Qubit measurements using nonreciprocal amplifiers

In recent years, there has been a lot of efforts toward designing nonreciprocal parametric amplifiers, i.e. parametric amplifiers that are inherently directional and do not require the use of external microwave circulators. Unsurprisingly, the bulk of these efforts started with the amplifiers themselves, usually measured in a clean broadband 50 Ohm environment. However, this dissipative environment is far from the reactive, non-dissipative load presented by the qubits and readout cavities that are intended to be measured. In practice, directly coupling these circuits creates a larger quantum system that combines the single-photon nonlinearity of qubits with strongly amplified signals, presenting both theoretical and experimental challenges. In addition, the extreme sensitivity of qubits to cavity population places stringent constraints on both the isolation and thermalization of the nonreciprocal amplifiers.

In this talk, I will show recent progress toward measuring qubits using two types of nonreciprocal amplifiers, a Field-Programable Josephson Amplifier (FPJA) and a Traveling Wave Parametric Amplifier and Converter (TWPAC). I will present new theoretical tools for understanding amplifier backaction, discuss the concept of qubit/amplifier co-design and address the challenges ahead.