  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 DQ. JQF consists of
a qubit strongly coupled to the control line to DQ, and its working principle
is a subradiance effect characteristic to waveguide quantum electrodynamics
setups. JQF is a passive circuit element and is therefore suitable for
integration in a scalable superconducting qubit system.

Quantum Physics