A silicon integrated source of hyper-entangled photon pairs with rate exceeding 1 billion pairs per second

Optical losses and environmental noise present significant challenges for the coherent propagation of entangled states of light over large distances in quantum communication protocols. To cope with optical losses, one possible strategy is to increase the emission rate as much as possible. Decoherence, related to thermal and pressure fluctuation and to the presence of parasitic processes, can be overcome by encoding information via hyper-entanglement, i.e. entanglement over multiple degrees of freedom (DoFs). In this work we experimentally demonstrate a silicon-integrated device implementing both strategies, demonstrating the emission of spectrally multiplexed and hyper-entangled photon pairs. The pair generation rate exceeds 109 pairs per second at the output of the device over an 8 THz-wide comb of multiplexed frequency modes. Hyperentanglement is proved by showing entanglement in the frequency-bin and time-energy DoFs, along with high purity of the states when tracing over the time-energy degree of freedom. Quantum state tomography shows both fidelity and purity to be greater than 95% in the frequency domain, while, in the time-domain encoding, Bell’s inequality is violated by more than 10 standard deviations.