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arXiv:1112.4551 · Physical Review A

Compact source of narrow-band counterpropagating polarization-entangled photon pairs using a single dual-periodically-poled crystal

Yan-Xiao Gong, Zhenda Xie, Ping Xu, Xiaoqiang Yu, Peng Xue, Shining N. Zhu

Published Nov 14, 2011• 47 citations
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Abstract

We propose a scheme for the generation of counterpropagating polarization-entangled photon pairs from a dual-periodically-poled crystal. Compared with the usual forward-wave-type source, this source, in the backward-wave way, has a much narrower bandwidth. With a 2-cm-long bulk crystal, the bandwidths of the example sources are estimated to be $3.6$ GHz, and the spectral brightnesses are more than 100 pairs$/$(s GHz mW). Two concurrent quasi-phase-matched spontaneous parametric down-conversion processes in a single crystal enable our source to be compact and stable. This scheme does not rely on any state projection and applies to both degenerate and nondegenerate cases, facilitating applications of the entangled photons.

DOI
10.1103/physreva.84.053825

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We propose a scheme for the generation of counterpropagating polarization-entangled photon pairs from a dual-periodically-poled crystal. Compared with the usual forward-wave-type source, this source, in the backward-wave way, has a much narrower bandwidth. With a 2-cm-long bulk crystal, the bandwidths of the example sources are estimated to be 3.63.63.6 GHz, and the spectral brightnesses are more than 100 pairs///(s GHz mW). Two concurrent quasi-phase-matched spontaneous parametric down-conversion processes in a single crystal enable our source to be compact and stable. This scheme does not rely on any state projection and applies to both degenerate and nondegenerate cases, facilitating applications of the entangled photons.
Entanglement enables many promising applications in quantum technology. Devising alternative generation methods and harnessing entanglement are prerequisites for practical applications. Here, we realize a distinct polarization-entangled source by simultaneously achieving type-0 and type-I backward quasiphase matching (BQPM) through spontaneous parametric down-conversion in a single bulk crystal, which is different from all previous entangled-source configurations. Pumping the crystal with a single polarized beam generates a nonmaximally polarization-entangled state, which can be further projected to a maximal Bell state with a pair of Brewster windows. Hong-Ou-Mandel interference experiments are done on polarization-degenerate photon pairs for both type-0 and type-I BQPM processes for the first time. The emitted photons in both processes have a bandwidth as narrow as 15.7 GHz. The high quality of this source is characterized by various methods. The rather simple configuration, narrow bandwidth, and high entanglement quality make the source very promising for many quantum information tasks.
We propose a scheme for the generation of counterpropagating polarization-entangled photon pairs from a periodically poled lithium niobate on insulator waveguide. The waveguide is designed to enable a special dispersion relationship between the T M 00 and T E 00 modes that allows two concurrent counterpropagating quasi-phase-matching type-II spontaneous parametric downconversion processes with a single reciprocal wave vector. Due to the counterpropagating phase-matching geometry, the source has a narrow bandwidth in the order of several gigahertz. The compact source opens up a new method for integrated photonic technologies.
We demonstrate a broadband entangled photon source based on a single-pass, type-0 phase-matched, non-collinear spontaneous parametric down-conversion (SPDC) process. By spatially multiplexing the intrinsically random transverse momentum states of biphotons emitted in an annular spatial distribution from a single-frequency-pumped, 2-mm-long periodically poled KTP crystal, we generate polarization-entangled pair photons with a spectral bandwidth of ∼130 nm centered at 810 nm. The negligible group-velocity mismatch of the co-polarized near-degenerate photons in single-pass and low birefringence of standard optical components preserves indistinguishability across the broadband emission without phase compensation, enabling high-quality polarization entanglement over the entire emission bandwidth. The source achieves a brightness of 0.57 Mpairs/s/mW and a two-photon coherence length of 6.97 μm, while exhibiting an interference visibility exceeding 95%, a Bell–CHSH parameter of 2.526 ± 0.005 that remains stable over 12 h of continuous operation and violates the Bell inequality by more than 100 standard deviations, and a state fidelity of 96.3%. The generic scheme providing the broadest bandwidth entangled biphotons at 810 nm, to our knowledge, can readily be used for other wavelengths as required for multiparty quantum networks and quantum sensing applications.
We demonstrate a broadband entangled photon source based on a single-pass, type-0 phase-matched, non-collinear spontaneous parametric down-conversion (SPDC) process. By spatially multiplexing the intrinsically random transverse momentum states of biphotons emitted in an annular spatial distribution from a single-frequency-pumped, 2-mm-long periodically poled KTP crystal, we generate polarization-entangled pair photons with a spectral bandwidth of ∼130 nm centered at 810 nm. The negligible group-velocity mismatch of the co-polarized near-degenerate photons in single-pass and low birefringence of standard optical components preserves indistinguishability across the broadband emission without phase compensation, enabling high-quality polarization entanglement over the entire emission bandwidth. The source achieves a brightness of 0.57 Mpairs/s/mW and a two-photon coherence length of 6.97 μm, while exhibiting an interference visibility exceeding 95%, a Bell–CHSH parameter of 2.526 ± 0.005 that remains stable over 12 h of continuous operation and violates the Bell inequality by more than 100 standard deviations, and a state fidelity of 96.3%. The generic scheme providing the broadest bandwidth entangled biphotons at 810 nm, to our knowledge, can readily be used for other wavelengths as required for multiparty quantum networks and quantum sensing applications.
Ultra-bright source of entangled photons is an essential component in optical quantum information processing. Here we propose a counterpropagating path-entangled photon pair sources using a quasi-periodic modulated lithium niobate crystal. The nonlinear crystal designed by a dual-grid method, simultaneously phase-matched two spontaneous parametric down-conversion processes. Signal and idler modes have opposite propagation directions in the two spontaneous parametric down-conversion processes, which is the key to generating path-entangled photon pairs. Compared to copropagating entangled sources, the counterpropagating path-entangled sources result in a much narrower spectrum. The quantum state of the path-entanglement source is not only suited for quantum coding, but also to allow the implementation of complex quantum algorithms on a photonic chip.
A periodically poled lithium niobate (PPLN) crystal has been used as an efficient source of non-collinearly generated polarization-entangled photon pairs at 810 and 1550 nm. The PPLN crystal was endowed with a specially designed poling pattern and the entangled photons were generated via the nonlinear optical process of spontaneous parametric down conversion (SPDC). A novel design based on overlapping two concurrent type-I quasi-phase-matching structures in a single PPLN crystals produced correlated pairs of alternatively polarized photons in largely separated spectral regions. The phase of the resulting two-photon state is directly linked to parameters of the nonlinear grating. Continuous tunability of the generated Bell state, from Phi(+) to Phi(-), has been demonstrated by translating a slightly wedged crystal perpendicular to the pump beam.