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Optics Express

Quantum nondemolition measurement of photon number via optical Kerr effect in an ultra-high-Q microtoroid cavity

Yun‐Feng Xiao, Şahin Kaya Özdemir, Venkat Gaddam, Chun-Hua Dong, Nobuyuki Imoto, Lan Yang

Published Dec 15, 2008• 89 citations
Publisher

Abstract

We theoretically investigate a quantum nondemolition (QND) measurement with optical Kerr effect in an ultra-high-Q microtoroidal system. The analytical and numerical results predict that the present QND measurement scheme possesses a high sensitivity, which allows for detecting few photons or even single photons. Ultra-high-Q toroidal microcavity may provide a novel experimental platform to study quantum physics with nonlinear optics at low light levels.

DOI
10.1364/oe.16.021462

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We revisit the idea of quantum nondemolition measurement (QND) of optical quanta via a resonantly enhanced Kerr nonlinearity taking into account quantum backaction. We show that the monolithic microcavities enable QND measurement of the number of quanta in a weak signal field using a classical probe field spatially overlapping with the signal. The phase of the probe field acquires information about the signal number of quanta without altering it due to the cross-phase modulation effect. We find the exact solution to the Heisenberg equations of motion of this system and calculate the measurement error, accounting for the optical losses in the measurement path. We identify a realistic approximation to obtain the explicit form of the final conditional quantum state of the signal field, accounting for the undesirable self-phase modulation effect and designing the optimal homodyne measurement of the probe beam to evade this effect. We show that the best modern monolithic microcavities allow achieving the measurement imprecision several times better than the standard quantum limit.
A photon-number quantum nondemolition (QND) measurement theory that takes dissipative losses into account is presented. A QND measurement criterion is developed to determine the feasibility of QND measurements in a lossy optical medium. Using an analysis based on loss-error characteristics, we examine a lossy Kerr medium. We obtain restrictions on the losses and requirements for the nonlinearity and signal and probe powers necessary to observe the QND effect. We calculate the expected results for an experimental system meeting such requirements. We conclude that QND measurements are possible in existing media.
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