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Doubling the quantum vacuum

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Doubling the quantum vacuum of a superconductor resonator

Is it possible to have two different quantum electrodynamical vacua (ground states) with the same energy in a solid-state chip ? Is such a vacuum ‘degeneracy’ robust with respect to fluctuations of the circuit parameters ? In a PRL paper by P. Nataf and C. Ciuti, it shown that these fascinating vacuum properties are indeed possible in a superconducting transmission line resonator embedding a chain of N artificial ‘fluxonium’ atoms (based on Josephson junctions). The authors show that the type of the fluxonium-resonator coupling and its ultrastrong caracter are the key physical reasons. The vacuum degeneracy is exact in the limit of a large number of fluxoniums and/or for large values of the coupling per atom. In practise, in the finite-size case, the lifting of the degeneracy can be made exponentially small by increasing the number of artificial fluxonium atoms. Such exponential ‘protection’ is shown also to be resilient in presence of random fluctuations of the energy of the individual fluxonium atoms in the chain. The present study may also have profound implications for a novel encoding of quantum information in two degenerate quantum electrodynamical vacua, forming a sort of robust vacuum qubit.

- P. Nataf, C. Ciuti, Vacuum degeneracy of a circuit-QED system in the ultrastrong coupling regime,
Phys. Rev. Lett. 104, 023601 (2010); arXiv:0909.3505

Contact: C. Ciuti