Quantum entanglement is a unique phenomenon whereby a specific interaction of particles unifies their characteristics, and makes them entangled. This means that when you change a characterisitc of one, the other simultaneously changes no matter how far apart they are. This would be an ideal kind of military or interstellar communication because:
1) It’s instantaneous. There’s no lag for the signal to travel to, say, mars for example. If the technology was small and durable enough we could run remote control probes that would take decades for radio signals to reach.
2) It’s secret. because the particles are connected in some mysterious way, there’s no way for an enemy to intercept a message. If information gets out, you know it has to be a mole.
3) It’s a 1:1 reaction. There’s no ambiguity, or way for the signal to be corrupted. Quantum entanglement is a proven particle phenomenon, even though we don’t really understand it.
Unfortunately, the equipment necessary for this is large and sensitive. Not an ideal “battlefield” kind of thing. Regardless, it could easily explain why, if aliens exist, we haven’t run into any signals.