Figure 2: Phases and fibres
When using optical fibres for quantum key distribution, the bit values are usually encoded in the phases of individual photons by way of an interferometer. Photons generated by Alice can travel by one of two paths through her interferometer, and similarly through Bob's apparatus. As the path (green) through the short loop of Alice's interferometer and the long loop of Bob's is almost exactly the same length as the alternative route (purple) through Alice's long loop and Bob's short loop, the paths undergo optical interference. By applying a phase delay to each of the two paths, Alice and Bob can determine in tandem the probability that a photon will exit at either of Bob's detectors – corresponding to "0" and "1". For example, if Bob sets a phase delay of 0°, Alice can cause the photon to exit at "0" or "1" by applying phase delays to her modulator of 0° or 180°, respectively. To implement the BB84 protocol (see box on the BB84 protocol), Alice applies one of four possible phase delays (&min;90°, 0°, 90°, 180°) to her modulator, in which a phase of 0° or 90° represents "0" and a phase of &min;90° or 180° represents "1". Meanwhile, Bob chooses a phase of either 0° or 90° with which to make his measurement. If the difference between Alice and Bob's phases is 0° or 180° then their choices are compatible, while if it is ±90° they are incompatible and Bob will measure a random bit value. Using a classical communication channel, Bob and Alice can then post-select their compatible choices to form a shared secret key.