Abhishek Mishra

Transmitter-device-dependence is a longstanding but often implicit problem in quantum key distribution (QKD), as compared to measurement-device-dependence. Quantum steering conceptually designates the sender and receiver in entanglement certification, offering an intuitive solution to transmitter-sided dependence. The well-known one-sided device-independent (1sDI) protocols exploit quantum steering as a resource to relax the security conditions of device-independent (DI) framework, yet provokes underlying signaling loophole. Here we formalize transmitter-device-independence based on the faithful quantum steering and propose a transmitter-device-independent (TDI) QKD protocol that closes the signaling loophole, thereby defending against common transmitter-side attacks. In a proof-of-principle experiment we validate our proposal obtaining a key-rate in the asymptotic limit of 1~kbps at a fiber transmission of 27~km. By implementing TDI security while maintaining strong loss tolerance, our approach bridges the gap between security and practicality for real-world QKD deployments.

Bridging the gap between practical implementation and rigorous security, semi-device-independent quantum key distribution (QKD) offers a compelling middle ground—requiring minimal trust assumptions while achieving key rates competitive with fully characterized protocols. We investigate two conceptually distinct constraints on the preparation device: the information constraint and restricted distrust. We identify protocols that remain secure under both assumptions and demonstrate their robustness to experimental noise. Notably, the widely used BB84 and B92 protocols fail to provide security under the information constraint. To obtain secure key rates, we showcase a numerical framework combining tracial non-commutative polynomial optimization with entropy relaxation techniques. Our approach extends naturally to broader classes of preparation constraints and QKD protocols.

Our motivation is to exploit the partial commutation structure between the variables in non-commutative polynomial optimisation problems to boost the performance. We provide an efficient normal form for free words in partially commuting letters based on the maximal cliques of the non-commutation graph between the letters. We adapt several non-commutative computations to the partially commuting setting exploiting this additional structure. In particular, we provide an algorithm to compute Grobner bases for polynomial ideals in partially commuting variables that overcomes some difficulties appearing in the non-commutative cases: sometimes infinite Grobner basis can be avoided using the normal form based on these cliques.