Multiplexed Readout of Superconducting Qubits in 3D cQED Architecture Using Impedance Engineered Broadband JPA*

We propose and demonstrate a frequency-multiplexed readout scheme in 3D cQED architecture. We use four transmon qubits coupled to individual rectangular cavities which are aperture-coupled to a single rectangular waveguide feedline. A coaxial to waveguide transformer at the other end of the feedline allows to launch and collect the multiplexed signal. The reflected readout signal is amplified by an impedance engineered broadband parametric amplifier with 380 MHz of bandwidth. This provides us high fidelity single-shot readout of multiple qubits using compact microwave circuitry, an efficient way for scaling up to more qubits in 3D cQED. We also discuss possible designs for multiplexing larger number of qubits. Finally, we discuss the saturation properties of our broadband JPA and explore a few approaches to improve them to increase the number of qubits that can be measured simultaneously.