Cited by 272Advanced Thermodynamics and Statistical MechanicsAdvanced Thermodynamic Systems and EnginesQuantum Mechanics and Applications

The DOI matches its Crossref record; 61 of the 62 references it cites were found in a registry or an index; none of the 25 sources its claims cite is checked for support yet.

Markovian master equations for quantum thermal machines: local versus global approach

Open access
Hofer, Patrick P, Perarnau-Llobet, Martí, Miranda, L David M, Haack, Géraldine, Silva, Ralph, Brask, Jonatan Bohr, Brunner, Nicolas·2017·New Journal of Physics

Abstract

The study of quantum thermal machines, and more generally of open quantum systems, often relies on master equations. Two approaches are mainly followed. On the one hand, there is the widely used, but often criticized, local approach, where machine sub-systems locally couple to thermal baths. On the other hand, in the more established global approach, thermal baths couple to global degrees of freedom of the machine. There has been debate as to which of these two conceptually different approaches should be used in situations out of thermal equilibrium. Here we compare the local and global approaches against an exact solution for a particular class of thermal machines. We consider thermodynamically relevant observables, such as heat currents, as well as the quantum state of the machine. Our results show that the use of a local master equation is generally well justified. In particular, for weak inter-system coupling, the local approach agrees with the exact solution, whereas the global approach fails for non-equilibrium situations. For intermediate coupling, the local and the global approach both agree with the exact solution and for strong coupling, the global approach is preferable. These results are backed by detailed derivations of the regimes of validity for the respective approaches.

Overview

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TL;DR

The paper benchmarks local versus global Markovian master equations for quantum thermal machines against exact numerics for a quadratic heat engine. It finds the local approach is generally well justified for weakly interacting subsystems, while the global approach is preferable for strong inter-system coupling and in equilibrium.

Key findings

  • For the harmonic-oscillator heat engine, the local master equation agrees well with exact numerics when the inter-system coupling satisfies \(g \lesssim \Omega_a\), whereas the global approach fails out of equilibrium when \(g \lesssim \kappa_a\), even predicting a finite heat current as \(g \to 0\).
  • At intermediate coupling both approaches agree with exact numerics; at strong coupling the global approach becomes preferable. In equilibrium, the global approach outperforms the local approach except in the limit \(g \to 0\).
  • The local approach gives the heat-engine efficiency \(\eta = 1 - \Omega_c/\Omega_h\) and, with an external drive, does not violate thermodynamic laws. The global approach requires dressed-state definitions of heat currents; using local-like definitions instead gives \(P=0\) and \(J_h=-J_c\).
  • For the two-qubit entangler, the global approach again predicts a constant heat current as \(g \to 0\), while the local model predicts vanishing current; the two models agree well for intermediate coupling. No exact benchmark is available for this case.

Methods

  • The main model is a heat engine of two harmonic oscillators with frequencies \(\Omega_c\) and \(\Omega_h\), each coupled to a thermal bath and coupled to each other via an external drive. The full Hamiltonian is quadratic, so exact unitary dynamics of system plus finite baths is simulated at the level of covariance matrices.
  • Exact numerics use baths of about 400 oscillators, cutoff \(\omega_c=3\), and evolution time \(t \approx 20\,\tau_{\rm eq}\). These are compared with local and global Markovian master equations for steady-state heat currents, power, efficiency, and fidelity of reduced Gaussian states.
  • A finite-dimensional two-qubit entangler is also analyzed by comparing local and global GKSL master equations, without an exact benchmark.

Limitations

  • Stated: the exact benchmark is available only for the harmonic-oscillator heat engine; for the qubit entangler, conclusions are conjectural because no exact solution is provided.
  • Stated: the Hamiltonian renormalization is neglected, which causes a dip in local-approach fidelity near \(g \approx |\Sigma_h|\) and a small dependence on the bath cutoff in the appendix.
  • Stated/evident: the validity regimes are asymptotic and model-specific, derived for bosonic Ohmic baths; the local approach fails at strong coupling, the global approach fails at weak coupling and out of equilibrium, and analogous fermionic-bath conditions are noted as usually not fulfilled at low temperatures.

