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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Extra phase noise from thermal fluctuations in non-linear optical crystals

A. S. Coelho, J. E. S. César, K. N. Cassemiro, Alessandro S. Villar, P. Nussenzveig, M. Martinelli

Published Jun 1, 2009• 9 citations
Publisher

Abstract

Recent experiments with optical parametric oscillators have shown that an unpredicted extra noise on the phase of the fields reduces the level of squeezing or quantum correlations. In some cases, it can hinder the attempts to observe entanglement in those systems. It is probably the main reason why signal and idler entanglement above threshold remained unobserved by almost 20 years after its prediction. This paper investigates that the extra noise can be explained by phonon scattering of the central carrier of the intense field. In this Brillouin process, the scattered light acts as an extra noise source in the sideband where squeezing is generally observed. With this model, the linear dependence of the excess noise with the intracavity power and its behavior with temperature can be explained. Adding this noise source to the analysis of the quantum features in the OPO above threshold, recent results in the quest for tripartite entanglement in the OPO above threshold can be explained.

DOI
10.1109/cleoe-eqec.2009.5192015

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Recent experiments with optical parametric oscillators have shown that an unpredicted extra noise on the phase of the fields reduces the level of squeezing or quantum correlations. In some cases, it can hinder the attempts to observe entanglement in those systems. It is probably the main reason why signal and idler entanglement above threshold remained unobserved by almost 20 years after its prediction. This paper investigates that the extra noise can be explained by phonon scattering of the central carrier of the intense field. In this Brillouin process, the scattered light acts as an extra noise source in the sideband where squeezing is generally observed. With this model, the linear dependence of the excess noise with the intracavity power and its behavior with temperature can be explained. Adding this noise source to the analysis of the quantum features in the OPO above threshold, recent results in the quest for tripartite entanglement in the OPO above threshold can be explained.
This paper demonstrates the nonlinear system of the smallest order for direct tripartite entanglement production. An optical parametric oscillator operating above threshold produces an inseparable state of pump, signal and idler modes. Previous attempts had been frustrated by the presence of an extra noise source, which is shown to be generated by photon scattering of light due to thermal phonons of the crystal. Reduction of this noise is achieved by cooling the KTP crystal of OPO down to -10deg C. Reflected pump field, signal and idler fields are measured after reflection by empty cavities (enabling the measurement of amplitude or phase quadrature) using efficient photodiodes, allowing the complete measurement of the covariance matrix. The photocurrent component at 21 MHz, used in the analysis, was registered with the help of an analog-to-digital card and a computer.
We show theoretically and experimentally that scattered light by thermal phonons inside a second-order nonlinear crystal is the source of additional phase noise observed in optical parametric oscillators. This additional phase noise reduces the quantum correlations and has hitherto hindered the direct production of multipartite entanglement in a single nonlinear optical system. We cooled the nonlinear crystal and observed a reduction in the extra noise. Our treatment of this noise can be successfully applied to different systems in the literature.
We review the measurement of entanglement in the above-threshold optical parametric oscillator, an experimental challenge that lasted 17 years. The challenge was in part technical, owing to the difficulty of measuring phase noise, but also related to unexpected noise features that remain unexplained. We describe the experimental strategies used to measure phase noise and to circumvent the "extra" noise. The theoretical prediction of a higher order of entanglement is presented, as well as measurements of bright three-color quantum correlations.
Quantum states with non-Gaussian statistics generated by optical parametric oscillators (OPO) with fluctuating parameters are studied by means of the kurtosis excess of the external field quadratures. The field generated is viewed as the response of a nonlinear device to the fluctuations of pump laser amplitude and frequency, crystal temperature, and cavity detuning, in addition to quantum noise sources. The kurtosis excess has been evaluated perturbatively up to the third order in the strength of the crystal nonlinear coupling factor and the second order in the classical fluctuating parameters. Applied to the device described in Opt. Expr. 13, 948 (2005), the model has given values of the kurtosis excess in good agreement with the measured ones.
We experimentally investigate quadrature correlations between pump, signal, and idler fields in an above-threshold optical parametric oscillator. We observe new quantum correlations among the pump and signal or idler beams, as well as among the pump and a combined quadrature of signal and idler beams. A further investigation of unforeseen classical noise observed in this system is presented, which hinders the observation of the recently predicted tripartite entanglement. In spite of this noise, current results approach the limit required to demonstrate three-color entanglement.