Extensively degenerate superradiant phase in a giant-atom quantum Rabi ring

@misc{he2026extensively,
  title = {{Extensively degenerate superradiant phase in a giant-atom quantum Rabi ring}},
  author = {Shu He and Li-Wei Duan},
  howpublished = {arXiv},
  year = {2026},
  month = oct,
  doi = {10.48550/arxiv.2610.09880},
  eprint = {2610.09880},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2610.09880},
}

TY  - UNPB
TI  - Extensively degenerate superradiant phase in a giant-atom quantum Rabi ring
AU  - Shu He
AU  - Li-Wei Duan
PY  - 2026
DA  - 2026/10/07
PB  - arXiv
DO  - 10.48550/arxiv.2610.09880
UR  - https://arxiv.org/abs/2610.09880
AB  - We show that an N-site quantum Rabi ring with matched giant-atom couplings has 2^N degenerate superradiant configurations as the atomic-to-cavity frequency ratio η=Δ/ω tends to infinity. Each atom couples to two cavities two sites apart, and the photon hopping carries a phase π/2. Tuning the ratio of contact strengths to the photon hopping reduces the mean-field energy to a sum of single-site Rabi energies, with a common critical coupling for all momentum-pair sectors. The degeneracy survives quadratic quantum fluctuations despite a dispersive excitation spectrum. At finite η, second-order processes involving the local cubic anharmonicity generate a same-sublattice Ising coupling of order ω/η, selecting ferro- and antiferro-superradiant configurations. This quantum order-by-disorder effect has no classical counterpart. Four-site calculations retaining all spin states quantitatively support the predicted energy coefficients. The separation between the induced coupling and the excitation gap allows a broad temperature interval with macroscopic photon occupation and short-range correlations between displacement signs.
ER  -