Quantum Embedded Superstates

Optical supercavity modes (superstates), i.e., hybrid modes emerging from the strong coupling of two modes of an open cavity, can support ultranarrow lines in their 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, classical quasi‐BIC sensors are fundamentally limited by the shot‐noise limit, which may be overcome in quantum sensors. Here, it is shown that a three‐level quantum system (e.g., atom, quantum dot, superconducting qubit) can be tailored to support the quantum analog of superstates with an unboundedly narrow emission line. Remarkably, it is demonstrated that the coupling of such a system with a cavity (e.g., plasmonic or dielectric nanoparticle, microcavity, microwave resonator) enables sensing properties with excellent statistical features. The results can be applied to a plethora of quantum platforms, from superconducting circuits to cold atoms and quantum dots, opening exciting opportunities for quantum sensing and computing. The work reports a quantum analogue of an embedded superstate with an infinitely narrow emission line arising in a three‐level quantum system with quantum interference between multiple atomic transition pathways. The coupling of such a three‐level V‐type quantum system with a resonator provides narrow emission spectra, giving rise to superb sensing properties with excellent statistical characteristics.