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Integrity signals

Registry

Crossref · 10.1088/1367-2630/aa964f

Cites

61 of 62 references verified0 citation errors · 1 not checkable

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Figures

Figure 1 Sketch of the heat engine. The system consists of two harmonic oscillators with different frequencies. Each oscillator couples to a thermal bath modeled by a collection of harmonic oscillators. An external fi eld mediates a weak coupling between the two oscillators. A thermal gradient can result in a heat fl ow from the hot to the cold bath, injecting some of the energy into the external fi eld which can in principle be used to charge a battery. A thermoelectric implementation of this machine in superconducting circuits is proposed in [ 44 ] . In this work, we compare different Markovian master equations for the description of this machine, using exact numerics as a benchmark.
Figure 1 Sketch of the heat engine. The system consists of two harmonic oscillators with different frequencies. Each oscillator couples to a thermal bath modeled by a collection of harmonic oscillators. An external fi eld mediates a weak coupling between the two oscillators. A thermal gradient can result in a heat fl ow from the hot to the cold bath, injecting some of the energy into the external fi eld which can in principle be used to charge a battery. A thermoelectric implementation of this machine in superconducting circuits is proposed in [ 44 ] . In this work, we compare different Markovian master equations for the description of this machine, using exact numerics as a benchmark.CC-BY · From the PDF
Figure 2 Comparison of steady states in equilibrium obtained from the local master equation ( see equation ( 12 )) , the global master equation ( see equation ( 23 )) , and exact numerics. ( ) a Fidelity between the states obtained from the master equation and the state obtained from exact numerics. ( ) b Other observables as a function of interaction strength. In the equilibrium case, = {ˆ ˆ } † a a Im 0 h c . Solid: local master equation, dashed: global master equation, dashed -dotted: exact numerics. The global master equation always outperforms the local master equation, except for the limit  g 0 , where the two approaches result in the same steady state. Parameters: W = W = 1 h c , k k = = 0.05 h c , = = k T k T 0.5 B c B h . Parameters numerics: w = 3 c , n = 400, k = t 20 , where t is the time we let the whole compound evolve to equilibrate.
Figure 2 Comparison of steady states in equilibrium obtained from the local master equation ( see equation ( 12 )) , the global master equation ( see equation ( 23 )) , and exact numerics. ( ) a Fidelity between the states obtained from the master equation and the state obtained from exact numerics. ( ) b Other observables as a function of interaction strength. In the equilibrium case, = {ˆ ˆ } † a a Im 0 h c . Solid: local master equation, dashed: global master equation, dashed -dotted: exact numerics. The global master equation always outperforms the local master equation, except for the limit  g 0 , where the two approaches result in the same steady state. Parameters: W = W = 1 h c , k k = = 0.05 h c , = = k T k T 0.5 B c B h . Parameters numerics: w = 3 c , n = 400, k = t 20 , where t is the time we let the whole compound evolve to equilibrate.CC-BY · From the PDF
Figure 3 Comparison of heat currents and other observables obtained from the local master equation ( see equation ( 12 )) , the global master equation ( see equation ( 23 )) , and exact numerics. ( ) a Heat current as a function of interaction strength. The local master equation performs very well even up to interaction strength k = W = g 2 10 c h . The global master equation breaks down for small g where the secular approximation is no longer justi fi ed. For higher values of g , the global master equation yields similar results to the local master equation and the exact numerics. The inset shows the fi delity between the state obtained from the master equations and the state obtained from exact numerics. ( ) b Other observables as a function of interaction strength. Solid: local master equation, dashed: global master equation, dashed -dotted: exact numerics. Again we observe the breakdown of the global approach for small g . Parameters: W = W = 1 h c , k k = = 0.05 h c , = k T 0.5 B c , = k T 5 B h . Parameters numerics: w = 3 c , n = 400, k = t 20 .
Figure 3 Comparison of heat currents and other observables obtained from the local master equation ( see equation ( 12 )) , the global master equation ( see equation ( 23 )) , and exact numerics. ( ) a Heat current as a function of interaction strength. The local master equation performs very well even up to interaction strength k = W = g 2 10 c h . The global master equation breaks down for small g where the secular approximation is no longer justi fi ed. For higher values of g , the global master equation yields similar results to the local master equation and the exact numerics. The inset shows the fi delity between the state obtained from the master equations and the state obtained from exact numerics. ( ) b Other observables as a function of interaction strength. Solid: local master equation, dashed: global master equation, dashed -dotted: exact numerics. Again we observe the breakdown of the global approach for small g . Parameters: W = W = 1 h c , k k = = 0.05 h c , = k T 0.5 B c , = k T 5 B h . Parameters numerics: w = 3 c , n = 400, k = t 20 .CC-BY · From the PDF
Image 4
CC-BY · From the PDF
Image 5
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Image 6
CC-BY · From the PDF
Figure 7 Heat engine performance as a function of W h . We fi nd good agreement between the local approach, the global approach, and the exact numerics except for ef fi ciencies close to the Carnot ef fi ciency ( here h = 0.5 C ) . The large differences in ef fi ciencies result from small differences in power and heat currents. Parameters: W = 1 c , k k = = 0.05 h c , g = 0.1, = k T 0.5 B c , = k T 1 B h . Parameters numerics: w = 3 c , n = 400, k = t 20 .
Figure 7 Heat engine performance as a function of W h . We fi nd good agreement between the local approach, the global approach, and the exact numerics except for ef fi ciencies close to the Carnot ef fi ciency ( here h = 0.5 C ) . The large differences in ef fi ciencies result from small differences in power and heat currents. Parameters: W = 1 c , k k = = 0.05 h c , g = 0.1, = k T 0.5 B c , = k T 1 B h . Parameters numerics: w = 3 c , n = 400, k = t 20 .CC-BY · From the PDF
Figure 8 Thermal machine consisting of two interacting qubits coupled to bosonic thermal baths.
Figure 8 Thermal machine consisting of two interacting qubits coupled to bosonic thermal baths.CC-BY · From the PDF
Image 9
CC-BY · From the PDF
Figure A1. Dependence of the numerical results on the choice of n and w c . Parameters: W = 1 c , W = 1 h k k = = 0.05 h c , w = 3 c , = k T 0.5 B c , = k T 5 B h . Horizontal lines show the predictions of the local master equation in equation ( 12 ) .
Figure A1. Dependence of the numerical results on the choice of n and w c . Parameters: W = 1 c , W = 1 h k k = = 0.05 h c , w = 3 c , = k T 0.5 B c , = k T 5 B h . Horizontal lines show the predictions of the local master equation in equation ( 12 ) .CC-BY · From the PDF

