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J. Gemmer M. Michel G. Mahler

Quantum Thermodynamics
Emergence of Thermodynamic Behavior
Within Composite Quantum Systems

123


Authors
J. Gemmer
Universit¨at Osnabr¨uck
FB Physik
Barbarastr. 7
49069 Osnabr¨uck, Germany

M. Michel
G. Mahler
Universit¨at Stuttgart
Pfaffenwaldring 57
70550 Stuttgart, Germany

J. Gemmer M. Michel G. Mahler , Quantum Thermodynamics, Lect. Notes Phys. 657
(Springer, Berlin Heidelberg 2005), DOI 10.1007/b98082

Library of Congress Control Number: 2004110894
ISSN 0075-8450
ISBN 3-540-22911-6 Springer Berlin Heidelberg New York
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Preface

This monograph views thermodynamics as an incomplete description of many
freedom quantum systems. Left unaccounted for may be an environment with
which the system of interest interacts; closed systems can be described incompletely by focussing on any subsystem with fewer particles and declaring the
remainder as the environment. Any interaction with the environment brings
the open system to a mixed quantum state, even if the closed compound state
is pure. Moreover, observables (and sometimes even the density operator) of
an open system may relax to equilibrium values, while the closed compound
state keeps evolving unitarily a` la Schr¨
odinger forever.
The view thus taken can hardly be controversial for our generation of
physicists. And yet, the authors offer surprises. Approach to equilibrium,
with equilibrium characterized by maximum ignorance about the open system of interest, does not require excessively many particles: some dozens suffice! Moreover, the precise way of partitioning which might reflect subjective
choices is immaterial for the salient features of equilibrium and equilibration.
And what is nicest, quantum effects are at work in bringing about universal thermodynamic behavior of modest size open systems. Von Neumann’s
concept of entropy thus appears as being much more widely useful than sometimes feared, way beyond truely macroscopic systems in equilibrium.
The authors have written numerous papers on their quantum view of
thermodynamics, and the present monograph is a most welcome coherent
review.

Essen,
June 2004

Fritz Haake


Acknowledgements

The authors thank Dipl. Phys. Peter Borowski (MPI Dresden) for the first numerical simulations to test our theoretical considerations and for contributing
several figures as well as some text material, and Dipl. Phys. Michael Hartmann (DLR Stuttgart) for contributing some sections. Furthermore, we thank

Cand. Phys. Markus Henrich for helping us to design some diagrams and,
both M. Henrich and Cand. Phys. Christos Kostoglou (Institut f¨
ur theoretische Physik, Universit¨
at Stuttgart) for supplying us with numerical data. We
have profited a lot from fruitful discussions with Dipl. Phys. Harry Schmidt,
Dipl. Phys. Marcus Stollsteimer and Dipl. Phys. Friedemann Tonner (Institut f¨
ur theoretische Physik, Universit¨
at Stuttgart) and Prof. Dr. Klaus
B¨arwinkel, Prof. Dr. Heinz-J¨
urgen Schmidt and Prof. Dr. J¨
urgen Schnack
(Fachbereich Physik, Universit¨
at Osnabr¨
uck). We benefitted much from conversations with Prof. Dr. Wolfram Brenig and Dipl. Phys. Fabian HeidrichMeisner (Technische Universit¨at Braunschweig) as well as Dr. Alexander Otte
and Dr. Heinrich Michel (Stuttgart). It is a pleasure to thank Springer-Verlag,
especially Dr. Christian Caron, for continuous encouragement and excellent
cooperation. This cooperation has garanteed a rapid and smooth progress of
the project. Financial support by the “Deutsche Forschungsgesellschaft” and
the “Landesstiftung Baden-W¨
urttemberg” is gratefully acknowledged. Last
but not least, we would like to thank Bj¨
orn Butscher, Kirsi Weber and Hendrik Weimer for helping us with typesetting the manuscript, proof-reading
and preparing some of the figures.


Contents

Part I Background
1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3

2

Basics of Quantum Mechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1 Introductory Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Operator Representations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1 Transition Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2 Pauli Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.3 State Representation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.4 Purity and von Neumann Entropy . . . . . . . . . . . . . . . . . .
2.2.5 Bipartite Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.6 Multi-Partite Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3 Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4 Invariants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5 Time-Dependent Perturbation Theory . . . . . . . . . . . . . . . . . . . . .
2.5.1 Interaction Picture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.2 Series Expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7
7
8
8
9
9
10
12
14

15
16
18
19
20

3

Basics of Thermodynamics and Statistics . . . . . . . . . . . . . . . . .
3.1 Phenomenological Thermodynamics . . . . . . . . . . . . . . . . . . . . . . .
3.1.1 Basic Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.2 Fundamental Laws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.3 Gibbsian Fundamental Form . . . . . . . . . . . . . . . . . . . . . . .
3.1.4 Thermodynamic Potentials . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Linear Irreversible Thermodynamics . . . . . . . . . . . . . . . . . . . . . .
3.3 Statistics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.1 Boltzmann’s Principle, A Priori Postulate . . . . . . . . . . .
3.3.2 Microcanonical Ensemble . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.3 Statistical Entropy, Maximum Principle . . . . . . . . . . . . .

