  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.

Quantum Physics