Fast random numbers from semiconductor lasers

Familiar software-based “random number generators” rely on deterministic algorithms, so their outputs are not actually random. For some applications, such as Monte Carlo simulations or randomized music playlists, that’s not a problem. But for others, such as secure communications or online gaming, it’s important to use numbers that are truly unguessable, such as can be generated from measurements of stochastic physical processes. One candidate is the chaotic output of a semiconductor laser: When a certain fraction of laser light is fed back into the laser, small intensity fluctuations (thought to be quantum in origin) are amplified into large, irregular, subnanosecond oscillations, as shown in the figure. Last year, Atsushi Uchida (Takushoku University, Tokyo) and colleagues used digitized measurements of two semiconductor lasers to generate random binary sequences at a rate of 1.7 billion bits per second—much faster than any previous method based on a physical system (A. Uchida et al., Nat. Photonics 2, 728, 2008 ). Small drifts in the lasers’ average intensities can throw off the balance of ones and zeros, but with the lasers suitably tuned, the sequences meet statistical criteria for randomness. Now, Michael Rosenbluh , Ido Kanter , and their students at Bar-Ilan University in Ramat-Gan, Israel, have developed a new method, using just one laser, that avoids the problem of intensity drift. From the differences between successive intensity measurements, they can generate 12.5 billion random bits per second. (I. Reidler et al., Phys. Rev. Lett., in press.) --Johanna Miller