Unconditionally Secure Device-Independent Quantum Key Distribution With Only Two Devices

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Device-independent quantum key distribution is the task of using uncharacterized quantum devices to establish a shared key between two users. If a protocol is secure regardless of the device behaviour, it can be used to generate a shared key even if the supplier of the devices is malicious. To date, all device-independent quantum key distribution protocols that are known to be secure require separate isolated devices for each entangled pair, which is a significant practical limitation. The authors introduce a protocol that requires Alice and Bob to have only one device each. Although inefficient, their protocol is unconditionally secure against an adversarial supplier limited only by locally enforced signalling constraints.