Quantum cryptography by polarization ambiguity is generally used but it involves polarization-maintained fibers. This invention proposes an alternative: quantum cryptography by ambiguity in time. It comprises the conversion of K bits to be transmitted into a train of K pulses of particle flows of time width .DELTA.T and whose frequency Tb, is predetermined knowing that each of the K pulses being shifted or not in time such that the k.sup.th pulse is shifted by a duration t0 respectively t1, with respect to the initial instant of the period depending on the value "0", respectively "1" of the k.sup.th bit, where k is an integer such that 0.ltoreq.k