Exceptional Point and Cross-Relaxation Effect in a Hybrid Quantum System

TL;DR

This paper reports the experimental observation of an exceptional point in a hybrid quantum system of nitrogen centers in diamond, demonstrating the role of cross relaxation and a new method to verify it.

Contribution

The study introduces a novel experimental approach to observe exceptional points in a hybrid quantum system, highlighting the robustness of EP due to cross relaxation effects.

Findings

EP observed in a hybrid quantum system with nitrogen centers

EP robustness against pumping demonstrates cross relaxation role

New method to verify cross relaxation via EP

Abstract

Exceptional points (EPs) are exotic degeneracies of non-Hermitian systems, where the eigenvalues and the corresponding eigenvectors simultaneously coalesce in parameter space, and these degeneracies are sensitive to tiny perturbations on the system. Here we report an experimental observation of the EP in a hybrid quantum system consisting of dense nitrogen (P1) centers in diamond coupled to a coplanar-waveguide resonator. These P1 centers can be divided into three subensembles of spins, and cross relaxation occurs among them. As a new method to demonstrate this EP, we pump a given spin subensemble with a drive field to tune the magnon-photon coupling in a wide range. We observe the EP in the middle spin subensemble coupled to the resonator mode, irrespective of which spin subensemble is actually driven. This robustness of the EP against pumping reveals the key role of the cross relaxation in P1 centers. It offers a novel way to convincingly prove the existence of the cross-relaxation effect via the EP.