Exceptional entanglement transition: non-Hermiticity meeting non-classicality

The non-Hermitian (NH) extension of quantum-mechanical Hamiltonians stands as a transformative breakthrough in physics. Over the last two decades, a plethora of intriguing NH phenomena have come to light, observable in both quantum and classical systems. This convergence prompts a pivotal inquiry: what NH signature distinctly separates quantum mechanics from its classical counterpart? Addressing this query is pivotal for authentic NH quantum effects, a realm yet unexplored. This study bridges the gap by unveiling unique entanglement phenomena, including exceptional entanglement transition, manifesting at the EP of an NH interacting quantum system. We present concrete evidence of these purely quantum NH effects using a naturally-dissipative light-matter scheme, meticulously engineered within a superconducting circuit QED platform. Our findings establish the groundwork for the exploration of genuinely quantum-mechanical NH physics, marked by EP-enabled entanglement behaviors. This breakthrough expands our comprehension of quantum mechanics and promises innovative applications in various fields of science and technology.