Directional readout resonator with interference Purcell filter for scalable and modular qubit readout*

In transmission-based readout of superconducting qubits, a weakly-coupled port is often used at the input of the readout bus to provide directionality close to unity for the readout microwave signal. However, this weakly-coupled port often requires the addition of large and high-magnetic field circulators and isolators for impedance matching, posing a significant challenge to quantum error correction, for which the number of qubits is expected to scale to thousands to millions. Moreover, the weakly-coupled port creates spatial dependence of the couplings to the readout resonators and limits the modularity of typical qubit readout design. In this work, we present a design for "directional readout", which avoids using a weakly-coupled port while preserving near-unity directionality. We also include in our design an "interference Purcell filter," a new form of bandstop Purcell suppression compatible with directional readout. We present progress towards an experimental implementation of directional readout designed to have near-unity directionality and high-fidelity readout of a transmon qubit. This design is expected to facilitate more scalable and modular qubit readout and design.