We present a superconducting device that realizes the sequential measurement of a transmon qubit. The unitary evolution between system and probe is indeed separated in time and spacefrom the measurement of the probe itself. The device disables common limitations of dispersive readout such as Purcell effect or transients in the cavity mode by tuning the coupling to the measurement channel on demand. The probe is initially stored in a memory mode and coupled to the qubit until a microwave pump releases it into an output line in a characteristic time as low as 10 ns, which is 400 times shorter than the memory lifetime. The Wigner function of the memory allows us to characterize the non-Gaussian nature of the probe and its dynamics. A direct measurement of the released probe field quadratures demonstrates a readout fidelity of 97.5 % in a total measurement time of 220 ns.
Strong microwave drives, referred to as pumps, are widely applied to superconducting circuits incorporating Josephson junctions in order to induce couplings between electromagneticmodes. This offers a variety of applications, from quantum-limited amplification, to quantum state and manifold stabilization. These couplings scale with the pump power, therefore, seeking stronger couplings requires a detailed understanding of the behavior of such circuits in the presence of stronger pumps. In this work, we probe the dynamics of a transmon qubit in a 3D cavity, for various pump powers and frequencies. For all pump frequencies, we find a critical pump power above which the transmon is driven into highly excited states, beyond the first seven states which we individually resolve through cavity spectroscopy. This observation is compatible with our theory describing the escape of the transmon state out of its Josephson potential well, into states resembling those of a free particle which does not induce any non-linear couplings.