  Superconducting qubits are a leading platform for scalable quantum computing
and quantum error correction. One feature of this platform is the ability to
perform projective measurements orders of magnitude more quickly than qubit
decoherence times. Such measurements are enabled by the use of quantum-limited
parametric amplifiers in conjunction with ferrite circulators - magnetic
devices which provide isolation from noise and decoherence due to amplifier
backaction. Because these non-reciprocal elements have limited performance and
are not easily integrated on-chip, it has been a longstanding goal to replace
them with a scalable alternative. Here, we demonstrate a solution to this
problem by using a superconducting switch to control the coupling between a
qubit and amplifier. Doing so, we measure a transmon qubit using a single,
chip-scale device to provide both parametric amplification and isolation from
the bulk of amplifier backaction. This measurement is also fast, high fidelity,
and has 70% efficiency, comparable to the best that has been reported in any
superconducting qubit measurement. As such, this work constitutes a
high-quality platform for the scalable measurement of superconducting qubits.
