Integrating a nonreciprocal amplifier with a 3D transmon qubit

In conventional superconducting qubit dispersive readout schemes, the measurement signal is routed from the readout cavity to the amplification chain using magnetic circulators. These circulators allow for a modular approach in which the amplifiers and qubit-cavity system can be developed separately, but their intrinsic loss and the required wiring significantly reduce the measurement efficiency.

Improvement in measurement efficiency can be achieved by integrating the qubit-cavity system with a nonreciprocal amplifier [1]. In the absence of an intermediate circulator, the cavity and amplifier merge into a single quantum system in which the fragile qubit state must coexist with a large amplified readout signal.

Here we discuss experimental progress toward further integration of a 3D transmon qubit with a multi-mode resonant circuit that acts as a readout cavity with built-in signal gain. We will focus on the delicate balance between large gain and low backaction and between fast readout and long qubit lifetime.
[1] F. Lecocq, et al, ArXiv:2009.08863