  We analyze a multiqubit circuit QED system in the regime where the
qubit-photon coupling dominates over the system's bare energy scales. Under
such conditions a manifold of low-energy states with a high degree of
entanglement emerges. Here we describe a time-dependent protocol for extracting
these quantum correlations and converting them into well-defined multipartite
entangled states of noninteracting qubits. Based on a combination of various
ultrastrong-coupling effects, the protocol can be operated in a fast and robust
manner, while still being consistent with experimental constraints on switching
times and typical energy scales encountered in superconducting circuits.
Therefore, our scheme can serve as a probe for otherwise inaccessible
correlations in strongly coupled circuit QED systems. It also shows how such
correlations can potentially be exploited as a resource for entanglement-based
applications.
