Plug-and-play QKD device under attack.
Alice’s side includes a 90:10 optical coupler (C) that diverts part of the light for monitoring to a series of classical detectors, and the quantum part with a variable attenuator (VOA), delay line (DL), phase modulator (PM), and Faraday mirror (FM). Alice uses the PM to introduce a secret phase and makes sure that the total attenuation guarantees that the mean photon number per pulse going back to the fiber channel is less than the value prescribed by the protocol. The coupling path in our attack is shown in green (gray): a ventilation hole in Alice’s case gives light an access to the fiber spool of the DL. From there, the injected photons can get to the phase modulator and carry unwanted information to the outside fiber channel.