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Physical Review A

Anticorrelation effect in femtosecond-pulse pumped type-II spontaneous parametric down-conversion

Yoon-Ho Kim, V. Berardi, Maria V. Chekhova, Yanhua H. Shih

Published Jun 1, 2001• 17 citations
Publisher

Abstract

We report an experimental demonstration of an anticorrelation effect in femtosecond-pulse pumped type-II spontaneous parametric down-conversion. Our experimental data, which is different from that of Atat\"ure et al. [Phys. Rev. Lett. 83, 1323 (2000)], confirmed the shallow symmetric ``dip'' that is predicted theoretically by Keller and Rubin [Phys. Rev. A 56, 1534 (1997)] and Grice and Walmsley [Phys. Rev. A 56, 1627 (1997)]. We show in this paper that the asymmetric dip observed in the literature is an artifact, which is caused by the asymmetric optical elements introduced into the beam path after the down-conversion process. The ``partial distinguishability'' theory suggested by Atat\"ure et al. is therefore incorrect.

DOI
10.1103/physreva.64.011801

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We report an experimental demonstration of an anticorrelation effect in femtosecond-pulse pumped type-II spontaneous parametric down-conversion. Our experimental data, which is different from that of Atat"ure et al. [Phys. Rev. Lett. 83, 1323 (2000)], confirmed the shallow symmetric dip'' that is predicted theoretically by Keller and Rubin [Phys. Rev. A 56, 1534 (1997)] and Grice and Walmsley [Phys. Rev. A 56, 1627 (1997)]. We show in this paper that the asymmetric dip observed in the literature is an artifact, which is caused by the asymmetric optical elements introduced into the beam path after the down-conversion process. The partial distinguishability'' theory suggested by Atat"ure et al. is therefore incorrect.
Quantum interference is observed using femtosecond parametric down-conversion in the absence of spectral filtering of the down-converted light. The well-known symmetric dip observed in cw polarization-interferometry experiments becomes less pronounced and substantially asymmetric under such pulsed excitation. We show that this asymmetry arises from the partial distinguishability of contributions from space-time segments within the down-conversion crystal. We develop a theory that achieves agreement with experiment by coherently adding intrasegment contributions while incoherently adding intersegment contributions.
We investigate the spatiotemporal structure of the biphoton correlation in type-II parametric down-conversion (PDC). As in type-I PDC [Phys. Rev. Lett. 102, 223601 (2009)], we find that the correlation is nonfactorizable in space and time. Differently from type I, the type-II correlation in the spontaneous regime displays an asymmetric V-shaped structure in any cross section including time and one transverse dimension. This asymmetry along the temporal coordinate originates from the signal-idler group velocity mismatch and tends to disappear as the parametric gain is raised. We observe a progressive transition toward a symmetric X-shaped geometry similar to that found in type I when stimulated PDC becomes dominant. We also give quantitative evaluations of the localization in space and in time of the correlation, analyze its behavior for different crystal tuning angles, and underline qualitative differences with respect to type-I PDC.
Generation of polarization-entangled photon pairs with a precise timing through down-conversion of femtosecond pulses is often faced with a degraded polarization entanglement quality. In a previous experiment, we have shown that this degradation is induced by spectral distinguishability between the two decay paths, in accordance with theoretical predictions. Here, we present an experimental study of the spectral compensation scheme proposed and first implemented by Kim and Grice [J. Mod. Opt. 49, 2309 (2002)]. By measuring the joint spectral properties of the polarization correlations of the photon pairs, we show that the spectral distinguishability between the down-converted components is eliminated. This scheme results in a polarization visibility of $97.9\ifmmode\pm\else\textpm\fi{}0.5%$ without any spectral filtering.
A model is presented to describe spontaneous type-II parametric down-conversion pumped by a broadband source. This process differs from the familiar cw-pumped down-conversion in that a broader range of pump energies is available for down-conversion. The properties of the nonlinear crystal determine how these energies are distributed into the down-converted photons. Because the two photons are polarized along different crystal axes, they have different spectral characteristics and are no longer exactly anticorrelated. As the pump bandwidth is increased, this effect becomes more pronounced. A fourth-order interference experiment is proposed, illustrating some of the features of broadband pumped down-conversion.