  Randomized benchmarking is a powerful technique to efficiently estimate the
performance and reliability of quantum gates, circuits and devices. Here we
propose to perform randomized benchmarking in a coherent way, where
superpositions of different random sequences rather than independent samples
are used. We show that this leads to a uniform and simple protocol with
significant advantages with respect to gates that can be benchmarked, and in
terms of efficiency and scalability. We show that e.g. universal gate sets, the
set of $n-$qudit Pauli operators or more general sets including arbitrary
unitaries, as well as a particular $n-$qudit Clifford gate using only the Pauli
set, can be efficiently benchmarked. The price to pay is an additional
complexity to add control to the involved quantum operations. However we
demonstrate that this can be done by using auxiliary degrees of freedom that
are naturally available in basically any physical realization, and are
independent of the gates to be tested.
