Simultaneous gates in frequency-crowded multilevel systems using fast, robust, analytic control shapes

TL;DR

This paper introduces a set of analytic pulse shapes for simultaneous, high-fidelity single-qubit rotations in frequency-crowded multilevel quantum systems, outperforming adiabatic methods and approaching a quantum speed limit.

Contribution

It develops a new analytic pulse ansatz for precise, simultaneous control of multilevel qubits, including a generalization of the WahWah method for faster gate times.

Findings

Achieves gate errors below 10^{-4} with smooth, composite pulses.

Demonstrates robustness against system variations and filtering effects.

Identifies a quantum speed limit slightly below 2π/δ for gate time.

Abstract

We present a few-parameter ansatz for pulses to implement a broad set of simultaneous single-qubit rotations in frequency-crowded multilevel systems. Specifically, we consider a system of two qutrits whose working and leakage transitions suffer from spectral crowding (detuned by $\delta$). In order to achieve precise controllability, we make use of two driving fields (each having two quadratures) at two different tones to implement arbitrary simultaneous rotations. Expanding the waveforms in terms of Hanning windows, we show how analytic pulses containing smooth and composite-pulse features can easily achieve gate errors less than $10^{-4}$ and considerably outperform known adiabatic techniques. Moreover, we find a generalization of the WahWah method by Schutjens et al. [Phys. Rev. A 88, 052330 (2013)] that allows precise separate single-qubit rotations for all gate times beyond a quantum speed limit. We find in all cases a quantum speed limit slightly below $2\pi/\delta$ for the gate time and show that our pulses are robust against variations in system parameters and filtering due to transfer functions, making them suitable for experimental implementations.