Supersensitive quantum sensing jointly enhanced by PT symmetry and entanglement in a spin-boson system

Extension of the Hamiltonian dynamics from Hermitian to non-Hermitian domain has significantly pushed forward physical sciences, and offered promising possibilities for cutting-edge technologies. Recent years have witnessed many achievements in exploring non-Hermitian (NH) physics and related technological applications, but mainly restricted to classical (non-entangled) cases. Several impressive progresses have been achieved for demonstrating semiclassical NH models with a single qubit driven by a classical field, but a realization at the fully quantum-mechanical level is still lacking, although which is indispensible for understanding ubiquitous NH quantum phenomena and for relevant quantum technological applications. We here study both theoretically and experimentally the non-Hermitian physics of a fully quantum light-matter system, composed of a superconducting qubit coupled to a microwave resonator, as well as to an artificial reservoir. We uncover an entanglement singular behavior at the exceptional point, where the concurrence in each eigenvector exhibits a discontinuous derivative. This unique light-matter entanglement transition is confirmed by quantum state tomography. We further demonstrate a sensing scheme, where the signal is encoded in the NH entanglement, which promises an improved sensitivity and a stronger robustness against some local noises compared to population-encoded sensing scenarios.