Device-independent quantum cryptography: disproving the sufficiency of Bell nonlocality and verifying the noise-robustness of current implementations

Date: Monday, November 7, 2022
Time: 14:15
Location: Quantum Chaos and Quantum Information (Jagiellonian University)
Passcode: subspace

Speaker: Jan Kolodynski (Center of New Technologies, University of Warsaw) 

Abstract Device-independent quantum key distribution (DIQKD) constitutes the most pragmatic approach to quantum cryptography that does not put any trust in the inner workings of the devices. This is possible by constructing security proofs at the level of correlations being shared by the end-users, leveraging from the phenomenon of Bell nonlocality. In particular, quantum nonlocality allows one then to lower-bound the asymptotically achievable key rates, even in the presence of the most general eavesdropping attacks. However, only recently first proof-of-principle implementations of DIQKD have been demonstrated, as the device-independent framework imposes very stringent requirements on the noise tolerance, even in the absence of any eavesdroppers. In our works, we follow a complementary approach in which we propose an easy-to-optimise attack on any DIQKD protocol, with help of which we construct upper bounds on the asymptotic key. On one hand, it allows us to disprove a long-standing conjecture that any form of Bell nonlocality is sufficient for distributing secret keys in a device-independent manner. On the other, it allows us to verify that current state-of-the-art implementations already operate very close to the ultimate noise thresholds, and cannot be thus improved by resorting to more profound security-proof techniques.