In the quiet depths of matter, where electrons dance to the laws of quantum physics and magnetic fields are more than invisible forces, a remarkable phenomenon has just been observed—one that flips the script on symmetry and challenges long-held notions in condensed matter physics. At the Center for Quantum Materials Science, nestled within the School of Physics, He Qinglin and his team have made history by reporting the first-ever observation of non-reciprocal Coulomb drag in a Chern insulator. Their discovery, published in Nature Communications, does more than add a new feather in the cap of quantum physics—it opens a promising gateway to exotic topological behaviors and novel quantum technologies.
When Currents Whisper Across the Void
To understand this breakthrough, we must journey into the world of Coulomb drag. Imagine two sheets of material, so close that the electrons in one can sense the presence of electrons in the other—yet so well-insulated that no charge ever tunnels between them. In this ethereal proximity, when a current flows in the “drive” layer, the electric field it generates nudges the electrons in the “drag” layer, creating a measurable voltage without any direct contact. This ghost-like interaction is mediated solely by long-range Coulomb forces, the very forces that make like charges repel.
Traditionally, Coulomb drag behaves like a mirrored dance—what goes forward in one direction is echoed back identically in the other. This reciprocal behavior is a signature of systems that obey time-reversal and spatial symmetry. But what if the mirror cracks? What if this delicate balance is broken, not by chance, but by the very topology of the material itself?
That’s precisely where Chern insulators enter the scene.
Chern Insulators: The Topological Mavericks
Chern insulators are not your everyday materials. These are magnetic topological insulators—materials that, due to their internal magnetization, exhibit a quantum Hall effect without the need for any external magnetic field. Their secret lies in their chiral edge states—unidirectional highways that electrons can traverse with zero resistance. Unlike traditional conductors, these edge states are protected by topology, a kind of mathematical robustness that makes them impervious to impurities or defects.
Named after the Chern number, a topological invariant, these insulators behave more like quantum playgrounds than solid matter. But until now, their role in Coulomb drag experiments had been limited, especially in terms of non-reciprocal behavior. That changed with the meticulous work of He Qinglin’s group.
Cracking the Code of Non-Reciprocity
The term non-reciprocal Coulomb drag refers to a striking asymmetry: the voltage induced in one direction doesn’t mirror the one in the reverse. It’s akin to whispering into a canyon and hearing a different echo depending on which direction you face. In classical systems, such asymmetry is rare and typically requires external magnetic fields or broken symmetries. But in the magnetic topology of a Chern insulator, this asymmetry arises naturally—baked into the quantum landscape.
He Qinglin’s team pulled off this feat using a carefully engineered Chern insulator: a vanadium-doped (Bi,Sb)₂Te₃ film grown via Molecular Beam Epitaxy (MBE). This material was optimized for the quantum anomalous Hall (QAH) effect at relatively high temperatures—a rare and prized trait. To isolate the Coulomb interaction from other interference, the researchers employed a dual Hall bar geometry, separated by a nanoscale vacuum gap. This gap ensured pure Coulomb coupling, free from any tunneling effects that might cloud the measurements.
But the real magic began under extreme conditions: temperatures as low as 20 millikelvin, just above absolute zero, and finely tuned perpendicular magnetic fields. Here, quantum fluctuations reign supreme, and the subtleties of topological behavior come into sharp relief.
Listening to the Quantum Drag
What the team observed was nothing short of remarkable. They detected both longitudinal (Vₓₓ) and transverse (Vₓᵧ) drag voltages. But these were not symmetrical mirror images—they were direction-dependent, revealing a clear break from reciprocity.
In the longitudinal drag, the polarity of the induced voltage remained fixed, no matter how the direction of current or magnetic field was flipped. This suggested a rectification-like behavior, where the system behaves more like a diode than a mirror. Such rectification in Coulomb drag has no precedent in traditional materials.
The transverse drag—the voltage measured perpendicular to the direction of current—proved even more intriguing. It varied with the direction of magnetization, strongly implicating the chiral edge states in this phenomenon. These edge states, unique to Chern insulators, appear to interact across the vacuum gap in a directional manner, effectively creating a one-way street for electron influence.
Beyond the Data: Mechanisms and Implications
To dissect the underlying physics, the researchers dove into the current-voltage (I-V) characteristics, temperature scaling, and noise analysis. They discovered that mesoscopic fluctuations—random but repeatable quantum variations that occur in small systems—dominated the drag signal at ultra-low temperatures. These fluctuations obeyed a T² (temperature squared) scaling law, reinforcing the quantum origin of the behavior.
At higher bias currents, shot noise—a hallmark of discrete electron charge—began to assert itself, introducing non-linearities. These two regimes—fluctuation-dominated at low energy and shot-noise-dominated at high energy—painted a complex but coherent picture of quantum interaction across the vacuum.
The implications of this work go far beyond exotic voltage readings. For one, this establishes Chern insulators as viable platforms for studying non-reciprocal transport phenomena, a field of immense theoretical and practical interest. More importantly, the observed drag offers a non-contact method to probe quantum states, a critical capability for the next generation of topological quantum computers.
Quantum Computing’s New Ally
Quantum computing demands not only stability but readability—the ability to detect and manipulate qubit states without destroying them. In topological quantum computing, qubits may be encoded in elusive particles like Majorana fermions, which require topological protection and precise detection methods. Non-reciprocal Coulomb drag provides just such a tool: a way to measure quantum states without physical contact, reducing noise and interference.
Moreover, because this effect hinges on magnetization and topology, it offers a pathway to chiral qubit interferometry, a scheme where quantum interference patterns reveal the presence or manipulation of topologically protected states. This could prove essential in building robust, fault-tolerant quantum architectures—a major step toward scalable quantum machines.
A Blueprint for Future Electronics
Beyond quantum computing, this research touches on the future of low-power electronics. The ability to guide electrons in one direction without energy loss—through rectification and chiral transport—holds promise for energy-efficient circuit design. It also opens new avenues for spintronic devices, where electron spin, rather than charge, encodes information. Here, magnetization dynamics and edge-state engineering could lead to devices that are faster, cooler, and more versatile than anything silicon can offer.
A New Frontier of Quantum Topology
He Qinglin’s group has not only reported a first-of-its-kind observation—they’ve cracked open a door to an unexplored quantum frontier. Their work demonstrates that non-reciprocity is not merely a quirk of asymmetrical systems but can be a designed feature of topological quantum materials.
This revelation ties together threads from multiple domains: fundamental physics, material science, and quantum engineering. It shows that by harnessing the unique features of Chern insulators—internal magnetization, chiral edge states, and topological invariance—we can build systems that violate symmetry with purpose, achieving behaviors once thought impossible.
Conclusion: Rewriting the Quantum Playbook
Science progresses not just through new answers but by asking bolder questions. The observation of non-reciprocal Coulomb drag in Chern insulators does both. It challenges foundational assumptions about symmetry and interaction, and it lays the groundwork for technologies that can see, compute, and communicate using the quantum rules of topology.
He Qinglin’s discovery is not merely a technical triumph—it is a conceptual leap. It suggests that the future of quantum materials will not be bound by the constraints of reciprocity or contact but shaped by engineered asymmetries, entangled distances, and the elegant twists of topology.
In the strange and beautiful theater of quantum matter, the echoes are no longer equal—they are directional, deliberate, and full of meaning. And we are only beginning to listen.
Reference: Yu Fu et al, Non-reciprocal Coulomb drag between Chern insulators, Nature Communications (2025). DOI: 10.1038/s41467-025-58401-5
Loved this? Help us spread the word and support independent science! Share now.