  The Kondo effect, a hallmark of strong correlation physics, is characterized
by the formation of an extended cloud of singlet states around magnetic
impurities at low temperatures. While many implications of the Kondo cloud's
existence have been verified, the existence of the singlet cloud itself has not
been directly demonstrated. We suggest a route for such a demonstration by
considering an observable that has no classical analog, but is still
experimentally measurable: "singlet weights", or projections onto particular
entangled two-particle states. Using approximate theoretical arguments, we show
that it is possible to construct highly specific energy- and position-resolved
probes of Kondo correlations. Furthermore, we consider a quantum transport
setup that can be driven away from equilibrium by a bias voltage. There, we
show that singlet weights are enhanced by voltage even as the Kondo effect is
weakened by it. This exposes a patently nonequilibrium mechanism for the
generation of Kondo-like entanglement that is inherently different from its
equilibrium counterpart.
