Scientists Achieve First Experimental Observation of Dirac Exceptional Points

For over a century, exceptional points (EPs) have intrigued physicists, offering glimpses into the peculiar behaviors of non-Hermitian systems—systems where energy is not conserved and imaginary components enter the normally clean world of physics equations. These strange points represent places where energy levels not only cross but become fundamentally inseparable, opening doors to exotic behaviors in materials like Dirac and Weyl semimetals. Yet despite decades of theory, experimentalists have only managed to observe two types of EPs—until now.

In a groundbreaking study recently published in Physical Review Letters, researchers at the University of Science and Technology of China, led by Xing Rong, achieved what had long eluded the scientific community: the first experimental observation of a new class of EPs known as Dirac Exceptional Points (Dirac EPs). Their success not only marks a historic moment in quantum physics but also lays down fresh tracks for future technologies powered by precise quantum control.

Exceptional Points: The Quantum Crossroads

To appreciate the magnitude of this achievement, it helps to first understand the terrain. In traditional quantum mechanics, systems are described by Hermitian Hamiltonians—mathematical operators that ensure real, stable energy values. Hermitian systems behave predictably, conserving probability and satisfying the tidy expectations of physicists.

Non-Hermitian systems, by contrast, are messy. They permit gain and loss, leakage and absorption. Despite their unruly nature, these systems are rich with new phenomena. Exceptional points arise within them when two or more energy levels not only meet but also merge their associated quantum states, causing the system’s behavior to become drastically more sensitive to external perturbations.

Historically, observed EPs have fueled discoveries in topological physics, optics, and quantum sensing. Yet all known EPs followed a predictable rule: their energy values became complex—involving both real and imaginary parts, signaling energy gain or loss.

What makes Dirac EPs astonishingly different is that they defy this rule.

Blending Two Worlds: Hermitian Dirac Points Meet Non-Hermitian EPs

Dirac EPs are a strange hybrid, blending features from two separate realms of physics. On one hand, they inherit traits from Dirac points—special points found in Hermitian systems like graphene, where energy bands touch with remarkable symmetry and no energy loss. On the other hand, they spring from the chaotic, dissipative world of non-Hermitian EPs.

What Rong and his team proposed—and then demonstrated—is that by carefully engineering the system, it’s possible to produce Dirac-like behavior (real energy values, symmetry) within a fundamentally non-Hermitian (lossy) framework. The result is a new kind of exceptional point that behaves with a real-valued energy spectrum in its vicinity, unlike anything physicists had seen before.

Engineering the Impossible: How They Did It

Turning theory into experimental reality required a bold leap of ingenuity. The researchers looked to a platform that has become the darling of quantum science: nitrogen-vacancy (NV) centers in diamond.

An NV center is essentially a tiny defect in diamond where a nitrogen atom replaces a carbon atom adjacent to a missing carbon, creating a system that behaves like a controllable, atomic-scale quantum bit (qubit). These defects offer rare access to manipulate and measure quantum states at room temperature, making them an ideal testbed for ambitious experiments.

Rong’s team began by designing a specialized non-Hermitian Hamiltonian—the mathematical heartbeat of their system. They introduced a new term, a spin-squared operator (Sz²), into a three-level NV system. This Sz² term altered the energy landscape, effectively molding the system into one capable of hosting a Dirac EP.

To faithfully implement this Hamiltonian in the lab, they used an ingenious trick known as dilation, a method the team had previously developed. Dilation enables non-Hermitian dynamics to be realized within a larger Hermitian space by embedding the messy behaviors into a broader, controlled environment.

The final step was direct observation: measuring eigenvalues and eigenstates around the predicted Dirac EP. And indeed, they found what they were looking for—a real-valued energy spectrum close to the Dirac EP, and degeneracy (overlapping of states) exactly at the EP, confirming that they had successfully entered this new, uncharted territory.

Breaking Free from Dissipation: Why Dirac EPs Matter

The implications of Dirac EPs stretch far beyond pure scientific curiosity.

Traditional exceptional points are tantalizing but fragile. Their complex energy values mean that systems suffer from unavoidable dissipation—energy loss that degrades quantum information and complicates experimental control. In contrast, Dirac EPs sidestep this major obstacle.

By maintaining real energy spectra near the EP, Dirac EPs allow for adiabatic evolution—the slow, steady change of a quantum system without sudden jumps or losses, a foundational requirement for reliable quantum manipulation. This feature opens a new highway for designing topologically protected quantum systems and crafting robust quantum control protocols.

Imagine quantum computers that could transition states more smoothly, quantum sensors that could operate with far greater sensitivity, and quantum materials that could host exotic new phases of matter—all thanks to Dirac EPs.

As Xing Rong put it, “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.”

In other words, the rules of the quantum game just changed.

Future Horizons: Quantum Phases, Geometric Effects, and Beyond

The success of Rong and his team does not mark an endpoint—it marks a beginning.

One of the most exciting opportunities unlocked by Dirac EPs is the potential to study geometric phases in non-Hermitian systems. In quantum mechanics, geometric phases (also known as Berry phases) represent a kind of “memory” that quantum states accumulate as they evolve around special points like EPs. However, traditional EPs, marred by dissipation, have made these effects difficult to observe experimentally.

Now, with the clean, real spectra near Dirac EPs, researchers can probe geometric phases in ways that were once purely theoretical. This could lead to new classes of quantum devices that exploit these phases for ultra-precise control.

Moreover, because Dirac EPs allow non-adiabatic transitions—sudden shifts in quantum states—to be avoided, they make the experimental landscape more stable and predictable. This stability is a key ingredient for pushing quantum technology from the lab into real-world applications.

As Rong emphasized, “Through Dirac EPs, non-adiabatic transitions associated with typical EPs can be avoided, which will make the experimental investigation of complex geometric phases possible.”

A New Chapter in Quantum Physics

What makes this moment truly historic is that it bridges two once-separate worlds: the elegant symmetry of Hermitian quantum mechanics and the chaotic openness of non-Hermitian physics. The emergence of Dirac EPs suggests that the barriers between Hermitian and non-Hermitian systems are more permeable than we thought.

It invites us to rethink how quantum states can be manipulated, protected, and evolved—not in spite of complexity, but by embracing it in a newly ordered way.

The discovery also serves as a testament to the creativity and persistence of the human spirit in science. Turning a theoretical curiosity into experimental fact requires not only technical brilliance but also vision—a willingness to challenge what is “known” and reach into the unknown.

As new experimental groups build upon these results, the next few years could see an explosion of discoveries: new quantum materials, novel phases of matter, more resilient quantum devices, and perhaps even revolutionary advances in quantum computing.

Dirac EPs are not just another line in the textbook. They are a new chapter in our ongoing quest to understand and harness the weirdness of the quantum world.

And as history shows, when physicists open new doors into the fabric of reality, the future is never the same again.

Reference: Yang Wu et al, Experimental Observation of Dirac Exceptional Points, Physical Review Letters (2025). DOI: 10.1103/PhysRevLett.134.153601. On arXiv: DOI: 10.48550/arxiv.2503.08436