# Enhancing the dipolar coupling of a $S$-$T_0$ qubit with a transverse sweet spot

## TL;DR

This paper introduces a transverse sweet spot in singlet-triplet qubits that enhances dipolar coupling, enables direct state transitions, and offers improved noise protection, facilitating high-fidelity two-qubit gates.

## Contribution

The work demonstrates a new transverse sweet spot that allows strong, tunable coupling and noise suppression in singlet-triplet qubits, expanding their operational capabilities.

## Key findings

- Achieves >99% fidelity for two-qubit gates with realistic noise models.
- Enables direct transitions between qubit states at the transverse sweet spot.
- Provides a method for coupling qubits to microwave cavities with noise resilience.

## Abstract

A fundamental design challenge for quantum dot spin qubits is to extend the strength and range of qubit interactions while suppressing their coupling to the environment, since both effects have electrical origins. Key tools include the ability to retune the qubits, to take advantage of physical resources in different operating regimes, and to access optimal working points, or "sweet spots," where dephasing is minimized. Here, we explore an important, new resource for singlet-triplet qubits: a transverse sweet spot (TSS) that enables (i) direct transitions between qubit states, (ii) a strong, charge-like qubit coupling, and (iii) leading-order protection from electrical fluctuations. Of particular interest is the possibility of transitioning between the TSS and symmetric operating points while remaining continuously protected. This arrangement is ideal for coupling qubits to a microwave cavity, because it combines maximal tunability of the coupling strength with leading-order noise suppression. We perform simulations with $1/f$-type electrical noise, demonstrating that two-qubit gates mediated by a resonator can achieve fidelities $>99$\% under realistic conditions. These results greatly expand the toolbox for singlet-triplet qubits.