Augmenting a train of bright phase-modulated laser-light pulses of a coherent communications system with infinitesimally small quantum photons per pulse—entangled across several time bins—prepared by splitting squeezed light in a temporal-mode interferometer can dramatically enhance a homodyne receiver’s ability to detect a sudden change in the channel loss, by up to a factor that is the inverse of the pre-change loss, without affecting the communications rate. We discuss the quantum limit of quickest change detection, and the problem of joint communications and change detection that our Letter opens up.

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• Quantum communication
• Quantum information theory
• Quantum parameter estimation