Toward the integration of a nonreciprocal amplifier with a transmon qubit

In typical 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 amplifier and qubit-cavity system can be developed separately. However, their intrinsic loss and the required wiring significantly reduce the measurement efficiency and increase the system complexity and footprint. 

These issues can be circumvented by integrating the qubit-cavity system with a nonreciprocal amplifier [1]. Here we will discuss the design, fabrication and measurement of a 3D transmon integrated with a multi-mode resonant circuit that acts as a readout cavity with built-in signal gain. In the absence of an intermediate circulator, the cavity and amplifier merge into a single quantum system that balances fast and efficient readout with low backaction and long qubit lifetime. 

[1] F. Lecocq, et al. PRL 126, 020502 (2021)