Multi-terminal quantum Hall devices with non-Hermitian topology are emerging as highly sensitive cryogenic sensors whose quantized response remains robust against imperfections. In work carried out with RIKEN-CEMS and IFW Dresden, researchers reported record sensitivity for quantum sensors built from magnetic topological insulators, with operation at higher temperatures than devices relying only on Chern-insulator topology.

The devices are based on magnetic topological insulators, also called Chern insulators, which support dissipationless transport through ballistic, spin-polarized one-dimensional edge states. These chiral states arise from the combination of magnetism and non-trivial band topology, producing a quantum Hall effect without an external magnetic field. Their chirality can be reversed by switching the magnetization in small magnetic fields, a feature relevant both for quantum metrology, such as resistance standards, and for topological electronics, including Chern networks.

The new generation of devices adds an intrinsic non-Hermitian topology tied to the device architecture itself. In these quantum sensors, chiral edge states are interconnected in a quantum Hall system, producing a non-Hermitian skin effect: the wave function becomes exponentially localized at one end of an open quantum chain. The team showed record performance for non-Hermitian devices made from magnetic topological insulators in the quantum anomalous Hall regime, without external magnetic fields or electrical gating, and under non-metrology-grade, industry-relevant conditions.

The sensors operate at liquid-helium temperature, where the relevant invariant remains well quantized and the localization is unusually strong. Unlike the Chern invariant of conventional quantum anomalous Hall devices, the non-Hermitian invariant associated with 1D quantum chains is more temperature robust and still shows good quantization at liquid-helium temperature. The ability to switch spin chirality with small magnetic fields also gives these devices a new function as highly sensitive cryogenic magnetic sensors.

The research was conducted within the CNRS international research project CITRON. It involved the Topological spintronics team, with collaboration from RIKEN-CEMS and IFW Dresden. Funding came from the H2020 FET Proactive project TOCHA (No. 824140) and CNRS IRP CITRON. Further reading is the paper “Non-Hermitian topological devices with Chern insulators” by K. Ochkan, M. Wissmann, L. Veyrat, L. Tai, M. Kawamura, Y. Tokura, V. Könye, B. Büchner, J. van den Brink, I.C. Fulga, J. Dufouleur, and R. Giraud, published in Newton 2, 100630 (2026), with open access at hal-05528954. Contact: Romain Giraud.

Quantum spintronic devices with non-Hermitian topology

Multi-terminal quantum Hall devices with non-Hermitian topology are highly-sensitive cryogenics sensors with quantized properties robust against imperfections. In collaboration with RIKEN-CEMS & IFW Dresden, we have shown the record sensitivity of such quantum sensors built from magnetic topological insulators, operating at higher temperatures than devices solely based on the materials’ topology of Chern insulators.

Multi-terminal quantum sensor built from the interconnexion of chiral spin-polarized edge states (blue arrows) propagating in a Cr-based (Bi,Sb)2Te3 heterostructure (light grey), along a disk set into the quantum anomalous Hall regime by the magnetization M, fixing the device chirality.

Magnetic topological insulators are quantum materials, also known as Chern insulators, with dissipationless transport properties related to ballistic spin-polarized 1D edge states. These chiral electronic states emerge from the interplay between magnetism and non-trivial band topology, realizing the quantum Hall effect in zero magnetic field, and chirality can be switched at will by the magnetization reversal in small magnetic fields. The quantized response of Chern insulators is of interest for quantum metrology (resistance standard) or topological electronics (Chern networks). Recently, a new generation of topological devices was discovered, with some intrinsic non-Hermitian topology specific to the device itself.

These quantum sensors are based on the interconnexion of chiral 1D edge states in quantum Hall systems, with a topological response (the non-Hermitian skin effect) related to the exponential localization of the wave function at the end of an open quantum chain. In this work, we have revealed the record performance of non-Hermitian devices built from magnetic topological insulators in the quantum anomalous Hall regime, without requiring external magnetic fields or electrical gating, in a non-metrology grade setup – that is, in industry-relevant conditions.

These quantum sensors can operate at liquid-Helium temperature, with a good quantization of their invariant and show some unprecedented degree of localization. Different from the Chern invariant of quantum anomalous Hall devices, the non-Hermitian invariant specific to 1D quantum chains is found more robust in temperature, with a rather good quantization at liquid-helium temperature. In addition, the spin-chirality switching in small magnetic fields gives a novel functionality to these topological devices, as highly-sensitive cryogenic magnetic sensors.

This research was conducted within the framework of the CNRS international research project “CITRON”.

Team: Topological spintronics

Collaboration: RIKEN-CEMS, IFW Dresden

Funding: H2020 FET Proactive project TOCHA (No. 824140); CNRS IRP CITRON

Further reading: Non-Hermitian topological devices with Chern insulators, K. Ochkan, M. Wissmann, L. Veyrat, L. Tai, M. Kawamura, Y. Tokura, V. Könye, B. Büchner, J. van den Brink, I.C. Fulga, J. Dufouleur, R. Giraud, Newton 2, 100630 (2026).

Open access: hal-05528954

Contact: Romain Giraud

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