Parametric architecture with qubit-resonator coupling mediated by a tunable bus for initialization, leakage reduction and readout

The ability to precisely and consistently prepare an initial qubit state, ensuring that the information remains in the computational states and to accurately measure the qubits are key requirements for the operation of a quantum processor. When using superconducting qubits thermal excitations of the initial state, leakage into non-computational states during gate operation along with/as well as unwanted dephasing and induced relaxation via the qubits dispersive coupling to a readout mode are major sources of errors. Here, we investigate an architecture utilizing a tunable coupler mediating interactions between qubits and a rapidly decaying resonator that is used as a dump for unwanted excitations. In this architecture the qubits are protected from resonator-induced noise, when the coupling is turned off. With the parametric couplings switched on we experimentally demonstrate a reset operation that unconditionally prepares the qubit ground state with a fidelity of 99.8(0.02) % and a leakage recovery operation causing a 98.5(0.3) % reduction in leakage. Furthermore, we implement a coupler-driven readout with an assignment fidelity of 88(0.4) % by parametrically tuning the qubit state-dependent dispersive shift of the resonator by two orders of magnitude. Completing this set of elementary operation with tunable-bus mediated qubit-qubit gates reduces the system complexity and may facilitate the implementation of scalable quantum processors.