  We present an uncertainty-relation-type quantum benchmark for
continuous-variable (CV) quantum channels that works with an input ensemble of
Gaussian distributed coherent states and homodyne measurements. It determines
an optimal trade-off relation between canonical quadrature noises unbeatable by
entanglement breaking channels and refines the notion of two quantum duties
introduced in the original papers of CV quantum teleportation. It can verify
the quantum-domain performance for all one-mode Gaussian channels.We also
address the case of stochastic channels and the effect of asymmetric gains.
