Polarization preserving quantum nondemolition photodetector

A polarization preserving quantum nondemolition photodetector is proposed based on nonlinearities obtainable through quantum coherence effects. An atomic level scheme is devised such that in the presence of strong linearly polarized drive field a coherent weak probe field acquires a phase proportional to the number of photons in the signal mode immaterial of its polarization state. It is also shown that the unavoidable phase-kicks resulting due to the measurement process are insensitive to the polarization state of the incoming signal photon. It is envisioned that such a device would have tremendous applicability in photonic quantum information proposals where quantum information in the polarization qubit is to be protected.