QUROPE Quantum Information Processing and Communication in Europe

B. W. Reichardt, F. Unger and U. Vazirani
Nature 496, 456–460 (2013)

Quantum computation and cryptography both involve scenarios in which a user interacts with a quantum
system which is either imperfectly characterised or even "untrusted" (i.e. possibly supplied by an adversary
or eavesdropper). It is thus both fundamental and practical to devise ways to reveal whether the system
behaves as desired. For example, Bell inequalities allow one to certify that a system is behaving quantum
mechanically and not classically.

In their work Reichardt and co-authors extend this basic idea to enable the characterization of a large
quantum system. The show how to determine the initial state of a system and to classically command it to
evolve according to desired dynamics, relying only on the assumption that system is quantum, even if the
system is designed to behave mischievously.

Among its main applications, their scheme allows one to test whether a claimed quantum computer is truly
quantum and advances towards a key goal of "device independent" quantum cryptography, to be able to
establish a shared random key between two parties when both parties use devices potentially provided by an
eavesdropper.