Quantum Random Number Generators

Random numbers are vital for a wide variety of applications including lotteries, statistical sampling, computer simulations and cryptography. They can also be used to make random decisions required in both quantum key distribution (QKD) and fundamental tests of the foundations of quantum mechanics.

For each application the requirements on the random numbers may be different. For some applications, such as computer simulations, it is sufficient for the numbers to have the required statistical properties without any need for them to be unpredictable. However for many applications, especially cryptography, it is essential for the numbers to be unpredictable too.

A.Kerckhoffs showed that the problem of secure communications could be reduced to the generation of random numbers for use as keys. Consequently, the security of modern cryptography rests on the random numbers used for secret keys, public key generation, session identifiers and more. The random number generators in these systems are therefore a potentially catastrophic security weakness. There has been a steady stream of revelations of weaknesses in existing random number generators (RNGs) and there are concerns that some can be exploited using backdoors introduced by systematically weakening them. The underlying problem is an insufficient supply of trustworthy entropy. In response to this regulators, standards bodies and certifications for cryptographic products are placing growing emphasis on validating claims for random number generation.

There exist a wide range of techniques for generating random numbers. Pseudorandom number generators (PRNGs), which use deterministic algorithms to generate random numbers, are not suitable for use in cryptography due to their predictability. We therefore turn to truly random number generators (TRNGs) which measure the outcome of an unpredictable or at least very hard to predict physical process and use the results to generate random numbers.

Providing entropy which cannot be known or manipulated by third parties using a TRNG is remarkably challenging. In this respect, quantum random number generators (QRNGs) which generate numbers based on measurements of quantum mechanical processes offer a clear advantage over TRNGs based on measuring classical processes in that their randomness source is typically a well defined quantum phenomenon. Consequently a precise description of the randomness source can be used to derive bounds on the entropy, even in the presence of additional classical noise and potential eavesdroppers.

Information theoretically secure random numbers can then be extracted by post-processing the raw bits produced by the measurements. The main aim of this project will be to develop a fast and secure photonic QRNG suitable for use in cryptographic applications.