2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/125705A 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.12pages, 4figures, Submitted to Phys. Rev. AQuantum Physicstext