  Transport at a quantum critical point depends sensitively on the relative
magnitudes of temperature, frequency and electric field. Here we used the
gauge/gravity correspondence to compute the full temperature and, generally
nonlinear, electric field dependence of the electrical conductivity for some
model critical theories. In the special case of 2+1 dimensions we are also able
to find the full time-dependent response of the system to an arbitrary time
dependent external electric field. The response of the system is instantaneous,
implying a frequency independent conductivity. We describe a mechanism that
rationalizes the instantaneous response.
