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Optica Quantum

Photon-pair production and frequency translation using backward-wave spontaneous parametric downconversion

Paulina S. Kuo, Dileep V. Reddy, Varun Boehm Verma, Sae Woo Nam, Andrius Žukauskas, Carlota Canalias

Published Oct 12, 2023• 16 citations

Abstract

Backward-wave three-wave mixing is a difficult χ (2) interaction to observe, because it requires ultrashort poling periods to achieve phasematching. Having realized sub-micrometer periods in periodically poled KTiOPO 4 (PPKTP), we demonstrate for the first time first-order quasi-phasematched, backward-wave spontaneous parametric downconversion (BW SPDC). We pumped the PPKTP crystal at 800 nm and obtained a forward-wave signal at 1400 nm and a backward-wave idler at 1868 nm. We estimated an internal pair production rate of 4.0 × 10 4 pairs/s/mW. The backward-wave phasematching constraints lead to the unique tuning property that spectral features of the pump are transferred to the forward-wave signal photons, which makes BW SPDC an attractive source of spectrally shaped, heralded single photons. These spectrally shaped photons are useful for quantum computing and quantum interconnects. For the first time, we experimentally show this effect by observing frequency translation between a spectrally shaped pump beam and the BW SPDC signal photons. Due to their unique properties, BW-SPDC-based devices will be important building blocks for quantum information processing.

DOI: 10.1364/opticaq.500021

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A nonlinear process based on backward quasi-phase matching (BQPM) can be used to realize mirrorless optical parametric oscillation, the generation of paired photons with a separable joint spectral amplitude and narrow wavelength bandwidth, and the preparation of counterpropagating polarization-entangled photons, which shows distinct advantages over some applications based on forward quasi-phase matching. In this work, three types of BQPM in a bulk periodically poled potassium titanyl phosphate crystal with a single period are theoretically analyzed. Experimentally, the harmonic wave generated by second-harmonic generation in type 0 and type I exhibits a narrow bandwidth of 15.5 GHz. Furthermore, photon pairs generated by spontaneous parametric downconversion in all types of BQPM (type 0, type I, and type II) at 7th order are observed and characterized. Their coincidence-to-accidental ratios are all greater than 5 × 10 3 in the pump power range from 10 mW to 500 mW. This research lays the foundation for further applications of BQPM in nonlinear optics, quantum optics, and quantum information processing.

Spontaneous parametric downconversion (SPDC) using periodically poled nonlinear optical crystals under the quasiphase- matching condition has found wide use in quantum optics. High efficiencies and good coupling to single-mode fibers resulted from using channel waveguides in crystals. It is often desirable to have a very narrow bandwidth for the signal and idler photons, but under the typical operating conditions, phase matching dictates the bandwidth of the SPDC to be of the order of <1 nm. This occurs because the co-propagating signal and idler photons are entangled, and an increase of the signal wave-vector is compensated by a decrease of the idler wave-vector. One way to reduce the bandwidth is by forming either external or internal cavities. Additionally, bandwidth reduction is possible without cavities when the signal and idler are counter-propagating, and the changes in the wave-vector with frequency are additive. To accomplish this a domain inversion on the wavelength scale is required. In this work, we experimentally demonstrate SPDC in one-dimensional KTP-based waveguides with sub-micron poling for forward and backward interactions. Some of the spectral features of the generated light are accounted for by mode coupling theory in periodically poled waveguides but other features are as yet not explained.

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We explore a promising method of generating pure heralded single photons. Our approach is based on parametric down-conversion in a periodically-poled waveguide. However, unlike conventional downconversion sources, the photon pairs are counter-propagating: one travels with the pump beam in the forward direction while the other is backpropagating towards the laser source. Our calculations reveal that these downconverted two-photon states carry minimal spectral correlations within each photon-pair. This approach offers the possibility to employ a new range of downconversion processes and materials like PPLN (previously considered unsuitable due to its unfavorable phasematching properties) to produce heralded pure single photons over a broad frequency range.

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