21
21
21
23
26
26
28
30
31
32

34

4

Brief Review of Pertinent Concepts . . . . . . . . . . . . . . . . . . . . . . .
4.1 Boltzmann’s Equation and H-Theorem . . . . . . . . . . . . . . . . . . . .
4.2 Ergodicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3 Ensemble Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

37
38
42
43


X

Contents

4.4
4.5
4.6
4.7
4.8

Macroscopic Cell Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The Problem of Adiabatic State Change . . . . . . . . . . . . . . . . . . .
Shannon Entropy, Jaynes’ Principle . . . . . . . . . . . . . . . . . . . . . . .
Time-Averaged Density Matrix Approach . . . . . . . . . . . . . . . . .
Open System Approach and Master Equation . . . . . . . . . . . . . .

4.8.1 Classical Domain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.8.2 Quantum Domain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

45
48
50
52
53
53
54

Part II Quantum Approach to Thermodynamics
5

The Program for the Foundation of Thermodynamics . . . . . 61
5.1 Basic Checklist: Equilibrium Thermodynamics . . . . . . . . . . . . . 61
5.2 Supplementary Checklist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

6

Outline of the Present Approach . . . . . . . . . . . . . . . . . . . . . . . . .
6.1 Compound Systems, Entropy and Entanglement . . . . . . . . . . . .
6.2 Fundamental and Subjective Lack of Knowledge . . . . . . . . . . . .
6.3 The Natural Cell Structure of Hilbert Space . . . . . . . . . . . . . . .

65
65
67
67

7

System and Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.1 Partition of the System and Basic Quantities . . . . . . . . . . . . . . .
7.2 Weak Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3 Effective Potential, Example for a Bipartite System . . . . . . . . .

71
71
73
74

8

Structure of Hilbert Space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.1 Representation of Hilbert Space . . . . . . . . . . . . . . . . . . . . . . . . . .
8.2 Hilbert Space Average . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.3 Hilbert Space Variance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.4 Purity and Local Entropy in Product Hilbert Space . . . . . . . . .
8.4.1 Unitary Invariant Distribution of Pure States . . . . . . . .
8.4.2 Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

79
79
82
85
86
86
88

9

Quantum Thermodynamic Equilibrium . . . . . . . . . . . . . . . . . . .
9.1 Microcanonical Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.1.1 Accessible Region (AR) . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.1.2 The “Landscape” of P g in the Accessible Region . . . . .
9.1.3 The Minimum Purity State . . . . . . . . . . . . . . . . . . . . . . . .
9.1.4 The Hilbert Space Average of P g . . . . . . . . . . . . . . . . . . .
9.1.5 Microcanonical Equilibrium . . . . . . . . . . . . . . . . . . . . . . . .
9.2 Energy Exchange Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.2.1 The Accessible and the Dominant Regions . . . . . . . . . . .
9.2.2 Identification of the Dominant Region . . . . . . . . . . . . . . .
9.2.3 Analysis of the Size of the Dominant Region . . . . . . . . .
9.2.4 The Equilibrium State . . . . . . . . . . . . . . . . . . . . . . . . . . . .

91
91
91
93
93
95
97
97
98
99
101
102


Contents

XI

9.3 Canonical Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
9.4 Fluctuations of Occupation Probabilities WA . . . . . . . . . . . . . . . 104
10 Interim Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.1 Equilibrium Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.1.1 Microcanonical Contact . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.1.2 Energy Exchange Contact, Canonical Contact . . . . . . . .
10.2 Local Equilibrium States and Ergodicity . . . . . . . . . . . . . . . . . .

109
109
109
110
112

11 Typical Spectra of Large Systems . . . . . . . . . . . . . . . . . . . . . . . . .
11.1 The Extensitivity of Entropy . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.2 Spectra of Modular Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.3 Entropy of an Ideal Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.4 The Boltzmann Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.5 Beyond the Boltzmann Distribution? . . . . . . . . . . . . . . . . . . . . . .

113
113
114
118
119
121

12 Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.1 Definition of Spectral Temperature . . . . . . . . . . . . . . . . . . . . . . .
12.2 The Equality of Spectral Temperatures in Equilibrium . . . . . .
12.3 Spectral Temperature as the Derivative of Energy . . . . . . . . . .
12.3.1 Contact with a Hotter System . . . . . . . . . . . . . . . . . . . . .
12.3.2 Energy Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

123
124
125
127
128
129

13 Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
13.1 On the Concept of Adiabatic Processes . . . . . . . . . . . . . . . . . . . . 133
13.2 The Equality of Pressures in Equilibrium . . . . . . . . . . . . . . . . . . 139
14 Quantum Mechanical and Classical State Densities . . . . . . .
14.1 Bohr–Sommerfeld Quantization . . . . . . . . . . . . . . . . . . . . . . . . . .
14.2 Partition Function Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.3 Minimum Uncertainty Wave Package Approach . . . . . . . . . . . .
14.4 Implications of the Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.5 Correspondence Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

