a, b Concept of the JQF. The JQF reflects single photons emitted from the qubit, suppressing the qubit radiative decay. In contrast, when a large-amplitude control pulse is applied to the qubit, the JQF saturates and transmits the pulse, enabling fast qubit control. c, d False-colored optical image and equivalent circuit of the fabricated superconducting circuit. A fixed-frequency transmon qubit acting as the data qubit (green) is connected to a control coplanar-waveguide with an ancillary tunable-frequency transmon qubit with a SQUID acting as the JQF (blue). The data qubit is coupled to a resonator (red) for fast dispersive readout via a readout line with a Purcell filter (purple). Air-bridges are fabricated on the waveguides to suppress spurious modes. The Josephson junctions are indicated by magenta crosses.