  A simple and efficient quantum non-demolition measurement (QND) scheme is
proposed in which the arrival of a signal photon is detected without affecting
the qubit state. The proposed QND scheme functions even if the ancillary photon
is replaced with weak light composed of vacuum and one-photon states. Although
the detection scheme is designed for entanglement sharing applications, it is
also suitable for general purification of a single photon state.
