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Video abstract for the article 'After-gate attack on a quantum cryptosystem ' by C Wiechers, L Lydersen, C Wittmann, D Elser, J Skaar, Ch Marquardt, V Makarov and G Leuchs (C Wiechers et al 2011 New J. Phys. 13 013043). Read the full article in New Journal of Physics at http://iopscience.iop.org/1367-2630/13/1/013043/fulltext/. GENERAL SCIENTIFIC SUMMARY Introduction and background. Quantum key distribution (QKD) systems provide communication with proven unconditional security and have recently become commercial products. Security proofs have been provided for different scenarios, assuming idealized models of these systems. Main results. We present a new method to exploit a discrepancy between the model assumptions and the real device properties of single-photon detectors. The vulnerability is based on the fact that telecommunication avalanche photodiodes operate in a gated mode. Outside of the activation time, they operate as linear classical photodiodes with a classical sensitivity threshold. Launching a bright light pulse just after the gate causes a controllable detection. This allows one to execute an intercept-resend attack, delivering a full secret key to the eavesdropper. However, the bright pulses produce a side effect in the form of random detection events (afterpulses) in subsequent gates. The resulting increase in the quantum bit error rate (QBER) might reveal the eavesdropper. Nevertheless, we show that the QBER caused by our attack is sufficiently small to allow for undetected eavesdropping in some parameter ranges (see figure). Wider implications. In the future, it will be necessary to extend security proofs to more general and realistic scenarios, carefully including deviations from implementations from the model.