title: Quantum receiver beyond the standard quantum limit of coherent optical communication.
abstract: The most efficient modern optical communication is known as coherent communication, and its standard quantum limit is almost reachable with current technology. Though it has been predicted for a long time that this standard quantum limit could be overcome via quantum mechanically optimized receivers, such a performance has not been experimentally realized so far. Here we demonstrate the first unconditional evidence surpassing the standard quantum limit of coherent optical communication. We implement a quantum receiver with a simple linear optics configuration and achieve more than 90% of the total detection efficiency of the system. Such an efficient quantum receiver will provide a new way of extending the distance of amplification-free channels, as well as of realizing quantum information protocols based on coherent states and the loophole-free test of quantum mechanics.
authors.affiliation: National Institute of Information and Communications Technology, 4-2-1 Nukui-kitamachi, Koganei, Tokyo 184-8795, Japan.
authors.forename: Kenji
authors.initials: K
authors.lastname: Tsujino
authors.forename: Daiji
authors.initials: D
authors.lastname: Fukuda
authors.forename: Go
authors.initials: G
authors.lastname: Fujii
authors.forename: Shuichiro
authors.initials: S
authors.lastname: Inoue
authors.forename: Mikio
authors.initials: M
authors.lastname: Fujiwara
authors.forename: Masahiro
authors.initials: M
authors.lastname: Takeoka
authors.forename: Masahide
authors.initials: M
authors.lastname: Sasaki