Comment on "measuring a photonic qubit without destroying it.

quantum nondemolition measurements Recently, Pryde et al. reported the demonstration of a quantum nondemolition (QND) scheme for single-photon polarization states with linear optics and projective measurements. In this experiment, a single photon with a specific polarization in the signal mode interacts on a beam splitter with a second polarized photon in the meter mode. A destructive polarization measurement of the meter photon then sometimes reveals the polarization of the signal photon without the need for direct detection of the signal mode. This allows the signal photon to propagate freely. Hence the interpretation of this experiment as a single-photon QND measurement.