Indistinguishable MHz-narrow heralded photon pairs from a whispering gallery resonator

TL;DR

This paper demonstrates MHz-bandwidth, indistinguishable heralded photon pairs generated from a whispering gallery resonator, enabling scalable quantum photonic applications with high efficiency and tunability.

Contribution

It introduces a method to produce indistinguishable photon pairs from independent sources using whispering gallery resonators, with high conversion efficiency and MHz bandwidth.

Findings

Achieved 74% Hong-Ou-Mandel dip contrast.

Photon bandwidth of about 1 MHz, tunable and suitable for quantum systems.

Used only 50 nW pump power per direction, demonstrating high efficiency.

Abstract

Hong-Ou-Mandel interference plays a vital role in many quantum optical applications where indistinguishability of two photons is important. Such photon pairs are commonly generated as the signal and idler in the frequency and polarization-degenerate spontaneous parametric down conversion~(SPDC). To scale this approach to a larger number of photons we demonstrate how two independent signal photons radiated into different spatial modes can be rendered conditionally indistinguishable by a heralding measurement performed on their respective idlers. We use the SPDC in a whispering gallery resonator, which is already proven to be versatile sources of quantum states. Its extreme conversion efficiency allowed us to perform our measurements with only \qty{50}{nW} of in-coupled pump power in each propagation direction. The Hong-Ou-Mandel interference of two counter-propagating signal photons manifested itself in the four-fold coincidence rate, where the two idler photons detection heralds a pair of signal photons with a desired temporal overlap. We achieved the Hong-Ou-Mandel dip contrast of \(74\pm 5\%\). Importantly, the optical bandwidth of all involved photons is of the order of a MHz and is continuously tunable. This, on the one hand, makes it possible to achieve the necessary temporal measurements resolution with standard electronics, and on the other hand, creates a quantum states source compatible with other candidates for qubit implementation, such as optical transitions in solid-state or vaporous systems. We also discuss the possibility of generating photon pairs with similar temporal modes from two different whispering gallery resonators.