  A class of Adaptive Decoders (AD's) for coherent-state sequences is studied,
including in particular the most common technology for optical-signal
processing, e.g., interferometers, coherent displacements and photon-counting
detectors. More generally we consider AD's comprising adaptive procedures based
on passive multi-mode Gaussian unitaries and arbitrary single-mode destructive
measurements. For classical communication on quantum phase-insensitive Gaussian
channels with a coherent-state encoding, we show that the AD's optimal
information transmission rate is not greater than that of a single-mode
decoder. Our result also implies that the ultimate classical capacity of
quantum phase-insensitive Gaussian channels is unlikely to be achieved with the
considered class of AD's.
