Quantum encryption is a form of encryption that has the potential to be super secure, and gives the user notice of any attempt to decode or intercept data. It works because whenever photons are produced in pairs, angular momentum is conserved. Photons have a form of angular momentum called 'spin' - either positive or negative (up or down). We can pair up with 1 and down with zero and send information using standard encryption methods. If two photons are produced from a particle antiparticle interaction with total original angular angular momentum zero, the only possible combination for the spin of the photons produced is for each particle to have equal and opposite spin.
The photons produced then move away, one with positive spin and one with negative spin - or spin up and spin down - perhaps never to meet again. The spin of the particles add up to zero, but it is a foundation of quantum physics that the spin of a particle - whether it is positive or negative - cannot be known until it is measured. Once it is measured the spin is know. I we measure the spin of one photon, and we measure that photon as having positive spin, then the other photon produced must have negative spin so that angular momentum is conserved.
Suppose then that some intercepts the data. The photons are put into a definite spin state and this 'collapse' of the photons into definite states can be detected