Protection of a Qubit via Subradiance: A Josephson Quantum Filter

The coupling between a superconducting qubit and a control line inevitably results in radiative decay of the qubit into the line. We propose a Josephson quantum filter (JQF), which protects the data qubit (DQ) from radiative decay through the control line without reducing the gate speed on the DQ. The JQF consists of a qubit strongly coupled to the control line of the DQ and its working principle is a subradiance effect that is characteristic of waveguide quantum electrodynamics setups. The JQF is a passive circuit element and is therefore suitable for integration in a scalable superconducting qubit system.