New Superconductor Could Pave Way to Practical Quantum Computers

Scientists at the University of Texas at Austin have developed a new superconducting material that might allow the construction of quantum computers that are more resistant to outside noise, such as electromagnetic interference.

Many of the quantum systems that have been produced for quantum computers are very fragile. Small effects from the outside world easily disturb them, introducing errors into calculations or turning results into junk. Scientists have proposed a design for quantum computers based on majoranas, exotic particles that are each both matter and antimatter, which would be more resilient to outside noise. That's because majoranas only interact very weakly with their surroundings.

In a paper published in the Proceedings of the National Academy of Sciences, Ken Shih and Allan McDonald, both UT Austin physics professors, along with colleagues from Louisiana State University, Ohio State University and Princeton University, describe a new material that could produce the elusive majorana particle.

MacDonald and his colleagues at UT Austin and Princeton University had observed a majorana particle before, but with this new material it would be possible to intentionally produce (and destroy) such a particle, instead of simply observing their existence, as in the previous work.

Creating these particles in a controlled way has always been difficult. For one thing, the majorana particle can only exist in a special superconducting material placed in a magnetic field, which poses a challenge, since magnetic fields tend to destroy superconductors. When a material superconducts, it must have a very orderly flow of electrons.

"Like a hundred couples dancing in a ballroom," says Shih, "the electrons all have to move in sync."

When a magnet enters the picture, it usually disrupts the orderly flow of electrons -- like a rowdy group of people walking across the dance floor, interfering with the perfect synchronicity of the dance.

Shih's group, in previous experiments, had found that a very thin superconductor, hundreds of times thinner than delicate gold foil, could circumvent these effects, and actually allow magnetism to increase the order of the electrons. This is because a thin material could have a magnetic field running parallel to its surface, encouraging electrons to move in an orderly way, like dancers all moving the same direction on a dance floor. The new superconductor is made of lead only a couple of atoms thick, which forms a neat, crystal-like pattern that can withstand a magnetic field.

Making a thin superconductor is not without challenges, since any imperfections, especially on a surface so thin that it is nearly 2-dimensional, would be like placing bulky furniture on the dance floor. Through innovations in the synthesis process—whereby the ultrathin lead is grown on a silicone sheet, much like a crystal is grown—the team produced a material that is extremely pure, allowing it to superconduct even in a magnetic field.

A second criterion for producing a majorana particle was that the electrons in the material exhibit so-called "spin orbit coupling," which is an effect of electrons moving close to the speed of light in an atom. This is only achieved in very large atoms, since the force of more protons is pulling at the electrons in orbit, making them orbit more quickly. For this reason, Shih and MacDonald chose lead, a heavy metal with a large nucleus and whose electrons exhibit spin orbit coupling, for their superconductor.