Tunable Superconducting Flux Qubits with Long Coherence Times

In this work, we study a series of tunable flux qubits inductively coupled to a coplanar waveguide resonator fabricated on a sapphire substrate. Each qubit includes an asymmetric superconducting quantum interference device, which is controlled by the application of an external magnetic field and acts as a tunable Josephson junction. The tunability of the qubits is typically ±3.5GHz around their central gap frequency. The measured relaxation times are limited by dielectric losses in the substrate and can attain T1∼8μs. The echo dephasing times are limited by flux noise even at optimal points and reach T2E∼4μs, almost an order of magnitude longer than state of the art.