Versatile Chip-Scale Platform for High-Rate Entanglement

Integrated photonic microresonators have become an essential resource for generating photonic qubits for quantum information processing, entanglement distribution and networking, and quantum communications. The pair-generation rate is enhanced by reducing the microresonator radius, but this comes at the cost of increasing the frequency-mode spacing and reducing the quantum information spectral density. Here, we circumvent this rate-density trade-off in an Al⁢Ga⁢As-on-insulator photonic device by multiplexing an array of 20 small-radius microresonators, each producing a 650-GHz-spaced comb of time-energy entangled-photon pairs. The resonators can be independently tuned via integrated thermo-optic heaters, enabling control of the mode spacing from degeneracy up to a full free spectral range. We demonstrate simultaneous pumping of five resonators with up to 50-GHz relative comb offsets, where each resonator produces pairs exhibiting time-energy entanglement visibilities up to 95%, coincidence-to-accidental ratios exceeding 5000, and an on-chip pair rate up to 2.6 G⁡Hz/mW2 per comb line—an improvement over prior work by more than a factor of 40. As a demonstration, we generate frequency-bin qubits in a maximally entangled two-qubit Bell state with fidelity exceeding 87% (90% with background correction) and detected frequency-bin entanglement rates up to 7 kHz (an approximately 70 MHz on-chip pair rate) using a pump power of approximately 250 μ⁢W. Multiplexing small-radius microresonators combines the key capabilities required for programmable and dense photonic qubit encoding while retaining high pair-generation rates, heralded single-photon purity, and entanglement fidelity.