  Quantum coherence allows the computation of an arbitrary number of distinct
computational paths in parallel. Based on quantum parallelism it has been
conjectured that exponential or even larger speedups of computations are
possible. Here it is shown that, although in principle correct, any speedup is
accompanied by an associated attenuation of detection rates. Thus, on the
average, no effective speedup is obtained relative to classical
(nondeterministic) devices.