143
144
146
147
156

157

15 Sufficient Conditions for a Thermodynamic Behavior . . . . .
15.1 Weak Coupling Limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.2 Microcanonical Equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.3 Energy Exchange Equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4 Canonical Equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.5 Spectral Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.6 Parametric Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.7 Extensitivity of Entropy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

159
159
160
161
161
162
162
163


XII

Contents

16 Theories of Relaxation Behavior . . . . . . . . . . . . . . . . . . . . . . . . . .
16.1 Fermi’s Golden Rule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16.2 Weisskopf–Wigner Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16.3 Open Quantum Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

165
165
166
167

17 The Route to Equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.1 System and a Large Environment . . . . . . . . . . . . . . . . . . . . . . . . .
17.2 Time Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.3 Hilbert Space Average . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.4 Short Time Step Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.5 Derivation of a Rate Equation . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.6 Solution of the Rate Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.7 Hilbert Space Variances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.8 Numerical Results for the Relaxation Period . . . . . . . . . . . . . . .

169
169
171
173
174
178
179
180
181

Part III Applications and Models
18 Equilibrium Properties of Model Systems . . . . . . . . . . . . . . . . .
18.1 Entropy Under Microcanonical Conditions . . . . . . . . . . . . . . . . .
18.2 Occupation Probabilities Under Canonical Conditions . . . . . . .
18.3 Probability Fluctuations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

18.4 Spin Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18.4.1 Global Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18.4.2 Local Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18.4.3 Chain Coupled Locally to a Bath . . . . . . . . . . . . . . . . . . .
18.5 On the Existence of Local Temperatures . . . . . . . . . . . . . . . . . . .
18.5.1 Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18.5.2 Global Thermal State in the Product Basis . . . . . . . . . .
18.5.3 Conditions for Local Thermal States . . . . . . . . . . . . . . . .
18.5.4 Spin Chain in a Transverse Field . . . . . . . . . . . . . . . . . . .
18.6 Quantum Thermometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18.6.1 The Classical Picture . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18.6.2 The Szilard Picture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18.6.3 Temperature Fluctuations . . . . . . . . . . . . . . . . . . . . . . . . .
18.6.4 Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18.6.5 Thermodynamic “Uncertainty Relation” . . . . . . . . . . . . .
18.7 Quantum Manometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18.7.1 Eigenspectrum of System and Manometer . . . . . . . . . . .
18.7.2 The Total Scenario . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18.7.3 Classical Picture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18.8 Adiabatic Following and Adiabatic Process . . . . . . . . . . . . . . . .

185
185
188
192
193
194
195
198
200

202
203
204
207
209
209
209
210
210
212
212
212
214
215
216


Contents

XIII

19 Heat Conduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19.1 Theories of Heat Conduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19.1.1 Linear Response Theory . . . . . . . . . . . . . . . . . . . . . . . . . . .
19.1.2 Quasi-Particle Approach . . . . . . . . . . . . . . . . . . . . . . . . . .
19.2 Quantum Heat Conduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19.2.1 Model Hamiltonian and Environment . . . . . . . . . . . . . . .
19.2.2 Current Operator and Fourier’s Law . . . . . . . . . . . . . . . .
19.2.3 Linear Stability Analysis . . . . . . . . . . . . . . . . . . . . . . . . . .
19.2.4 Heat Conduction in Low Dimensional Systems . . . . . . .

19.2.5 Fourier’s Law for a Heisenberg Chain . . . . . . . . . . . . . . .
19.2.6 Implications of These Investigations . . . . . . . . . . . . . . . .

221
221
222
223
224
225
227
229
230
231
232

20 Quantum Thermodynamic Machines . . . . . . . . . . . . . . . . . . . . . .
20.1 Tri-Partite System: Sudden Changes of Embedding . . . . . . . . .
20.2 Work Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
20.3 Carnot Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
20.4 Generalized Control Cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

235
235
237
237
239

21 Summary and Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241

Part IV Appendices

A

Hyperspheres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.1 Surface of a Hypersphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.2 Integration of a Function on a Hypersphere . . . . . . . . . . . . . . . .
A.3 Sizes of Zones on a Hypersphere . . . . . . . . . . . . . . . . . . . . . . . . . .

247
247
248
250

B

Hilbert Space Average under Microcanonical Conditions . 253

C

Hilbert Space Averages and Variances . . . . . . . . . . . . . . . . . . . .
C.1 General Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C.2 Special Hilbert Space Averages . . . . . . . . . . . . . . . . . . . . . . . . . . .
C.3 Special Hilbert Space Variances . . . . . . . . . . . . . . . . . . . . . . . . . .

