  The negatively charged tin-vacancy center in diamond (SnV$^-$) is an emerging
platform for building the next generation of long-distance quantum networks.
This is due to the SnV$^-$'s favorable optical and spin properties including
bright emission, insensitivity to electronic noise, and long spin coherence
times at temperatures above 1 Kelvin. Here, we demonstrate measurement of a
single SnV$^-$ electronic spin with a single-shot readout fidelity of $87.4\%$,
which can be further improved to $98.5\%$ by conditioning on multiple readouts.
We show this performance is compatible with rapid microwave spin control,
demonstrating that the trade-off between optical readout and spin control
inherent to group-IV centers in diamond can be overcome for the SnV$^-$.
Finally, we use weak quantum measurement to study measurement induced
dephasing; this illuminates the fundamental interplay between measurement and
decoherence in quantum mechanics, and makes use of the qubit's spin coherence
as a metrological tool. Taken together, these results overcome an important
hurdle in the development of the SnV$^-$ based quantum technologies, and in the
process, develop techniques and understanding broadly applicable to the study
of solid-state quantum emitters.
