High fidelity gates in a transmon using bath engineering for passive leakage reset

Abstract

Leakage to non-computational states is one of the limiting factors which prevents fault-tolerant quantum computing. This is especially true for superconducting circuit-based architectures, where the qubits are often weakly non-linear oscillators and unwanted population transfer to higher levels cannot be ignored on energetic grounds alone. Although several proposals have been introduced to mitigate the effect of leakage, here we theoretically explore and experimentally demonstrate a passive filtering approach to leakage in a standard transmon. By coupling the qubit to a structured environment, we engineer the decay rate of the f-state to be large and e-state to be small; any erroneous f-state population (or higher leakage population) is quickly brought back to the qubit subspace, whereas the e-state remains long-lived. Considering only energetically relevant levels, we simulate the full coupled transmon-filter system and demonstrate that it is possible to obtain both a large f-state decay rate and a high averaged gate fidelity. Our work demonstrates the possibility of passive leakage reduction as a useful tool for fault-tolerant quantum computation. *This work was undertaken thanks in part to funding from NSERC, Canada First Research Excellence Fund, and the Ministère de l'Économie et de l'Innovation du Québec.