The entanglement trick
You generate pairs of entangled photons. Keep one (the 'idler') in your lab. Send the other (the 'signal') out at the target. When the signal returns, you compare it with its idler twin. Even buried in noise, entangled pairs correlate in a way classical noise cannot fake — so a stealth target with a tiny RCS becomes detectable.
Why it's hard
Entangled photons in the microwave band only exist at temperatures near absolute zero. You need a superconducting source, a cryogenic detector, and you have to store the idler for the entire round-trip time without losing coherence. At 30 km that's 200 µs — an eternity for a quantum state.
What's been demonstrated
Lab benchtop systems at ranges of metres, with modest sensitivity gains over classical microwave detection. The 2016 Chinese 100 km claim was never independently verified. The 2020 US AFRL Quantum 2-Way Ranging program demonstrated only short ranges in controlled conditions.
Verdict
Quantum radar is a real and active research field. It is not a deployable system today, and probably not for at least another decade. Classical radar engineering — better waveforms, AESA, MIMO — keeps closing the stealth gap faster.