  Spin-photon interfaces based on solid-state atomic defects have enabled a
variety of key applications in quantum information processing. To maximize the
light-matter coupling strength, defects are often placed inside nanoscale
devices. Efficiently coupling light and microwave radiation into these
structures is an experimental challenge, especially in cryogenic or high vacuum
environments with limited sample access. In this work, we demonstrate a
fiber-based scanning probe that simultaneously couples light into a planar
photonic circuit and delivers high power microwaves for driving electron spin
transitions. The optical portion achieves 46% one-way coupling efficiency,
while the microwave portion supplies an AC magnetic field with strength up to 9
Gauss. The entire probe can be scanned across a large number of devices inside
a $^3$He cryostat without free-space optical access. We demonstrate this
technique with silicon nanophotonic circuits coupled to single Er$^{3+}$ ions.
