Purcell Filtering for Qubit Readout Using Intrinsic Geometric Multi-modes of the Transmon

Over the past two decades, superconducting circuits have become pivotal in quantum computing, achieving quantum logic gates with minimal errors. As we address the challenges of scaling, the role of microwave engineering, particularly in enhancing the lifetime and coherence of transmons at a reduced system complexity, becomes crucial. Here, we report a novel Purcel filtering technique in cQED by ustilising the instristic multiple geometric modes of the quantum circuit. The precise control of the coupling strenght and the relative phase between the transmon and these higher order geometric modes can result in constructive or destructive interference effects at prescribed frequencies, e.g. the qubit frequency. This can impact the effective coupling strength and energy exchange dynamics between the transmon, the readout resonator, and the external driving ports, and it can be utilised to engineer Purcel filtering without introducing additional circuitry to the quantum processor such as a quarter-wavelenght bandstop filter or an always active multiple drives.