Sending a one-time pad code with perfect privacy has been achieved. A one-time pad message can be sent securely using quantum mechanics and now also send the original message can be sent securely.
Physicists could check quantum states to see if a one time pad message had been overheard. The leakage is detected after it has occurred. An eavesdropper would already have the information by the time physicists found out.
For a one-time pad, a set of random numbers that can be used to encrypt a message. If the one-time pad is overheard, physicists simply disregard it and send another, until they can be sure that the process was completely private.