  Quantum computing based on spins in the solid state allows for densely-packed
arrays of quantum bits. While high-fidelity operation of single qubits has been
demonstrated with individual control pulses, the operation of large-scale
quantum processors requires a shift in paradigm towards global control
solutions. Here we report the experimental implementation of a new type of
qubit protocol - the SMART (Sinusoidally Modulated, Always Rotating and
Tailored) protocol. As with a dressed qubit, we resonantly drive a two-level
system with a continuous microwave field, but here we add a tailored modulation
to the dressing field to achieve increased robustness to detuning noise and
microwave amplitude fluctuations. We implement this new protocol to control a
single spin confined in a silicon quantum dot and confirm the optimal
modulation conditions predicted from theory. Universal control of a single
qubit is demonstrated using modulated Stark shift control via the local gate
electrodes. We measure an extended coherence time of $2$ ms and an average
Clifford gate fidelity $>99$ $\%$ despite the relatively long qubit gate times
($>15$ $\unicode[serif]{x03BC}$s, $20$ times longer than a conventional square
pulse gate), constituting a significant improvement over a conventional spin
qubit and a dressed qubit. This work shows that future scalable spin qubit
arrays could be operated using global microwave control and local gate
addressability, while maintaining robustness to relevant experimental
inhomogeneities.
