  We present an experimental approach to construct a dephrasure channel, which
contains both dephasing and erasure noises, and can be used as an efficient
tool to study the superadditivity of coherent information. By using a
three-fold dephrasure channel, the superadditivity of coherent information is
observed, and a substantial gap is found between the zero single-letter
coherent information and zero quantum capacity. Particularly, we find that when
the coherent information of n channel uses is zero, in the case of larger
number of channel uses, it will become positive. These phenomena exhibit a more
obvious superadditivity of coherent information than previous works, and
demonstrate a higher threshold for non-zero quantum capacity. Such novel
channels built in our experiment also can provide a useful platform to study
the non-additive properties of coherent information and quantum channel
capacity.
