Nature 474, 627–630 (2011)

The Kondo effect — a phenomenon that involves an interaction between a confined spin and the spins of an electronic reservoir — is commonly used to shape how the electrical resistivity of a metal changes with temperature, particularly at very low temperatures. The scattering of conduction charges creates a resistance minimum at a non-zero temperature and ensures finite resistance at absolute zero temperature. Christian Latta and collaborators from Switzerland, Germany and the USA have now shown the first optical signatures of the Kondo effect using a charge-tunable quantum dot. Using a gate voltage to tune the charge state of the dot, the researchers observed photoluminescence spectra, an absorption line shape and a voltage dependence of the peak absorption energy — all of which agree with theoretical predictions. They also found that photon absorption can turn off the exchange interaction between the quantum dot electron and the electron reservoir; that is, when the local scattering potential for the reservoir is turned off, the Kondo effect is quantum-quenched.