  I derive a universal upper bound on the capacity of any communication channel
between two distant systems. The Holevo quantity, and hence the mutual
information, is at most of order $E \Delta t / \hbar$, where $E$ is the average
energy of the signal, and $\Delta t$ is the amount of time for which detectors
operate. The bound does not depend on the size or mass of the emitting and
receiving systems, nor on the nature of the signal. No restrictions on
preparing and processing the signal are imposed.
  As an example, I consider the encoding of information in the transverse or
angular position of a signal emitted and received by systems of arbitrarily
large cross-section. In the limit of a large message space, quantum effects
become important even if individual signals are classical, and the bound is
upheld.
