The entanglement between two particles is among those phenomena in quantum physics that are hard to reconcile with everyday experiences. If entangled, certain properties of the two particles are closely linked, even when far apart. Albert Einstein described entanglement as a “spooky action at a distance”. Research on entanglement between light particles (photons) was awarded this year's Nobel Prize in Physics. Two electrons can be entangled as well – for example in their spins. In a superconductor, the electrons form so-called Cooper pairs responsible for the lossless electrical currents and in which the individual spins are entangled. The team of Christian Schönenberger and Andreas Baumgartner from University of Basel, in collaboration with Lucia Sorba and Valentina Zannier from Cnr Nano and Scuola Normale Superiore (SNS) in Pisa, have now been able to experimentally demonstrate what has long been expected theoretically: electrons from a superconductor always emerge in pairs with opposite spins. Results are published in the journal Nature. Using an innovative experimental setup, the physicists were able to measure that the spin of one electron points upwards when the other is pointing downwards, and vice versa. “We have thus experimentally proven a negative correlation between the spins of paired electrons,” explains project leader Andreas Baumgartner. The researchers exploited the unique structure of a single InAs nanowire with built-in InP barriers, grown at the Chemical Beam Epitaxy facility of Cnr Nano and SNS. In such a nanostructure the two segments of the nanowire - below and above the InP barriers - act as two quantum dots, each of which only allows single electrons to pass. To extract the electron pair researchers used a spin filter, developed at the Department of Physics, University of Basel. Using tiny magnets, they generated individually adjustable magnetic fields in each of the two quantum dots that separate the Cooper pair electrons. Since the spin also determines the magnetic moment of an electron, only one particular type of spin is allowed through at a time.

“We can adjust both quantum dots so that mainly electrons with a certain spin pass through them,” explains first author 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 both quantum dots are set to pass only the same spins, the electric currents in both quantum dots are reduced, even though an individual electron may well pass through a single quantum dot.” "The high quality of the nanowires, in terms of morphology, aspect ratio, crystal structure and chemical composition has been crucial to realize the strong quantum dot confinement that allowed to measure single electron spin polarizations," adds Valentina Zannier. “With this method, we were able to detect such negative correlations between electron spins from a superconductor for the first time,” Andreas Baumgartner concludes. “Our experiments are a first step, but not yet a definitive proof of entangled electron spins, since we cannot set the orientation of the spin filters arbitrarily – but we are working on it.” The research, which was recently published in Nature, is considered an important step toward further experimental investigations of quantum mechanical phenomena, such as the entanglement of particles in solids, which is also a key component of quantum computers. Original publication Arunav Bordoloi, Valentina Zannier, Lucia Sorba, Christian Schönenberger, Andreas Baumgartner Spin Cross-Correlation Experiments in an Electron Entangler, Nature (2022), doi: 10.1038/s41586-022-05436-z [Image: University of Basel, Department of Physics/Scixel]