n atom as a physical bit then quantum mechanics tells us that apart from the two distinct electronic states the atom can be also prepared in a coherent superposition of the two states. This means that the atom is both in state 0 and state 1. To get used to the idea that a quantum object can be in `two states at once' it is helpful to consider the following experiment (Fig.A and B) Let us try to reflect a single photon off a half-silvered mirror i.e. a mirror which reflects exactly half of the light which impinges upon it, while the remaining half is transmitted directly through it (Fig. A). Where do you think the photon is after its encounter with the mirror --- is it in the reflected or in the transmitted beam? It seems that it would be sensible to say that the photon is either in the transmitted or in the reflected beam with the same probability. That is one might expect the photon to take one of the two paths choosing randomly which way to go. Indeed, if we place two photodetectors behind the half-silvered mirror in direct lines of the two beams, the photon will be registered with the same probability either in the detector 1 or in the detector 2. Does it really mean that after the half-silvered mirror the photon travels in either reflected or transmitted beam with the same probability 50%? No, it does not ! In fact the photon takes `two paths at once'. This can be demonstrated by recombining the two beams with the help of two fully silvered mirrors and placing another half-silvered mirror at their meeting point, with two photodectors in direct lines of the two beams (Fig. B). With this set up we can observe a truly amazing quantum interference phenomenon. If it were merely the case that there were a 50% chance that the photon followed one path and a 50% chance that it followed the other, then we should find a 50% probability that one of the detectors registers the photon and a 50% probability that the other one does. However, that i...
