  Electron spins are amongst the most coherent solid-state systems known,
however, to be used in devices for quantum sensing and information processing
applications, they must be typically placed near interfaces. Understanding and
mitigating the impacts of such interfaces on the coherence and spectral
properties of electron spins is critical to realize such applications, but is
also challenging: inferring such data from single-spin studies requires many
measurements to obtain meaningful results, while ensemble measurements
typically give averaged results that hide critical information. Here, we report
a comprehensive study of the coherence of near-surface bismuth donor spins in
28-silicon at millikelvin temperatures. In particular, we use strain-induced
frequency shifts caused by a metallic electrode to make spatial maps of spin
coherence as a function of depth and position relative to the electrode. By
measuring magnetic-field-insensitive clock transitions we separate magnetic
noise caused by surface spins from charge noise. Our results include
quantitative models of the strain-split spin resonance spectra and extraction
of paramagnetic impurity concentrations at the silicon surface. The interplay
of these decoherence mechanisms for such near-surface electron spins is
critical for their application in quantum technologies, while the combination
of the strain splitting and clock transition extends the coherence lifetimes by
up to two orders of magnitude, reaching up to 300 ms at a mean depth of only
100nm. The technique we introduce here to spatially map coherence in
near-surface ensembles is directly applicable to other spin systems of active
interest, such as defects in diamond, silicon carbide, and rare earth ions in
optical crystals.
