M. S. Kim

Harvesting multi-qubit entanglement from ultrastrong interactions in circuit QED

  1. Federico Armata,
  2. Giuseppe Calajo,
  3. Tuomas Jaako,
  4. M. S. Kim,
  5. and Peter Rabl
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.
arxiv pdf

Recurrent delocalization and quasi-equilibration of photons in coupled circuit QED systems

  1. Myung-Joong Hwang,
  2. M. S. Kim,
  3. and Mahn-Soo Choi
We explore the photon population dynamics in two coupled circuit QED systems. For a sufficiently weak inter-cavity photon hopping, as the photon-cavity coupling increases, the dynamics
undergoes double transitions first from a delocalized to a localized phase and then from the localized to another delocalized phase. The latter delocalized phase is distinguished from the former one; instead of oscillating between the two cavities, the photons rapidly quasi-equilibrate over the two cavities. These intrigues are attributed to an interplay between two qualitatively distinctive nonlinear behaviors of the circuit QED systems in the utrastrong coupling regime, whose distinction has been widely overlooked.
arxiv pdf