  A causal relation between quantum agents, say Alice and Bob, is necessarily
mediated by an interaction. Modelling the last one as a reversible quantum
channel, an intervention of Alice can have causal influence on Bob's system,
modifying correlations between Alice and Bob's systems. Causal influence
between quantum systems necessarily allows for signalling. Here we prove a
mismatch between causal influence and signalling via direct computation of the
two quantities for the Cnot gate. Finally we show a continuity theorem for
causal effects of unitary channels: a channel has small causal influence iff it
allows for small signalling.
