Unconditionally secure device-independent quantum key distribution with only two devices

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 behavior, 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. We introduce a protocol that requires Alice and Bob to have only one device each. Although inefficient and unable to tolerate reasonable levels of noise, our protocol is unconditionally secure against an adversarial supplier limited only by locally enforced signaling constraints.