A polarization modulation method of photonic pulses, in particular for generating quantum cryptographic keys, ensures an optimum stability of the outgoing polarization states, and comprises the steps of: generating a plurality of photonic pulses with an unspecified polarization state obtained by the overlapping of both the horizontal and vertical polarization modes thereof, and routing them in a first polarization-maintaining fiber; splitting said horizontal and vertical polarization modes and routing them in respective terminals of a second polarization-maintaining fiber forming a ring, whereby they travel such ring clockwise and counter-clockwise respectively, or vice versa; inducing a respective phase modulation (oe, ol) of both said polarization modes, in one point of said ring which is spaced from said terminals by optical paths having different length along said ring, thereby a polarization-maintaining fiber delay line is determined; and recombining said polarization modes in one single photonic pulse beam, and routing the resulting beam through said first polarization-maintaining fiber, by obtaining, at the output thereof, a polarization state of the pulses depending upon the difference between said phase modulations (oe, ol).