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arXiv:quant-ph/0305092 · Physical Review A

High-flux source of polarization-entangled photons from a periodically poled KTiOPO 4 parametric down-converter

Christopher E. Kuklewicz, Marco Fiorentino B, Gaétan Messin, Franco N. C. Wong, Jeffrey H. Shapiro

Published Jan 15, 2004• 153 citations
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Abstract

We have demonstrated a high-flux source of polarization-entangled photons using a type-II phase-matched periodically poled ${\mathrm{KTiOPO}}_{4}$ parametric down-converter in a collinearly propagating configuration. We have observed quantum interference between the single-beam down-converted photons with a visibility of 99% together with a measured coincidence flux of $300{\mathrm{s}}^{\ensuremath{-}1}/\mathrm{mW}$ of pump. The Clauser-Horne-Shimony-Holt version of Bell's inequality was violated with a value of $2.711\ifmmode\pm\else\textpm\fi{}0.017.$

DOI
10.1103/physreva.69.013807

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We have demonstrated a high-flux source of polarization-entangled photons using a type-II phase-matched periodically poled KTiOPO4{\mathrm{KTiOPO}}_{4}KTiOPO4​ parametric down-converter in a collinearly propagating configuration. We have observed quantum interference between the single-beam down-converted photons with a visibility of 99% together with a measured coincidence flux of 300s\ensuremath−1/mW300{\mathrm{s}}^{\ensuremath{-}1}/\mathrm{mW}300s\ensuremath−1/mW of pump. The Clauser-Horne-Shimony-Holt version of Bell's inequality was violated with a value of 2.711\ifmmode±\else\textpm\fi0.017.2.711\ifmmode\pm\else\textpm\fi{}0.017.2.711\ifmmode±\else\textpm\fi0.017.
We present a simple polarization entanglemenet source based on type-II phase-matched parametric down-conversion in periodically poled KTiOPO4. By use of noncritical phase matching in a noncollinear geometry the single-crystal source emits a cone of polarization-entangled photons. Two beams on opposite sides of the cone are selected for measurements to yield an observed flux of 820 pairs/s per mW of pump power with a two-photon fringe visibility of 96%. With this source we measure a violation of Bell's inequality of 140 standard deviations in 160 s.
We report a source of polarization-entangled photon pairs in the 1550-nm telecommunication band, which is based on non-collinear spontaneous parametric down-conversion in a periodically poled lithium niobate crystal pumped by picosecond pulses. This source is realized utilizing a polarization-based Sagnac interferometer employing a type-0 non-collinear quasi-phase-matching configuration. Polarization entanglement is verified through measurement of the polarization-correlation interference fringes with visibility >96% and by testing the experimental violation of the Clauser-Horne-Shimony-Holt (CHSH) form of Bell's inequality. The CHSH-Bell parameter S is found to be 2.72 ± 0.04, with 18 standard deviations from the statistical uncertainty.
We review the use of bidirectional pumping and collinear propagation in type-II phase-matched periodically poled KTiOPO 4 to achieve efficient generation of polarization-entangled photons via spontaneous parametric downconversion. The advantages of this approach are fully exploited in our optimized polarization Sagnac interferometric source. Its measured flux of 4700 polarization-entangled photon pairs/s per milliwatt of pump power in a 1-nm bandwidth exhibited 99% quantum-interference visibility. We also present a plane-wave Gaussian state theory for continuous-wave spontaneous parametric downconversion. This formulation affords the complete description of multiple-pair emissions that is needed for quantum communication analyses while reducing to the usual biphoton-state description at low generation rates.
We demonstrate pulsed operation of a bidirectionally pumped polarization Sagnac interferometric down-conversion source and its generation of narrow-band, high-visibility polarization-entangled photons. Driven by a narrow-band, mode-locked pump at 390.35 nm, the phase-stable Sagnac source with a type-II phase-matched periodically poled KTiOPO4{\text{KTiOPO}}_{4}KTiOPO4​ crystal is capable of producing 0.01 entangled pair per pulse in a 0.15-nm bandwidth centered at 780.7 nm with 1 mW of average pump power at a repetition rate of 31.1 MHz. We have achieved a mean photon-pair generation rate of as high as 0.7 pair per pulse, at which multipair events dominate and significantly reduce the two-photon quantum-interference visibility. For low generation probability $\ensuremath{\alpha}$, the reduced visibility $V=1\ensuremath{-}\ensuremath{\alpha}$ is independent of the throughput efficiency and of the polarization analysis basis, which can be utilized to yield an accurate estimate of the generation rate $\ensuremath{\alpha}$. At low $\ensuremath{\alpha}$ we have characterized the source entanglement quality in three different ways: average quantum-interference visibility of 99%, the Clauser-Horne-Shimony-Holt SSS parameter of $2.739\ifmmode\pm\else\textpm\fi{}0.119$, and quantum state tomography with 98.85% singlet-state fidelity. The narrow-band pulsed Sagnac source of entangled photons is suitable for use in quantum information processing applications such as free-space quantum key distribution.
We demonstrate a high-quality source of polarization-entangled photons in aperiodically-poled KTiOPO 4 (KTP). The crystal is based on a phase-modulated domain-engineered design that allows two simultaneous downconversion processes, where the two processes correspond to the two terms of the entangled-photon state. We characterized the temperature tuning of both the spectral response and the polarization-fringe visibility for spontaneous parametric downconversion (SPDC) using this crystal. At the optimum temperature where wavelength distinguishability of the two SPDC processes is smallest, we obtained average polarization-entanglement visibility 0.984 ± 0.003 with a measured Clauser–Horne–Shimony–Holt (CHSH) parameter S = 2.801 ± 0.014. We also discuss the effects of the relative phase in the entangled-photon state and present a new way to compensate for these effects to obtain high polarization-entanglement visibility.