Key Findings
A research team led by the Wallenberg Centre for Quantum Technology (WACQT) at Chalmers University of Technology, in collaboration with Université Bourgogne Europe and Aalto University, has demonstrated a quantum control framework called Time-Rescaling Robust Inverse-Optimal (TRIO) pulses. In tests on a superconducting transmon processor, a 90-nanosecond TRIO X-gate reached an Interleaved Randomized Benchmarking (IRB) fidelity of 99.90%. The system maintained fidelities above 99.5% even when subjected to simultaneous qubit detuning shifts of up to 4 MHz and drive-amplitude variations of up to 20%.
Technical Details
The TRIO protocol addresses limitations found in classical Robust Inverse-Optimal (RIO) pulse designs, which often produce square-like waveforms that cause distortion in finite-bandwidth hardware and induce leakage into upper transmon states. TRIO utilizes a time-rescaling function g(τ) to map energy-optimal Bloch-sphere trajectories onto smooth, hyper-Gaussian envelopes (Ω TR (τ)), removing high-frequency spectral components while preserving trajectory geometry.
To further optimize performance, the framework integrates first-order Derivative Removal by Adiabatic Gate (DRAG) corrections to mitigate AC Stark shifts and higher-level transmon leakage. The resulting complex envelope is synthesized via in-phase and quadrature (IQ) microwave phase modulation.
Background and Context
The team expanded the framework to a universal single-qubit gate set, including X π/2 and Y -π/2 Hadamard-type operations. These operations demonstrated average fidelities of 99.62% and 99.69%, respectively. Additionally, numerical simulations suggest that applying TRIO control to systems with higher anharmonicity, such as 1 GHz fluxonium qubits, could result in gate errors below 10 -6.
Significance and Outlook
By expanding the parameter range for high-fidelity execution by an order of magnitude over standard DRAG Gaussian pulses, TRIO pulses provide a method to reduce the overhead associated with periodic re-calibration. This robustness against simultaneous control errors offers a pathway toward more stable operation in large-scale quantum processors.
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