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arXiv:0909.1230

Spontaneous symmetry breaking in thermalization and anti-thermalization

Jie‐Qiao Liao, Hang Dong, X. G. Wang, X. F. Liu, C. P. Sun

Published Sep 7, 2009• 0 citations
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Abstract

The phenomenon of spontaneous symmetry breaking is investigated in the dynamic thermalization of a degenerate quantum system. A three-level system interacting with a heat bath is carefully studied to this end. It is shown that the three-level system with degenerate ground states might have different behaviors depending on the details of the interaction with the heat bath when the temperature approaches zero. If we introduce an external field to break the degeneracy of the ground states and let it approach zero after letting the temperature approach zero, then two possibilities will arise: the steady state is a definite one of the degenerate states independent of the initial state, or the steady state is dependent on the initial state in a complicated way. The first possibility corresponds to a spontaneous symmetry breaking of the system and the second one implies that the heat bath could not totally erase the initial information in certain cases.

DOI
10.48550/arxiv.0909.1230

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The phenomenon of spontaneous symmetry breaking is investigated in the dynamic thermalization of a degenerate quantum system. A three-level system interacting with a heat bath is carefully studied to this end. It is shown that the three-level system with degenerate ground states might have different behaviors depending on the details of the interaction with the heat bath when the temperature approaches zero. If we introduce an external field to break the degeneracy of the ground states and let it approach zero after letting the temperature approach zero, then two possibilities will arise: the steady state is a definite one of the degenerate states independent of the initial state, or the steady state is dependent on the initial state in a complicated way. The first possibility corresponds to a spontaneous symmetry breaking of the system and the second one implies that the heat bath could not totally erase the initial information in certain cases.
Spontaneous symmetry breaking (SSB) in quantum systems, such as ferromagnets, is normally described as (or as arising from) degeneracy of the ground state; however, it is well established that this degeneracy only occurs in spatially infinite systems, and even better established that ferromagnets are not spatially infinite. I review this well-known paradox, and consider a popular solution where the symmetry is explicitly broken by some external field which goes to zero in the infinite-volume limit; although this is formally satisfactory, I argue that it must be rejected as a physical explanation of SSB since it fails to reproduce some important features of the phenomenology. Motivated by considerations from the analogous classical system, I argue that SSB in finite systems should be understood in terms of the approximate decoupling of the system's state space into dynamically-isolated sectors, related by a symmetry transformation; I use the formalism of decoherent histories to make this more precise and to quantify the effect, showing that it is more than sufficient to explain SSB in realistic systems and that it goes over in a smooth and natural way to the infinite limit.
Spontaneous symmetry breaking is a ubiquitous phenomenon in nature. One of the defining features of symmetry-broken phases is that the large system size limit and the vanishing external field limit do not commute. In this work, we study a family of extensions of the NNN-state clock model. We find that the exact symmetry and the ground state degeneracy under the periodic boundary condition heavily depend on the system size, although the model has the manifest translation symmetry. In particular, the ground state can be unique and all excitations are gapped even when the phase exhibits noncommutativity of the two limits. Our model hence poses a question on the standard understanding of spontaneous symmetry breaking.
Providing the microscopic behavior of a thermalization process has always been an intriguing issue. There are several models of thermalization, which often require interaction of the system under consideration with the microscopic constituents of the macroscopic heat bath. With the aim to simulate such a thermalization process, here we look at the thermalization of a two-level quantum system under the action of a Markovian master equation corresponding to memory-less action of a heat bath, kept at a certain temperature, using a single-qubit ancilla. A two-qubit interaction Hamiltonian ( H th , say) is then designed —with a single-qubit thermal state as the initial state of the ancilla— which gives rise to thermalization of the system qubit in the infinite time limit. Further, we study the general form of Hamiltonian, of which ours is a special case, and look for the conditions for thermalization to occur. We also derive a Lindblad-type non-Markovian master equation for the system dynamics under the general form of system-ancilla Hamiltonian.
We study analytically the quantum thermalization of two coupled two-level systems (TLSs), which are connected with either two independent heat baths (IHBs) or a common heat bath (CHB). We understand the quantum thermalization in eigenstate and bare-state representations when the coupling between the two TLSs is stronger and weaker than the TLS-bath couplings, respectively. In the IHB case, we find that, when the two IHBs have the same temperatures, the two coupled TLSs in eigenstate representation can be thermalized with the same temperature as those of the IHBs. However, in the case of two IHBs at different temperatures, just when the energy detuning between the two TLSs satisfies a special condition, the two coupled TLSs in eigenstate representation can be thermalized with an immediate temperature between those of the two IHBs. In bare-state representation, we find a counterintuitive phenomenon that, under some conditions, the temperature of the TLS connected with the high-temperature bath is lower than that of the other TLS, which is connected with the low-temperature bath. In the CHB case, the coupled TLSs in eigenstate representation can be thermalized with the same temperature as that of the CHB in nonresonant cases. In bare-state representation, the TLS with a larger energy separation can be thermalized to a thermal equilibrium with a lower temperature. In the resonant case, we find a phenomenon of antithermalization. We also study the steady-state entanglement between the two TLSs in both the IHB and CHB cases.
We show that the spontaneous symmetry breaking can be defined also for finite systems based on the properly defined jump probability between the ground states in the 2d and 3d Ising models on a square and a cubic lattice respectively. Our analysis reveals the existence of an interval in the temperature (control parameter) space within which the spontaneous symmetry breaking takes place. The upper limit of this region is the pseudocritical point where the symmetric vacuum bifurcates in energetically degenerate non-symmetric vacua, initiating the spontaneous symmetry breaking process. The lower limit, identified as the temperature value at which the spontaneous symmetry breaking is completed, is characterized by maximal characteristic time for the decay of magnetization (order parameter) auto-correlations. We argue that this anomalous enhancement of auto-correlations is attributed to the transition from type I to on-off intermittency in the order parameter dynamics. Possible phenomenological implications of this behaviour are briefly discussed.
We propose a generalization of the eigenstate thermalization hypothesis accountin g for the emergence of symmetry-breaking phases. It consists of two conditions that any system with a degenerate spectrum must fulfill in order to thermalize. The failure of each condition generates a different nonthermalizing scenario. One is due to the absence of chaos and may indicate that extra constants of motion are required to describe equilibrium states. The other one implies the existence of initial conditions evolving toward symmetry-breaking equilibrium states. If it spreads across an entire spectral region, then this region gives rise to a symmetry-breaking phase. We explore the applicability of this formalism by means of numerical experiments on a three-site Bose-Hubbard model with two noncommuting discrete symmetries.