Qubit dephasing by spectrally diffusing quantum two-level systems

We investigate the pure dephasing of a Josephson qubit due to the spectral diffusion of two-level systems that are close to resonance with the qubit. We identify the parameter regime in which this pure dephasing rate can be of the order of the energy relaxation rate and, thus, the relation $T_2=2T_1$ is violated for the qubit. This regime is reached if the dynamics of the thermal TLSs responsible for the spectral diffusion is sufficiently slower than the energy relaxation of the qubit. By adding periodic bias modulating the qubit frequency or TLS excitation energies we show that this pure dephasing mechanism can be mitigated, allowing enhancement of superconducting qubits' coherence time. Mitigating pure dephasing, even if it is subdominant, is of special significance in view of recent suggestions for converting the dominant relaxation process ($T_1$) into erasure errors, leaving pure dephasing as the bottleneck for efficient quantum computation.