Cracking quantum cryptography by cheating

Nature : With the use of lasers, hackers have now found a way to fake the quantum property of entanglement at the heart of cryptographic systems, reports Nature‘s Zeeya Merali. Entanglement here refers to the relationship between two photons that are connected in such a way that measuring the polarization state of one instantaneously modifies the polarization state of its partner. Each of the two entangled photons is assigned to an entity, say Alice and Bob. Any attempt by a third party to eavesdrop by intercepting either Alice’s or Bob’s photon will destroy the entanglement. To check the entanglement is secure a technique called the Bell test is used..Christian Kurtsiefer of the National University of Singapore and colleagues, however, discovered one can cheat the Bell test by blinding Bob’s detectors with a laser beam and intercepting its photons. While blinded, the detector can be tricked into registering the correct value whenever the hacker fires an additional laser pulse at it.The theoretical version of the Bell test would pick up on this deception, but in the real world, the equipment compensates for inperfections and hence treats the signal as valid.