  Electronic coherence is of utmost importance for the access and control of
quantum-mechanical solid-state properties. Using a purely electronic
observable, the photocurrent, we measure an electronic coherence time of 22 +/-
4 fs in graphene. The photocurrent is ideally suited to measure electronic
coherence as it is a direct result of quantum path interference, controlled by
the delay between two ultrashort two-color laser pulses. The maximum delay for
which interference between the population amplitude injected by the first pulse
interferes with that generated by the second pulse determines the electronic
coherence time. In particular, numerical simulations reveal that the
experimental data yield a lower boundary on the electronic coherence time and
that coherent dephasing masks a lower coherence time. We expect that our
results will significantly advance the understanding of coherent
quantum-control in solid-state systems ranging from excitation with weak fields
to strongly driven systems.
