  We study classical capacities of quantum multi-access channels in geometric
terms revealing breaking of additivity of Holevo-like capacity. This effect is
purely quantum since, as one points out, any classical multi-access channels
have their regions additive. The observed non-additivity in quantum version
presented here seems to be the first effect of this type with no additional
resources like side classical or quantum information (or entanglement)
involved. The simplicity of quantum channels involved resembles butterfly
effect in case of classical channel with two senders and two receivers.
