Loss-tolerant quantum key distribution with a twist

TL;DR

This paper extends the loss-tolerant security proof technique for measurement device-independent quantum key distribution (MDI QKD) to mixed signal states, introducing a semidefinite programming method to optimize key rates.

Contribution

It generalizes the loss-tolerant protocol to mixed states and provides a practical optimization approach for improving key rates in MDI QKD.

Findings

The protocol can be applied to mixed signal states in MDI QKD.

A semidefinite programming method enhances key rate optimization.

Example calculations demonstrate achievable key rates with polarization errors.

Abstract

The security of measurement device-independent quantum key distribution (MDI QKD) relies on a thorough characterization of one's optical source output, especially any noise in the state preparation process. Here, we provide an extension of the loss-tolerant protocol [Phys. Rev. A 90, 052314 (2014)], a leading proof technique for analyzing the security of QKD, to MDI QKD protocols that employ mixed signal states. We first reframe the core of the proof technique, noting its generalization to treat $d$-dimensional signal encodings. Concentrating on the qubit signal state case, we find that the mixed states can be interpreted as providing Alice and Bob with a virtual shield system they can employ to reduce Eve's knowledge of the secret key. We then introduce a simple semidefinite programming method for optimizing the virtual operations they can perform on the shield system to yield a higher key rate, along with an example calculation of fundamentally achievable key rates in the case of random polarization modulation error.