[Article] From Earth to Mars: Transporting spin information at the speed of light !

Abstract

In spintronics, recognized by its success with magnetic computer hard drives and magnetic memories, information is represented by electron spin and by its proxy, the direction of magnetization. Ferromagnets such as iron or cobalt have a finite magnetization, because unequal numbers of their electrons’ spins are oriented either along or against the magnetization axis, representing binary information 0 and 1. While electrons with spin along the magnetization travel smoothly across a ferromagnet, those with opposite spin orientation are bounced around, just as a player in a football match dealing with the member of his own or the opposing team. The resulting change of the resistance with the spin orientation-magnetoresistance, is the key principle for spintronic devices. As we know, the fridge magnet does not need power to remain stuck to the fridge door. Its magnetic state, which can be considered as stored information, is maintained indefinitely.

However, by taking electrons out of the ferromagnet, akin to taking fish out of the water, the spin information is quickly lost and cannot travel far. This major limitation can be overcome by utilizing light through its circular polarization or handedness, also known as helicity*, as another spin carrier. Just as recognized for centuries that written information would be carried faster and farther by pigeons, the trick would be to transfer electron spin to photon spin, the quantum of light. The presence of spin-orbit coupling, which is also responsible for the spin information loss outside of the ferromagnet, makes such transfer of spin from electron to photon possible. The crucial missing link is then to electrically modulate the magnetization and thereby change the helicity of the emitted light.

The group of scientists from the Jean Lamour Institute (CNRS/University of Lorraine, France) in collaboration with Laboratoire Albert Fert (France), Université de Toulouse (France), Université Paris-Saclay (France), Ruhr-Universität Bochum (Germany), Institute of Semiconductors and Institute of Physics (Chinese Academy of Sciences), National Institute of Advanced Industrial Science and Technology (Japan), University of Minnesota (USA), National Renewable Energy Laboratory (USA), and University at Buffalo (USA) have chosen circular polarization of light to carry the electron's spin information. They successfully switched the magnetization of a spin injector by an electrical pulse using the spin-orbit torque. The electron's spin is rapidly converted into information contained in the helicity of the emitted photons enabling a seamless integration of magnetization dynamics with photonic technologies.

This electrically controlled spin-photon conversion is now achieved in the electroluminescence of light-emitting diodes. In the future, through the implementation in semiconductor laser diodes, so-called spin-lasers, this highly-efficient information encoding could pave the way for rapid communication over interplanetary distances since polarization of light can be conserved in space propagation, potentially making it as the fastest mode of communication between Earth and Mars. It will also greatly benefit the development of various advanced technologies on Earth, such as optical quantum communication and computation, neuromorphic computing for artificial intelligence, ultrafast and high-efficient optical transmitters for data centers or Light-Fidelity (LiFi) applications. More perspectives can be found in the journal Nature.

* It represents the rotation direction of the electrical component of light (clockwise or counterclockwise) around its propagation axis and is related to the spin state of the photons.

Reporter: CARTIER DIT MOULIN Christophe (christophe.cartier@cnrs-dir.fr)

Authors :

Christophe CARTIER DIT MOULIN (Journalist, CNRS)
Igor Zutic (Professor of University at Buffalo, USA)
Nils Gerhardt (Professor at Ruhr University in Bochum, Germany)
Yuan Lu (CNRS researcher, Institut Jean Lamour)

References

Pambiang Abel Dainone et al.
Controlling the helicity of light by electrical magnetization switching
Nature 2024, https://www.nature.com/articles/s41586-024-07125-5

Contact

Yuan Lu, CNRS researcher at l’Institut Jean Lamour (CNRS/Université de Lorraine), Email: yuan.lu@univ-lorraine.fr