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Quantum Computing, Communication, and Simulation

Simulation and demonstration of non-degenerate high purity biphoton state generation by SPDC in PPKTP

Hugo da Nóbrega, Giacomo Corrielli, Andrea Barone, Daniele Bajoni, Marco Liscidini, Mattéo Galli, Roberto Osellame

Published Mar 12, 2024• 1 citation
Publisher

Abstract

Single photons generated by spontaneous parametric down conversion are often utilized in heralded schemes, in which case the heralded photon purity must be maximized to preserve the indistinguishability of the photons that carry out quantum computing. In this work, we present software designed for accurate calculation of such purity, which was optimized to minimize computational resources and time, allowing for maximum purity searches in broad wavelength ranges. Lithium niobate and potassium titanyl phosphate (KTP) were studied in three phase-matching conditions, and an ultra-high purity configuration was found. The configuration was experimentally realized through an ion-exchanged KTP waveguide, exploiting flexibility in pump and phase matching temperature to compensate fabrication deviations, and purities as high as 0.923 were found, with very high generation rate of 40.7 MHz/mW. The experimental purity is significantly degraded by the multimode guiding at the desired wavelengths, which we propose to address by engineering with femtosecond laser writing, of which preliminary results are presented.

DOI
10.1117/12.3002457

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Single photons generated by spontaneous parametric down conversion are often utilized in heralded schemes, in which case the heralded photon purity must be maximized to preserve the indistinguishability of the photons that carry out quantum computing. In this work, we present software designed for accurate calculation of such purity, which was optimized to minimize computational resources and time, allowing for maximum purity searches in broad wavelength ranges. Lithium niobate and potassium titanyl phosphate (KTP) were studied in three phase-matching conditions, and an ultra-high purity configuration was found. The configuration was experimentally realized through an ion-exchanged KTP waveguide, exploiting flexibility in pump and phase matching temperature to compensate fabrication deviations, and purities as high as 0.923 were found, with very high generation rate of 40.7 MHz/mW. The experimental purity is significantly degraded by the multimode guiding at the desired wavelengths, which we propose to address by engineering with femtosecond laser writing, of which preliminary results are presented.
We investigated the high-purity entangled photon-pair generation in five kinds of “non-poled” potassium titanyl phosphate (KTP) isomorphs (i.e., KTiOPO4, RbTiOPO4, KTiOAsO4, RbTiOAsO4, and CsTiOAsO4). The technique is based on the spontaneous parametric down-conversion (SPDC) under Type II extended phase matching (EPM), where the phase matching and the group velocity matching are simultaneously achieved between the interacting photons in non-poled crystals rather than periodically poled (PP) KTPs that are widely used for quantum experiments. We discussed both theoretically and numerically all aspects required to generate photon pairs in non-poled KTP isomorphs, in terms of the range of the beam propagation direction (or the spectral range of photons) and the corresponding effective nonlinearities and beam walk-offs. We showed that the SPDC efficiency can be increased in non-poled KTP isomorphs by 29% to 77% compared to PPKTP cases. The joint spectral analyses showed that photon pairs can be generated with high purities of 0.995–0.997 with proper pump filtering. In contrast to the PPKTP case, where the EPM is achieved only at one specific wavelength, the spectral position of photon pairs in the non-poled KTP isomorphs can be chosen over the wide range of 1883.8–2068.1 nm.
Spontaneous parametric down-conversion (SPDC) is a reliable and robust source of photons for quantum information applications. For applications that involve operations such as entanglement swapping or single-photon heralding, two-photon states are required to be factorable (uncorrelated) in their spectral and spatial degrees of freedom. We report the design and experimental characterization of an SPDC source that has been optimized for high spectral and spatial purity. The source is pumped by the 776 nm output of a mode-locked Ti:Sapphire laser and consists of a periodically-poled Potassium Titanyl Phosphate (PPKTP) crystal phase-matched for collinear type-II SPDC. The dispersive properties of PPKTP at these wavelengths is such that it is possible to minimize the spectral entanglement by matching the widths of the pump to the spectral phase-matching function. The spatial entanglement is minimized through careful control of the pump focus, yielding nearly single-mode emission. An advantage of this approach is that the emission rate into the collection modes is very high, resulting in a very bright SPDC source. We also report a scheme that employs the output of collinear sources such as these to produce polarization-entangled photon pairs. The scheme, which requires only simple polarization elements, can be scaled to N-photon GHZ states.
We report on the generation of an indistinguishable heralded single-photon state, using highly nondegenerate spontaneous parametric downconversion (SPDC). Spectrally factorable photon pairs can be generated by incorporating a broadband pump pulse and a group-velocity matching (GVM) condition in a periodically-poled potassium titanyl phosphate (PPKTP) crystal. The heralding photon is in the near IR, close to the peak detection efficiency of off-the-shelf Si single-photon detectors; meanwhile, the heralded photon is in the telecom L-band where fiber losses are at a minimum. We observe spectral factorability of the SPDC source and consequently high purity (90%) of the produced heralded single photons by several different techniques. Because this source can also realize a high heralding efficiency (> 90%), it would be suitable for time-multiplexing techniques, enabling a pseudo-deterministic single-photon source, a critical resource for optical quantum information and communication technology.
The properties of the backward-wave spontaneous parametric down-conversion (SPDC) in a periodically poled potassium titanyl phosphate (KTP) waveguide are studied in the context of creating narrowband heralded sources of single-photon states. The effective index of refraction and spatial profile of different waveguide modes, efficiency of different SPDC processes and purity of heralded photons are calculated numerically for a given waveguide. Compared to the usual co-propagating SPDC, spectral narrowing of the backward-wave SPDC was observed as should be expected. Generation biphoton states in backward-wave regime is experimentally observed in two-photon detection scheme.
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.
We report on fiber-coupled nonlinear optical waveguides in periodically poled potassium titanyl phosphate (PPKTP), fabricated via femtosecond laser direct writing for efficient quantum light generation through spontaneous parametric down-conversion. The platform achieves high nonlinear conversion efficiency with fiber integration for compact, alignment-free operation. We discuss applications including enhanced performance for low-efficiency parametric processes and high-visibility polarization-entangled photon pair generation. This work demonstrates laser-written PPKTP waveguides as promising building blocks for in-fiber integrated quantum photonic circuits and quantum technologies.