Generalized fast quasi-adiabatic population transfer for improved qubit readout, shuttling, and noise mitigation.

Population-transfer schemes are commonly used to convert information robustly stored in some quantum system for manipulation and memory into more macroscopic degrees of freedom for measurement. These schemes may include, e.g., spin-to-charge conversion for spins in quantum dots, detuning of charge qubits between a noise-insensitive operating point and a measurement point, and parity-to-charge conversion schemes for qubits based on Majorana zero modes. A common strategy is to use a slow (adiabatic) conversion. However, in an adiabatic scheme, the adiabaticity conditions on the one hand, and accumulation of errors through dephasing, leakage, and energy relaxation processes on the other hand, limit the fidelity that can be achieved. We give explicit fast quasi-adiabatic (fast-QUAD) conversion strategies (pulse shapes) beyond the adiabatic approximation that allow for optimal state conversion. We account for a general source of classical Gaussian noise, and give analytic descriptions for the noise-induced transfer errors, allowing for noise-error mitigation strategies. We confirm the analytical results for generic models with numerical simulations. Further, we show that for a Pauli spin-blockade readout a fast-QUAD pulse should lead to a single-shot fidelity of better than 99.9 %, an improvement of approximately two orders of magnitude over what can be achieved with a t-linear detuning ramp under comparable conditions.