  Quantum coherence is the key resource for quantum technology, with
applications in quantum optics, information processing, metrology and
cryptography. Yet, there is no universally efficient method for quantifying
coherence either in theoretical or in experimental practice. I introduce a
framework for measuring quantum coherence in finite dimensional systems. I
define a theoretical measure which satisfies the reliability criteria
established in the context of quantum resource theories. Then, I present an
experimental scheme implementable with current technology which evaluates the
quantum coherence of an unknown state of a d-dimensional system by performing
two programmable measurements on an ancillary qubit, in place of the $O(d^2)$
direct measurements required by full state reconstruction. The result yields a
benchmark for monitoring quantum effects in complex systems, e.g. certifying
non-classicality in quantum communication and simulation protocols and probing
the quantum behaviour of biological complexes.
