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Quantum Computing, Communication, and Simulation

Polarization-entangled photon pairs generated in aperiodically-poled KTP

Paulina S. Kuo, CH. S. S. Pavan Kumar, Anouar Rahmouni, Dileep V. Reddy, Ori Levin, Noa Voloch Bloch

Published Mar 5, 2026• 0 citations
Publisher

Abstract

We demonstrate polarization-entangled photon-pair generation in aperiodically poled KTiOPO4 (aPPKTP). The aperiodic poling allows two simultaneous downconversion processes corresponding to |HV ⟩ and |V H⟩ signal-idler states. Using a 775 nm pump, we obtained signal and idler at 1530.6 nm and 1569.8 nm, respectively. After compensating for the relative phase between the |HV ⟩ and |V H⟩ states, we obtain polarization entanglement visibilities greater than 0.98 using both pulsed and continuous-wave pumping. This compensation process is important because the relative phase is fixed by the poling pattern. These polarization-entangled photons are useful for distribution of entanglement across a quantum network.

DOI
10.1117/12.3080937

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We demonstrate polarization-entangled photon-pair generation in aperiodically poled KTiOPO4 (aPPKTP). The aperiodic poling allows two simultaneous downconversion processes corresponding to |HV ⟩ and |V H⟩ signal-idler states. Using a 775 nm pump, we obtained signal and idler at 1530.6 nm and 1569.8 nm, respectively. After compensating for the relative phase between the |HV ⟩ and |V H⟩ states, we obtain polarization entanglement visibilities greater than 0.98 using both pulsed and continuous-wave pumping. This compensation process is important because the relative phase is fixed by the poling pattern. These polarization-entangled photons are useful for distribution of entanglement across a quantum network.
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.
Polarization entangled photon pair source is widely used in many quantum information processing applications such as teleportation, quantum communications, quantum computation and high precision quantum metrology. We report on the generation of a continuous-wave pumped 1550 nm polarization entangled photon pair source at telecom wavelength using a type-II periodically poled KTiOPO(4) (PPKTP) crystal in a Sagnac interferometer. Hong-Ou-Mandel (HOM) interference measurement yields signal and idler photon bandwidth of 2.4 nm. High quality of entanglement is verified by various kinds of measurements, for example two-photon interference fringes, Bell inequality and quantum states tomography. The source can be tuned over a broad range against temperature or pump power without loss of visibilities. This source will be used in our future experiments such as generation of orbital angular momentum entangled source at telecom wavelength for quantum frequency up-conversion, entanglement based quantum key distributions and many other quantum optics experiments at telecom wavelengths.
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.