Characterizing the materials for next-generation quantum computers with nonlinear optical spectroscopy

Researchers from Universität Hamburg’s Department of Physics and the Cluster of Excellence “CUI: Advanced Imaging of Matter” and the University of California, Irvine have recently proposed a new method for characterising topological superconductors using multi-THz pulse experiments.

This paves the way for unambiguously identifying predicted exotic states of matter and can aid in the design of novel materials for future quantum information-carrying and processing devices.

Scientists from all over the world are collaborating to develop scalable quantum computers based on solid-state matter. Topological superconductors are one such class of material. They are said to contain a specific type of collective quantum state, non-abelian anyons in the form of Majorana fermions, at their boundaries. Researchers can build logical quantum gates, the building blocks of quantum computers, by moving these quasiparticles around in networks of quantum wires.

Instead of boundary properties, use bulk propertie

Early evidence for the existence of Majoranas was reported using quantum transport measurements, but these studies were later found to be unreliable because Majoranas are easily confused with trivial boundary excitations. The new theory takes an unconventional approach. The bulk material is investigated rather than the Majoranas at the device’s boundaries. Majoranas are inextricably linked to the topology of the bulk band structure of the superconductor due to the so-called “bulk-boundary correspondence.” The particle excitations in the bulk material “twist” in some ways with the Majoranas at the boundaries. Two-dimensional THz spectroscopy, a technique widely used in molecules and bulk matter, can be used to investigate this strong interlinking.

“Unlike ‘linear’ absorption spectroscopy, nonlinear multi-pulse experiments allow us to study the optical response of excited particles and thus help to reveal this ‘twisting’ clearly, with unique signatures of the exotic topological state in the 2D spectra,” says Prof. Dr. Michael Thorwart of Universität Hamburg, a Cluster of Excellence scientist.

The theory proposal, which appears in Physical Review Letters, formulates an important step between detecting the most basic but not fully characterising properties of Majoranas and the still too ambitious demonstration of logical gate operations with non-abelian anyons in the form of braiding of Majorana states. “Such optical techniques provide spectroscopic information in addition to imaging and enable unquestionable characterization of topological materials. As a result, they may be able to build a bridge to their distant applications in quantum technologies “Felix Gerken, lead author and Ph.D. student at the Cluster of Excellence’s CUI-Graduate School, adds