New Voltage-Based Method Enables Precise Control of Individual Molecular Qubits

Moving Beyond Magnetic Field Control

Quantum computing relies on the manipulation of quantum states within tiny particles, typically atoms or molecules, known as qubits. For years, the standard approach to controlling these states has been the application of magnetic fields. However, this method presents significant engineering challenges as systems scale up. Because magnetic fields inevitably affect the surrounding space, packing multiple qubits closely together makes it difficult to address a specific unit without disturbing its neighbors.

A joint research team from the Center for Quantum Nanoscience and the Karlsruhe Institute of Technology has identified a way to bypass this limitation. By utilizing voltage rather than magnetism, the team demonstrated that they could exert nanoscale control over individual qubits. This development, which was announced on the 16th, suggests a path toward more scalable, densely packed quantum architectures.

Manipulating Iron Phthalocyanine Molecules

To test their theory, the researchers constructed a molecular complex consisting of a single iron phthalocyanine molecule paired with an iron atom, placed on a magnesium oxide surface. Iron phthalocyanine is characterized by an iron atom held within a stable, ring-shaped organic structure. The team monitored the quantum spin of these molecules using a specialized instrument that integrates scanning tunneling microscopy (STM) with electron spin resonance (ESR) techniques.

The researchers discovered that by adjusting the voltage applied to the tip of the STM probe, they could significantly alter the spin resonance frequency of the qubit. The team observed that the intrinsic frequency—the rate at which quantum spins change state in response to a stimulus—responded directly to electrical input. When the voltage crossed a certain threshold, the resonance frequency shifted in a nonlinear fashion, changing by as much as 30%. The researchers noted this effect is roughly 30 times larger than previously documented electric control methods for molecular spins.

For more on this story, see Researchers Control Molecular Qubits Using Voltage Instead of Magnetic Fields.

Exchange Interaction as a New Mechanism

The team clarified that these shifts are not the result of physical movement within the molecular structure caused by the electric field. Instead, the effect is driven by changes in the exchange interaction, a process where the spins of nearby electrons influence one another. As voltage increases, the exchange interaction between the probe and the molecule strengthens, which in turn modifies the qubit’s spin energy.

The research team identified a new principle for controlling a single molecular qubit by using the exchange interaction, in which nearby electrons influence one another.

This mechanism offers a critical advantage for multi-qubit systems. In experiments involving two bonded molecules, the team demonstrated that they could selectively control the qubit located directly under the probe. By tuning the voltage, the researchers successfully shifted the resonance of one spin while leaving adjacent spins unaffected, a capability that remains difficult to achieve with conventional magnetic field manipulation.

Future Implications for Quantum Information Processing

The ability to address individual qubits using only voltage addresses one of the most persistent hurdles in quantum hardware design: the interference caused by magnetic fields in dense, multi-qubit arrays. According to the researchers, this principle is expected to play a central role in the development of next-generation quantum computers and sensors.

“This achievement will become a key technology for implementing molecule-based quantum devices integrating multiple qubits in the future,” adding, “We expect it to serve as an important turning point for the development of next-generation quantum computers, quantum sensors, and quantum information processing technologies.”

Christoph Wolf, corresponding author of the study and research fellow

As the field moves toward integrating multiple qubits into functional devices, the reliance on voltage-based control could simplify the architecture of quantum processors. While the current results are confined to laboratory settings using STM and ESR instrumentation, the team’s demonstration of selective control in bonded structures provides a foundational proof of concept for future quantum information processing applications.