Hacking the Bell test using classical light in energy-time entanglement–based quantum key distribution

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.

基于能量-时间纠缠（energy-time entanglement）的光子系统，已被提出用于借助贝尔不等式（Bell inequality）检验定域实在论。该不等式的违背通常亦可验证设备无关量子密钥分发（device-independent quantum key distribution, QKD）的安全性，确保攻击者无法窃听或控制系统。我们证明，在采用标准雪崩光电探测器（avalanche photodetectors）的能量-时间纠缠系统中，该安全性检验可被绕过，使攻击者能够在不留下任何痕迹的前提下入侵该系统。我们通过定制化经典光脉冲，在伪造探测器效率达97.6%时实现了最高3.63的贝尔值，这一结果甚至超越了量子理论的预测。这是首个兼具可调谐违背性与高伪造探测器效率的违背伪造源的演示实验。其影响后果严峻：基于弗兰森配置（Franson’s configuration）的能量-时间纠缠实验装置，无法通过标准克劳瑟-霍恩-希米尼-霍尔特不等式（Clauser-Horne-Shimony-Holt inequality）证明设备无关安全性。不过设备无关安全性仍可被重建，我们在文末列出了若干可抵御当前及未来此类攻击的改进型检验方案与实验装置。