  Viscosity is an important property of out-of-equilibrium systems such as
active biological materials and driven non-Newtonian fluids, and for fields
ranging from biomaterials to geology, energy technologies and medicine.
However, noninvasive viscosity measurements typically require integration times
of seconds. Here we demonstrate a four orders-of-magnitude improvement in
speed, down to twenty microseconds, with uncertainty dominated by fundamental
thermal noise for the first time. We achieve this using the instantaneous
velocity of a trapped particle in an optical tweezer. To resolve the
instantaneous velocity we develop a structured-light detection system that
allows particle tracking with megahertz bandwidths. Our results translate
viscosity from a static averaged property, to one that may be dynamically
tracked on the timescales of active dynamics. This opens a pathway to new
discoveries in out-of-equilibrium systems, from the fast dynamics of phase
transitions, to energy dissipation in motor molecule stepping, to violations of
fluctuation laws of equilibrium thermodynamics.
