  It is universally accepted that noise and decoherence affecting the
performance of superconducting quantum circuits are consistent with the
presence of spurious two-level systems (TLS). In recent years bulk defects have
been generally ruled out as the dominant source, and the search has focused on
surfaces and interfaces. Despite a wide range of theoretical models and
experimental efforts, the origin of these surface TLS still remains largely
unknown, making further mitigation of TLS induced decoherence extremely
challenging. Here we use a recently developed on-chip electron spin resonance
(ESR) technique that allows us to detect spins with a very low surface
coverage. We combine this technique with various surface treatments
specifically to reveal the nature of native surface spins on Al$_2$O$_3$ -- the
mainstay of almost all solid state quantum devices. On a large number of
samples we resolve three ESR peaks with the measured total paramagnetic spin
density $n=2.2\times 10^{17}$m$^{-2}$, which matches the density inferred from
the flux noise in SQUIDs. We show that two of these peaks originate from
physisorbed atomic hydrogen which appears on the surface as a by-product of
water dissociation. We suggest that the third peak is due to molecular oxygen
on the Al$_2$O$_3$ surface captured at strong Lewis base defect sites,
producing charged O$_2^-$. These results provide important information towards
the origin of charge and flux noise in quantum circuits. Our findings open up a
whole new approach to identification and controlled reduction of paramagnetic
sources of noise in solid state quantum devices.
