Exceeding the classical capacity limit in a quantum optical channel

The theory of capacity generally gives no guidance on how to construct codes that approach the capacity. The practical problem is to find good codes for small blocks. Although several coding schemes are proposed to exhibit superadditive coding gain, little attention is paid to this topic so far, and no experimental work is reported yet. This paper experimentally demonstrates the superadditive coding gain by designing a coding circuit for a quantum channel consisting of the ternary symmetric states in a two-state system (qubit) of a single photon.