Researchers at ETH Zurich have achieved a significant breakthrough in quantum physics by demonstrating that quantum states of single electron spins can be controlled using spin-polarized electron currents. This pioneering method holds potential for future applications in electronic circuit elements, potentially revolutionizing data storage and processing.
The Spin of Electrons
Electrons possess an intrinsic angular momentum known as spin, allowing them to align along a magnetic field similar to how a compass needle aligns with the Earth's magnetic field. Traditionally, the behavior of electrons in electronic circuits is governed by their electric charge. However, electron spin is now increasingly being harnessed for data storage and processing, paving the way for advancements in spintronics.
One practical application of electron spin is already available in the form of MRAM (magnetic random access memories). In MRAM devices, information is stored using small magnets composed of numerous electron spins. These memory elements utilize spin-polarized currents, where electrons with aligned spins can alter the magnetization at specific points in the material.
Controlling Single Electron Spins
Pietro Gambardella and his team at ETH Zurich have taken this concept further by showing that spin-polarized currents can also control the quantum states of single electron spins. Their findings, recently published in the journal Science, suggest new possibilities for quantum technologies, including the manipulation of quantum bits (qubits).
Investigating Quantum Processes
To delve deeper into the quantum mechanical processes underlying this phenomenon, the researchers prepared pentacene molecules (a type of aromatic hydrocarbon) on a silver substrate, coated with a thin insulating layer of magnesium oxide. This insulating layer ensures that the electrons in the pentacene molecules behave similarly to electrons in free space.
Using a scanning tunneling microscope (STM), the researchers characterized the electron clouds within the pentacene molecules. The STM technique measures the current generated when electrons tunnel quantum mechanically from a tungsten needle tip to the molecule. While classical physics dictates that electrons lack the energy to bridge this gap, quantum mechanics allows them to “tunnel” through, creating a measurable current.
Creating a Spin-Polarized Tunnel Current
To achieve spin polarization, the researchers utilized the tungsten tip to pick up a few iron atoms from the insulating layer, forming a miniature magnet. When a tunnel current flows through this magnetized tip, the electron spins align parallel to its magnetization.
By applying both a constant voltage and a fast-oscillating voltage to the magnetized tungsten tip, the researchers measured the resulting tunnel current. Adjusting the strength of these voltages and the oscillation frequency enabled them to observe characteristic resonances in the tunnel current. These resonances provided insights into the interactions between the tunneling electrons and those in the pentacene molecule.
Future Implications
This research opens up new avenues for controlling quantum states in single electron spins, which could lead to significant advancements in quantum computing and other technologies. The ability to manipulate quantum bits using spin-polarized currents offers a promising alternative to traditional methods that rely on electromagnetic fields, such as radio-frequency waves or microwaves.
As the field of spintronics continues to evolve, the work of Gambardella and his team at ETH Zurich highlights the potential for integrating quantum control techniques into practical electronic devices. This could ultimately result in faster, more efficient, and more secure data processing technologies, marking a significant step forward in the ongoing quest to harness the power of quantum mechanics.