  We demonstrate a sub-shot-noise-limit discrimination of on-off keyed coherent
signals by an optimal displacement quantum receiver in which a superconducting
transition edge sensor is installed. Use of a transition edge sensor and a
fiber beam splitter realizes high total detection efficiency and high
interference visibility of the receiver and the observed average error
surpasses the shot-noise-limit in a wider range of the signal power. Our
technique opens up a new technology for the sub-shot-noise-limit detection of
coherent signals in optical communication channels.
