A QUANTUM RANDOM-NUMBER GENERATOR

It isn’t easy at all to understand how a bit that is both a digital one and a digital zero at the same time could be used to convey or process data. In fact, the perfectly random outcome of measuring such a bit is now used to generate random streams of numbers.

The random-number function in your computer produces only a random-looking sequence of numbers through a specific algorithm. If you know the algorithm and the position in the sequence, you can accurately predict what will be the next number in the sequence. While this may be good enough for home computer games, a wide variety of high-security applications—from online casinos to jamming enemy communications—require truly random sequences of numbers that cannot be guessed through any means.

Traditionally, true random-number generators have relied on measuring events produced by random processes, such as thermal noise in electronic components or the timing between radioactive decays of an isotope. However, the measurement equipment itself somewhat biases the random-number sequence, which allows a level of prediction by very sophisticated users, opening up vulnerabilities in very sensitive applications.

Quantum random-number generators overcome these biases and are now commercially available for use in critical applications. One popular scheme is shown in Figure 151. Here, a photon polarized at a 45° angle is in quantum superposition, and will be detected in one or the other port of the PBS with 50%/50% probability. The 45° photons are produced by an entangled-photon source, but the entanglement in itself is not used. Instead, the idler photon, which is generated at exactly the same time as the signal photon, is used to prompt the system that an event detected by the SPCMs looking at the outputs of the PBS is the result of an actual 45° signal photon, and not the result of a stray photon or dark count.*** In 1999, Thomas Jennewein from Anton Zeilinger’s group in Austria demonstrated that no prior method has produced a higher-quality random-number sequence than the one produced in this manner.⁵⁶

Figure 151 A popular quantum random-number generator produces a true random sequence of digital ones and zeros by detecting photons polarized at 45° through a PBS. The photons are produced by an entangled-photon source, although the entanglement in itself is not used. Instead, the idler photon is used to prompt the system that an event detected by the SPCMs looking at the outputs of the PBS is the result of an actual 45° signal photon, and not the result of a stray photon or dark count.