New material promises to make qubits more resilient

Technology News | February 27, 2019

By Rich Pell

Currently, qubits – the core information carriers in quantum computing – are too fragile and sensitive to their surroundings to work well enough to build a practical quantum computer. The new material, say the researchers, promises to make qubits more resilient by “immunizing” them against noise – such as heat and other parts of a computer – that interferes with how well they hold information.

The researchers engineered the material into a “topological insulator” nanoribbon, which conducts electrical current on its surface but not on the inside, with two superconductor electrical leads attached to form a “Josephson junction” – where a thin layer of a nonsuperconducting material is sandwiched between two layers of superconducting material.

Such a topological-insulator nanoribbon Josephson junction device is one of many options researchers have been investigating for building more resilient qubits. The resilience, say the researchers, could come from special properties created by conducting a supercurrent on the surface of a topological insulator, where an electron’s quantum mechanical spin is locked to momentum.

The problem so far is that a supercurrent tends to leak into the inside of topological insulators, preventing it from flowing completely on the surface. To become more resilient, topological qubits need supercurrents to flow through the surface channels of topological insulators.

“We have developed a material that is really clean, in the sense that there are no conducting states in the bulk of the topological insulator,” says Yong Chen, a Purdue professor of physics and astronomy and of electrical and computer engineering, and the director of the Purdue Quantum Science and Engineering Institute. “Superconductivity on the surface is the first step for building these topological quantum computing devices based on topological insulators.”

In experiments, the researchers could show that the supercurrent wraps all the way around the new topological insulator nanoribbon at temperatures 20% lower than the “critical temperature,” when the junction becomes superconducting.

“It’s known that as the temperature lowers, the superconductivity is enhanced,” says Chen. “The fact that much more supercurrent flowed at even lower temperatures for our device was evidence that it is flowing around these protective surfaces.”

For more, see “Anomalous Low-Temperature Enhancement of Supercurrent in Topological-Insulator Nanoribbon Josephson Junctions: Evidence for Low-Energy Andreev Bound States.”