | PDF | Favorites | |
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