Pure narrowband photon-pair generation in a monolithic cavity

TL;DR

This work addresses the need for efficient, pure heralded single photons for quantum technologies by implementing a heralded SPDC source inside a monolithic nonlinear crystal cavity. The theoretical framework shows how cavity-driven JSA shaping and a double-pass pump yield a central spectral mode with high purity, aided by pump-pulse engineering and mode clustering. Experimentally, the source emits telecom-band photons at 1540 nm with a 168 MHz bandwidth, achieving a maximum heralding efficiency of 84% and a central-mode spectral purity of (96.2 ± 2.7)%, with Hong-Ou-Mandel indistinguishability of (91.2 ± 9.3)%. These results demonstrate a compact, robust, high-purity photon source suitable for fiber-based quantum networks and interfacing with atomic or solid-state systems, with clear paths to further improvements via cavity design.

Abstract

Photonic quantum technologies require efficient sources of pure single photons. Here we present a heralded SPDC single-photon source in a monolithic cavity optimized for high spectral and spatial purity. The source heralds single-photons at a wavelength of 1540 nm and a spectral bandwidth of 168 MHz with a maximum heralding efficiency of 84%, while keeping the multi-photon contamination below 3%. The cavity enhancement generates photons mainly in the central cavity mode with 96.2% spectral purity.

Figures (6)

• Figure 1: Left: Schematic of the monolithic cavity. The nonlinear crystal with coated facets forms a cavity for the signal and idler photons. The pump photon at 775 nm is non-resonantly reflected back, while the signal and idler photons at 1540 and 1560 nm resonate within it and exit through the second facet. HR: highly-reflective coating. AR: anti-reflective coating. Right: Signal and idler cavity JSI for a 0.4 ns-long pump pulse with the corresponding JSI marginal of the signal photon. The signal (idler) photons have 150 MHz (158 MHz) linewidths and 19.3 GHz (20.2 GHz) FSRs.
• Figure 2: Signal and idler cavity JSI of the central mode as a function of the frequency detuning of signal and idler for different pump pulse lengths $\tau_p$. The Schmidt number $K$ and spectral purity $P$ for each pump pulse length are indicated, showing that shorter pulses decrease frequency correlations.
• Figure 3: Experimental setup. The setup consists of the generation of pump pulses at 775 nm via second harmonic generation, and of the generation of signal and idler photon pairs at 1540 nm and 1560 nm, respectively, via SPDC in the monolithic crystal cavity shown in Fig. \ref{['fig:monolithiccavityandJSI']}. EOM: electro-optic modulator, EPG: electrical pulse generator, SHG: second harmonic generation, SPDM: shortpass dichroic mirror, LPDM: longpass dichroic mirror, SPF: shortpass filter, LPF: longpass filter, PBS: polarizing beamsplitter, HWP: half-wave plate, SNSPD: superconducting nanowire single-photon detector, TDC: time-to-digital converter
• Figure 4: Idler spectrum measured via difference frequency generation. The weak higher-order modes are magnified for clarity at the bottom of each plot. Top: free-space detection. Bottom: spatially filtered detection (after a single-mode fiber).
• Figure 5: Left: Measured signal and idler crosscorrelation over their temporal separation for 2.03 mW pump power. The exponential temporal function of each photon is fit to the data. The integration window of 6 ns for all subsequent measurements is shown in gray. Right: Measured heralded-signal autocorrelation as a function of the pump average power with the corresponding linear fit. The error bars are derived from Poissonian photon counting statistics.