An Alternative Method is Proposed to Achieving Topological Superconductivity

April 29, 2020 by Luke James

Researchers at the Center for Quantum Devices, Niels Bohr Institute of the University of Copenhagen have used a pencil-shaped semiconductor to “unravel a new path” to Majorana zero modes and topological superconductivity.

According to the team’s research, which was published in the journal Science, this “conceptual breakthrough” was achieved by collaborating with Microsoft Quantum researchers. Together, the research team used the phase winding around the circumference of a cylindrical superconductor that surrounds a semiconductor. 

Charles Marcus of the Niels Bohr Institute said, “The result may provide a useful route toward the use of Majorana zero modes as a basis of protected qubits for quantum information. We do not know if these wires themselves will be useful, or if just the ideas will be useful.”


A Possible Sighting

The research paper explains that Majorana zero modes, quasiparticles that are predicted to occur in topological superconductors, hold promise as a building block for topological quantum computing. Today, there are two frontrunners for the physical implementation of Marjoranas—hybrid semiconductor-supercomputer nanowires and topological insulators that are in contact with a superconductor. 

The paper introduces a platform that combines elements of both: a semiconductor nanowire that is wrapped by a superconductor. By combining theoretical calculations with their own experiments, the research team presents its evidence that is reportedly consistent with the emergence of Majorana zero modes.


Majorana fingerprints in full-shell nanowires.

Figures detailing majorana fingerprints in full-shell nanowires. Image credited to the University of Copenhagen, Niels Bohr Institute


Combining Two Known Ideas

The research introduces a conceptually distinct approach to generating Majorana zero modes and utilizes two already known ideas that have already been used in the world of quantum mechanics: Vortex-based topological superconductors and one-dimensional topological superconductivity in nanowires. 

By using these two ideas, the team’s result was the unveiling of “a new path” with findings that can be traced back around 50 years to a piece of physics known as the Little-Parks effect.

Essentially, the Little-Parks effect says that a superconductor in the shape of a cylindrical shell will adjust to an external magnetic field, threading the cylinder by jumping to a “vortex state” where the quantum wavefunction around the cylinder carries a twist of its phase. 


A Path Forward for Quantum Computing

To reveal this new path, a special type of material that combined semiconductor nanowires and superconducting aluminum was needed. These were developed in the Center for Quantum Devices, as were the special wires with the superconducting shell fully surrounding the semiconductor. 

"Our motivation to look at this in the first place was that it seemed interesting and we didn't know what would happen", said Marcus about the experimental discovery, which was confirmed theoretically in the same research paper. Nonetheless, the idea may indicate a path forward for quantum computing.