Multiphoton quantum interference at ultracompact inverse-designed multiport beam splitter

In photonic quantum technologies, interferometers are one of the key components. They set the stage for essential quantum multiphoton interference, generating and controlling the desired quantum states. Inverse design method revolutionizes such devices by significantly downsizing the interfering region without sacrificing the device performance, offering a viable route to a disruptive photonic architecture of high integration density and low circuit complexity. However, the capability of the inverse-designed interferometers for quantum applications has not yet been fully explored despite the constant development. In this work, we demonstrate multiphoton quantum interference using a topology-optimized tritter with a size of merely 8.0 µm × 4.5 µm. We characterize the tritter and reconstruct its transfer matrix by means of single- and two-photon statistics. We also perform heralded three-photon quantum interference with the tritter. The measured four-fold coincidence features a peak with visibility of (−47.9±8.6)%, which is in fair agreement with the prediction of −55.8% estimated from the reconstructed transfer matrix. Our work confirms successful multiphoton quantum interference at an ultracompact interferometer and demonstrates the possibility of utilizing topology-optimized multiport interferometers for various quantum technologies.