Scientists have developed a new method of controlling quantum states using magnetic qubits

Spin-polarized currents can also be used to control the quantum states of individual electronic spins. The results of the research, published in the scientific journal Science, may be used in various technologies in the future, for example in controlling the quantum states of magnetic qubits

Researchers from the Zurich Institute of Technology (ETH Zurich) led by Pietro Gambardella have developed a method to control the quantum states of individual electronic spins using spin-polarized currents. This method may improve the technologies in the field of quantum computing. The new technique offers more precise and focused control compared to traditional methods using electromagnetic fields, which may improve the ability to control quantum states in devices like qubits.

Electrons possess an internal angular momentum called spin, which allows them to align with a magnetic field, similar to the action of a compass needle. In addition to their electrical charge, the spin of electrons is now increasingly used for data storage and processing.

Already today you can purchase MRAM memory components (Magnetic Random Access Memory), where the information is stored in very small but still classic magnets, which contain many electron spins. The MRAM is based on streams of electrons with parallel spins that can change the magnetization at a certain point in the material.

Pietro Gambardella and his colleagues from the Zurich Institute of Technology have shown that spin-polarized currents can also be used to control the quantum states of individual electronic spins. The results of the research, published in the scientific journal Science, may be used in various technologies in the future, for example in controlling the quantum states of magnetic qubits.

Tunneling currents in single molecules

"Traditionally, electron spins are manipulated using electromagnetic fields such as radio waves or microwaves," says Dr. Sebastian Stefanov, a senior scientist in Gambardella's lab. This technique, also known as electron paramagnetic resonance, was developed as early as the mid-40s in 20th century and is used in various fields such as materials research, chemistry and biophysics "several years ago, it was proven that electronic paramagnetic resonance can be induced in individual atoms; However, until now the exact mechanism for this was not clear," adds Stefanov.

To investigate the quantum processes behind this mechanism, the researchers prepared pentacene molecules (an aromatic hydrocarbon) on a silver substrate. A thin insulating layer of magnesium oxide was placed on the substrate. This layer ensures that the electrons in the molecule behave more or less as they would in free space.

Using a scanning tunneling microscope, the researchers first characterized the electron clouds in the molecule. This involves measuring the current created when the electrons quantumly stream from the tip of a tungsten needle into the molecule. According to the laws of classical physics, the electrons should not be able to jump the gap between the tip of the needle and the molecule due to a lack of the necessary energy. However, quantum mechanics allows electrons to "flow" through the gap despite the lack of energy, leading to a measurable current.

A miniature magnet at the tip of the needle

The tunnel current can be polarized using the tungsten needle to pick up a number of iron atoms that are also on the insulating layer. On the tip of the needle, the iron atoms form a kind of miniature magnet. When a tunneling current flows through this magnet, the spins of the electrons in the current align parallel to its magnetization.

Now, the researchers applied both a constant voltage and a rapidly oscillating voltage to the magnetized tungsten tip, and measured the resulting tunneling current. By changing the magnitude of the voltage and changing the frequency of the oscillating voltage, they were able to observe characteristic resonances in the tunneling current. The exact shape of these resonances allowed them to draw conclusions about the processes that took place between the tunneling electrons and those of the molecule.

Direct spin control by polarized currents

From the data, Stefanov and his colleagues were able to discover two findings. First, the electron spins in the pentacene molecule responded to the electromagnetic field created by the oscillating voltage in a manner similar to normal electron paramagnetic resonance. Second, the shape of the resonance indicated that there was another process that affected the electron spins in the molecule.

"This process is the so-called 'spin transfer torque,' and the pentacene molecule has an ideal model system for this," says PhD student Stefan Kobaric. Spin transfer torque is an effect in which the spin of a molecule changes under the influence of a spin-polarized current without the direct action of an electromagnetic field. The researchers from the Zurich Institute of Technology demonstrated that it is also possible to create quantum superposition states of molecular spin in this way. Such superposition states are used, for example, in quantum technologies.

"Spin control by spin-polarized currents at the quantum level opens up various application possibilities," Kobarik says. Unlike electromagnetic fields, spin-polarized currents act very locally and can be directed with a precision of less than a nanometer. Such currents can be used to address electronic circuit components in quantum devices with great precision and thus control the quantum states of magnetic qubits.

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