Truly random numbers, which are needed for cryptographic uses such as quantum-key distribution (QKD), are not as easy to generate as one might think. In fact, the “random” numbers generated by computers using even complex algorithms are only pseudorandom. If a pseudorandom number is what is standing between a secure information system and a hacker who wants into the system, then the hacker, if sophisticated, has a chance of breaking in. To create truly random numbers, a physical, preferably quantum-mechanical, process must be part of the random-number generator. Photonic systems can be created that generate truly random numbers—for example, a distributed-feedback (DFB) laser can be modulated with a radio-frequency (RF) signal to bring the laser below its threshold with a period equal to the RF signal, and each laser pulse that occurs as threshold is again exceeded has a phase independent of that of the other pulses. The phase independence is ascribed to fluctuations in the quantum vacuum. The problem with approaches like this is that they require bulk optics and lab electronics, making them large, costly, and finicky.
Scientists at Toshiba Research Europe (Cambridge, England) and the University of Leeds (Leeds, England) have solved this problem by creating a photonic integrated-circuit (PIC)-based quantum random number generator with simple electronics. Operating at a pulse clock rate of 1 GHz and a data rate of 8 Gbit/s, the device produces truly random numbers that pass all tests performed within the National Institute for Standards and Technology (NIST) test suite. The thermoelectrically cooled indium phosphide (InP)-based PIC includes two DFB lasers, which are driven in the gain-switching regime. When their outputs are combined interferometrically on-chip, the result is a train of pulses with a random intensity level, allowing measurement and extraction of random numbers via a photodetector, a field programmable gate array, and a 10-bit digitizer. The entire device can be packaged within a standard optical module—future devices will fit entirely within a single printed-circuit board (PCB). Reference: T. Roger et al., J. Opt. Soc. Am. B (2019); https://doi.org/10.1364/josab.36.00b137.
