  Optical supercavity modes (superstates), i.e., hybrid modes emerging from the
strong coupling of two nonorthogonal modes of an open cavity, can support
ultranarrow lines in scattering spectra associated with quasi bound states in
the continuum (quasi-BIC). These modes are of great interest for sensing
applications as they enable compact systems with unprecedented sensitivity.
However, these quasi-BIC sensors obey the shot-noise limit, which may be
overcome only in quantum sensors. Here, we unveil that a three-level quantum
system (e.g., atom, quantum dot, superconducting qubit) can be tailored to
support the quantum analog of an embedded superstate with an unboundedly narrow
emission line in the strong coupling regime. Remarkably, we demonstrate that
the coupling of such a system with a cavity (e.g., plasmonic or dielectric
nanoparticle, microcavity, microwave resonator) enables sensing properties with
significantly reduced noise. Our results can be applied to a plethora of
quantum platforms from microwave superconductors to cold atoms and quantum
dots, opening interesting opportunities for quantum sensing and computing.
