  We propose quantum protection circuit (quantum lock), based on dark states of
ensembles of two-level atoms in optical cavity. The secret key is the splitting
of atoms into pairs, and publicly accessible part of the lock is the tensor
product of EPR singlets, corresponding to the given splitting. To open the lock
one must move synchronously pairs of atoms from the correct splitting to the
other cavity; the lock will open if atoms do not emit photons. This scheme has
perfect secrecy: it is impossible to hack it, even with effective solutions of
any classical computational problems, in contrast to the RSA scheme. The method
of obtaining dark states through Stark shift of atomic excitation energy is
also proposed. This scheme makes possible to create secret keys of a few tens
of atoms that is sufficient for the most practical applications.
