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arXiv:2502.18752 · ACS Photonics

Highly Nondegenerate Polarization-Entangled Photon Pairs Produced through Noncritical Phasematching in Single-Domain KTiOPO4

Jia Boon Chin, Diane Prato, Alexander Ling

Published Sep 17, 2025• 2 citations

Abstract

High Resolution Image Download MS PowerPoint Slide Photon-pair sources are useful for entanglement distribution. The most mature of these are spontaneous parametric downconversion (SPDC) sources, most of which achieve phasematching via engineering the domains in poled crystals or the angle between the optic axis and the pump beam. For multichannel entanglement distribution of photon pairs, where one photon is transmitted through free-space and the other photon is transmitted through fiber, it is beneficial to use highly nondegenerate photon-pair sources. The currently accepted approach in such sources is quasi-phasematching. In this paper, a simpler, more stable alternative is presented for producing highly nondegenerate photon pairs. A source of polarization-entangled photon pairs with low temperature sensitivity based on noncritical phasematched SPDC in single-domain potassium titanyl phosphate was demonstrated. Over a crystal temperature range of 75 °C, the center wavelength of the idler photons was observed to change by 10.8(20) nm while the average entanglement visibility was maintained above 98%. With the signal photons detected locally, the idler photons were transmitted through 62 km and 93 km of deployed telecom fibers with average raw visibilities of 98.2(1)% and 95.6(3)% respectively.

DOI: 10.1021/acsphotonics.5c00880

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High Resolution Image Download MS PowerPoint Slide Photon-pair sources are useful for entanglement distribution. The most mature of these are spontaneous parametric downconversion (SPDC) sources, most of which achieve phasematching via engineering the domains in poled crystals or the angle between the optic axis and the pump beam. For multichannel entanglement distribution of photon pairs, where one photon is transmitted through free-space and the other photon is transmitted through fiber, it is beneficial to use highly nondegenerate photon-pair sources. The currently accepted approach in such sources is quasi-phasematching. In this paper, a simpler, more stable alternative is presented for producing highly nondegenerate photon pairs. A source of polarization-entangled photon pairs with low temperature sensitivity based on noncritical phasematched SPDC in single-domain potassium titanyl phosphate was demonstrated. Over a crystal temperature range of 75 °C, the center wavelength of the idler photons was observed to change by 10.8(20) nm while the average entanglement visibility was maintained above 98%. With the signal photons detected locally, the idler photons were transmitted through 62 km and 93 km of deployed telecom fibers with average raw visibilities of 98.2(1)% and 95.6(3)% respectively.

Noncritical phasematched (NCPM) Type-II spontaneous parametric downconversion (SPDC) was used to produce highly nondegenerate polarization-entangled photon pairs in a 50mm single-domain potassium titanyl phosphate (KTP) crystal. The center wavelength of the idler photon and high entanglement quality of the source were observed to have a low temperature dependence. The high temperature stability makes this approach of generating and distributing polarization-entangled photon pairs suitable for field deployment without temperature control. In addition to the wavelengths of the signal and idler photons being in the visible and telecom bands respectively, the photons have narrow linewidths, making the NCPM approach of generating entangled photon pairs suitable for multi-channel entanglement distribution where the signal photon can be transmitted through free space and the idler photon can be transmitted through fiber.

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.

The temperature dependence of the refractive indices of potassium titanyl phosphate (KTP) is shown to enable quasi-phasematched type II spontaneous parametric downconversion (SPDC) with low temperature sensitivity. Calculations show the effect to be maximized for emission of photons at around 1165 nm, as well as producing similar plateaus for wavelengths throughout the telecommunications bands. We experimentally demonstrate the effect, observing temperature-insensitive degenerate emission between 20 °C and 100 °C at 1327 nm, within the telecommunications O band. This result has practical implications for the development of entangled photon sources for resource-constrained environments, and we demonstrate a simple polarization entangled source as a proof of concept.

We present a simple but highly efficient source of polarization-entangled photons based on spontaneous parametric down-conversion (SPDC) in bulk periodically poled potassium titanyl phosphate crystals (PPKTP) pumped by a 405 nm laser diode. Utilizing one of the highest available nonlinear coefficients in a non-degenerate, collinear type-0 phase-matching configuration, we generate polarization entanglement via the crossed-crystal scheme and detect 0.64 million photon pair events/s/mW, while maintaining an overlap fidelity with the ideal Bell state of 0.98 at a pump power of 0.025 mW.

We present a simple but highly efficient source of polarization-entangled photons based on spontaneous parametric down-conversion (SPDC) in bulk periodically poled potassium titanyl phosphate crystals (PPKTP) pumped by a 405 nm laser diode. Utilizing one of the highest available nonlinear coefficients in a non-degenerate, collinear type-0 phase-matching configuration, we generate polarization entanglement via the crossed-crystal scheme and detect 0.64 million photon pair events/s/mW, while maintaining an overlap fidelity with the ideal Bell state of 0.98 at a pump power of 0.025 mW.