Observation of the Kondo screening cloud

When a magnetic impurity exists in a metal, conduction electrons form a spin cloud that screens the impurity spin. This basic phenomenon is called the Kondo effect^1,2. Unlike electric-charge screening, the spin-screening cloud^3-6 occurs quantum coherently, forming spin-singlet entanglement with the impurity. Although the spins interact locally around the impurity, the Kondo cloud can theoretically spread out over several micrometres. The cloud has not so far been detected, and so its physical existence-a fundamental aspect of the Kondo effect-remains controversial^7,8. Here we present experimental evidence of a Kondo cloud extending over a length of micrometres, comparable to the theoretical length ξ_K. In our device, a Kondo impurity is formed in a quantum dot^2,9-11, coupling on one side to a quasi-one-dimensional channel¹² that houses a Fabry-Pérot interferometer of various gate-defined lengths L exceeding one micrometre. When we sweep a voltage on the interferometer end gate-separated by L from the quantum dot-to induce Fabry-Pérot oscillations in conductance we observe oscillations in the measured Kondo temperature T_K, which is a signature of the Kondo cloud at distance L. When L is less than ξ_K the T_K oscillation amplitude becomes larger as L becomes smaller, obeying a scaling function of a single parameter L/ξ_K, whereas when L is greater than ξ_K the oscillation is much weaker. Our results reveal that ξ_K is the only length parameter associated with the Kondo effect, and that the cloud lies mostly within a length of ξ_K. Our experimental method offers a way of detecting the spatial distribution of exotic non-Fermi liquids formed by multiple magnetic impurities or multiple screening channels^13-16 and of studying spin-correlated systems.