Breaking the trade-off between gate and relaxation times of a superconducting qubit with a Josephson quantum filter: Experiment*

The rapid development in designs and fabrication techniques of superconducting qubits is making coherence times of qubits longer and longer. In the near future, however, the radiation decay of the qubit (data qubit) into the control line will be a fundamental limitation, imposing a trade-off between the gate and relaxation times. Here, we successfully break the trade-off by strongly coupling another superconducting qubit, or a Josephson quantum filter (JQF), along the control line connected to the data qubit. The JQF prevents the data qubit from emitting a microwave photon and thus suppresses the relaxation, while transmitting faithfully large-amplitude control microwave pulses through the saturation of the quantum filter. In this talk, we will present the circuit design and demonstrate the effect of JQF on the qubit coherence.