Superconducting spin cycle.
Superconducting spin cycle.

Researchers from Harvard have solved one of the main problems of quantum computing: how to transmit spin information through superconducting materials. Their work lays the foundations for quantum superconducting devices.

The concept of electron spin as an additional dimension to the concept of electronics gives rise to the notion of spintronics. Here, the electron's spin (up or down) as well its charge or the absence of an electron, can be used to encode information. However, whereas the presence of an electron can represent a bit of data, 1 or 0, and its absence (a positive hole) represent the opposite, a 0 or a 1, using the charge of the electron as a quantum bit poses significant challenges since it interacts very easily with its surroundings. The spin of the electron, on the other hand, can also be used to encode quantum information offering superposition of spin states that could be exploited in a so-called quantum computer that alights on the answer to a problem by "testing" all solutions simultaneously.

Physicists hope that they will be able to use superconducting materials through which electrons can move without loss of energy, to construct quantum devices. Unfortunately, the tunnel is blocked for this train of thought because superconductors cannot transmit spin. Any electron pairs that pass through a superconductor emerge with a combined spin of zero. Harvard physicists have rerouted the problem and found an answer that points to a way to transmit spin information through superconducting materials and so take advantage of their potential low energy demands.

"We now have a way to control the spin of the transmitted electrons in simple superconducting devices," explains senior author Amir Yacoby. [Yacoby et al., Nature Physics (2016) DOI: 10.1038/nphys3877]. The solution is based on the particular way in which electrons move as Cooper pairs through certain semiconductors that are in contact with conventional superconductors.

In principle, this changes their momentum so that it is asymmetric which precludes the spin state from being symmetric so it is not lost on transmission. In practice, the team has used a superconducting material in close proximity to a non-superconducting material - two superconductors sandwiching a filling of mercury telluride in fact wherein quantum and relativistic effects come into play because of the heavy atoms and the fast moving electrons. "Because the atoms are so heavy, you have electrons that occupy high-speed orbitals," explains team member Hechen Ren. "When an electron is moving this fast, its electric field turns into a magnetic field which then couples with the spin of the electron. This magnetic field acts on the spin and gives one spin a higher energy than another."

The end result is that when the Cooper pairs hit this filling material, their spins turn breaking the symmetry of the orbital and this means that the spin can now be a non-zero value as it is carried through the system.

"This discovery opens up new possibilities for storing quantum information. Using the underlying physics behind this discovery provides also new possibilities for exploring the underlying nature of superconductivity in novel quantum materials," adds Yacoby.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".