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Quantum 2.0 Conference and Exhibition

Spectrally Pure Heralded Single Photons at Telecommunication O-Band from a Custom-Poled KTiPO4 Crystal

Wu‐Hao Cai, Rui‐Bo Jin, Keiichi Edamatsu, Fumihiro Kaneda

Published Jan 1, 2023• 0 citations
Publisher

Abstract

We design a custom periodically poled KTiOPO4 crystal to generate a high spectral purity (99.99%) heralded single photons at the telecom O-band. Our designed source is feasible with currently available pump lasers and single-photon detectors.
DOI
10.1364/quantum.2023.qw2a.33

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We theoretically and experimentally investigate the spectral tunability and purity of photon pairs generated from spontaneous parametric down conversion in periodically poled KTiOPO(4) crystal with group-velocity matching condition. The numerical simulation predicts that the spectral purity can be kept higher than 0.81 when the wavelength is tuned from 1460 nm to 1675 nm, which covers the S-, C-, L-, and U-band in telecommunication wavelengths. We also experimentally measured the joint spectral intensity at 1565 nm, 1584 nm and 1565 nm, yielding Schmidt numbers of 1.01, 1.02 and 1.04, respectively. Such a photon source is useful for quantum information and communication systems.
ABSTRACT We report on optimizing the spectral purity of heralded single photons in the telecom O‐band, where single photons can be propagated with low loss and low dispersion in a standard telecom optical fiber. We numerically searched for various group‐velocity‐matching conditions and corresponding optimal poling structures of a potassium titanyl phosphate crystal for spontaneous parametric downconversion. Our poling optimization results using phase‐matching coherence‐length and sub‐coherence‐length modulation schemes show % spectral purity with pump wavelengths ranging from 603.8 to 887.3 nm. Some optimized configurations are feasible with off‐the‐shelf lasers and single‐photon detectors. Moreover, by investigating noise photon spectra for different poling optimization methods, we show that, in practice, appropriate, gentle spectral filtering helps achieve high purity. This study will pave the way for developing practical quantum sources for quantum information applications at the telecom O‐band.
Optical quantum information processing requires individual photons prepared in well defined quantum states. For high brightness, the key ingredient is a crystal with suitable nonlinear optical properties. The authors identify a family of four crystal systems that yield single photons of significantly higher spectral purity than those from the familiar PPKTP crystal. Furthermore, the wavelengths of these photons may be tuned in a wide band around 1550 nm, ideally matched to current telecommunication protocols.
We theoretically investigated spectrally uncorrelated biphotons generated in a counter-propagating spontaneous parametric downconversion (CP-SPDC) from periodically poled M T i O X O 4 ( M = K , Rb, Cs; X = P , As) crystals. By numerical calculation, it was found that the five crystals from the KTP family can be used to generate heralded single photons with high spectral purity and wide tunability. Under the type-0 phase-matching condition, the purity at 1550 nm was between 0.91 and 0.92, and the purity can be maintained over 0.90 from 1500 nm to 2000 nm wavelength. Under the type-II phase-matching condition, the purity at 1550 nm was 0.96, 0.97, 0.97, 0.98, and 0.98 for PPKTP, PPRTP, PPKTA, PPRTA, and PPCTA, respectively; furthermore, the purity can be kept over 0.96 for more than 600 nm wavelength range. We also simulated the Hong–Ou–Mandel interference between independent photon sources for PPRTP crystals at 1550 nm, and interference visibility was 92% (97%) under the type-0 (type-II) phase-matching condition. This study may provide spectrally pure narrowband single-photon sources for quantum memories and quantum networks at telecom wavelengths.