An alternative way to manipulate quantum states

Researchers at ETH Zurich have shown that the quantum states of single-electron spins can be controlled by streams of electrons whose spins are evenly aligned. In the future, this method can be used in electronic circuit elements.

Electrons have an intrinsic angular momentum called spin, which means they can align themselves along a magnetic field, much like a compass needle. In addition to the electric charge of electrons, which determines their behavior in electronic circuits, their spin is increasingly used to store and process data.

Currently, it is possible to buy MRAM (Magnetic Random Access Memory) memory elements, where information is stored in very small but still classical magnets – that is, they contain very many electron spins. MRAMs are based on electron flows with parallel spins that can change the magnetization at a specific point in a material.

Pietro Gambardella and colleagues at ETH Zurich now show that such spin-polarized currents can also be used to control the quantum states of single-electron spins. Their results have recently been published in the journal Sciencecan be used in various technologies in the future, for example in controlling the quantum states of quantum bits (qubits).

Tunneling currents in single molecules

“Traditionally, electron spins have been manipulated using electromagnetic fields such as radio frequency waves or microwaves,” said Sebastian Stepano, a senior scientist in Gambardella’s lab. This technique, also known as electron paramagnetic resonance, was developed in the mid-1940s and has since been used in various fields such as materials research, chemistry, and biophysics.

“A few years ago, it was shown that electron paramagnetic resonance could be induced in single atoms,” says Stepano. However, until now, the exact mechanism of this has been unclear.

To study the quantum mechanical processes behind this mechanism in more detail, the researchers prepared pentacene molecules (an aromatic hydrocarbon) on a silver substrate. A thin insulating layer of magnesium oxide was previously deposited on the substrate. This layer ensures that the electrons in the molecule behave more or less like free space.

Using a scanning tunneling microscope, the researchers first identified the electron clouds in the molecule. This involves measuring the current that is created when electrons quantum mechanically tunnel from the tip of a tungsten needle into the molecule. According to the laws of classical physics, electrons should not be able to pass through the gap between the needle tip and the molecule because they do not have the necessary energy. However, quantum mechanics allows electrons to “tunnel” through the gap despite this deficiency, resulting in a measurable current.

A miniature magnet at the tip of the needle

This tunnel current can be spin-polarized by using a tungsten tip to remove a few iron atoms, which are also on the insulating layer. At its tip, the iron atoms form a miniature magnet. When a tunneling current passes through this magnet, the spins of the electrons in the current are all aligned parallel to its magnetization.

The researchers applied a constant voltage as well as a rapidly oscillating voltage to the magnetic tungsten tip and measured the resulting tunnel current. By varying the strength of both the voltage and the frequency of the oscillating voltage, they were able to observe characteristic resonances in the tunnel current. The exact shape of these resonances allowed them to draw conclusions about the processes that occur between the tunneling electrons and the electrons of the molecule.

Direct spin control by polarized currents

From the data, Stepano and his colleagues were able to glean two insights. On the one hand, the electron spins in the pentacene molecule respond to the electromagnetic field created by the alternating voltage in the same way as in normal electron paramagnetic resonance. On the other hand, the shape of the resonances suggests that there is an additional process that also affects the spins of the electrons in the molecule.

“This is the so-called spin-transfer torque process, for which the pentacene molecule is an ideal model system,” says Dr. Ph.D. Student of Stepan Kovarik. 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 effect of an electromagnetic field. ETH researchers showed that quantum mechanical superposition states of molecular spin can also be created in this way. Such superposition modes are used, for example, in quantum technologies.

“This control of spin by polar spin currents at the quantum level opens up many possible applications,” says Kovarik. Unlike electromagnetic fields, polar spin currents are highly localized and can be steered with a precision of less than a nanometer. Such currents can be used to address electronic circuit elements in highly precise quantum devices and thus, for example, control the quantum states of magnetic qubits.