Fast, high fidelity, on demand qubit initialization out of a latched state using baseband pulses

Latched schemes provide high-fidelity readout of quantum dot qubits by exploiting long-lived, nonequilibrium states with easily measured charge occupations. While long-lived latches simplify readout, fast re-initialization is also desirable and cannot be accomplished by relying on the natural decay process of the latched state. Here, we demonstrate fast, high-fidelity reset of a latched quantum dot hybrid qubit that can be deployed on demand, using only baseband pulses. Our protocol circumnavigates the slow decay process of the latched state by pulsing to a region in gate-voltage space in which the latched state resets in a fast, two-step process. The key step is a charge relaxation event, and we find the T₁ for that decay can be 100 ns or shorter, mediated by inelastic tunneling across the interdot barrier. In this way, we achieve reset fidelities as high as 98% with reload times as short as 2μs. Our work provides a steppingstone to robust quantum error correction in quantum dot qubits, where the readout and initialization times should be comparable to gate times.