Near-unity quantum interference of transverse spatial modes in an ultra-compact inverse-designed photonic device

Transverse spatial modes as a degree of freedom is an untapped resource for scaling up integrated photonic quantum computing. To be practically useful for improving scalability, reliable and high-visibility quantum interference between transverse spatial modes on-chip needs to be demonstrated. We show repeatable quantum interference using inverse-designed transverse mode beamsplitters that have an ultra-compact footprint of 3 μm × 3 μm - the smallest transverse mode beamsplitters for 1550 nm photons to date. We measure a Hong-Ou-Mandel visibility of up to 99.56{plus minus}0.62 % from a single device, with an average visibility across three identical devices of 99.38{plus minus}0.41 %, indicating a high degree of reproducibility. Our work demonstrates that inverse-designed components are suitable for engineering quantum interference on-chip of multimode devices, paving the way for future compact integrated quantum photonic devices that exploit transverse spatial modes as a degree of freedom for high-dimensional quantum information.