D

Power of a Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265

E

Local Temperature Conditions for a Spin Chain . . . . . . . . . . 267

259
259
262
263

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279


List of Symbols

ˆ
1
ˆ
1(µ)
∇
δij
δ(. . . )
⊗
dA
∆A
δA
CN {f }
f
O(R, n)
ˆ B
ˆ
A,
F{f }

f
i|j
{ηi , ξi }
AB AB
{ηab
, ξab }
{ηiJ , ξiJ }
{r, φi }
{rJ , φJi }
{rAB , φAB
i }
Aˆ
Aˆ
Aˆ(µ)
Aˆ†
AˆH (t)
AˆI
|A, a
A
a

Unit operator
Unit operator in Hilbert space of subsystem µ
Nabla operator
Kronecker delta
Dirac δ-function
Dyadic product
Infinitesimal change of an integrable quantity, complete
differential
Finite change of A

Infinitesimal change of a non-integrable quantity
Convolution of N identical functions f
Hilbert space average of a quantity f
Surface of an n-dimensional hypersphere with radius R
ˆ
Commutator of Aˆ with B
Fourier transformation of f
Mean value of f
Scalar product in Hilbert space
Set of all coordinates i = 1, . . . n
Set of coordinates in subspace AB
Subset of coordinates in subspace J
Generalized spherical coordinates
Spherical coordinates of subspace J
Spherical coordinates of subspace AB
Expectation value of operator Aˆ
Operator
Operator in subspace µ
Adjoint operator
Time-dependent operator in Heisenberg picture
Operator in the interaction picture
Basis state of the gas system g
Index of the energy subspace of the gas system with
g
energy EA
Index of the degenerate eigenstates belonging to one energy subspace A of the gas system


XVI

List of Symbols

A, B/E
Aij
A∗ij
AR
|B, b
B
b
c
Dρ2ˆρˆ
D
d
dA
Ei
E
E0
Ec
c
EB
Eg
g
EA
E
Fρˆρˆ
F (T, V )
F
FJ
FAB
F

G(T, p)
g(E)
g(γ, E)
c
Gc (EB
)
g
g
G (EA
)
G(E)
g
H(S, p)
H(q, p)
ˆ
H
ˆ (µ)
H
loc
ˆ (µ,µ+1)
H
F
ˆ (µ,µ+1)
H
R
ˆ (µ,µ+1)
H
NR
ˆ0
H

= h/2π

g
c
Index operation under the constraint EA
+ EB
=E
ˆ
Matrix elements of an operator A
Complex conjugate matrix elements of an operator Aˆ
Accessible region
Basis state of the container system c
Index of the energy subspace of the container system
c
with energy EB
Index of the degenerate eigenstates belonging to one energy subspace B of the container system
Label for container
Distance measure (Bures metric)
Diagonal deviation matrix
Dimension of the Liouville space
Dimension of special subspace A
Energy of the state |i
Energy
Zero-point energy
Total energy of container system
Energy eigenvalues of the container system
Total Energy of gas system g
Energy eigenvalues of the gas system g
Off-diagonal part of a matrix, deviation matrix
Fidelity between ρˆ and ρˆ

Free energy
Jacobian matrix (functional matrix)
Jacobian matrix in subspace J
See FJ
Force
Gibbs free energy
State density of subsystems
Energy spectrum of wave package |γ
State density of the container system
State density of the gas system
State density at energy E
Label for gas subsystem
Enthalpy
Hamilton function
Hamiltonian
Local Hamiltonian of subsystem µ
Next neighbor F¨
orster coupling
Next neighbor random coupling
Next neighbor non-resonant coupling
Unperturbed Hamiltonian
Planck’s constant


List of Symbols

ˆc
H
ˆg
H

H
H(µ)
i
Iˆ
Iˆgc
|i
i|
|i, j, . . .
|i, t
|i ⊗ |j = |ij
J
j
ju
js
Jˆ
Jˆ(µ,µ+1)
kB
Lˆ
Lˆcoh
Lˆ1,2
inc
L
m
N0c
N1c
c
N c (EB
)
N (E)
g

N g (EA
)
nvar
n(µ)
ntot
N
n
NAB = NA NB
NA
NB
Xi
p
pµ
ˆµ
p
Pˆ ex

XVII

Hamiltonian of the container system c
Hamiltonian of the gas system g
Hilbert space
Hilbert space of subsystem µ
Imaginary unit
Interaction operator
Interaction between gas system and container system
Basis state
Adjoint basis state
Product state of several subsystems
Time-dependent state

Product state of two subsystems
Label for a side condition
Current
Energy current
Entropy current
Current operator
Local current operator between subsystem µ and µ + 1
Boltzmann constant
Super-operator acting on operators of the Liouville space
(Lindblad operator)
Coherent part of Lindblad super-operator
Incoherent part of Lindblad super-operator
Transport coefficient (in general a matrix)
Number of micro states accessible for a system; mass
Number of levels in lower band
Number of levels in upper band
Total number of states in a subspace B, degeneracy
Number of levels under the constraint A, B/E
Total number of states in a subspace A, degeneracy
Number of macro variables
Number of levels of subsystem µ
Dimension of the total Hilbert space
Number of subsystems, particle number
Number of levels of a subsystem
Number of states in subspace AB
g
See N g (EA
)
c
c

