  We present a superconducting device that realizes the sequential measurement
of a transmon qubit. The device disables common limitations of dispersive
readout such as Purcell effect or transients in the cavity mode by turning on
and off the coupling to the measurement channel on demand. The qubit
measurement begins by loading a readout resonator that is coupled to the qubit.
After an optimal interaction time with negligible loss, a microwave pump
releases the content of the readout mode by upconversion into a measurement
line in a characteristic time as low as 10~ns, which is 400 times shorter than
the lifetime of the readout resonator. A direct measurement of the released
field quadratures demonstrates a readout fidelity of $97.5~\%$ in a total
measurement time of $220~\mathrm{ns}$. The Wigner tomography of the readout
mode allows us to characterize the non-Gaussian nature of the readout mode and
its dynamics.
