The generation of random bits is of enormous importance in modern informationscience. Cryptographic security is based on random numbers which require aphysical process for their generation. This is commonly performed by hardwarerandom number generators. These exhibit often a number of problems, namelyexperimental bias, memory in the system, and other technical subtleties, whichreduce the reliability in the entropy estimation. Further, the generatedoutcome has to be post-processed to "iron out" such spurious effects. Here, wepresent a purely optical randomness generator, based on the bi-stable output ofan optical parametric oscillator. Detector noise plays no role and no furtherpost-processing is required. Upon entering the bi-stable regime, initially theresulting output phase depends on vacuum fluctuations. Later, the phase isrigidly locked and can be well determined versus a pulse train, which isderived from the pump laser. This delivers an ambiguity-free output, which isreliably detected and associated with a binary outcome. The resulting randombit stream resembles a perfect coin toss and passes all relevant randomnessmeasures. The random nature of the generated binary outcome is furthermoreconfirmed by an analysis of resulting conditional entropies.
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