Harnessing Physical Disorder for Photonic Security Applications

Disorder and scattering in photonic systems have long been considered a nuisance that should be circumvented. Recently, disorder has been harnessed for a rapidly growing number of applications, including imaging, sensing, and spectroscopy. The chaotic dynamics and extreme sensitivity to external perturbations make random media particularly well-suited for optical cryptography and security applications. In this talk, Hui Cao presents two examples. The first one is massive-parallel ultrafast random bit generation using a multimode laser. Random numbers are widely used for information security, cryptography, stochastic modeling, and quantum simulations. Key technical challenges for physical random number generation are speed and scalability. She and her team demonstrate a new method for ultrafast generation of hundreds of random bit streams in parallel with a single laser diode. Spatio-temporal interference of many lasing modes in a specially designed cavity is introduced as a scheme for greatly accelerated random bit generation. Spontaneous emission, caused by quantum fluctuations, produces stochastic noise that makes the bit streams unpredictable. They achieve a total bit rate of 250~Tb/s with off-line post-processing, which is more than two orders of magnitude higher than the current post-processing record. Their approach is robust, compact, energy efficient, and should impact applications in secure communication and high-performance computation. The second example is remote key establishment using a long optical fiber. Using random media for distribution of secret keys between remote users is challenging since it requires the users have access to the same scattering sample. Hui Cao and her team utilize random mode mixing in a long multimode fiber to generate and distribute keys simultaneously. Fast fluctuations in fiber mode mixing provide the source of randomness for key generation, and optical reciprocity guarantees that the keys at the two ends of the fiber are identical. They experimentally demonstrate the scheme using classical light and off-the-shelf components, opening the door for a practically secure key establishment at the physical layer of fiber-optic networks.