The Rabi model is a paradigm for interacting quantum systems.It couples a bosonic mode to the smallest possible quantum model, a two-level system.I present the analytical solution which allows us to consider the question of integrability for quantum systems that do not possess a classical limit.A criterion for quantum integrability is proposed which shows that the Rabi model is integrable due to the presence of a discrete symmetry.Moreover, I introduce a generalization with no symmetries; the generalized Rabi model is the first example of a nonintegrable but exactly solvable system.
The Rabi model is a paradigm for interacting quantum systems. It couples a bosonic mode to the smallest possible quantum model, a two-level system. I present the analytical solution which allows us to consider the question of integrability for quantum systems that do not possess a classical limit. A criterion for quantum integrability is proposed which shows that the Rabi model is integrable due to the presence of a discrete symmetry. Moreover, I introduce a generalization with no symmetries; the generalized Rabi model is the first example of a nonintegrable but exactly solvable system.
Quantum integrability is a cornerstone of the exact theory of interacting quantum spin chains. In its standard formulation, however, one starts from R-matrices satisfying the Yang--Baxter equation, rather than from the Hamiltonian itself. It has therefore remained unclear how Yang--Baxter solvability can be characterized directly at the Hamiltonian level, and how it is related to the existence of local conservation laws. Here we prove that, in a broad standard setting, the Reshetikhin condition is not only necessary but also sufficient for Yang--Baxter integrability, thereby reducing the hidden algebraic structure of integrability to a Hamiltonian-level conservation law. Since the Reshetikhin condition is equivalent to conservation of the total energy current, this Hamiltonian-level criterion is also experimentally accessible. This result establishes a quantum counterpart of the Liouville--Arnold theorem for isotropic spin chains, stating that Yang--Baxter solvability is equivalent to an infinite hierarchy of local conserved quantities. Our result also simplifies substantially the search for integrable spin chains by replacing the search for R-matrices with a direct criterion on local Hamiltonians.
Abstract We investigate the integrability of the Rabi model, which is traditionally viewed as not Yang-Baxter integrable despite its solvability. Building on efforts by Bogoliubov, Amico, Batchelor, and Zhou, who explored special limiting cases of the model, we develop a spin-boson interaction Hamiltonian under more general boundary conditions, particularly focusing on open boundary conditions with off-diagonal terms. Our approach maintains the direction of the spin in the
z direction, and also preserves the boson particle number operator
a†a, marking a progression beyond previous efforts that have primarily explored reduced forms of the Rabi model from Yang-Baxter algebra. We also address the presence of ``unwanted" quadratic boson terms
a2 and
a†2, which share coefficients with the boson particle number operator. Interestingly, these terms vanish when spectral parameter
u=±θs, simplifying the model to a limiting case of operator-valued twists, a scenario previously discussed by Batchelor and Zhou.
The simplified two-qubit Rabi model is proposed and its analytical solution is presented. There are no level crossings in the spectral graph of the model, which indicates that it is not integrable. The criterion of integrability for the Rabi model proposed by Braak (2011 Phys. Rev. Lett. 107 100401) is also used for the simplified two-qubit Rabi model and the same conclusion, consistent with what the spectral graph shows, can be drawn, which indicates that the criterion remains valid when applied to the two-qubit case. The simplified two-qubit Rabi model is another example of a non-integrable but exactly solvable system except for the generalized Rabi model.