Harvesting Multiqubit Entanglement from Ultrastrong Interactions in Circuit Quantum Electrodynamics

Abstract

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.

14 pages, 10 figures

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• Efficient tensor-network simulation for the few-atom multimode Dicke model via coupling-matrix transformation
• Eigenstates of two-level systems in a single-mode quantum field: from quantum Rabi model to $N$-atom Dicke model
• Arbitrary gauge quantisation of light-matter theories with time-dependent constraints
• Tripartite Entanglement in Multimode Cavity Quantum Electrodynamics