We analyze a multi-qubit 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 multi-partite entangled
states of non-interacting 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.