1. An apparatus for distributing a sequence of symbols between an emitter station and a receiver station connected by a quantum channel and a conventional channel, the apparatus comprising: an emitter station having a control means so as to enable the emitter station to command and control its own components in cooperation with associated components of the receiver station and a quantum source for generating a stream of separate quantum systems, wherein the separate quantum systems have a coherent phase relationship between systems located in nearby positions of the stream, wherein some of the separate quantum systems are prepared in a quantum state belonging to a first set of quantum states, this set comprising at least two non-orthogonal quantum states, and the quantum states of this first set being associated with symbol values, and wherein the emitter station comprises a quantum system source controlled by a processing unit having a memory, as well as a data transmission and communications mechanism permitting communications with other components of the apparatus, in which the quantum system source generates a stream of qubits, wherein each qubit is made up of a pair comprising a first and a second pulsed weak coherent state of an electromagnetic field, each qubit including a time bin of duration t, the center of each of the time bin is separated by a time Ti, with t being smaller than T 1 , wherein the center of the second pulsed weak coherent state of a qubit is separated from the center of the first pulsed weak coherent state of the following qubit by a time T 2 , with t being smaller than T 2 and T 1 is not equal to T 2 and wherein a quantum non-demolition measurement across two weak coherent states belonging to a single qubit destroys the phase coherence with adjacent weak coherent states thereby inducing warning counts with non-zero probability when one weak coherent states of an attacked qubit by an eavesdropper is superposed with a weak coherent state of a neighboring qubit.