Free-Space key distribution method comprising exchanging information between an emitter (100) and a receiver (200) based on the physical layer wiretap channel model, comprising the steps of randomly preparing (710), at the emitter (100), one qubit encoded with one of two possible non-identical quantum states, sending (720) the encoded qubit to the receiver (200) through a physical layer quantum-enhanced wiretap channel (500), such that an eavesdropper (300) tapping said channel is provided with partial information about the said states only, detecting and measuring (730) the received quantum states, key sifting (740) between the emitter and the receiver through a classical channel, calculating (750, 760) an amount of information available to any eavesdropper (300) based on the detected and received quantum states.