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

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 δ). In order to achieve precise controllability, we make use of two driving fields (each having two quadratures) at two different tones to simultaneously apply arbitrary combinations of rotations about axes in the X-Y plane to both qubits. 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 (Weak AnHarmonicity With Average Hamiltonian) method by Schutjens et al. [R. Schutjens, F. A. Dagga, D. J. Egger, and F. K. Wilhelm, 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 2π/δ 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.