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Subshot noise intensity fluctuations in four-wave mixing

Weiping Zhang, Daniel F. Walls

Published May 21, 1990• 0 citations
OpenAlex

Abstract

Recent experiments by Heidman et al.1 on parametric oscillation have shown that intensity fluctuations reduced below the shot noise may be obtained in the difference of the signal and idler intensities. This quantum noise reduction in the intensity difference is a consequence of the fact that the signal and idler photons are produced in pairs. We study quantum noise reduction in the intensity difference of the two weak fields generated in four-wave mixing.

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Recent experiments by Heidman et al.1 on parametric oscillation have shown that intensity fluctuations reduced below the shot noise may be obtained in the difference of the signal and idler intensities. This quantum noise reduction in the intensity difference is a consequence of the fact that the signal and idler photons are produced in pairs. We study quantum noise reduction in the intensity difference of the two weak fields generated in four-wave mixing.
We present an exact nonlinear derivation of the spectrum of fluctuations in the difference of output intensities for the signal and idler beams generated by intracavity two-photon gain, when the cavity decay rates are the same for the two modes, and taking into account the atomic fluctuations. For the field inside the cavity, we show that the variance in the difference of intensities may be up to 50% below the shot-noise value. We also consider the case of different decay rates, and show that the noise in the output-intensity difference vanishes at zero frequency, for single-ported cavities, and as long as the only source of field decay is the transmission through the mirror. Within this general approach, we show that for the two-photon laser there is a strong correlation between signal and idler output beams when the relay-level detuning is larger than the power broadening. We calculate, in lowest order, the correction to the noise spectrum of the difference of intensities due to the population of the relay level.
Nondegenerate optical parametric oscillators generate above-threshold signal and idler beams that have intensity fluctuations correlated at the quantum level (twin beams). We describe what is to our knowledge the first high-sensitivity spectroscopy experiment using twin beams emitted by a cw optical parametric oscillator: a very weak two-photon absorption signal, in the 10(-7) range, is recorded on the 4S(1/2)-5S(1/2) transition of atomic potassium with a noise background that is reduced by 1.9 dB with respect to the shot-noise limit of the light used in the experiment.