The first experimental observation of Dirac exceptional points

Exceptional points (EPs) are unique types of energy-level degeneracies that occur in non-Hermitian systems. Since their existence was first proposed more than a century ago, physicists have only been able to experimentally observe two types of EPs, both of which were found to give rise to exotic phases of matter in various materials, including Dirac and Weyl semimetals.

Building on recent theoretical studies, researchers at the University of Science and Technology of China recently set out to experimentally observe a new class of EPs, known as Dirac EPs. Their paper, published in Physical Review Letters, could open new exciting possibilities for the study of non-Hermitian dynamics and for the development of protocols to reliably control quantum systems.

“Our inspiration stemmed from a prior theoretical study that proposed a type of exceptional point (EP) termed Dirac EPs,” Xing Rong, senior author of the paper, told Phys.org. “We realized that this novel type of EP is distinct from all experimentally observed EPs over the past half-century. Our work aimed to transform this theoretical prediction into experimental reality.”

Dirac EPs are degeneracies theorized to blend two different physical concepts, namely Dirac points observed in Hermitian systems and EPs, which pertain to non-Hermitian systems. The primary objective of the recent work by Rong and his colleagues was to successfully construct and observe these energy-level degeneracies leveraging nitrogen-vacancy defects in diamond, which are atomic-scale quantum systems within a solid-state material.

“We engineered a non-Hermitian Hamiltonian hosting Dirac EPs by introducing a spin-squared operator term (S_z²) into a three-level non-Hermitian system,” explained Rong. “Leveraging our previously developed dilation method, we implemented this Hamiltonian experimentally using nitrogen-vacancy defects in diamond. The existence of Dirac EP is confirmed by observing real eigenvalues near the Dirac EP and eigenstate degeneracy at the EP itself.”

This research group’s successful observation of Dirac EPs, which are brought by Hermitian-like symmetry into non-Hermitian systems, could soon inspire new experimental work in the field. These efforts could help us better understand these new energy-level degeneracies, potentially informing the development of new, reliable approaches to controlling quantum states.

“The Dirac EP exhibits a unique real-valued eigenvalue spectrum in its vicinity, challenging the conventional understanding that typical EPs are inherently accompanied by complex eigenvalues,” said Rong. “This characteristic enables adiabatic evolution in non-Hermitian systems while overcoming dissipative effects, holding significant promise for applications in EP-related topological physics and quantum control protocols.”

Rong and his colleagues hope that their study will spark further advances in the field of non-Hermitian and quantum physics. In the future, their experimental observation of Dirac EPs could provide quantum physicists and engineers with a promising avenue to achieve greater control over various cutting-edge quantum technologies, including quantum sensors and computers.

“Through Dirac EPs, non-adiabatic transitions associated with typical EPs can be avoided, which will make the experimental investigation of complex geometric phases possible,” added Rong. “Thus, the successful observation of Dirac EP in our work paves the way toward the experimental study of geometric phase in non-Hermitian systems.”

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