Message encryption, Monte Carlo simulations, and electronic gambling machines all rely on random number generators. With a computer algorithm, it is only possible to generate numbers in a pseudorandom way, since once one figures out the algorithm itself, the sequence of numbers can be known. For applications that require higher security, means of generating true, or nondeterministic, random numbers become necessary.

The intensity fluctuation in light from a chaotic laser, made chaotic by external optical feedback, has an unpredictable output, which approximately repeats itself at the round trip time of the external cavity. If this quasiperiodicity can be eliminated, the signal can be used to rapidly generate sequences of nondeterministic random bits. In a paper appearing in *Physical Review Letters*, Igor Reidler, Yaara Aviad, Michael Rosenbluh, and Ido Kanter from the Bar-Ilan University in Israel use a simple edge-emitting semiconductor laser to create a chaotic signal with a broad frequency spectrum and short, spiking intensity fluctuations. Reidler *et al.* sample the output laser intensity at a rate of $2.5\phantom{\rule{0.333em}{0ex}}\text{GHz}$ and store the measured value of the signal as $8\phantom{\rule{0.333em}{0ex}}\text{bits}$, which they subtract from the previous value, and then truncate to obtain a random bit string. The differentiation and truncation eliminate the quasiperiodicity of the optical signal and the concatenated bit strings stream out at a rate of $12.5\phantom{\rule{0.333em}{0ex}}\text{Gbits/s}$.

The generated stream passes the NIST and Diehard tests for randomness and is currently the fastest random number generator based on a physical process. – *Sonja Grondalski*