Necessary detection efficiencies for secure quantum key distribution and bound randomness

TL;DR

This paper analyzes the critical detection efficiencies needed for secure quantum key distribution, revealing vulnerabilities in current protocols and introducing the concept of bound randomness related to non-local correlations and non-signalling eavesdroppers.

Contribution

It presents a general attack applicable to many quantum cryptography protocols, establishing bounds on detection efficiencies and demonstrating the existence of bound randomness.

Findings

Most partly device-independent protocols require detection efficiencies similar to fully device-independent ones.

The attack exposes vulnerabilities in quantum cryptography implementations due to losses and detection tuning.

Bound randomness implies non-local correlations can reveal measurement results to non-signalling eavesdroppers.

Abstract

In recent years, several hacking attacks have broken the security of quantum cryptography implementations by exploiting the presence of losses and the ability of the eavesdropper to tune detection efficiencies. We present a simple attack of this form that applies to any protocol in which the key is constructed from the results of untrusted measurements performed on particles coming from an insecure source or channel. Because of its generality, the attack applies to a large class of protocols, from standard prepare-and-measure to device-independent schemes. Our attack gives bounds on the critical detection efficiencies necessary for secure quantum distribution, which show that the implementation of most partly device independent solutions is, from the point of view of detection efficiency, almost as demanding as fully device-independent ones. We also show how our attack implies the existence of a form of bound randomness, namely non-local correlations in which a non-signalling eavesdropper can find out a posteriori the result of any implemented measurement.