The ability to generate genuinely random numbers is crucial for digital security, and typically relies on hardware design to introduce randomness. The emergence of light-based circuits for data processing means that an all-optical method for generating random sequences would be very useful. Tobias Steinle and collaborators have accomplished this, using a technique with several practical advantages.
An optical parametric oscillator produces single photons by down-converting a pumped state into one of two possible states that are either in phase or out of phase with a reference signal. Which state emerges is determined by, among other effects, the randomness of the quantum vacuum fluctuations in the optical cavity. The generated state can be interpreted as a binary bit, or the head and tail of a coin toss.
Analysis of the output shows that the generated bit stream is random with perfect entropy for up to 105 samples. Importantly, the non-degeneracy of the two output states means that the generation scheme is robust against fluctuations in the pump power, and the state detection via phase difference introduces less noise than other detection schemes.
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Abergel, D. A quantum coin toss. Nature Phys 14, 7 (2018). https://doi.org/10.1038/nphys4342