Fast unconditional reset and leakage reduction of a tunable superconducting qubit via an engineered dissipative bath

Rapid and accurate initialization of qubits, known as reset, is a crucial building block for various tasks in quantum information processing, such as quantum error correction and estimation of the statistics of noisy quantum devices with many qubits. We demonstrate unconditional reset of a frequency-tunable transmon qubit that simultaneously resets multiple excited states using a metamaterial waveguide engineered to provide a cold bath over a wide spectral range, while providing strong protection against Purcell decay of the qubit. We report a reset error below 0.13% (0.16%) when prepared in the first (second) excited state of the transmon within 88 ns. In addition, through the sharp roll-off in the density of states of the metamaterial waveguide, we implement a leakage-reduction unit that selectively resets the transmon's second excited state to 0.285(3)% residual population within 44 ns while acting trivially in the computational subspace as an identity operation that preserves encoded information with an infidelity of 0.72(1)%.