  We study the dynamical decoherence of a qubit weakly coupled to a two-body
random interaction model (TBRIM) describing a quantum dot of interacting
fermions or the Sachdev-Ye-Kitaev (SYK) black hole model. We determine the
rates of qubit relaxation and dephasing for regimes of dynamical thermalization
of the quantum dot or of quantum chaos in the SYK model. These rates are found
to correspond to the Fermi golden rule and quantum Zeno regimes depending on
the qubit-fermion coupling strength. An unusual regime is found where these
rates are practically independent of TBRIM parameters. We push forward an
analogy between TBRIM and quantum small-world networks with an explosive
spreading over exponentially large number of states in a finite time being
similar to six degrees of separation in small-world social networks. We find
that the SYK model has approximately two-three degrees of separation.
