Demonstration of quantum-optimum entanglement-enhanced covert sensing*

The laws of quantum physics endow unconditional security for information processing. However, the executions of many quantum cryptography protocols such as quantum key distribution are detectable by an adversary, who may subsequently launch, e.g., a denial-of-service attack to disrupt the data integrity. Quantum covert protocols aim to meet an elevated security criterion: the executions of the very protocols are undetectable, thereby not only protecting the secrecy but also concealing the processed information. Here, we report the theory and experiment for entanglement-enhanced covert sensing using a high-efficiency broadband entanglement source and an unconventional phase-conjugate quantum receiver. We show that entanglement offers a performance boost in estimating the imparted phase by a probed object, as compared to a classical protocol at the same covertness level. The implemented entanglement-enhanced covert sensing protocol operates close to the fundamental quantum limit by virtue of its near-optimum entanglement source and quantum receiver. Our work is expected to create ample opportunities for quantum information processing at unprecedented security and performance levels.