Fast reset process of a superconducting qubit with autonomous feedback system

Quantum algorithm requires a high-fidelity state reservoir, which can be achieved by a reset process of qubits. A natural relaxation of a qubit has been used without utilizing an artificial reset process, which demands unwanted extension of processing time as the relaxation time(T1) is longer. We demonstrate a new reset process in superconducting quantum circuits. The process uses qubit state-dependent cavity transmission and frequency conversion by a Josephson mixer. One cavity mode is designed to have a high transmission for a cavity-dressed state frequency at undesired qubit states. A transmitted pulse with the dressed frequency derives the undesired qubit states to Rabi-oscillate after the frequency conversion. The autonomous feedback process stops when a qubit has no population in undesired states. The reset pulse length depends on Rabi-oscillation frequency, not on the relaxation time of a qubit, so that a sub-microsecond reset can be available in our system although a qubit has a much longer relaxation time.