Mitigating Dynamic Crosstalk with Optimal Control

Abstract

The prevalence of quantum crosstalk is an important barrier to scaling frequency-addressable qubit architectures, with dynamic crosstalk being particularly difficult to detect and suppress. This form of crosstalk refers to unintended interactions driven by the gate control fields themselves. Here, we minimize dynamic crosstalk using quantum optimal control based on the perfect entangler spectrum, where spectral peaks signal unwanted entanglement with spectator qubits. Focusing on parametric gates in tunable coupler systems, we derive pulse shapes that eliminate dynamic crosstalk. Remarkably, only minimal pulse modifications are required to mitigate the form of crosstalk that is otherwise most difficult to predict. The ability to suppress dynamic crosstalk via the perfect entangler spectrum establishes a generalizable control principle for eliminating unwanted interactions in quantum hardware.

Figures (3)

• Figure 1: Pulse for the original $\sqrt{i\mathrm{SWAP}}$ protocol from Ref. McKayPRA16 (red) and optimized pulses (blue) for $\omega_3=4.464\,\mathrm{GHz}$ (a,b) and $\omega_3=5.568\,\mathrm{GHz}$ (c,d). The tables list the optimization error $\varepsilon_\mathrm{PE}$ and the average gate error $\varepsilon_\mathrm{avg}$.
• Figure 2: (a) Optimization errors (red crosses) obtained with gradient-free optimization for different values of $\omega_3$ for the $\sqrt{i\mathrm{SWAP}}$ gate; the PE spectrum of the original protocol McKayPRA16 is shown in gray. Vertical blue lines indicate "static" resonances where a transition in the spectator qubit matches a transition in one of the targeted qubits. (b) Optimized PE spectrum (dark green) with the pulse offset $\Theta$ as the only optimization parameter. The inset depicts the shift of the three peaks near $\omega_3=4.436\mathrm{GHz}$ as $\Theta$ is varied. (c) Optimized PE spectrum $\mathcal{J}_\mathrm{PE}^{\omega_\phi\text{-shift}}$ (light red) when adjusting only $\omega_\phi$. The dark red curve shows the minimum between $\mathcal{J}_\mathrm{PE}^{\omega_\phi\text{-shift}}$ and the original spectrum (gray).
• Figure 3: Same as Fig. \ref{['fig:mckay-optimized']} but for the CZ gate with the original protocol found in Ref. GanzhornPRR20.