See N (EB )
Extensive macro variable
Vector of all momentum coordinates
Momentum of the µth particle
Momentum operator of the µth particle
Projector projecting out some part ex of the total state
space


XVIII List of Symbols

p
(µ)
Pˆii
Pˆij
P
Q
ˆµ
q
q
q
qµ
{q cν }
{q gµ }
r
RJ
RAB
S
S tot
Slin

s(q, t)
s(U, V )
sph(n)
T (q, t)
T
T (µ)
t
Tr {. . . }
Trµ {. . . }
u(q, t)
U
ˆ
U
ˆ
U0 (t, t0 )
Uij
ˆ (t)
U
ˆ (µ)(t)
U
v
Vˆ
VˆI (t)
V
v
g
c
, EB
)
W (EA

c
W (EB )
c
W d (EB
)
g
d
W (EA
)
W (E)
g
)
W (EA

Pressure
Projector within subspace µ
Transition operator, projector for i = j
Purity
Heat
Position operator of the µth particle
Position
Vector of position coordinates of N particles
Position of the µth particle
Set of position coordinates of all container particles
Set of position coordinates of all gas particles
Radial coordinate of a hypersphere
Radius of hypersphere in subspace J
See RJ
Entropy
Entropy of system and environment

Linearized von Neumann entropy
Entropy density
Specific entropy
n-dimensional hypersphere
Temperature field
Temperature
Local temperature of subsystem µ
Time
Trace operation
Partial trace over subsystem µ
Energy density
Internal energy
Unitary transformation
Unitary transformation into the interaction picture
Unitary matrix
Time evolution operator
Time evolution operator of subsystem µ
Velocity vector
Potential
Potential in the interaction picture
Volume
Hilbert space velocity
See WAB
See WB
c
Dominant probability of finding the container in EB
g
Dominant probability of finding the gas in EA
Probability of finding the complete system at energy E
See WA

List of Symbols

W (q, p, t)
(12)

Wij

W1→0
Wij (t)
Wi = ρii
WAB
WB
d
WAB
WA
{WAB }
d
{WAB
}
d
WA
WBd
|x
Zi
Z
α
β
γi = (pi , qi )

|γ
δ
∆E0c
∆E1c
∆2H (f )
∆WA
εi
εAB
η
ηi
κ
λ
λ0
µ
ξi
g
ρˆeq
ρˆ
ρˆ(µ)
ρˆirrel

XIX

Probability of finding a point in phase space at position
(q, p) at time t
Probability of finding subsystem 1 in state i and subsystem 2 in state j
Rate for a decay from |1 to |0
Transition probability from state j into state i
Statistical weight of, or probability of finding the system
in the pure state Pˆii

g
Joint probability of finding the gas system at energy EA
c
and the container system at energy EB
c
Probability of finding the container system at energy EB
Dominant probability distribution for the whole system
g
Probability of finding the gas system at energy EA
Set of all probabilities WAB , probability distribution
Set of dominant probabilities
Dominant probability distribution for the gas system,
g
equivalent to W d (EA
)
Dominant probability distribution for the container sysc
tem, equivalent to W d (EB
)
State vector indexed by position x
Macro variables
Canonical partition function
Lagrange parameter or exponent for spectra
Lagrange parameter; inverse temperature
Wavepackage coordinates
Complete but not necessarily orthogonal basis
Band width
Energy level spacing in lower band
Energy level spacing in upper band
Hilbert space variance of a quantity f
Variance of WA

Deviation, small quantity
d
Deviation from WAB
Carnot efficiency
Real part of ψi
Heat conductivity
Coupling constant
Mean absolute value of interaction matrix element
Subsystem index; chemical potential
Imaginary part of ψi ; intensive macro variable
Equilibrium state of subsystem g
Density operator
Reduced density operator subsystem µ
Irrelevant part of density operator


XX

List of Symbols

ρˆrel
ρirrel(q, p)
ρrel(q, p)
ρ(q, p)
ρij
ρˆ
g
ρˆmin
σ
ˆ±

σ
ˆi
σ
|ψ
|ψI (t)
|ψ (µ)
AB
ψab
ψaA
ψbB
ψij
ψi
ωji
Ω(E)
V ({WAB })
Vd

Relevant part of density operator
Irrelevant part of the prob. dist. func. ρ(q, p)
Relevant part of the prob. dist. func. ρ(q, p)
Probability distribution function
Density matrix
Density operator
Minimum purity state of the gas g
Raising/lowering operator
Pauli operator
Standard deviation
Wave function, arbitrary pure state
Arbitrary wave function in the interaction picture
Pure state of subsystem µ

