Quantum technologies promise revolutionary advances in computing, sensing, and information processing. However, controlling individual quantum bits (qubits) at the atomic scale remains a major challenge because conventional approaches rely on magnetic fields, which are difficult to confine to a single molecule.
A research team at the Center for Quantum Nanoscience (QNS) led by Director Andreas HEINRICH within the Institute for Basic Science (IBS), together with collaborators at Karlsruhe Institute of Technology (KIT), has now demonstrated that the quantum state of an individual magnetic molecule can instead be controlled electrically using a newly identified exchange-mediated mechanism. The study provides a new strategy for electrically controlling molecular quantum systems and could help pave the way toward more scalable quantum technologies.
Magnetic molecules are considered attractive building blocks for future quantum technologies because they are only a few nanometers in size, can self-assemble into ordered structures, and can be chemically tailored to possess desired quantum properties. These characteristics make them promising candidates for molecular quantum computing, quantum sensing, and spintronic applications. Yet one major obstacle has remained: researchers have struggled to manipulate the spin state of individual molecules with sufficient precision while avoiding unwanted effects on neighboring molecules. Although previous studies showed that electric fields could tune molecular spins, the effect was generally too weak for practical quantum operations.
To overcome this limitation, the KIT team led by QNS alumnus Philip WILLKE investigated individual iron phthalocyanine (FePc) molecules and related molecular spin complexes using electron spin resonance combined with scanning tunnelling microscopy (ESR-STM). By precisely varying the applied voltage, they discovered that electrical control becomes dramatically stronger through an exchange-mediated interaction between the molecule and the magnetic STM tip.
Instead of changing linearly with voltage, the molecular spin exhibited a pronounced nonlinear response as the applied voltage approached one of the molecule's electronic energy levels. The newly identified mechanism produced resonance-frequency shifts approaching 30% — an order of magnitude larger than most previously reported electrical tuning effects in molecular spin systems. The newly identified mechanism also provides experimental validation of a theoretical framework previously proposed by researchers at QNS, demonstrating that voltage-controlled exchange interactions can generate a highly localized effective magnetic field for spin control.
The researchers further demonstrated that this new mechanism could do more than simply shift spin energy levels — it also enabled coherent quantum control. Through Rabi oscillation measurements, they showed that individual molecular spins could be selectively controlled using electrical voltages rather than changes in magnetic fields. They also demonstrated electrical control of coupled molecular spin systems, allowing one spin to be tuned without disturbing a neighboring spin.
These results represent an important step toward performing quantum operations within molecular-scale devices.
Unlike previously proposed mechanisms, the exchange-mediated effect does not rely on physically deforming the molecule. Instead, it arises from interactions between the molecular spin and a nearby magnetic electrode, making the effect highly localized and potentially applicable to many other quantum systems, including quantum dots and solid-state spin defects.
Because electrical signals are much easier to localize and integrate into electronic devices than magnetic fields, the approach offers a practical strategy for building future molecular quantum technologies.
“Electrical control is one of the key requirements for building scalable quantum technologies,” said Christoph WOLF, corresponding author of the study. “Our work demonstrates a fundamentally new mechanism that allows individual molecular spins to be manipulated with unprecedented efficiency and nanoscale precision.”
The findings establish a new strategy for electrically controlling molecular quantum systems and provide a foundation for future atom- and molecule-based quantum technologies, including quantum computing, quantum sensing, and quantum information processing.