Heralding efficiency and brightness optimization of a micro-ring resonator via tunable coupling

Efficient and bright single photon sources on photonic chips are key to scaling quantum technologies. Spontaneous four wave mixing in micro-ring resonators creates excellent narrowband and tunable photon sources. We experimentally demonstrate the optimization of heralding efficiency and brightness by tuning the coupling of the pump, signal and idler modes into the resonator whilst operating in a pulsed pump regime. We observe a maximum detected pair rate of over 93,000~coincidences per second in a moderately over-coupled regime, alongside a high intrinsic idler heralding efficiency of 97.87$\pm$8.97% when operating close to maximal over-coupling. The measured dependence on coupling strength is in strong agreement with theoretical predictions, experimentally validating the predicted trade-off between brightness and heralding efficiency.

💡 Research Summary

This paper presents a systematic experimental study of how the coupling strength between a bus waveguide and a silicon micro‑ring resonator (MRR) influences two key performance metrics of a heralded single‑photon source (HSPS): heralding efficiency and brightness. The authors use spontaneous four‑wave mixing (SFWM) in a 699 µm radius ring fabricated on a 250 nm silicon‑on‑insulator (SOI) platform. The ring is coupled to the bus waveguide through a Mach‑Zehnder interferometer (MZI) whose phase shifter (Φ₁) can be tuned electrically, allowing continuous adjustment of the external coupling rate Γ while keeping the intrinsic loss rate M essentially constant.

A femtosecond fiber laser (1550.12 nm, 50 MHz repetition, 158 pm spectral width) provides pulsed pump light. The pump power is attenuated with a variable optical attenuator and monitored via a 1 % tap. After the chip, a cascade of dense‑wavelength‑division‑multiplexing (DWDM) filters isolates the pump (channel 34) and the generated signal (channel 40) and idler (channel 28) photons. Superconducting nanowire single‑photon detectors (SNSPDs, 88 % efficiency) and a time‑tagger record singles and coincidence events.

The authors model the SFWM process with the following equations:

C_{s,i}(P)=η_{s,i}