Defeating Active Eavesdropping with Quantum Illumination

Quantum illumination is a paradigm for using entanglement to gain a performance advantage—in comparison with classical‐state systems of the same optical power—over lossy, noisy channels that destroy entanglement. Previous work has shown how it can be used to defeat passive eavesdropping on a two‐way Alice‐to‐Bob‐to‐Alice communication protocol, in which the eavesdropper, Eve, merely listens to Alice and Bob’s transmissions. This paper extends that analysis to consider how Alice and Bob can minimize their vulnerability to Eve’s doing active eavesdropping, i.e., when she injects her own light into the channel.