An international research group — in which Professor Heung-Sun Sim of KAIST’s Department of Physics collaborated with researchers from RIKEN, City University of Hong Kong, University of Tokyo, and Ruhr-University of Bochum — have observed the Kondo screening cloud for the first time, ending a 50-year quest to prove its existence.
On March 11, the research was published in Nature titled “Observation of the Kondo Screening Cloud”, presenting experimental evidence of the Kondo effect. This quantum phenomenon is a spin cloud that masks magnetic impurities in a metal. The spins of conduction electrons form spin-singlet entanglement with the localized magnetic impurities, effectively screening them. Although this phenomenon is important in many physical phenomena, such as high-temperature superconductivity, its physical existence had not been detected despite various approaches.
Professor Sim proposed a new method of using electrostatics to detect the Kondo cloud. In the research, a Kondo impurity was formed in a quantum dot, a type of “artificial atom”, and restricted to exist inside a quasi-one-dimensional channel. A Fabry-Pérot interferometer, which is a type of optical device used to control and measure the wavelengths of radiation, was used to observe changes in the Kondo temperature. This temperature is correlated to the region’s conductance, and therefore the strength and size of the spin coupling cloud around the impurity. When a voltage was applied at different points along the channel, a controllable wave of scattered electrons returned to the quantum dot impurity and interfered with it. The induced oscillations observed in the measured Kondo temperature were a sign of the quantum Kondo screening cloud, which acts to preserve the wave nature of electrons inside that particular region.
After analyzing the oscillations, which were in the region of only 100mK (millikelvin), the research team concluded that the observed length of the Kondo cloud was comparable to the theoretical length that had been predicted, several micrometers, and that the shape of the cloud was universally proportional to the inverse of the Kondo temperature.
Professor Sim commented that “it is remarkable in a fundamental and technical point of view that such a large quantum object can now be created, controlled, and detected” and expressed hope that “electrostatically controlling Kondo clouds will pave the way to understanding other magnetic phenomena or utilizing quantum entanglement to develop future quantum information processing devices.”