Photon Number Non-Demolition Measurement in Quantum-Enhanced Telescopy*

Quantum-enhanced long-baseline interferometry aims to go beyond what is classically achievable in telescopy. We propose a quantum non-demolition measurement-based scheme that verifies which spatio-temporal modes are occupied without destroying the information about the visibility function. It can be applied in examining weak thermal sources and used to improve several existing quantum telescopy schemes. Our scheme is optimal in terms of the ancilla resources needed to perform it. We show that a successful measurement requires a maximum degree of entanglement in the ancilla state.