On-chip quantum interference of indistinguishable single photons from integrated independent molecules

On-chip integration of independent channels of indistinguishable single photons is a prerequisite for scalable optical quantum information processing. This requires separate solid-state single-photon emitters to exhibit identical lifetime-limited transitions. This challenging task is usually further exacerbated by spectral diffusion due to complex charge noise near material surfaces made by nanofabrication processes. Here we develop a molecular quantum photonic chip and demonstrate on-chip Hong-Ou-Mandel quantum interference of indistinguishable single photons from independent molecules. The molecules are embedded in a single-crystalline organic nanosheet and integrated with single-mode waveguides without nanofabrication, thereby ensuring stable, lifetime-limited transitions. With the aid of Stark tuning, we show how 100 waveguide-coupled molecules can be tuned to the same frequency and achieve on-chip Hong-Ou-Mandel interference visibilities exceeding 0.97 for 2 molecules separately coupled to 2 waveguides. For two molecules with a controlled frequency difference, we unveil over 100-µs-long quantum beating in the interference, showing both excellent single-photon purity (particle nature) and long coherence (wave nature) of the emission. Our results showcase a possible strategy towards constructing scalable optical universal quantum processors and a promising platform for studying waveguide quantum electrodynamics with identical single emitters wired via photonic circuits.