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Journal of Physics A Mathematical and General

Ground-state symmetry classification for a non-isolated tunnelling particle

G. Benivegna, ANTONINO MESSINA, Elisabetta Paladino

Published May 21, 1996• 3 citations
Publisher

Abstract

We demonstrate that the ground energy of a two-level system coupled with a bosonic environment can be labelled with a quantum number related to the total excitation number operator and independent of the coupling strength. Our approach is exact and is based on operator methods combining symmetry considerations with general properties of the lowest energy state. The relevance of our result in connection with the nature of the transition from the weak to strong coupling regime is briefly discussed.

DOI
10.1088/0305-4470/29/10/026

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We demonstrate that the ground energy of a two-level system coupled with a bosonic environment can be labelled with a quantum number related to the total excitation number operator and independent of the coupling strength. Our approach is exact and is based on operator methods combining symmetry considerations with general properties of the lowest energy state. The relevance of our result in connection with the nature of the transition from the weak to strong coupling regime is briefly discussed.
We present some characteristic properties of the ground state of a two-level system linearly coupled to a phonon field. Such properties are valid over the whole range of the parameters appearing in the Hamiltonian of the system. The knowledge of these exact results provides a physical basis for an accurate variational determination of the ground state. A physical picture of the self-trapping transition is discussed in terms of particle-field correlations. Our treatment is based on operator methods combining symmetry considerations with some general properties of the lowest energy state.
We present some characteristic properties of the ground state of a two-level unit linearly coupled to a bosonic field, exactly deduced without previously solving the eigenvalue problem for the physical system. Such properties are valid in the whole range of the characteristic parameters appearing in the Hamiltonian modelling this system. Our treatment is based on operatorial methods which combine symmetry considerations with some general peculiarities of the lowest energy state. In particular we present rigorous conclusions concerning the expectation value of the two-level unit energy and the mean value of the interaction energy for arbitrary strenght of the coupling constant. The relevance of these rigorous results in connection with an approximate variational determination of the ground state of the system is evidenced.
By using a variation method we study the ground state of a tunneling particle coupled to many bosons in a double-well potential. The boson system is assumed to have dense low-energy excitations, just as taken by Chakravarty and Leggett to discuss the dissipative effect in macroscopic quantum phenomena. We examine reduction of the tunneling matrix and eventual localization of the particle due to boson excitations for arbitrary strengths of tunneling and coupling parameters. Ground state energy is evaluatied. Continuous as well as discontinuous changes from tunneling to localized states are found when the coupling parameter increases. The results are related to the orthogonality of many-particle ground states, as known for many-electron system as Anderson's theorem.
The introduction of an asymmetric term into the quantum Rabi model generally lifts energy-level degeneracies. However, when the asymmetry parameter takes specific multiples of the bosonic mode frequency, level degeneracies reappear−-−a phenomenon referred to as the hidden symmetry in the asymmetric quantum Rabi model. Identifying the origin of this hidden symmetry and its explicit operator form constitutes two central tasks in studying this system. Here, we investigate the origin of this hidden symmetry using the method of two successive diagonalizations, with a focus on physics in the regime where the ratio between the two-level splitting ΔΔΔ and the mode frequency ωωω satisfies Δ/ω≫1Δ/ω\gg 1Δ/ω≫1. We find that the hidden symmetry stems from energy-level matching within the asymmetric double-well potential, a picture strongly supported by the wavefunctions of both the ground and excited states. Moreover, the emergence of an excited-state quantum phase transition is identified and qualitatively discussed, which arises from the breaking and restoration of this hidden symmetry across different coupling regimes. Our results provide deeper insight into the physics of the asymmetric quantum Rabi model, particularly in the previously less-explored strong-coupling regime where Δ/ω≫1Δ/ω\gg 1Δ/ω≫1.
We study the ground state of the attractive one-dimensional Bose-Hubbard model, and in particular the nature of the crossover between the weak interaction and strong interaction regimes for finite system sizes. Indicator properties such as the gap between the ground and first excited energy levels, and the incremental ground-state wave function overlaps are used to locate different regimes. Using mean-field theory we predict that there are two distinct crossovers connected to spontaneous symmetry breaking of the ground state. The first crossover arises in an analysis valid for large LLL with finite NNN, where LLL is the number of lattice sites and NNN is the total particle number. An alternative approach valid for large NNN with finite LLL yields a second crossover. For small system sizes we numerically investigate the model and observe that there are signatures of both crossovers. We compare with exact results from Bethe ansatz methods in several limiting cases to explore the validity for these numerical and mean-field schemes. The results indicate that for finite attractive systems there are generically three ground-state phases of the model.
We determine the low-energy properties of a trapped Bose gas split in two by a potential barrier over the whole range of barrier heights and asymmetry between the wells. For either weak or strong coupling between the wells, our two-mode theory yields a two-site Bose-Hubbard Hamiltonian with the tunneling, interaction, and bias parameters calculated simply using an explicit form of two mode functions. When the potential barrier is relatively low, most of the particles occupy the condensate mode and our theory reduces to a two-mode version of the Bogoliubov theory, which gives a satisfactory estimate of the spatial shape and energy of the lowest collective excitation. When the barrier is high, our theory generalizes the standard two-site Bose-Hubbard model into the case of asymmetric modes, and correctly predicts a full separation of the modes in the limit of strong separation of the wells. We provide explicit analytic forms for the number squeezing and coherence as a function of particle number and temperature. We compare our theory to other two-mode theories for bosons in a double well and discuss their validity in different parameter regimes.