Operation of RSFQ Hardware Random Number Generator Comprising Two Josephson Oscillators

We describe the operation of a rapid-single-flux-quantum (RSFQ) hardware random number generator comprising two Josephson oscillators, an XOR gate, and a non-destructive read-out (NDRO) cell. Single-flux-quantum (SFQ) pulse trains fed to the input terminals of the XOR gate contain timing jitters due to thermal noises. Then, the output of the XOR gate becomes random, which is stored in the NDRO cell and fetched by the trigger signal. We experimentally evaluated the output randomness of a test circuit fabricated using a 10 kA/cm2 Nb/AlOx/Nb integration process. For evaluation, the NIST FIPS 140-2 test suite was used. 24 random number sequences of 20 kb length were acquired with the trigger signal of 1 MHz, and evaluated for each operation condition. Relationships between the output randomness and operation conditions were obtained. In addition, numerical simulation demonstrated true random number generation at 20 GHz.