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arXiv:quant-ph/0407211 · Physical Review Letters

Detection of Sub-Shot-Noise Spatial Correlation in High-Gain Parametric Down Conversion

Ottavia Jedrkiewicz, Yun-Kun Jiang, E. Brambilla, A. Gatti, Morten Bache, Luigi A. Lugiato, P. Di Trapani

Published Dec 6, 2004• 206 citations
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Abstract

Using a 1 GW, 1 ps pump laser pulse in high-gain parametric down conversion allows us to detect sub-shot-noise spatial quantum correlation with up to 100 photoelectrons per mode by means of a high efficiency charge coupled device. The statistics is performed in single shot over independent spatial replica of the system. Evident quantum correlations were observed between symmetrical signal and idler spatial areas in the far field. In accordance with the predictions of numerical calculations, the observed transition from the quantum to the classical regime is interpreted as a consequence of the narrowing of the down-converted beams in the very high-gain regime.

DOI
10.1103/physrevlett.93.243601

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Using a 1 GW, 1 ps pump laser pulse in high-gain parametric down conversion allows us to detect sub-shot-noise spatial quantum correlation with up to 100 photoelectrons per mode by means of a high efficiency charge coupled device. The statistics is performed in single shot over independent spatial replica of the system. Evident quantum correlations were observed between symmetrical signal and idler spatial areas in the far field. In accordance with the predictions of numerical calculations, the observed transition from the quantum to the classical regime is interpreted as a consequence of the narrowing of the down-converted beams in the very high-gain regime.
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We consider travelling-wave parametric down-conversion in the high-gain regime and present the experimental demonstration of the quantum character of the spatial fluctuations in the system. In addition to showing the presence of sub-shot noise fluctuations in the intensity difference, we demonstrate that the peak value of the normalized spatial correlations between signal and idler lies well above the line marking the boundary between the classical and the quantum domain. This effect is equivalent to the apparent violation of the Cauchy–Schwartz inequality, predicted by some of us years ago, which represents a spatial analogue of photon antibunching in time. Finally, we analyse numerically the transition from the quantum to the classical regime when the gain is increased and we emphasize the role of the inaccuracy in the determination of the symmetry centre of the signal–idler pattern in the far-field plane.
We review and compare the results of quantum spatial correlation measurements in parametric down-conversion in the high-gain pulsed regime, with preliminary measurements performed in the low-gain cw-pumped regime. The diagnostics is based on a high quantum efficiency CCD camera, and in the second case the radiation pattern is recorded after temporal integration of the "single-photon" spatial distribution. The effect of the detected number of temporal modes on the accessibility of the sub-shot noise regime is discussed, together with the identification of suitable regimes for weak image detection.
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