  Solids with topologically robust electronic states exhibit unusual electronic
and optical transport properties that do not exist in other materials. A
particularly interesting example is chiral charge pumping, the so-called chiral
anomaly, in recently discovered topological Weyl semimetals, where simultaneous
application of parallel DC electric and magnetic fields creates an imbalance in
the number of carriers of opposite topological charge (chirality). Here, using
time-resolved terahertz measurements on the Weyl semimetal TaAs in a magnetic
field, we optically interrogate the chiral anomaly by dynamically pumping the
chiral charges and monitoring their subsequent relaxation. Theory based on
Boltzmann transport shows that the observed effects originate from an optical
nonlinearity in the chiral charge pumping process. Our measurements reveal that
the chiral population relaxation time is much greater than 1 ns. The
observation of terahertz-controlled chiral carriers with long coherence times
and topological protection suggests the application of Weyl semimetals for
quantum optoelectronic technology.
