Manipulating the quantum dance of spinning electrons

In the world of spinning electrons and quantum states, an exciting realm that’s reshaping our everyday lives through our gadgets, researchers have made a discovery that promises even more powerful storage and processing capacities.

What makes an electron spin?

Just like a compass needle aligns itself to a magnetic field, electrons possess an inherent angular momentum, termed as ‘spin.’

Beyond their electric charge, which dictates behavior in electronic circuits, their spin has become pivotal for storing and processing data.

Our current gadgets, such as MRAM memory elements (magnetic random access memories), information is stored via small classical magnets.

These comprise a myriad of electron spins. The MRAM’s, in turn, operate on spin-aligned electron currents, which can shift magnetization at a certain point in a material.

Researcher Pietro Gambardella, and his team at ETH Zurich, discovered that spin-polarized currents can also govern the quantum states of single electron spins.

Their findings, freshly published in the scientific journal Science, promise great potential for controlling quantum states of quantum bits (qubits).

Decoding electron spins

“Electron spins have been traditionally manipulated utilizing electromagnetic fields like radio-frequency waves or microwaves,” explains Sebastian Stepanow, a Senior Scientist in Gambardella’s laboratory.

This established technique, known as electron paramagnetic resonance, traces back to the mid-1940’s and has found use in assorted fields such as material research, chemistry, and biophysics.

However, the exact mechanism of inducing electron paramagnetic resonance in singular atoms has remained hazy.

To delve deeper into the quantum mechanical processes behind this mechanism, the researchers readied pentacene molecules (an aromatic hydrocarbon) onto a silver substrate.

A thin insulating layer of magnesium oxide, previously deposited on the substrate, ensures that the electrons in the molecule behave more or less as they would in free space.

What’s the quantum trick?

The researchers used a scanning tunnelling microscope to measure the current created when the electrons tunnelled quantum mechanically from the tip of a tungsten needle to the molecule.

Classical physics would argue against this process, but quantum mechanics empowers the electrons to ‘tunnel’ through the gap, generating a measurable current.

By applying a constant voltage and a rapidly oscillating voltage to a magnetized tungsten tip, and subsequently measuring the resulting tunnel current, the team was able to observe characteristic resonances in the tunnel current.

The shape of these resonances allowed them to infer the processes between the tunnelling electrons and those of the molecule.

Critical insights gained

Through their data analysis, Stepanow and his team reaped two critical insights.

Firstly, the electron spins in the pentacene molecule reacted to the electromagnetic field created by the alternating voltage, similar to ordinary electron paramagnetic resonance.

Secondly, they found an additional process at play that also influenced the spins of the electrons in the molecule.

“This process is the so-called spin transfer torque,” says PhD student Stepan Kovarik. Under the influence of a spin-polarized current, the spin of the molecule is altered without any direct action of an electromagnetic field.

Spin control and the quantum future

The ETH researchers demonstrated that it’s possible to create quantum mechanical superposition states of the molecular electron spin, and these states are being used in quantum technologies.

“Spin control by spin-polarized currents at the quantum level gives way to numerous potential applications,” Kovarik predicts.

Contrary to electromagnetic fields, spin-polarized currents can act locally and be steered with a precision of less than a nanometer.

They could be deployed to address electronic circuit elements in quantum devices with extreme precision, thereby controlling the quantum states of magnetic qubits.

Time will tell how this exciting development will translate into practical applications in storing and processing data. But until then, thanks to the relentless curiosity of scientists like Gambardella, Stepanow, and Kovarik, our understanding of the quantum dance of electrons continues to evolve.

The full study was published in the journal Science.

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