Shubhayan Sarkar

Quantum key distribution (QKD) is the most explored application of quantum information theory. A central problem in entanglement-based QKD (EB-QKD), is whether every entangled state can be used to extract a key. We observe that entanglement is not sufficient for standard practical EB-QKD protocols where the input choices are announced by the parties that want to share a secure key, such as E91 or entanglement-based BB84 type protocols, when even an arbitrarily small amount of leakage of classical side information occurs. We do this by identifying a class of two-qubit isotropic states that are entangled but cannot be used to distil the key under such protocols for any possible measurement by the parties. Counter-intuitively, this gap persists even when the leakage occurs from the "junk" rounds of the protocol, i.e, rounds that cannot be used to generate any key. We then extend this result to arbitrary dimensions and parties by identifying a class of isotropic states that are not useful to extract a secure key under such protocols, even if they are entangled. Finally, we demonstrate that our approach provides a tool to upper-bound the scalability of repeater-based QKD architectures in a protocol-independent manner. Interestingly, we find that allowing for even a tiny noise in the preparation drastically reduces the scalability of the QKD network.

Quantum Key Distribution (QKD) enables provable secure communication but faces challenges in device characterization, posing potential security risks. Device-Independent (DI) QKD protocols overcome this issue by making minimal device assumptions but are limited in distance because they require high detection efficiencies, which refer to the ability of the experimental setup to detect quantum states. Our study explores an entanglement-based one-sided device-independent QKD scenario, where one party's device is semi-trusted, while the second is completely untrusted. We introduce specific assumptions about the semi-trusted device's measurements and assess the security of our protocol without post-selecting outcomes, thereby allowing one to prove security against coherent attacks. By applying the latest analytical and numerical methods, we established that our protocol can securely operate as long as the involved detection efficiencies exceed a minimal threshold of 50.1% specifically on the untrusted side. This is almost the theoretical limit achievable for protocols with two untrusted measurements and is within current experimental capabilities. Interestingly, we also show that, by placing the source of states close to the untrusted side, our protocol is secure over distances comparable to standard QKD protocols. Our findings not only reinforce the practicality of QKD systems under less stringent conditions but also serve as a feasible hybrid approach, bridging conventional QKD with DI QKD.