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, so-called spin, which means they can line up along a magnetic field, much like a compass needle. In addition to the electrical charge of electrons, which determines their behavior in electronic circuits, their spin is increasingly used for data storage and processing.

Now, you can buy MRAM memory elements (magnetic random access memories), in which information is stored in very small, but still classic magnets – that is, containing many electronic spins. MRAMs are based on electron streams with parallel-aligned spins that can change the magnetization at a given point in a material.

Pietro Gambardella and his collaborators at ETH Zurich now show that such spin-polarized currents can also be used to control the quantum states of an electron’s spins. Their results, which have just been published in the journal scienceit can 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 are manipulated using electromagnetic fields such as radio frequency waves or microwaves,” says Sebastian Stepanow, 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 fields as diverse as materials research, chemistry and biophysics.

“A few years ago, it was demonstrated that one can induce paramagnetic resonance of electrons in single atoms; however, until now the exact mechanism for this has been unclear,” says Stepanow.

To study more closely the quantum mechanical processes behind this mechanism, the researchers prepared molecules of pentacene (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 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 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 tip of the needle and the molecule because they lack the necessary energy. Quantum mechanics, however, allows electrons to “tunnel” through the gap despite the gap, leading to a measurable current.

Miniature magnet on the tip of a needle

This tunnel current can be spin polarized by first using the tungsten tip to capture some iron atoms, which are also in the insulating layer. At the tip, the iron atoms create a kind of miniature magnet. When a tunnel current flows through this magnet, the spins of the electrons in the current all line up parallel to its magnetization.

The researchers applied a constant voltage as well as a rapidly fluctuating voltage to the magnetized tungsten tip, and they measured the resulting tunnel current. By varying the strength of both voltages 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 occurred between the tunneling electrons and those of the molecule.

Direct control of rotation by polarized currents

From the data, Stepanow and his colleagues were able to glean two insights. On the one hand, the electron spins in the pentacene molecule reacted to the electromagnetic field created by the alternating voltage in the same way as in the ordinary paramagnetic resonance of electrons. On the other hand, the shape of the resonances suggested that there was an additional process that also affected the electron spins in the molecule.

“That process is the so-called spin-transfer torque, for which the pentacene molecule is an ideal model system,” says Ph.D. student Stepan Kovarik. Spin transfer torque is an effect in which the spin of a molecule is changed under the influence of a spin-polarized current without the direct action of an electromagnetic field. The ETH researchers demonstrated that it is also possible to create quantum mechanical superposition states of molecular spin in this way. Such superposition states are used, for example, in quantum technologies.

“This spin control by spin-polarized currents at the quantum level opens up various possible applications,” says Kovarik. In contrast to 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 elements in quantum devices with great precision and thus, for example, to control the quantum states of magnetic qubits.