  Exceptional points (EPs) in non-Hermitian systems have recently attracted
wide interests and spawned intriguing prospects for enhanced sensing. However,
EPs have not yet been realized in thermal atomic ensembles, which is one of the
most important platforms for quantum sensing. Here we experimentally observe
EPs in multi-level thermal atomic ensembles, and realize enhanced sensing of
magnetic field for one order of magnitude. We take advantage of the rich energy
levels of atoms and construct effective decays for selected energy levels by
employing laser coupling with the excited state, yielding unbalanced decay
rates for different energy levels, which finally results in the existence of
EPs. Furthermore, we propose the optical polarization rotation measurement
scheme to detect the splitting of the resonance peaks, which makes use of both
the absorption and dispersion properties, and shows advantage with enhanced
splitting compared with the conventional transmission measurement scheme.
Besides, in our system both the effective coupling strength and decay rates are
flexibly adjustable, and thus the position of the EPs are tunable, which
expands the measurement range. Our work not only provides a new controllable
platform for studying EPs and non-Hermitian physics, but also provide new ideas
for the design of EP-enhanced sensors and opens up realistic opportunities for
practical applications in the high-precision sensing of magnetic field and
other physical quantities.
