Sub-Megahertz Single Photon Source

TL;DR

This paper demonstrates a cavity-enhanced spontaneous parametric downconversion method to generate sub-MHz single photon pairs at the Rubidium D1 line with 100% duty cycle, suitable for quantum memory applications.

Contribution

It introduces a novel cavity design with a half-wave plate for triple resonance, achieving ultra-narrow bandwidth and continuous photon generation.

Findings

Bandwidth of 666 ± 16 kHz for single photons

Achieved 100% duty cycle of photon generation

Simplified locking scheme with cavity stabilization

Abstract

We report 100% duty cycle generation of sub-MHz single photon pairs at the Rubidium D$_1$ line using cavity-enhanced spontaneous parametric downconversion. The temporal intensity crosscorrelation function exhibits a bandwidth of $666 \pm 16$ kHz for the single photons, an order of magnitude below the natural linewidth of the target transition. A half-wave plate inside our cavity helps to achieve triple resonance between pump, signal and idler photon, reducing the bandwidth and simplifying the locking scheme. Additionally, stabilisation of the cavity to the pump frequency enables the 100% duty cycle. These photons are well-suited for storage in quantum memory schemes with sub-natural linewidths, such as gradient echo memories.