Quantum illumination
Generate two entangled photons. Send one ('signal') toward a target, keep the other ('idler') at the receiver. When the signal returns, perform a joint measurement against the idler. Theoretically, this gives a 6 dB SNR advantage over classical radar when the background is bright and the signal is weak.
The decoherence problem
Entanglement is fragile. By the time the signal photon has reflected off a target tens of kilometres away, the entanglement is almost certainly destroyed by interactions with atmospheric molecules. The 6 dB advantage collapses. Lab demonstrations work at sub-metre ranges in cryogenic environments — not exactly fighter-vs-radar conditions.
Microwave generation
Generating entangled microwave photons (not optical) requires Josephson parametric amplifiers cooled to 20 mK. The 'transmitter' is a dilution refrigerator the size of a wardrobe. You cannot put this on a destroyer, let alone a fighter.
What's actually plausible
Quantum-enhanced radar for short-range, low-power applications (medical imaging, single-photon LIDAR) is real. Long-range quantum radar against stealth aircraft is — for now — a press release, not a system. The advantage is real but small, the engineering is fierce, and classical AESA + ML keeps closing the same SNR gap by a different route.