Quantum hacking: Experimental demonstration of time-shift attack against practical quantum-key-distribution systems

Quantum-key-distribution (QKD) systems can send quantum signals over more than 100 km standard optical fiber and are widely believed to be secure. Here, we show experimentally a technologically feasible attack-namely, the time-shift attack-against a commercial QKD system. Our result shows that, contrary to popular belief, an eavesdropper, Eve, has a non-negligible probability (similar to 4%) to break the security of the system. Eve's success is due to the well-known detection efficiency loophole in the experimental testing of Bell's inequalities. Therefore, the detection efficiency loophole plays a key role not only in fundamental physics, but also in technological applications such as QKD systems.