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arXiv:2506.06460 · Optica Quantum

Direct polarization-entangled photon pair generation using domain-engineered nonlinear crystals

Anatoly Shukhin, Inbar Hurvitz, Leonid Vidro, Ady Arie, H. S. Eisenberg

Published Sep 3, 2025• 8 citations
arXivHTMLPDF

Abstract

A biphoton polarization-frequency entanglement source was realized by shaping the phase-matching function of a periodically poled KTP crystal. It provides a simple method to achieve polarization (or spectral) entanglement in a simple collinear setup, based on single-pass SPDC and a dichroic (or polarizing) beam splitter. This is a robust and cost-effective configuration that can be easily implemented outside the laboratory environment. We characterized the source using two approaches: reconstructing the density matrix of the generated state with quantum state tomography, by recording the coincidences across 16 mutual polarization settings, in addition to a new method based on quantifying the symmetry of the joint spectral intensity by swapping between the signal and idler wavelengths. The polarization-entangled source violates the Clauser–Horne–Shimony–Holt inequality with a measured S =2.747±0.004. We also measured the polarization entanglement visibility in two mutually unbiased polarization bases and evaluated the squeezing level by characterizing the reduction in visibility at high pump power.

DOI
10.1364/opticaq.567643

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A biphoton polarization-frequency entanglement source was realized by shaping the phase-matching function of a periodically poled KTP crystal. It provides a simple method to achieve polarization (or spectral) entanglement in a simple collinear setup, based on single-pass SPDC and a dichroic (or polarizing) beam splitter. This is a robust and cost-effective configuration that can be easily implemented outside the laboratory environment. We characterized the source using two approaches: reconstructing the density matrix of the generated state with quantum state tomography, by recording the coincidences across 16 mutual polarization settings, in addition to a new method based on quantifying the symmetry of the joint spectral intensity by swapping between the signal and idler wavelengths. The polarization-entangled source violates the Clauser–Horne–Shimony–Holt inequality with a measured S =2.747±0.004. We also measured the polarization entanglement visibility in two mutually unbiased polarization bases and evaluated the squeezing level by characterizing the reduction in visibility at high pump power.
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.
Polarization-entangled photons are indispensable to numerous quantum technologies and fundamental studies. In this paper, we propose and demonstrate what we believe to be a novel source that generates collinear polarization-entangled photons by simultaneously achieving two distinct types of phase-matching conditions (noncritically birefringent and quasi phase matching) in a periodically poled nonlinear crystal with a large poling period of 2 mm. The photon pairs are generated in a polarization-entangled state with a fidelity and concurrence of 0.998 and 0.935, respectively, and violate the Clauser-Horne-Shimony-Holt inequality by 84 standard deviations. The compact source does not require interferometer, delicate domain structures, or post selection, and is advantageous for scalable quantum computing and communication, where many replicas or chip-scale devices are needed.
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.
We develop and demonstrate a source of polarization-entangled photon pairs using spontaneous parametric down-conversion (SPDC) in domain-engineered, periodically poled lithium niobate (PPLN) at telecom wavelengths. Pumped at 775 nm, this domain-engineered type-II SPDC source produces non-degenerate signal and idler pairs at 1530 nm and 1569 nm. Because of birefringence, the photon pair with horizontally polarized signal and vertically polarized idler has a different phasematching condition than the pair with vertically polarized signal and horizontally polarized idler. Using phase-modulation of the domain structure, we produced a crystal that can simultaneously generate both states in a distributed fashion throughout a single crystal. Performing SPDC using this aperiodically poled crystal, we observed polarization entanglement visibility above 93%. We compare the phase-modulated crystal to other aperiodic structures, including dual-periodically-poled and interlaced biperiodic structures.
We present direct single-pass generation of polarization-entangled photon pairs near 810 nm in a domain-engineered nonlinear crystal pumped by a continuous-wave 405 nm laser. Despite operating outside the natural group-velocity matching regime, the source produces high-quality polarization entanglement with a Clauser–Horne–Shimony–Holt parameter of S =2.764±0.002. We further exploit the intrinsic phase sensitivity of the entangled state to perform nonlocal, wavelength-dependent retardance measurements of birefringent waveplates through two-photon polarization correlations. Our results establish a high-performance alternative to conventional polarization entanglement sources, combining room-temperature silicon-detector compatibility, compactness, and spectral flexibility, in a practical platform for phase-sensitive quantum metrology.