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Optics Letters

Compact polarization-entangled source near 810 nm and its application to nonlocal retardance measurement

Shani Izhak, Inbar Hurvitz, H. S. Eisenberg, Ady Arie

Published May 19, 2026• 0 citations
Publisher

Abstract

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.

DOI
10.1364/ol.600510

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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.
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.
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.
A compact source of polarization-entangled photon pairs is developed using a blue single-mode laser diode (/spl lambda/ = 402 nm, P = 30 mW) as the pump source of a beta-barium-borate (BBO) nonlinear optical crystal . Using the technique to optimize the collection efficiency, the created photon pairs are coupled into single-mode optical fibers, which are directly connected to quenched silicon avalanche photodiodes (Si APDs). The entanglement of the generated photon pairs was tested with the measurement of the polarization correlations in H/V and +45/spl deg// -45/spl deg/ basis and by violating the Clauser-Horne-Shimony-Holt (CHSH)-type Bell inequality. This compact, robust and economic source can be employed in any application of quantum information and quantum communication, in particular in practicable entanglement based quantum key distribution.
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.
Sources of entangled photon pairs are a key component in both fundamental tests of quantum theory and practical applications such as quantum key distribution and quantum computing. In this work, we describe and characterize a source of polarization entangled photon pairs based on two spontaneous parametric down-conversion crystals in a Sagnac interferometer. Our source is compact and produces high-quality entangled states in a very flexible manner. The wavelengths of the photon pairs (around 810 and 1550 nm), the phase between orthogonal components of the entangled state, and the tangle of the state are all independently adjustable. In addition to presenting basic characterization data, we present experimental violations of Clauser-Horne-Shimony-Holt and Leggett inequalities, as well as an instance of the ``beautiful'' Bell inequality, which has not previously been tested experimentally.