Readout of a Transmon Qubit using an All-Pass Readout Resonator with Interference Purcell Suppression*

Impedance mismatch in the readout bus is a leading cause of high variance in measurement rate κ in superconducting quantum processors. Moreover, the addition of bulky and high-magnetic field circulators and isolators is often needed for impedance matching. In this work, we demonstrate transmission-based readout of a transmon qubit using a directional readout resonator. Whereas a typical readout resonator would have a sharp dip in |S21| on resonance, our directional resonator demonstrates a dip of less than 1dB on resonance, thus closely preserving the 50-ohm readout bus. This both maximizes measurement efficiency and avoids needing a weakly-coupled port, a major source of impedance mismatch in many standard qubit readout schemes. To enable fast readout and reset, we propose a novel interference Purcell filter compatible with directional readout and demonstrate Purcell suppression by 2 orders of magnitude over a bandwidth of more than 600 MHz. This architecture is expected to facilitate more scalable and modular design of quantum processors.