Entanglement witness on a programmable photonic processor

Multiphoton quantum interference is a one of the two technology platforms in which a quantum advantage has been demonstrated [1]. In these systems, a large-scale quantum state forms due to repeated effective interactions of photons (through the generalized Hong-Ou-Mandel effect) at the nodes of a photonic processor, resulting in a state that intrinsically possesses quantum complexity. A key question is how to validate the formation of a large-scale quantum state. One aspect of this is to measure whether the output quantum state is entangled – a necessary but not sufficient criterion for complexity. In this work, we derive and demonstrate an entanglement witness for large-scale optical quantum interference, and experimentally use this to demonstrate the presence of entanglement in our optical processor [2]. [1] Zhong et al., Science 370, 1460 – 1463 (2020) [2] Taballione et al., ArXiv 2012.05673 (2021)