A method for exchanging a secure cryptographic key comprising a number of bits for a quantum cryptography apparatus employing non-ideal elementary quantum systems, wherein - the apparatus comprises an emitter and a receiver , being connected by a quantum channel and a conventional communication channel , - the emitter encodes each bit at random, using one of four states, onto a pair of non-orthogonal states belonging to two sets A = {|0 a 〉,|1 a )} and B = {|0 b 〉,|1 b 〉}, chosen such that |〈0 a |1 a 〉| = η a ≠ 0, |〈0 b |1 b 〉| = η b # 0 and wherein the overlap of the quantum states of any two states is not equal to 0, the states being |± x 〉 or |± y 〉 with the convention that |± x 〉 code for 0 and |± y 〉 code for 1, - the emitter sends the encoded bit along the quantum channel to the receiver , wherein weak coherent states are exchanged between the emitter and the receiver , - the receiver randomly chooses an analysis measurement within said two sets between F A = 1 1 + η | + x 〉 〈 1 a ⊥ | + | − x 〉 〈 0 a ⊥ | and F B = 1 1 + η | + x 〉 〈 1 b ⊥ | + | − x 〉 〈 0 b ⊥ | followed by a Von Neumann measurement distinguishing between |+ x 〉 and |- x 〉. - the emitter sends a set information along the conventional communication channel , announcing one of the four pairs of non-orthogonal states A ω,ω' ' = {| ω x 〉,| ω ' y 〉} with ω,ω' ∈ {+,-} and such that one of the states is the one which the emitter has sent by sending an appropriate message along the conventional communication channel , - the receiver discards all received encoded bits for which he has chosen a different analysis measurement incompatible with the set these encoded bits belonged to and where the measurement result he has obtained is possible for both states disclosed by the emitter and sends appropriate information to the emitter along the conventional communication channel , and - the receiver deduces the state actually sent by the emitter and adds the corresponding bit to the key. - the apparatus comprises an emitter and a receiver , being connected by a quantum channel and a conventional communication channel , - the emitter encodes each bit at random, using one of four states, onto a pair of non-orthogonal states belonging to two sets A = {|0 a 〉,|1 a )} and B = {|0 b 〉,|1 b 〉}, chosen such that |〈0 a |1 a 〉| = η a ≠ 0, |〈0 b |1 b 〉| = η b # 0 and wherein the overlap of the quantum states of any two states is not equal to 0, the states being |± x 〉 or |± y 〉 with the convention that |± x 〉 code for 0 and |± y 〉 code for 1, - the emitter sends the encoded bit along the quantum channel to the receiver , wherein weak coherent states are exchanged between the emitter and the receiver , - the receiver randomly chooses an analysis measurement within said two sets between F A = 1 1 + η | + x 〉 〈 1 a ⊥ | + | − x 〉 〈 0 a ⊥ | and F B = 1 1 + η | + x 〉 〈 1 b ⊥ | + | − x 〉 〈 0 b ⊥ | followed by a Von Neumann measurement distinguishing between |+ x 〉 and |- x 〉. - the emitter sends a set information along the conventional communication channel , announcing one of the four pairs of non-orthogonal states A ω,ω' ' = {| ω x 〉,| ω ' y 〉} with ω,ω' ∈ {+,-} and such that one of the states is the one which the emitter has sent by sending an appropriate message along the conventional communication channel , - the receiver discards all received encoded bits for which he has chosen a different analysis measurement incompatible with the set these encoded bits belonged to and where the measurement result he has obtained is possible for both states disclosed by the emitter and sends appropriate information to the emitter along the conventional communication channel , and - the receiver deduces the state actually sent by the emitter and adds the corresponding bit to the key. <EOS>