Physicists from the University of Basel have demonstrated experimentally for the first time that there is a negative correlation between the two spins of an entangled pair of electrons in a superconductor. For their study, the researchers used spin filters made of nanomagnets and quantum dots, as they report in the scientific journal Nature.
The entanglement between two particles is one of the phenomena of quantum physics that is difficult to reconcile with everyday experiences. If they are entangled, some properties of the two particles are closely related, even when they are far apart. Albert Einstein described entanglement as “frightening action at a distance”. Research on the entanglement between light particles (photons) received this year’s Nobel Prize in Physics.
Two electrons can also be entangled, for example in their spins. In a superconductor, the electrons form so-called Cooper pairs responsible for lossless electric currents and in which the individual spins are entangled.
For several years, researchers from the Swiss Nanoscience Institute and the Department of Physics at the University of Basel have been able to extract pairs of electrons from a superconductor and spatially separate the two electrons. This is achieved by means of two quantum dots – nanoelectronic structures connected in parallel, each of which only passes single electrons.
Opposite electron spins of Cooper pairs
The team of Prof. Dr. Christian Schönenberger and Dr. Andreas Baumgartner, in collaboration with researchers led by Prof. Dr. Lucia Sorba of the Istituto Nanoscienze-CNR and the Scuola Normale Superiore in Pisa, has now been able to demonstrate experimentally what has long been theoretical: the electrons of a superconductor always emerge in pairs with opposite spins.
Thanks to an innovative experimental setup, physicists were able to measure that the spin of one electron points up when the other points down, and vice versa. “We have thus experimentally proven a negative correlation between the spins of paired electrons”, explains project leader Andreas Baumgartner.
The researchers achieved this by using a spin filter they developed in their lab. Using tiny magnets, they generated individually adjustable magnetic fields in each of the two quantum dots that separate the electrons from the Cooper pair. Since spin also determines the magnetic moment of an electron, only one particular type of spin is allowed at a time.
“We can tune the two quantum dots so that mostly electrons with a certain spin pass through them,” says first author Dr Arunav Bordoloi. “For example, an electron with spin up passes through one quantum dot and an electron with spin down passes through the other quantum dot, or vice versa. If the two quantum dots are tuned to transmit only the same spins, the electric currents in both quantum dots the dots are reduced, even though an individual electron may very well pass through a single quantum dot.”
“With this method, we were able to detect such negative correlations between the electron spins of a superconductor for the first time”, concludes Andreas Baumgartner. “Our experiments are a first step, but not yet a definitive proof of entangled electron spins, as we cannot arbitrarily define the orientation of the spin filters, but we are working on it.”
The research, recently published in Nature, is seen as an important step towards further experimental research into quantum mechanical phenomena, such as particle entanglement in solids, which is also a key component of quantum computers.
Arunav Bordoloi, Spin cross-correlation experiments in electron entanglement, Nature (2022). DOI: 10.1038/s41586-022-05436-z. www.nature.com/articles/s41586-022-05436-z
Provided by the University of Basel
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