  Using a circuit QED device, we demonstrate a simple qubit measurement pulse
shape that yields fast ring-up and ring-down of the readout resonator
regardless of the qubit state. The pulse differs from a square pulse only by
the inclusion of additional constant-amplitude segments designed to effect a
rapid transition from one steady-state population to another. Using a Ramsey
experiment performed shortly after the measurement pulse to quantify the
residual population, we find that compared to a square pulse followed by a
delay, this pulse shape reduces the timescale for cavity ring-down by more than
twice the cavity time constant. At low drive powers, this performance is
achieved using pulse parameters calculated from a linear cavity model; at
higher powers, empirical optimization of the pulse parameters leads to similar
performance.
