Heralded single-photon source with on-chip heralding detector

Quantum interference between single-photons is a prerequisite of an optical quantum computing. This requirement means that we require bright, deterministic single-photon sources that can consistently produce single-photons which are simultaneously pure and indistinguishable into a single mode. Photonic integrated circuit (PIC) technology allows us to engineer nonlinear single-photon pair sources to satisfy all these requirements simultaneously except that they are still inherently probabilistic.

Nevertheless, the heralded nature of such sources allows us to multiplex an array of them to achieve a more deterministic logical source. Multiplexing, however, requires feedforwarding via electronics which is slow compared to the speed of photons, requiring us to use a long optical delay line, so that the heralding signal can be transduced to a switch controlling signal for the heralded photon in time.

The long delay line is undesirable as the longer it is, the more photons are loss in it, and hence less brightness. The first step towards reducing the length of the feedforwarding time delay, and thus the delay line, is to place the detector for the heralding photon as close as possible to the source, or, for PIC-based photon source, on the same chip.

This is unavoidable especially if the end goal is a fully integrated on-chip multiplexing. Furthermore, integrating the heralding detector on-chip would also boost the system heralding event probability as it would eliminate potential sources of the coupling loss between the source and the detector, e.g., chip-to-fibre coupler loss, fibre loss, etc.

Single-photons in general may be detected by many means but the only mean that can simultaneously achieve high efficiency, low dark count rate, low reset time, and low jitter is via superconducting nanowire single-photon detectors (SNSPD) with the only expense of having to operate it at cryogenic temperature. In our case, the heralding of single-photon source for multiplexing, however, requires all the aforementioned performances, making it our only choice.

In year 1 of this PhD, I completed the repair of many failed components of the magnetron sputtering deposition system that I will use in year 2 for making superconducting films which are required for making SNSPDs. Then, I will attempt to integrate SNSPDs directly on top of waveguides, allowing it to be close to the source. The promise of the boost in the system heralding event probability also requires that the filter for the heralding photon incur minimal loss.

In addition, to prevent false heralding which will impact the heralding efficiency (the probability of delivering a heralded single-photon given a heralding event), it also needs to maintain the high extinction ratio necessary to reject the noises from the pump and the unwanted non-linear processes that may be present in the chip. The development of a range of filtering strategies and designs-based on existing solutions in literature and developed jointly with the SNSPDs fabricated in-house-will be the focus of year 3.

If everything goes according to plan, in year 3, the final PIC for a single-photon source with on-chip heralding detector may be designed, fabricated and post-processed, using the components from the previous years, and hopefully, the on-chip heralding will be demonstrated.