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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Quantum noise of ultrastable bright twin beams

Olivier Pfister, Sheng Feng

Published May 25, 2004• 0 citations
Publisher

Abstract

We investigate experimentally the number and phase quantum noises of the light emitted by an ultrastable nondegenerate optical parametric oscillator (OPO) above threshold. Strong intensity correlations at the photon level -- number-difference squeezing or twin beams -- have already been observed by several groups. We have recently observed the twin-beam interference of a classically phase-locked OPO and have shown that the phase-difference is antisqueezed, as expected from the number-phase Heisenberg (in)equality. In this paper we describe the realization of a stable classical measurement of the joint quadratures of an OPO, as part of our ongoing effort to generate ultrastable EPR light beams simultaneously squeezed in number difference and phase sum.

DOI
10.1117/12.546130

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We investigate experimentally the number and phase quantum noises of the light emitted by an ultrastable nondegenerate optical parametric oscillator (OPO) above threshold. Strong intensity correlations at the photon level -- number-difference squeezing or twin beams -- have already been observed by several groups. We have recently observed the twin-beam interference of a classically phase-locked OPO and have shown that the phase-difference is antisqueezed, as expected from the number-phase Heisenberg (in)equality. In this paper we describe the realization of a stable classical measurement of the joint quadratures of an OPO, as part of our ongoing effort to generate ultrastable EPR light beams simultaneously squeezed in number difference and phase sum.
We report the first measurement of the quantum phase-difference noise of an ultrastable nondegenerate optical parametric oscillator that emits twin beams classically phase locked at exact frequency degeneracy. The measurement illustrates the property of a lossless balanced beam splitter to convert number-difference squeezing into phase-difference squeezing, and thus provides indirect evidence for Heisenberg-limited interferometry using twin beams. This experiment is a generalization of the Hong-Ou-Mandel interference effect for continuous variables and constitutes a milestone towards continuous-variable entanglement of bright, ultrastable nondegenerate beams.
We report the first measurement of the quantum phase-difference noise of an ultrastable nondegenerate optical parametric oscillator that emits twin beams classically phase-locked at exact frequency degeneracy. The measurement illustrates the property of a lossless balanced beam-splitter to convert number-difference squeezing into phase-difference squeezing and, thus, provides indirect evidence for Heisenberg-limited interferometry using twin beams. This experiment is a generalization of the Hong-Ou-Mandel interference effect for continuous variables, and constitutes a milestone towards continuous-variable entanglement of bright, ultrastable nondegenerate beams.
We report the realization of a classically phase-locked source of quantum twin beams. Theoretical work by Reid and Drummond, and more recently by van Loock and Braunstein, predicts that such a source is adequate for creating bipartite and multipartite continuous-variable entanglement. The source is a type-II optical parametric oscillator (OPO) above threshold. Its exceptional frequency and intensity stability is derived from careful design and from three servo loops. The OPO can stably emit on the frequency-degenerate mode, pumped a few percent above the threshold. We observe a preliminary number-difference squeezing of 5.5 dB (6.4 dB inferred) at 200 kHz. In order to enable future EPR measurements, the signal and idler fields must also be classically phase-locked: we measure a signal-idler-frequency-difference linewidth smaller than 300 Hz for our OPO.
We have generated bright squeezed light and quantum-correlated twin photon beams by using a cw semimonolithic KTP Optical Parametric Oscillator (OPO), which exhibits a low oscillation threshold and is capable of oscillating even in the free-running (no external-feedback) condition. The pump beam reflected by the OPO, after a cascading nonlinearity process in the crystal, appears as bright squeezed light. The squeezing observed on the reflected pump beam from the semimonolithic OPO was 30% (inferred squeezing: 48%). The signal and idler beams generated by the optical parametric oscillator are highly quantum-correlated twin photon beams. Since the squeezing property of the intensity difference between the twin photonbeams is robust in the condition where the two fields are balanced, the squeezing can be observed even in the freerunning condition of the OPO. This intensity-difference squeezing observed between the twin photon beams from the free-running semimonolithic OPO was 70% (inferred squeezing: 87%).
The signal and idler beams from a non-degenerate, above threshold, optical parametric oscillator have a strong quantum correlation, which are called twin beams. This correlation arises from the simultaneous generation of the signal and idler photons in the form of twin pairs of photons through the parametric down conversion, and the generated twin beams have exactly the same photon statistics. As a result, the fluctuations on the difference between the intensities of the twin beams are reduced with respect to the shot noise limit, that is, intensity-difference squeezing. We give a comprehensive overview of the generation of twin beams using the optical parametric oscillator, and the applications exploiting the quantum correlation of twin beams are presented.
A strong quantum correlation between twin-beams at wavelength was observed. The intensity difference noise of twin-beams, which is generated by a semimonolithic OPO cavity pumped by a frequency-doubled and stabilized Nd:YAP laser, is about 80% (7 dB) below the short noise limit at the measurement frequency of 1.5 MHz. The threshold of the optical parametric oscillator is about 20 mW. The parametric dependence of the quantum correlation is experimentally investigated. The experimental results agree with the theory well.