Universal stabilization of single-qubit states using a tunable coupler

We theoretically analyze a scheme for fast stabilization of arbitrary qubit states with high fidelities, extending a protocol recently demonstrated experimentally [Lu, Phys. Rev. Lett. 119, 150502 (2017)PRLTAO0031-900710.1103/PhysRevLett.119.150502]. That experiment utilized red and blue sideband transitions in a system composed of a fluxonium qubit, a low-Q LC oscillator, and a coupler enabling us to tune the interaction between them. Under parametric modulations of the coupling strength, the qubit can be steered into any desired pure or mixed single-qubit state. For realistic circuit parameters, we predict that stabilization can be achieved within 100ns. By varying the ratio between the oscillator's damping rate and the effective qubit-oscillator coupling strength, we can switch between underdamped, critically damped, and overdamped stabilization and find optimal working points. We further analyze the effect of thermal fluctuations and show that the stabilization scheme remains robust for realistic temperatures.