Phys. Rev. X 7, 041050 (2017)

Random bit sequences are important for cryptography, but hardware-based generators can introduce unwanted bias. Now, Tobias Steinle and collaborators in Germany have demonstrated an all-optical random bit generator by exploiting the bistable output of an optical parametric oscillator (OPO). In this scheme there are only two possible states, ‘distanced’ by more than 400 standard deviations, that can be regarded as 0 and 1 without ambiguity. The authors achieve the bistability using a lithium niobate-based OPO pumped by a mode-locked Yb:KGW oscillator (450 fs pulse duration, 1,032 nm centre wavelength) and the output was detected by a reverse-biased InGaAs photodiode. The system was pumped into the ‘period-2’ (P2) state (which gives pulses with alternating pulse energy, peak power and spectral properties) and the OPO was switched by an optical chopper for the random bit stream generation. At present, the random bit rate is limited by the 10 kHz chopper, but the relaxation time of the process is on the order of several hundred nanoseconds and by using a faster chopper, rates of 1 MHz, or even higher with a faster pump laser repetition rate, are possible.