Long Relaxation Times of a C-shunt Flux Qubit Coupled to a 3D Cavity*

We present measurements of relaxation times of a capacitively shunted flux qubit embedded in a 3D microwave cavity. Qubit energy-relaxation times T₁ were found to be in the range of 60-90 μs, and Hahn-echo coherence time T_2E was about 80 μs. Using the CPMG dynamical decoupling sequence, the T₁-limited coherence time T_2CPMG of 160 μs was reached. Qubit energy relaxation could be attributed to quasiparticle tunneling, while dephasing times were limited by charge and critical-current noises at the optimal flux bias point and by 1/f flux noise elsewhere. With their relaxation times being comparable or exceeding previously reported values for other types of flux qubits, 3D c-shunt flux qubits can be utilized for quantum annealing, quantum magnetometry and spin sensing.