cillating in quadrature), and the reverse. The above demonstration, albeit somewhat tedious, was very well worth it: it made it possible to reach a major conclusion: there exist four special states of polarization for the classical E-field, which form two orthogonal bases, namely {1, <>} and {n,U}, respectively. Furthermore, there exists a one-to-one correspondence between these two Classical and quantum cryptography bases and the quantum bases {|0), |1)} and {l+), |-)}, respectively. We must notice that in order to reach this conclusion we did not need to make any assumption about the quantum nature of light. Such an assumption will come into the picture later, when considering single-photon measurements. It will turn out, however, that the EM field associated with the photon is essentially described by the same {1, }, {n, U} bases and, therefore, the above conclusion remains fully valid at quantum scales. Next, I shall introduce an optical component referred to as a quarter-wave plate (QWP). As the name suggests, it is a piece of flat material, and it is transparent to light. The material, however, is a special type of crystal exhibiting the property of birefringence. A material is said to be birefringent if the speed of light varies according to the polarization orientation of the incident light ray (assumed linearly polarized). Thus, the speed of light is faster in some polarization direction (referred to as the fast axis) and slower in the orthogonal direction (referred to as the slow axis). If the incident light ray is parallel to either the fast or slow axes, the light polarization remains unchanged. However, if the fast (or slow) axis forms a 45° angle with the incident polarization, the E-field component that projects onto the fast axis propagates faster than the E-field component that projects onto the slow axis, thus, introducing a phase delay AQ between the two components and, hence, making the state of polarization of the ray evolve as it traverses the plate. The additional feature of the QWP is that