  Practical implementations of quantum key distribution (QKD) have been shown
to be subject to various detector side-channel attacks that compromise the
promised unconditional security. Most notable is a general class of attacks
adopting the use of faked-state photons as in the detector-control and, more
broadly, the intercept-resend attacks. In this paper, we present a simple
scheme to overcome such class of attacks: A legitimate user, Bob, uses a
polarization randomizer at his gateway to distort an ancillary polarization of
a phase-encoded photon in a bidirectional QKD configuration. Passing through
the randomizer once on the way to his partner, Alice, and again in the opposite
direction, the polarization qubit of the genuine photon is immune to
randomization. However, the polarization state of a photon from an intruder,
Eve, to Bob is randomized and hence directed to a detector in a different path,
whereupon it triggers an alert. We demonstrate theoretically and experimentally
that, using commercial off-the-shelf detectors, it can be made impossible for
Eve to avoid triggering the alert, no matter what faked-state of light she
uses.
