Chip-to-chip quantum photonic interconnect by path-polarization interconversion

Integrated photonics has enabled much progress toward quantum technologies. Many applications, e.g., quantum communication, sensing, and distributed cloud quantum computing, require coherent photonic interconnection between separate on-chip subsystems. Large-scale quantum computing architectures and systems may ultimately require quantum interconnects to enable scaling beyond the limits of a single wafer, and toward multi-chip systems. However, coherently connecting separate chips remains a challenge, due to the fragility of entangled quantum states. The distribution and manipulation of entanglement between multiple integrated devices is one of the strictest requirements of these systems. Here, we report, to the best of our knowledge, the first quantum photonic interconnect, demonstrating high-fidelity entanglement distribution and manipulation between two separate photonic chips, implemented using state-of-the-art silicon photonics. Path-entangled states are generated on one chip, and distributed to another chip by interconverting between path and polarization degrees of freedom, via a two-dimensional grating coupler on each chip. This path-to-polarization conversion allows entangled quantum states to be coherently distributed. We use integrated state analyzers to confirm a Bell-type violation of S = 2.638 ± 0.039 between the two chips. With further improvements in loss, this quantum photonic interconnect will provide new levels of flexibility in quantum systems and architectures.