Hacking energy-time entanglement-based systems with classical light

TL;DR

This paper demonstrates that classical light pulses can fake Bell inequality violations in energy-time entangled systems, undermining their security for device-independent quantum key distribution.

Contribution

It introduces a method using classical light to artificially violate Bell inequalities, revealing a security vulnerability in existing energy-time entanglement setups.

Findings

Classical light pulses can reach Bell values up to 3.63 with high detector efficiency.

Standard Bell tests can be fooled by tailored classical pulses, compromising security.

The violation-faking source allows tunable Bell violations and high detector efficiency.

Abstract

Photonic systems based on energy-time entanglement have been proposed to test local realism using the Bell inequality. A violation of this inequality normally also certifies security of device-independent quantum key distribution, so that an attacker cannot eavesdrop or control the system. Here, we show how this security test can be circumvented in energy-time entangled systems when using standard avalanche photodetectors, allowing an attacker to compromise the system without leaving a trace. With tailored pulses of classical light we reach Bell values up to 3.63 at 97.6% detector efficiency which is an extreme violation. This is the first demonstration of a violation-faking source that both gives tunable violation and high detector efficiency. The implications are severe: the standard Clauser-Horne-Shimony-Holt inequality cannot be used to show device-independent security for standard postselecting energy-time entanglement setups. We conclude with suggestions of improved tests and experimental setups that can re-establish device-independent security.