Attack with Hong–Ou–Mandel interferometer to quantum key distribution

We suggest a sophisticated attack protocol, which can be applied to the two-way deterministic quantum key distribution protocol. When more than two photons are used for a qubit, Eve picks out a photon each from the forward and the backward channel and measures whether the states of the two picked-out photons are orthogonal or the same by using the fourth order quantum interference. We describe the attack protocol, which uses an active arrangement of two Hong–Ou–Mandel interferometers combined with further quantum nondemolition measurements, beam splitters, and polarization rotator.