Device-Independent Quantum Key Activation

Device-independent quantum key distribution (DIQKD) allows two distant parties to establish a secret key, based only on the observed Bell nonlocal distribution. It remains unclear, however, what the minimal resources for enabling DIQKD are and how to maximize the key rate from a given distribution. In the present Letter, we consider a scenario where several copies of a given quantum distribution are jointly processed via a local and classical wiring operation. We find that, under few assumptions, it is possible to activate device-independent key, that is, starting from a distribution that is useless in a DIQKD protocol, we obtain a positive asymptotic key rate by wiring several copies together. We coin this effect device-independent key activation. Our analysis focuses on the standard DIQKD protocol with one-way postprocessing, and we resort to semidefinite programming techniques for computing lower bounds on the key rate.