  Claude Shannon proved in 1949 that information-theoretic-secure encryption is
possible if the encryption key is used only once, is random, and is at least as
long as the message itself. Notwithstanding, when information is encoded in a
quantum system, the phenomenon of quantum data locking allows one to encrypt a
message with a shorter key and still provide information-theoretic security. We
present one of the first feasible experimental demonstrations of quantum data
locking for direct communication and propose a scheme for a quantum enigma
machine that encrypts 6 bits per photon (containing messages, new encryption
keys, and forward error correction bits) with less than 6 bits per photon of
encryption key while remaining information-theoretically secure.
