True random-number generators are becoming essential for cryptographic data communications, and are also widely used in scientific experiments. So-called pseudo-random numbers can be generated by software algorithms, but cryptography built around such numbers can be deciphered with enough time because of the nonrandom basis of the number generation. In contrast, truly random numbers can be generated only by a truly random physical process—for example, one based on quantum-mechanics such as the random generation of photons or optical intensity and phase fluctuations. A number of such devices have been developed, but they tend to be bulky, as they rely on free-space or fiber optics.
Now, engineers at the University of Bristol (Bristol, England) and Xanadu (Toronto, ON, Canada) have created a quantum random-number generator that is based on phase fluctuations from a laser diode fiber-coupled to a millimeter-scale monolithic silicon-on-insulator (SOI) chip. The laser light is injected into a Mach-Zehnder interferometer (MZI) with a delay line in one of its arms—thus, rapid phase fluctuations in the laser light will cause fluctuations in the intensity of the light exiting the MZI. The phase noise of the laser actually has a quantum and a classical component, which can be separated out from each other and from the background electronic noise. The device has an estimated randomness-generation rate of about 2.8 Gbit/s, which, if a faster transimpedance amplifier were used, could be increased to beyond 10 Gbit/s, the researchers say. The device generates random numbers at submilliwatt optical power levels. Reference: F. Raffaelli et al., Opt. Express (2018); https://doi.org/10.1364/oe.26.019730.