The Kondo problem is a few-body problem

Quenching of local moments in metals generally induces long-range electronic correlations known as the Kondo screening cloud. We prove that this non-locality is not intrinsically many-body in character, but is rather encoded in the spatial dependence of the natural one-body orbitals that are eigenstates of the single-particle density matrix. Limiting electronic interactions to a few of these orbitals, we find that Kondo correlated ground states can be converged in the thermodynamic limit to exponential accuracy by numerical diagonalization of the resulting few-body problem. An emergent many-body character is only observed for exponentially large Kondo lengths, i.e. in close vicinity to a quantum critical point.