  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. Contrary to electric charge screening, the spin screening cloud occurs
quantum coherently, forming spin-singlet entanglement with the impurity.
Although the spins interact locally around the impurity, the cloud can spread
out over micrometers. The Kondo cloud has never been detected to date, and its
existence, a fundamental aspect of the Kondo effect, remains as a long-standing
controversial issue. Here we present experimental evidence of a Kondo cloud
extending over a length of micrometers comparable to the theoretical length
$\xi_\mathrm{K}$. In our device, a Kondo impurity is formed in a quantum dot
(QD), one-sided coupling to a quasi-one dimensional
channel~\cite{Theory_Proposal_HS} that houses a Fabry-Perot (FP) interferometer
of various gate-defined lengths $L > 1 \, \mu$m. When we sweep a voltage on the
interferometer end gate separated from the QD by the length $L$ to induce FP
oscillations in conductance, we observe oscillations in measured Kondo
temperature $T_\mathrm{K}$, a sign of the cloud at distance $L$. For $L
\lesssim \xi_\mathrm{K}$ the $T_\mathrm{K}$ oscillation amplitude becomes
larger for the smaller $L$, obeying a scaling function of a single parameter
$L/ \xi_\mathrm{K}$, while for $L>\xi_\mathrm{K}$ the oscillation is much
weaker. The result reveals that $\xi_\mathrm{K}$ is the only length parameter
associated with the Kondo effect, and that the cloud lies mostly inside the
length $\xi_\mathrm{K}$ which reaches microns. Our experimental method of using
electron interferometers offers a way of detecting the spatial distribution of
exotic non-Fermi liquids formed by multiple magnetic impurities or multiple
screening channels and solving long-standing issues of spin-correlated systems.
