  A quantum channel physically is a unitary interaction between the information
carrying system and an environment, which is initialized in a pure state before
the interaction. Conventionally, this state, as also the parameters of the
interaction, is assumed to be fixed and known to the sender and receiver. Here,
following the model introduced by us earlier [Karumanchi et al.,
arXiv[quant-ph]:1407.8160], we consider a benevolent third party, i.e. a
helper, controlling the environment state, and how the helper's presence
changes the communication game. In particular, we define and study the
classical capacity of a unitary interaction with helper, indeed two variants,
one where the helper can only prepare separable states across many channel
uses, and one without this restriction. Furthermore, the two even more powerful
scenarios of pre-shared entanglement between helper and receiver, and of
classical communication between sender and helper (making them conferencing
encoders) are considered.
