  Using micromagnets to enable electron spin manipulation in silicon qubits has
emerged as a very popular method, enabling single-qubit gate fidelities larger
than 99:9%. However, these micromagnets also apply stray magnetic field
gradients onto the qubits, making the spin states susceptible to electric field
noise and limiting their coherence times. We describe here a magnet design that
minimizes qubit dephasing, while allowing for fast qubit control and
addressability. Specifically, we design and optimize magnet dimensions and
position relative to the quantum dots, minimizing dephasing from magnetic field
gradients. The micromagnet-induced dephasing rates with this design are up to
3-orders of magnitude lower than state-of-the-art implementations, allowing for
long coherence times. This design is robust against fabrication errors, and can
be combined with a wide variety of silicon qubit device geometries, thereby
allowing exploration of coherence limiting factors and novel upscaling
approaches.
