Developing indistinguishable heralded single-photon sources in silicon photonics

High visibility quantum interference between multiple single-photons is a key prerequisite for linear optical quantum computing, networking and relevant technologies. To date, numerous experiments have investigated the interference between multiple on-chip silicon photonics sources, but all have been restricted to the generation of a single path-entangled photon pair as 2-fold coincidence measurements [1]. Interfering heralded singles photons generated from independent sources as 4-fold coincidence measurements—a true measure of indistinguishability—has been limited to waveguide sources [2-3]. However, the indistinguishability of heralded single photons from micro-ring resonators has not been measured in any photonic platform including silicon. Resonant enhancement in micro-ring resonators naturally enables brighter, purer and more indistinguishable single photon productions without any tight spectral filtering compared to waveguide sources. Here we report on-chip indistinguishability measurements of heralded single-photons at telecom wavelengths from independent micro-ring resonator sources. We take an important step in the realization of integrated linear optical quantum computing by combining on-chip generation, spectral demultiplexing and interference of heralded single-photons from independent sources. We measured the raw heralded two-photon interference fringe visibility as 72±3% from the 4-fold coincidences [5]. A low resolution joint spectral intensity measurement estimates the upper bound of purity to be 96% which agrees with our simulation. We found that the multi-pair emission is the primary cause of visibility degradation. In addition, we have measured the purity and indistinguishability of waveguide sources as a function of spectral filtering achieving 86±1% raw indistinguishability. Using these measurements, we compared and identified pathways to improve nanowire waveguides and micro-ring resonators single photon sources in terms of brightness, purity and indistinguishability.

1. Silverstone et al., Nat. Comm. 6, 7948 (2015).
2. Harada et al., New J. Phys. 13(6), 065005 (2011).
3. Zhang et al., Technologies 4(3), 25 (2016).
4. Adcock et al., arXiv:1811.03023v1 (2018).
5. Faruque et al., Opt. Express 26(16), 20379 (2018).