Amplitute of a product state |Aa ⊗ |Bb
Product state amplitude of gas system
Product state amplitude of container system
Amplitude of a basis product state |ij
Amplitude of basis state |i
Transition frequency from state j into state i
Total volume of phase space below the energy surface
H(q, p) = E
Size of region in Hilbert space with probability distribution {WAB }
Size of dominant region in Hilbert space


1 Introduction

Over the years enormous effort was invested in proving
ergodicity, but for a number of reasons, confidence in
the fruitfulness of this approach has waned.
— Y. Ben-Menahem and I. Pitowsky [11]

Originally, thermodynamics was a purely phenomenological science. Early
scientists (Galileo, Santorio, Celsius, Fahrenheit) tried to give definitions for
quantities which were intuitively obvious to the observer, like pressure or temperature, and studied their interconnections. The idea that these phenomena
might be linked to other fields of physics, like classical mechanics, e.g., was
not common in those days. Such a connection was basically introduced when
Joule calculated the heat equivalent in 1840 showing that heat was a form of
energy, just like kinetic or potential energy in the theory of mechanics.
At the end of the 19th century, when the atomic theory became popular,
researchers began to think of a gas as a huge amount of bouncing balls inside
a box. With this picture in mind it was tempting to try to reduce thermodynamics entirely to classical mechanics. This was exactly what Boltzmann
tried to do in 1866 [18], when he connected entropy, a quantity which so far

had only been defined phenomenologically, to the volume of a certain region
in phase space, an object defined within classical mechanics. This was an
enormous step forward, especially from a practical point of view. Taking this
connection for granted one could now calculate all sorts of thermodynamic
properties of a system from its Hamilton function. This gave rise to modern
thermodynamics, a theory the validity of which is beyond any doubt today.
Its results and predictions are a basic ingredient for the development of all
kinds of technical apparatuses ranging from refrigerators to superconductors.
Boltzmann himself, however, tried to prove the conjectured connection
between the phenomenlogical and the theoretical entropies, but did not succeed without invoking other assumptions like the famous ergodicity postulate
or the hypothesis of equal “a priori probabilities”. Later on, other physicists
(Gibbs [41], Birkhoff [14], Ehrenfest [32], von Neumann [88], etc.) tried to
prove those assumptions, but none of them seems to have solved the problem
satisfactorily. It has been pointed out, though, that there are more properties
of the entropy to be explained than its mere equivalence with the region in
phase space, before thermodynamics can be reduced to classical mechanics,
thus the discussion is still ongoing [11]. The vast majority of the work done
in this field is based on classical mechanics.
Meanwhile, quantum theory, also initially triggered by the atomic hypothesis, has made huge progress during the last century and is today believed
J. Gemmer, M. Michel, and G. Mahler, Quantum Thermodynamics, Lect. Notes Phys. 657, 3–5
(2004)
c Springer-Verlag Berlin Heidelberg 2004
/>

4

1 Introduction

to be more fundamental than classical mechanics. At the beginning of the
21st century it seems highly unlikely that a box with balls inside could be

anything more than a rough caricature of what a gas really is. Furthermore,
thermodynamic principles seem to be applicable to systems that cannot even
be described in classical phase space. Those developments make it necessary
to rethink the work done so far, whether it has led to the desired result
(e.g., demonstration of ergodicity) or not. The fact that a basically classical
approach apparently did so well may even be considered rather surprising.
Of course, there have been suggestions of how to approach the problem
on the basis of quantum theory [69, 70, 73, 87, 96, 116, 134, 136], but again,
none of them seems to have established the emergence of thermodynamics
from quantum mechanics as an underlying theory in a conclusive way.
The text at hand can be viewed as a contribution to this ongoing discussion. Thus, on the one hand, one might consider this work as a somewhat
new explanation for the emergence of thermodynamic behavior. This point
of view definitely leaves one question open: whether or not all macroscopic
thermodynamic systems belong to the class of systems that will be examined
in the following. The answer to this question is beyond the scope of this text.
Furthermore, this quantum approach to thermodynamics may turn out
not to be a one-way road. In fact, this delicate interplay between quantum
mechanics and thermodynamics could possibly shed new light on some interpretational problems within quantum mechanics: with the “exorcism” of
subjective ignorance as a guiding principle underlying thermodynamic states,
the general idea of quantum states representing subjective knowledge might
lose much of its credibility.
However, this book might be looked at also from another, less speculative
angle. Rather than asking how thermodynamic behavior of typical systems
might be explained, one can ask whether the principles of thermodynamics
are powerful tools for predictions and whether their descriptions might be
applicable to systems other than the pertinent large, many-particle systems.
It is a main conclusion of this work that the answer has to be positive. For
it turns out that a large class of small quantum systems without any restriction concerning size or particle number show thermodynamic behavior
with respect to an adequately defined set of thermodynamic variables. This
behavior (“nano-thermodynamics”) requires some embedding but is nevertheless established entirely on the basis of the Schr¨

odinger equation.
So it is left to the reader to decide whether there is room and need for a
foundation of thermodynamics on the basis of quantum theory or whether he
simply wants to gain insight into how the applicability of thermodynamics
can be extended down to the microscopical scale; in both cases we hope the
reading will be interesting and clarifying.
This book is not intended to be a review, not even of the most important contributions, which more or less point in a similar direction. Related
work includes, in particular, the so-called decoherence theory. We cannot


