Isolating a qubit from amplifier backaction by coordinated switching*

We demonstrate a cQED qubit measurement scheme in which the qubit is isolated from amplifier backaction without ferrite circulators. We swap a readout signal from the qubit cavity to an ancillary cavity by temporarily coupling them. The state of the ancillary cavity is then amplified by parametrically pumping past bifurcation into one of two large amplitude states that are correlated with the qubit state. Isolation and efficiency are comparable to readout using a ferrite circulator, but are achieved using a chip-scale device compatible with the integration of superconducting qubits. Because the measurement result is contained in the digital state of a superconducting bifurcation amplifier, it is possible to use this scheme to create autonomous feedback loops at the cryostat base temperature. The integrated nature of this readout suggests an avenue toward the scalable measurement of larger quantum networks.