Is it possible to “encrypt” laser beams such that they would need to be “decrypted” to be seen? I was playing a video game that had some guns equipped with laser sights. It had me think if it would be possible to transmit the laser beam in such a way that it is only visible to an observer through a vision system (e.g. goggles), tuned to the laser characteristics. Simply transmitting a v at a particular frequency (e.g. IR) would not ensure that only a specific observer could see the laser. Anyone who had the capability of seeing that frequency could see it. What I’m asking is if it is possible to somehow “encrypt” the transmission.
 A: What you describe is impossible.  You can always gather the energy coming off of the target and identify it.  However, there are two corner cases that you can research further.

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*Spread spectrum - seen typically in the RF world to avoid interference, spread spectrum transmitters "hop" frequencies in a pesudorandom order.  If you know the order, you see 100% of the signal.  If you don't know the order (such as if you are another transmitter that we want to avoid interfering with), you'll only see a fraction.  But the energy was still emitted, and can be detected.  There's a vague analogy to frogs and crickets making their chirping noises -- the frogs and crickets are attuned to the particular sound, but its hard for the predators to get a good fix on them.


*Quantum Encryption - This is a fascinating technique where you send out photons with particular polarization.  The observer looks at them with its own particular polarization.  Then, you compare notes and agree which photon polarization the receiver can determine.  Then those polarization are used to transmit the encrypted information.  An eavesdropper would indeed see that the laser was pointed at the target, but couldn't tell anything more than that.  This wouldn't be used in a "laser sight" type of situation, but it's at least interesting.
