Joseph F. Fitzsimons

One-time programs, computer programs which self-destruct after being run only once, are a powerful building block in cryptography and would allow for new forms of secure software distribution. However, ideal one-time programs have been proved to be unachievable using either classical or quantum resources. Here we relax the definition of one-time programs to allow some probability of error in the output and show that quantum mechanics offers security advantages over purely classical resources. We introduce a scheme for encoding probabilistic one-time programs as quantum states with prescribed measurement settings, explore their security, and experimentally demonstrate various one-time programs using measurements on single-photon states. These include classical logic gates, a program to solve Yao’s millionaires problem, and a one-time delegation of a digital signature. By combining quantum and classical technology, we demonstrate that quantum techniques can enhance computing capabilities even before full-scale quantum computers are available.

Delegating difficult computations to remote large computation facilities, with appropriate security guarantees, is a possible solution for the ever-growing needs of personal computing power. For delegated computation protocols to be usable in a larger context—or simply to securely run two protocols in parallel—the security definitions need to be composable. Here, we define composable security for delegated quantum computation, and prove that several known protocols are composable, including Broadbent, Fitzsimons and Kashefi’s Universal Blind Quantum Computation protocol.We distinguish between protocols which provide only blindness—the computation is hidden from the server—and those that are also verifiable—the client can check that it has received the correct result. We show that the composable security definition capturing both these notions can be reduced to a combination of two distinct stand-alone security definitions.