Quantum locking of classical correlations and quantum discord of classical-quantum states

TL;DR

This paper demonstrates that quantum discord quantifies the advantage of quantum locking protocols over classical ones for classical-quantum states, highlighting a quantum advantage independent of entanglement.

Contribution

It establishes a direct link between quantum discord and the security advantage in quantum locking of classical correlations.

Findings

Quantum discord measures the advantage in quantum locking protocols.

Quantum locking allows smaller keys compared to classical methods.

Entanglement is not necessary for the quantum advantage.

Abstract

A locking protocol between two parties is as follows: Alice gives an encrypted classical message to Bob which she does not want Bob to be able to read until she gives him the key. If Alice is using classical resources, and she wants to approach unconditional security, then the key and the message must have comparable sizes. But if Alice prepares a quantum state, the size of the key can be comparatively negligible. This effect is called quantum locking. Entanglement does not play a role in this quantum advantage. We show that, in this scenario, the quantum discord quantifies the advantage of the quantum protocol over the corresponding classical one for any classical-quantum state.