y, the QKD protocols. Before proceeding to a formal description of such protocols, it is useful to consider a communication link between Alice and Bob, and how they may be able to communicate "secret" information by means of X- and Z-basis photon-state preparations and measurements; this is described in the next section. A secure photon communication channel In this section, we assume that Alice and Bob can send light signals to each other by means of free-space-optics or an optical fiber link, which we refer to as a photon communication channel. Both have single-photon sources that are capable of producing photons in any polarization states (1, <->, n, U or |0),|1),|+), 1-)), and independent photon-measurement apparatus in the X and Z bases. What is possible for Alice is the same as for Bob, SO here we only need to consider the case of Alice transmitting photons to Bob. As a starting point, Alice generates a random 16-bit sequence from her end, say, for instance: 1010 0011 1110 0100. Then Alice prepares a sequence of 16 photons, which she randomly polarizes according to the following selection rule (for instance): For the 1 bit, use either I or n polarizations at random; For the 0 bit, use either or U polarizations at random. Consistently with the above bit sequence and random-selection rule, Alice's preparation yields (for instance) the following polarized-photon sequence: Alice then inputs the above photon sequence through the link. At the other end, Bob receives the photon sequence and makes 16 corresponding measurements, according to his own random choice of X and Z basis. Table 25.8 shows Bob's basis choices and the outcome of each of Bob's measurements. Conventionally, and unless otherwise specified, a +1 eigenvalue measurement will be now called a 1 bit of information, and a -1 measurement will be called a 0 bit of information, respectively. The table shows that Bob's random choices of X and Z measurement bases are either "right" or "wrong." The right basis choices correspond to measuring I, with Z, and n