Researchers Control Molecular Qubits Using Voltage Instead of Magnetic Fields

IBS and Karlsruhe Institute Researchers Replace Magnetic Fields with Voltage

The quest for scalable quantum computing has long been hindered by a fundamental hardware limitation: the reliance on magnetic fields to manipulate qubits, which are the information-processing units of a quantum computer. Because magnetic fields influence the surrounding space, packing multiple qubits closely together often leads to unintended interference, making it difficult to selectively control only a specific qubit. 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, thereby bypassing the need for magnetic fields.

Iron Phthalocyanine Molecules Enable 30% Spin Resonance Frequency Shifts

To test this principle, the research team fabricated a molecular complex by combining a single iron phthalocyanine molecule—a stable molecule in which an iron atom is surrounded by a ring-shaped organic molecule—and an iron atom on a magnesium oxide surface. The team observed the quantum spin of individual molecules in the complex using an instrument that combines scanning tunneling microscopy (STM) and electron spin resonance (ESR) techniques. Analysis showed that as the voltage applied to the tip of the STM probe increased, the qubit’s spin resonance frequency changed significantly. The change in resonance frequency reached up to about 30%, which is roughly 30 times larger than previously reported electric control effects of molecular spins.

The research team clarified that this phenomenon does not arise from movement of the molecular structure caused by the electric field, but rather from changes in the exchange interaction between the probe and the molecule depending on the voltage. Above a certain voltage, a nonlinear phenomenon appeared in which the resonance frequency did not increase linearly in proportion to the voltage but changed abruptly in specific ranges. As the voltage increases, the exchange interaction becomes stronger, changing the qubit spin energy and thereby altering the resonance frequency.

The team also confirmed that, even in a structure where two molecules are bonded together, only the qubit located under the probe can be selectively controlled. By varying the voltage, they succeeded in shifting the resonance condition of one spin without affecting the other spins. This demonstrates that it is possible to control only the desired qubit even in environments where multiple qubits are connected.

Christoph Wolf, corresponding author of the study, research fellow, and leader of the theory team at the Center for Quantum Nanoscience, stated, This achievement will become a key technology for implementing molecule-based quantum devices integrating multiple qubits in the future. He added, 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.

Organic Carbene Molecules Achieve Single-Photon Quantum Control

Separate research in the field of molecular qubits has also demonstrated the potential of organic carbene molecules to achieve single-photon quantum control. By embedding an organic carbene molecule in a specially engineered crystal, researchers have created a system that can maintain stable optical signals and long-lived quantum states. This allows for the initialization, control, and readout of the quantum state of an individual molecule. These researchers achieved optical line widths as narrow as 38 megahertz for single molecules, with spectral stability lasting over an hour with fluctuations of only a few megahertz. Furthermore, this molecular qubit was able to maintain its quantum information for milliseconds at ultra-cold temperatures.

Unlike many leading quantum computing architectures that rely on top-down fabrication methods, molecular systems use bottom-up synthesis. This approach allows researchers to design qubits atom by atom through chemistry, offering the possibility of engineering quantum systems with tunable optical transitions, customized spin properties, and intentionally placed nuclear spins. From a commercial perspective, the compatibility of molecular systems with photonic integrated circuits based on materials such as silicon nitride and lithium niobate is noted as a potential path for on-chip photon routing and quantum repeater nodes.

While these developments are significant, researchers acknowledge that technical hurdles remain. Experiments currently require cryogenic temperatures and highly controlled optical setups. Additionally, researchers in the organic carbene study have not yet demonstrated entanglement between multiple molecular qubits or scalable quantum processing architectures. These factors remain key areas for future work before molecular spin-photon systems reach commercial viability as quantum computers.

Sources: 동아사이언스, m.dongascience.com.