1 Introduction

5

do justice to the numerous investigations; we merely give a few references
[21, 42, 45, 135]. It might be worth mentioning here that decoherence has,
during the last years, mainly been discussed as one of the main obstacles in
the implementation of large-scale quantum computers [90], possibly neglecting other aspects of the phenomenon.
Last but not least a short “manual” for reading this book shall be given
here.
Chapters 2 and 3 are not meant to serve as a full-fledged introduction,
more as a reminder of the central topics in quantum mechanics and thermodynamics. They may very well be skipped by a reader familiar with these
subjects. Chapter 4 is a collection of historical approaches to thermodynamics
with a focus on their insufficiencies hereby neglecting their undoubtable brilliance. Again this chapter is not imperative for the understanding of Part II.
Chapter 5 lists the properties of thermodynamic quantities that need to
be derived from an underlying theory (quantum mechanics). This derivation
is then given in the remainder of Part II. In Chap. 6 the central ideas of
this quantum approach to thermodynamics are explained in plain text (no
formulas). For a quick survey it might be read without referring to anything
else. Starting with Chap. 7 and throughout Part II, these ideas are derived

in detail, and will probably only be enlightening if read from the beginning.
Exceptions are Chap. 11 and Chap. 14, which are important for the general
picture, but have their own “selfcontained” messages.
Chapter 18 mainly consists of numerical illustrations of the analytically
derived principles in Part II. In order to get an idea of the benefits of the
theories derived in Part II, it might be read as a “stand alone” chapter. In
Chap. 19 recent results on quantum heat conduction are presented, it may
also be read individually, for it is only loosely connected to the rest of the
book as far as mathematical techniques are concerned.


2 Basics of Quantum Mechanics
Indeed my favourite key to understanding quantum
mechanics is that a subsystem cannot be isolated
by tracing from an enveloping pure state without
generating impurity: the probability associated with
measurement develops because the observer must
implicitly trace himself away from the observed system.
— E. Lubkin [76]

Before we can start with the quantum mechanical approach to thermodynamics we have to introduce some fundamental terms and definitions of
standard quantum mechanics for later reference. This chapter should introduce the reader only to some indispensible concepts of quantum mechanics
necessary for the text at hand, but is far from being a complete overview
of this subject. For a complete introduction we refer to standard textbooks
[9, 70, 79, 89, 113, 126].

2.1 Introductory Remarks
The shortcomings of classical theories had become apparent by the end of the
19th century. Interestingly enough, one of the first applications of quantum
ideas has been within thermodynamics: Planck’s famous formula for black

body radiation was based on the hypothesis that the exchange of energy
between the container walls and the radiation field should occur in terms of
fixed energy quanta only. Later on, this idea has been put on firmer ground
by Einstein postulating his now well known rate equations [75].
Meanwhile quantum mechanics has become a theory of unprecedented
success. So far, its predictions have always been confirmed by experiment.
Quantum mechanics is usually defined in terms of some loosely connected
axioms and rules. Such a foundation is far from the beauty of, e.g., the “principles” underlying classical mechanics. Motivated, in addition, by notorious
interpretation problems, there have been numerous attempts to modify or
“complete” quantum mechanics.
A first attempt was based on so-called “hidden variables” [10]. Its proponents essentially tried to expel the non-classical nature of quantum mechanics.
More recent proposals intend to “complete” quantum mechanics not within
mechanics, but on a higher level: by means of a combination with gravitation
theory (Penrose [102]), with psychology (Stapp [122]) or with (quantum-)
information theory [26, 38].
While the emergence of classicality from an underlying quantum substrate
has enjoyed much attention recently, it has so far not been appreciated that
J. Gemmer, M. Michel, and G. Mahler, Quantum Thermodynamics, Lect. Notes Phys. 657,
7–20 (2004)
c Springer-Verlag Berlin Heidelberg 2004
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8

2 Basics of Quantum Mechanics

the understanding of quantum mechanics may benefit also from subjects like
quantum thermodynamics.

2.2 Operator Representations

In quantum mechanics we deal with systems (Hamilton models), observables,
and states. They all are represented by Hermitian operators. Their respective specification requires data (parameters), which have to be defined with
respect to an appropriate reference frame. These frames are operator representations. Let us in the following consider some aspects of these operator
representations in detail. First of all we will concentrate on simple systems
and their representation.
2.2.1 Transition Operators
If we restrict ourselves to systems living in a finite and discrete Hilbert space
H (a complex vector space of dimension ntot ), we may introduce a set of
orthonormal state vectors |i ∈ H. From this orthonormal and complete set
of state vectors with
i|j = δij ,
we can define

i, j = 1, 2, . . . , ntot ,

n2tot

Pˆij = |i j| ,

(2.1)

transition operators (in general non-Hermitian)
Pˆij† = Pˆji .

