In a first, scientists identify origin of ‘beating’ signal hidden in quantum material

Researchers at the Korea Research Institute of Standards and Science (KRISS) and the Gwangju Institute of Science and Technology (GIST) have solved the years-long mystery about the origins of the ‘beating’ signal in the topological insulator (TI) nanowires. The discovery paves the way for better interpretation of signals from topological quantum devices and attaining their desired electronic states. Emerging quantum technologies like quantum computing and sensing hold the promise of a new era of devices with capabilities that are hard to believe could be real. Quantum computers could beat the fastest supercomputers of the day in just a few seconds, while advanced sensors can measure changes in electric or magnetic fields even at nanoscales. For all their advantages, quantum technologies are extremely sensitive, and these devices can be thrown out of gear by the slightest changes in temperature or environmental noise. Topological devices leverage mathematical properties of topology to protect quantum information and ensure that data remains coherent for longer and resilient to external noise. Beating signal in TI Topological insulators (TI) are materials that are insulating in their interiors but can conduct electrons on their edges or outer surfaces via special electronic states. Scientists use this materials to make nanowires where electrons travel along the perimeter. When a magnetic field is applied to these wires, electron waves passing through different paths interfere, producing Aharonov-Bohm (AB) oscillations. Here, the conductance of the material changes at regular intervals depending on the intensity of the magnetic field. When a TI is doped, it can create a thin layer beneath the surface where the electrons can flow. However, whether this layer participates in the AB oscillations was not well known. When working with antimony (Sb)-doped bismuth selenide (Bi₂Se₃) nanowires, researchers found a ‘beating’ signal where oscillations of slightly different periods and signal intensities were observed, confirming the presence of another oscillation. What causes the beating signal? The researchers analyzed electrical conductance data from previous years of research working with TI and confirmed that this beating signal had been present all along. The team attributed this to the oscillations emerging from the ‘Topological Surface State’ (TSS), the electronic state on the surface of the TI, and the ‘Two-Dimensional Electron Gas’ (2DEG), where electrons gather and move under the surface. Electrons passing through the two paths pass through the conduction states and through slightly different cross-sectional areas of the nanowire, causing their oscillations at different periods, thereby producing the beating pattern. While this seems straightforward, it was difficult to distinguish these oscillation components using conventional frequency analysis. So, the research team, with help from Song Taegeun, a professor at the Kongju National University and used machine learning to differentiate oscillation components, even when beating patterns changed with the gate voltage. “This achievement shows that electrons can undergo quantum interference by moving through not only topological states but also ordinary electronic states,” said Bae Myung-Ho, Principal Research Scientist in the Quantum Device Group at KRISS in a press release. “To make use of only the desired topological state, it is important to precisely control doping and the gate so that the ordinary conduction state does not intervene.” “The principle of understanding and controlling the interference between different electronic states could also be applied to the design of topological quantum devices in the future,” concluded Choi Sang-Jun, a professor at the Department of Physics and Photon Science at the GIST in the press release. The research findings were published in the journal Nano Letters. Get the latest in engineering, tech, space & science - delivered daily to your inbox.Ameya is a science writer based in Hyderabad, India. A Molecular Biologist at heart, he traded the micropipette to write about science during the pandemic and does not want to go back. He likes to write about genetics, microbes, technology, and public policy.