Verification

Claims the paper makes

24 Own interpretation · 23 Own result · 12 Not checked yet · 12 No basis given · 4 Own method

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  • It may state two separate claims as one. 21
  • The quote only partly states this claim. 2
  • Probably not a substantial claim. 4
25 of 25
  • The authors state this as an assumption.

    The baths are in thermal states.

    “where we assumed the baths to be in thermal states.”
  • The authors state this as an assumption.

    The renormalization of the Hamiltonian is neglected.

    “In the following, we will thus neglect this renormalization.”
    “As for the local approach, we will neglect the renormalization of the Hamiltonian in the following.”
  • The paper relies on this without support.

    If the sub-systems do not interact, each sub-system thermalizes to its respective bath.

    “Clearly, if the sub-systems do not interact, each sub-system should thermalize to its respective bath.”
  • The authors state this as an assumption.

    The dip in fidelity for the local approach arises from neglecting the renormalization of the Hamiltonian.

    “The dip in the fidelity for the local approach occurs at » S | | g h and is therefore assumed to arise from neglecting the renormalization of the Hamiltonian; see equation (16).”
  • The authors state this as an assumption.

    The first law of thermodynamics is enforced in the global approach.

    “We note that for the global approach, we enforce the first law while in the local approach, it follows from the expressions for heat currents and power.”
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Citations

Cites

Show all 61 references

Research works printed but not linked: 5

  1. Breuer H-P, 2002, The Theory of Open Quantum Systems
  2. 10.1016/bs.aamop.2015.07.002q3 · DOI
  3. Brask J B, 2015, New J. Phys., 17
  4. 10.1007/s10051-002-8904-5 · DOI
  5. Carmichael H, 1991, An Open Systems Approach to Quantum Optics

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Metadata

Authors
Hofer, P. P., Perarnau-Llobet, M., Miranda, L. D. M., Haack, G., Silva, R., Brask, J. B., & Brunner, N.
Affiliations
  • Max Planck Institute of Quantum Optics (DE)
  • University of Geneva (CH)
Venue
New Journal of Physics
Published
2017-12-20
Volume
19
Issue
12
Pages
123037
ISSN
1367-2630 (ISSN-L)
OpenAlex
W2740733769
Cited by
272
References
61
Citations by year
2026: 19 · 2025: 34 · 2024: 36 · 2023: 22 · 2022: 28 · 2021: 43 · 2020: 25 · 2019: 37 · 2018: 25 · 2017: 3
FWCI
12.75
Type
journal article
Language
en
Open access
Locations
  • New Journal of Physics (publishedVersion, cc-by)
  • arXiv (Cornell University) (publishedVersion)
  • edoc (University of Basel) (acceptedVersion)
  • DOAJ (DOAJ: Directory of Open Access Journals) (submittedVersion)
  • Lund University Publications (Lund University) (submittedVersion)
  • arXiv (Cornell University)
Funders
  • Alexander von Humboldt-Stiftung
  • European Cooperation in Science and Technology (Thermodynamics in the quantum regim)
  • Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung (200021_169002, Marie-Heim Vögtlin, 164466, QSIT, Starting Grant DIAQ)
Topics
  • Physical Sciences › Physics and Astronomy › Statistical and Nonlinear Physics › Advanced Thermodynamics and Statistical Mechanics
  • Physical Sciences › Engineering › Mechanical Engineering › Advanced Thermodynamic Systems and Engines
  • Physical Sciences › Physics and Astronomy › Atomic and Molecular Physics, and Optics › Quantum Mechanics and Applications
Keywords
quantum thermal machines, Markovian master equation, open quantum systems, heat current, non-equilibrium thermodynamics, weak and strong coupling, exact solution comparison

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