(2.2)

These operators are, again, orthonormal in the sense that
Tr Pˆij Pˆi† j

= δii δjj ,

(2.3)

where Tr {. . . } denotes the trace operation. Furthermore, they form a complete set in so-called Liouville space, into which any other operator Aˆ can be
expanded,
Aˆ =

Aij Pˆij ,

(2.4)

i,j

Aij = Tr Aˆ Pˆij†

ˆ .
= i|A|j

(2.5)

The n2tot parameters are, in general, complex (2n2tot real numbers). For Hermitian operators we have, with
A∗ij Pˆij† =

Aˆ† =
i,j

A∗ij = Aji ,

Aji Pˆji = Aˆ ,

(2.6)

i,j

(2.7)

i.e., we are left with n2tot independent real numbers. All these numbers must
ˆ
be given to uniquely specify any Hermitian operator A.


2.2 Operator Representations

9

2.2.2 Pauli Operators
There are many other possibilities to define basis operators, besides the transition operators. For ntot = 2 a convenient set is given by the so-called Pauli
operators σ
ˆi (i = 0, . . . , 3). The new basis operators can be expressed in terms
of transition operators
σ
ˆ1 = Pˆ12 − Pˆ21 ,
σ
ˆ = i(Pˆ21 − Pˆ12 ) ,

(2.8)
(2.9)

2

σ
ˆ3 = Pˆ11 − Pˆ22 ,
ˆ.
σ
ˆ0 = 1

(2.10)
(2.11)

These operators are Hermitian and – except for σ
ˆ0 – traceless. The Pauli operators satisfy several important relations: (ˆ
σi )2 = ˆ1 and [ˆ
σ1 , σ
ˆ2 ] = 2iˆ
σ3 and
their cyclic extensions. Since the Pauli operators form a complete orthonormal operator basis, it is possible to expand any operator in terms of these
basis operators. Furthermore we introduce raising and lowering operators, in
accordance with
ˆ1 + iˆ
σ2 ,
σ
ˆ+ = σ

σ
ˆ− = σ
ˆ1 − iˆ
σ2 .

(2.12)

Also for higher dimensional cases, ntot ≥ 2, one could use as a basis the
Hermitian generators of the SU(ntot ) group.
2.2.3 State Representation
The most general way to note the information about a state of a quantum
mechanical system is by its density matrix, ρij , which specifies the representation of the density operator,
ρij Pˆij

ρˆ =

(2.13)

i,j

subject to the condition
Tr {ˆ
ρ} =

ρii = 1 .

(2.14)

i

The expectation value for some observable Aˆ in state ρˆ is now given by
ˆρ} =
A = Tr{Aˆ

Aij ρij .

(2.15)

i,j

The density matrix ρij = i|ˆ
ρ|j is a positive definite and Hermitian matrix.
The number of independent real numbers needed to specify ρˆ is thus d =


10

2 Basics of Quantum Mechanics

n2tot − 1. For the density operator of an arbitrary pure state |ψ we have
ρˆ = |ψ ψ|. In the eigenrepresentation one finds, with Wi = ρii ,
Wi Pˆii ,

ρˆ =

(2.16)

i

which can be seen as a “mixture” of pure states Pˆii = |i i| with the statistical
weight Wi . From this object the probability W (|ψ ) to find the system in an
arbitrary pure state, expanded in the basis |i
|ψ =

ψi |i ,

(2.17)

i

can be calculated as
2

W (|ψ ) = ψ|ˆ
ρ|ψ =

|ψi | Wi .

(2.18)

i

To measure the distance of two arbitrary, not necessarily pure states,
given by ρˆ and ρˆ we define a “distance measure”
Dρ2ˆρˆ = Tr (ˆ
ρ − ρˆ )2

.

(2.19)

This commutative measure (sometimes called Bures metric) has a number
of convenient properties: Dρ2ˆρˆ ≥ 0 with the equal sign holding if and only if
ρˆ = ρˆ ; the triangle inequality holds as expected for a conventional distance
measure; for pure states
2
D|ψ

2

|ψ

= 2 1 − | ψ|ψ |

≤2

(2.20)

and D2 is invariant under unitary transformations. A second measure of
distance is the fidelity defined by [90]
1/2

Fρˆρˆ = Tr

ρˆ ρˆ

ρˆ

.

For pure states F is just the modulus of the overlap: F|ψ

(2.21)
|ψ

= | ψ|ψ |.

2.2.4 Purity and von Neumann Entropy
For a pure state all matrix elements in (2.13) of the density matrix are zero
except ρii = 1, say, i.e., the density operator ρˆ = Pˆii is a projection operator.
Obviously in this case ρˆ2 = ρˆ, due to the properties of the projection operator,
so that the so-called purity becomes
P = Tr ρˆ2 = 1 .
In general, we have

(2.22)