Forschungszentrum Jülich Demonstrates 6.8-Nanosecond Single-Qubit Gates with > 99.98% Fidelity via Multi-Photon Leakage Suppression

Physicists at Forschungszentrum Jülich, RWTH Aachen University and the University of Cologne have demonstrated single-qubit gates lasting 6.8 ns with average fidelities of 99.981% for X gates and 99.989% for X/2 gates on a superconducting transmon processor. The work, published in Nature Communications as “Ultrafast single-qubit gates through multi-photon transition removal”, introduces the “R2D” (Double Recursive DRAG) pulse-shaping method, which uses second-derivative filtering to notch out two-photon spectral components and suppresses leakage to |2⟩ and |3⟩ below 0.002%, a more than 20-fold reduction versus standard DRAG. Leakage rate is reported as < 2.0 × 10⁻⁵ per gate, measured with an Amplified Leakage Error (ALE) protocol that detects non-computational transitions down to 10⁻⁶. The gates operate at a normalized drive strength of Ω/|Δ| = 0.33, with coherence times T₁ = 40 µs and T₂ Echo = 56 µs. Scaling to multi-qubit lattices will require addressing spectator-qubit dephasing, microwave cross-talk and frequency crowding. ¹