Engineered broadband Purcell protection using a shared $\Pi$-filter for multiplexed superconducting qubits

TL;DR

This paper introduces a compact broadband Purcell-protection scheme using a shared $$-filter integrated into the feedline, significantly extending qubit relaxation times in superconducting quantum circuits.

Contribution

The authors design and validate a shared $$-filter that provides broadband Purcell suppression for multiple qubits with minimal hardware overhead.

Findings

Purcell-limited relaxation times exceeding 1 ms over 1.5 GHz bandwidth

Strong suppression of transmission within the target frequency band

Compatible with standard dispersive readout protocols

Abstract

We propose a broadband Purcell-protection scheme based on a single shared filter integrated directly into the feedline, enabling simultaneous protection of multiple qubits in a compact architecture with minimal hardware overhead. The filter consists of two open-ended stubs connected by an in-line transmission line, forming a $\Pi$ geometry, and operates via engineered passive microwave interference that suppresses the real part of the environmental admittance over a wide frequency window. Circuit simulations and finite-element modeling show strong suppression of transmission within the target band (the qubit's frequencies) while preserving the readout and reset modes of the multiplexed architecture. For realistic device parameters, the proposed design yields Purcell-limited relaxation times exceeding $1$ ms over a frequency span of approximately $1.5$ GHz, which can be further extended with straightforward modifications of the design. Our results establish the $\Pi$-filter as a compact and scalable solution for broadband impedance engineering in superconducting quantum circuits, compatible with standard dispersive readout protocols.