# Vacuum degeneracy of a circuit-QED system in the ultrastrong coupling regime

Abstract : We investigate theoretically the quantum vacuum properties of a chain of $N$ superconducting Josephson atoms inductively coupled to a transmission line resonator. We derive the quantum field Hamiltonian for such circuit-QED system, showing that, due to the type and strength of the interaction, a quantum phase transition can occur with a twice degenerate quantum vacuum above a critical coupling. In the finite-size case, the degeneracy is lifted, with an energy splitting decreasing exponentially with increasing values of $g^2 N^2$, where $g$ is the dimensionless vacuum Rabi coupling per artificial atom. We determine analytically the ultrastrong coupling asymptotic expression of the two degenerate vacua for an arbitrary number of artificial atoms and of resonator modes. In the ultrastrong coupling regime the degeneracy is protected with respect to random fluctuations of the transition energies of the Josephson elements.
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Journal articles
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Cited literature [7 references]

https://hal.archives-ouvertes.fr/hal-00418515
Contributor : Cristiano Ciuti <>
Submitted on : Friday, January 15, 2010 - 5:28:39 PM
Last modification on : Monday, November 9, 2020 - 11:35:19 AM
Long-term archiving on: : Thursday, September 23, 2010 - 6:06:16 PM

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### Citation

Pierre Nataf, Cristiano Ciuti. Vacuum degeneracy of a circuit-QED system in the ultrastrong coupling regime. Physical Review Letters, American Physical Society, 2010, 104, pp.023601. ⟨10.1103/PhysRevLett.104.023601⟩. ⟨hal-00418515v3⟩

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