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

TL;DR

Efficient and bright heralded single-photon sources on photonic chips are essential for scalable quantum technologies. The authors use spontaneous four-wave mixing in a micro-ring resonator with a tunable bus coupling implemented by a Mach-Zehnder interferometer to access different coupling regimes under a pulsed pump. They observe a maximum detected pair rate of over $9.3×10^4$ coincidences per second in a moderately over-coupled regime and an intrinsic heralding efficiency approaching $97.87%$ near maximal over-coupling, validating the predicted brightness–heralding efficiency trade-off between $Γ$ and $M$. The results, aligned with theory, demonstrate a tunable HSPS design that is advantageous for quantum networking and photonic quantum computing and provide a framework for optimizing micro-ring resonators across wavelength-multiplexed channels.

Abstract

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.

Figures (2)

• Figure 1: Experimental setup for ring resonator characterisation. (a) Optical setup used for device characterisation. (b) Schematic of the micro-ring resonator with tunable coupling. (c) Lorentzian resonance profiles in different coupling regimes.
• Figure 2: The optical response to ring resonator coupling variation. The top plot shows the Q-factor (blue triangles) and extinction (red inverted triangles) of the ring resonances. Q-factor error comes from fitting of resonances. The middle plot shows the detected coincidences per second (red squares). The lower plot shows the show extracted intrinsic heralding efficiencies $\eta_s$ (blue circles) and $\eta_i$ (red circles). Error bars arise from uncertainty in system losses and goodness of fit. Black dashed lines show theory predictions with shared region as the bounds. Shaded regions indicate the under-coupled regime (blue), the moderately over-coupled regime (yellow), and the strongly over-coupled regime (green). The black dotted line is critical coupling.