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, wherein the apparatus assesses the amount of information an eavesdropper having access to both of the channels can have obtained on the sequence, the apparatus comprising the 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 quantum systems, wherein the quantum systems have a coherent phase relationship between systems located in nearby positions of the stream, wherein some of the quantum systems are prepared in a quantum state belonging to a first sct 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 is adapted to produce some of the quantum systems in a quantum state belonging to a second set of quantum states, this second set comprising at least one state, the states of this second set being non-orthogonal to some of the states of the first set, the states of this second set not being a superposition of some of the states of the first set, the states of this second set being selected in such a way that they arc perturbed by a measurement adapted, when applied to a quantum system prepared in a state belonging to the first set, to determine at least in some cases in which state this quantum system was prepared wherein the quantum system source of the emitter station is adapted for; producing groups of at least two weak coherent states of the electromagnetic field each in a time bin of duration t, the center of each weak coherent slate being separated by a time T 1 from the center of its closest neighbors, with T 1 greater than t, the center of the last weak coherent state of one such group being separated from the center of the first weak coherent state of the next group by a time T 2 , with T 2 greater than t, wherein any two weak coherent states in a group are phase coherent and a weak coherent state in a group is phase coherent with at least some of the weak coherent states of another nearby group; producing for each of the symbol of the sequence to be transmitted one quantum system comprising one such weak coherent state with non-zero amplitude in one of the time bins and weak coherent states with zero amplitude in the other time bins; inserting quantum systems with at least two weak coherent states with non zero amplitude in at least two time bins between some of the quantum systems associated with symbols; and sending the quantum systems to the receiver station through an appropriate quantum channel.