title: Hacking the Bell test using classical light in energy-time entanglement-based quantum key distribution.
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 (QKD) so that an attacker cannot eavesdrop or control the system. 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. We reach Bell values up to 3.63 at 97.6% faked detector efficiency using tailored pulses of classical light, which exceeds even the quantum prediction. This is the first demonstration of a violation-faking source that gives both tunable violation and high faked detector efficiency. The implications are severe: the standard Clauser-Horne-Shimony-Holt inequality cannot be used to show device-independent security for energy-time entanglement setups based on Franson's configuration. However, device-independent security can be reestablished, and we conclude by listing a number of improved tests and experimental setups that would protect against all current and future attacks of this type.
authors.affiliation: Institutionen för Systemteknik, Linköpings Universitet, 581 83 Linköping, Sweden.
authors.forename: Jonathan
authors.identifier: 0000-0002-8032-1466
authors.initials: J
authors.lastname: Jogenfors
authors.affiliation: Department of Physics, Stockholm University, 106 91 Stockholm, Sweden.
authors.forename: Ashraf Mohamed
authors.identifier: 0000-0001-7562-7148
authors.initials: AM
authors.lastname: Elhassan
authors.affiliation: Department of Physics, Stockholm University, 106 91 Stockholm, Sweden.
authors.forename: Johan
authors.identifier: 0000-0001-8865-2270
authors.initials: J
authors.lastname: Ahrens
authors.affiliation: Department of Physics, Stockholm University, 106 91 Stockholm, Sweden.
authors.forename: Mohamed
authors.initials: M
authors.lastname: Bourennane
authors.affiliation: Institutionen för Systemteknik, Linköpings Universitet, 581 83 Linköping, Sweden.
authors.forename: Jan-Åke
authors.identifier: 0000-0002-1082-8325
authors.initials: JÅ
authors.lastname: Larsson
keywords: Bell’s Theorem; Energy-Time Entanglement; Quantum Hacking; Quantum Key Distribution
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