  The most efficient modern optical communication is known as coherent
communication and its standard quantum limit (SQL) is almost reachable with
current technology. Though it has been predicted for a long time that this SQL
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 SQL 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.
