So say researchers at the University of Vienna:
Imagine the computer tries to snoop on the qubits and see their entanglement, which could then be used to extract the information they carry. You’d be able to tell, because of the laws of quantum mechanics. The cat is both dead and alive until you check whether it’s dead or alive, and then it’s one or the other. If your photon has a specific state, you’d be able to tell that it was spied upon.
Although some have serious doubts about quantum cryptography’s practical security.