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Physical Review E

Lasing process in a closed bipartite quantum system: A thermodynamical analysis

G. Waldherr, G. Mahler

Published Jun 15, 2010• 3 citations
Publisher

Abstract

Closed weakly bound bipartite quantum systems typically exhibit relaxation behavior with respect to the smaller subsystem. Here, we investigate a model composed of a finite spin network with one interfacing spin being coupled to a single electromagnetic field mode via the Jaynes-Cummings interaction. The initial pure state of the system can be chosen such that the resulting thermodynamical relaxation process is lasing/nonlasing relaxation or energy back flow from the field mode. We examine the properties of the field mode with quantum optical methods. During the lasing process, the field mode is in a phase-diffused Glauber state with no optical coherence. The thermodynamical analysis of our system is consistent with this finding: The total energy exchanged between both subsystems is found to be heat only. Yet the mapping of this function onto a thermodynamic heat engine appears to be of limited value.

DOI
10.1103/physreve.81.061122

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Closed weakly bound bipartite quantum systems typically exhibit relaxation behavior with respect to the smaller subsystem. Here, we investigate a model composed of a finite spin network with one interfacing spin being coupled to a single electromagnetic field mode via the Jaynes-Cummings interaction. The initial pure state of the system can be chosen such that the resulting thermodynamical relaxation process is lasing/nonlasing relaxation or energy back flow from the field mode. We examine the properties of the field mode with quantum optical methods. During the lasing process, the field mode is in a phase-diffused Glauber state with no optical coherence. The thermodynamical analysis of our system is consistent with this finding: The total energy exchanged between both subsystems is found to be heat only. Yet the mapping of this function onto a thermodynamic heat engine appears to be of limited value.
The single-atom version of the quantum heat engine based on electromagnetically induced transparency is theoretically investigated in this paper. In contact with a hot and a cold bath, the atom trapped in an optical cavity is coherently coupled by a coherent field, thus generating photons as output through a process that mimics a heat engine. Using semiclassical theory, we obtain the conditions for the classical gain parameter being positive. Then by taking a full quantum approach, we discuss the statistical property of the generated photons depicted by the photon number distribution and Wigner function, and show that it can be effectively tuned by the coupling field. Investigations on the energy and entropy balances of the heat engine process lead to an interesting result that the power of the engine is simply the energy per time provided by the hot reservoir. The coupling field plays a critical role in supporting the heat-engine process. However, the energy that it provides is all dumped into the cold reservoir. The total energy exchange conforms to the first law of thermodynamics. The efficiency of the engine depends on the temperatures of the two reservoirs and the strength of the coupling field. The entropy change of the heat-engine process suggests that when working far above the threshold of lasing, the efficiency approaches that of its atomic-ensemble-based partner.
Thermodynamics of a three-level maser was studied in the pioneering work of Scovil and Schulz-DuBois [Phys. Rev. Lett. 2, 262 (1959)]. In this work we consider the same three-level model, but treat both the matter and the light quantum mechanically. Specifically, we analyze an extended (three-level) dissipative (ED) Jaynes-Cummings model (JCM) within the framework of a quantum heat engine, using formulas for heat flux and power in bipartite systems introduced in our previous work [E. Boukobza and D. J. Tannor Phys. Rev. A 74, 063823 (2006)] Amplification of the selected cavity mode occurs even in this simple model, as seen by a positive steady state power. However, initial field coherence is lost, as seen by the decaying off-diagonal field density matrix elements, and by the Husimi-Kano QQQ function. We show that after an initial transient time the field's entropy rises linearly during the operation of the engine, which we attribute to the dissipative nature of the evolution and not to matter-field entanglement. We show that the second law of thermodynamics is satisfied in two formulations (Clausius, Carnot) and that the efficiency of the ED JCM heat engine agrees with that defined intuitively by Scovil and Schulz-DuBois. Finally, we compare the steady state heat flux and power of the fully quantum model with the semiclassical counterpart of the ED JCM, and derive the engine efficiency formula of Scovil and Schulz-DuBois analytically from fundamental thermodynamic fluxes.
Heat and work for quantum systems governed by dissipative master equations with a time-dependent driving field were introduced in the pioneering work of Alicki [J. Phys. A 12, L103 (1979)]. Alicki's work was in the Schr\"odinger picture; here we extend these definitions to the Heisenberg and interaction pictures. We show that, in order to avoid consistency problems, the full time derivatives in the definitions for heat flux and power (work flux) should be replaced by partial time derivatives. We also present an alternative approach to the partitioning of the energy flux which differs from that of Alicki in that the instantaneous interaction energy with the external field is not included directly. We then proceed to generalize Alicki's definition of power by replacing the original system and its external driving field with a larger, bipartite system, governed by a time-independent Hamiltonian. Using the definition of heat flux and the generalized definition of power, we derive the first law of thermodynamics in differential form, for both the full bipartite system and the partially traced subsystems. Although the second law (Clausius formulation) is satisfied for the full bipartite system, we find that in general there is no rigorous formulation of the second law for the partially traced subsystem unless certain additional requirements are met. Once these requirements are satisfied, however, both the Carnot and the Clausius formulations of the second law are satisfied. We illustrate this thermodynamic analysis on both the simple Jaynes-Cummings model and an extended dissipative Jaynes-Cummings model, which is a model for a quantum amplifier.
We investigate a two-level system in contact with a larger quantum environment, often consisting of many two-level systems itself (spin environment) which may or may not interact. The total system is considered to be closed. The environment typically is in a canonical state with a given temperature initially. Depending on the precise spectral structure of the environment and the type of coupling between both systems, the smaller part may relax to a canonical state with the same temperature as the environment (i.e., thermal relaxation) or to some other quasiequilibrium state (nonthermal relaxation). The type of (quasi)equilibrium state can be related to the distribution of certain properties of the energy eigenvectors of the total system. We examine these distributions for several abstract and concrete (spin environment) Hamiltonian systems; the significant aspect of these distributions can be related to the relative strength of the local and interaction parts of the Hamiltonian.
Distinctive features in the sub-dynamics of an interacting bipartite system are explored. The sub-dynamic Heisenberg operators are introduced in a novel way leading to the quasi-particle-like concept in the respective subsystems. This formulation is illustrated using the known exact solutions of the generic (Jaynes - Cummings) model of interacting bipartite system of a spin 1/2 particle and a single mode electromagnetic field (more generally, boson field). The striking effects of interaction and entanglement are reflected in the dressing of the non-interacting number and spin representations, displaying quasi-particle-like properties. This development may be at the heart of the physics of back - action discussed in the recent literature.
We investigate the decoherence of a spin- 1/2 subsystem weakly coupled to an environment of many spins- 1/2 with and without mutual coupling. The total system is closed, its state is pure, and evolves under Schrödinger dynamics. Nevertheless, the considered spin typically reaches a quasistationary equilibrium state. Here we show that this state depends strongly on the coupling to the environment on the one hand and on the coupling within the environmental spins on the other. In particular we focus on spin star and spin ring-star geometries to investigate the effect of intra-environmental coupling on the central spin. By changing the spectrum of the environment, its effect as a bath on the central spin is also changed and may even be adjustable to some degree. We find that the relaxation behavior is related to the distribution of the energy eigenstates of the total system. For each of these relaxation modes, there is a dual mode for which the resulting subsystem approaches an inverted state occupation probability (negative temperature).