  Measurements of the persistent current in a ring containing a quantum dot
would afford a unique opportunity to finally detect the elusive Kondo screening
cloud. We present the first large-scale numerical results on this controversial
subject using exact diagonalization and density matrix renormalization group
(RG). These extremely challenging numerical calculations confirm RG arguments
for weak to strong coupling crossover with varying ring length and give results
on the universal scaling functions. We also study, analytically and
numerically, the important and surprising effects of particle-hole symmetry
breaking.
