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Jarzynski Relations for Quantum Systems and Some Applications

Statistical Mechanics · Submitted 17 Sep 2000 · 12 references

Pith's verdict: Correct, elementary algebra; the one new claim (compound-system entropy increase) is real but conditional on a final-equilibration premise the paper itself flags, and the conditionality runs in the safe direction.

T0 · AI review

How this review was made: An AI referee (selfhosted/deepseek-v4.1-flash) read the paper on October 29, 2026 and answered a fixed list of questions about it. T0 is the starting level every paper gets. The levels describe how deeply Pith has checked the paper, never the authors.

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  4. T3 Formal statement The claims written out in a formal math language a computer can read.
  5. T4 Proof Those claims proved, and the proof checked by computer.

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arXiv IDcond-mat/0009244 (version 2)
Pith ID6SANYA6S, Pith's permanent record of this paper and its review
CitationHal Tasaki. "Jarzynski Relations for Quantum Systems and Some Applications." arXiv:cond-mat/0009244 (2000). https://pith.science/paper/6SANYA6S
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Referee report
The paper

The paper claims

A quantum version of Jarzynski's equality is used to derive the strongest form of the second law, and to bound heat flow between two systems at different temperatures.

The reading

Tasaki asks whether Jarzynski's nonequilibrium work relation — an exact identity linking a fluctuating quantity to a ratio of partition functions — survives the move to quantum mechanics, and whether it can be pushed far enough to prove the second law in its strongest form. The answer he gives is yes on both counts. The central identity concerns a joint probability of finding the system in energy eigenstate $i$ of the initial Hamiltonian and eigenstate $j$ of the final one after unitary evolution; averaging $e^{βE_i - β̃E'_j}$ over that distribution collapses, by unitarity, to $Z'(β̃)/Z(β)$. Jensen's inequality then converts the equality into a family of linear inequalities in the average energies, one for each choice of $β̃$. The application the author considers new is the compound-system case: several subsystems, each in its own Gibbs state at its own temperature, separated by adiabatic walls, are manipulated together. The same averaging argument, now with one free parameter per final subsystem, yields the statement that the total entropy of the parts can never decrease.

What carries the argument

The load-bearing object is the joint distribution $p_{i,j} = (e^{-βE_i}/Z(β))|⟨ϕ'_j|U|ϕ_i⟩|^2$ — the probability of starting in the $i$-th eigenstate of the initial Hamiltonian and ending in the $j$-th eigenstate of the final one — together with its multi-temperature generalization $p_{i,j} = (∏_k e^{-β_k E^{(k)}_{i_k}}/Z_k(β_k))|⟨Φ'_j|U|Φ_i⟩|^2$. Its virtue is that the exponential weight in the initial Gibbs factor cancels exactly against the $e^{βE}$ inserted into the average, and unitarity then sums the remaining matrix elements to one, leaving only the ratio of partition functions. Two consequences follow mechanically: the equality itself, and, via the convexity of the exponential, an inequality for ordinary quantum expectation values of the subsystem Hamiltonians. The same maneuver, applied to a time-independent two-body Hamiltonian with a product initial state, produces the heat-flow results: $[e^{-s}]_t = 1$ where $s = β_1(E'^{(1)} - E^{(1)}) + β_2(E'^{(2)} - E^{(2)})$, hence $ΔS(t) ≥ 0$, and the sharper term-by-term representation $ΔS(t) = ∑ p_{i,j,ℓ,m}(t) d[s]$ with $d(x) = e^{-x} - 1 + x ≥ 0$.

Extended reading notes

Core claim

The paper's central claim is that the law of entropy increase for a general compound system — the statement that when $n$ simple systems at inverse temperatures $β_1, …, β_n$ are manipulated into $m$ simple systems, the sum of their entropies cannot decrease — follows from a multi-temperature quantum Jarzynski equality. The equality (Eq. 3.15) says that for an initial state that is a product of Gibbs states $⨂_k e^{-β_k H_k}/Z_k(β_k)$, and for any set of final inverse temperatures $β̃_1, …, β̃_m$, the average of $e^{∑_k β_k E^{(k)} - ∑_ℓ β̃_ℓ E'^{(ℓ)}}$ over the joint transition probabilities equals $∏_ℓ Z'_ℓ(β̃_ℓ) / ∏_k Z_k(β_k)$. Jensen's inequality turns this into $∑_k β_k(⟨H_k⟩ - F_k(β_k)) ≤ ∑_ℓ β̃_ℓ(⟨H'_ℓ⟩ - F'_ℓ(β̃_ℓ))$ for every choice of the $β̃_ℓ$; minimizing the right side over those parameters identifies it with the final entropy, and the left side is the initial entropy. The author notes that the ordinary, single-system entropy increase is a special case, and that the compound-system version is the one with real content, since it constrains operations that redistribute energy among parts rather than merely among states of one part.

Load-bearing premise

The step from the inequality to a statement about entropy requires that each subsystem at the end of the operation has actually settled into equilibrium at some temperature, so that its entropy can be read off as the Legendre transform of its free energy; the author assumes the final Hamiltonian is held fixed long enough for this to happen and notes separately that the derivation works only when the initial state is a product of exact Gibbs states.

Editorial extensions

If this is right

  • The minimum work needed to drive a quantum system between two Hamiltonians is bounded below by the free-energy difference, with the bound following from the equality rather than assumed.
  • The entropy of a compound system cannot decrease under any adiabatic operation that starts from a product of Gibbs states, even when walls are inserted or removed and the number of subsystems changes.
  • Heat flow between two quantum systems initially at different temperatures is one-directional in the sense that $ΔS(t) ≥ 0$ at every time, not merely in the long-time limit.
  • Because $ΔS(t)$ decomposes into a sum of individually nonnegative terms, a rigorous lower bound on heat flow can be obtained by discarding any unwanted terms and keeping only those that can be controlled in a specific model.
  • The distribution of the entropy-production-like variable $s$ obeys $e^{-s}P_t(s) = P_{-t}(-s)$, and reduces to $e^{-s}P_t(s) = P_t(-s)$ when the dynamics is time-reversal symmetric.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The term-by-term positivity of $ΔS(t)$ is the more useful half of the heat-transfer result: it suggests a general recipe for proving finite heat flow in concrete lattice or spin-bath models, where one would keep only the transitions between a few low-lying levels and bound $d(x)$ from below by a quadratic.
  • The author's own remark that the inequalities follow directly from positivity of quantum relative entropy suggests the equality is doing work only where the inequality is not enough — namely in the fluctuation-type symmetries and in the exact partition-function ratios, so a fair test of the method's value is whether those stronger statements get used elsewhere.
  • If the derivation extends to initial states that are only approximately Gibbs, the compound-system result would inherit the robustness that the author notes already exists for simple systems, which would make the entropy principle independent of the exact-canonical assumption.
  • The two-system heat-flow setup is a natural testbed for asking whether the bound degrades with system size in a way that matches the usual linear-response prediction for small temperature differences.
Original abstract

We derive quantum analogues of Jarzynski's relations, and discuss two applications, namely, a derivation of the law of entropy increase for general compound systems, and a preliminary analysis of heat transfer between two quantum systems at different temperatures. We believe that the derivation of the law of entropy increase is new and of importance.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13-20260917 via OpenRouter, question bank abstract-p1-20260923, 2026-09-23: 1 request, 2,287 input tokens, $0.0001 reported.

What the abstract itself states, as probabilities: claims first 0.42 · states limitation 0.52 · human participants 0.02 · controlled comparison 0.02 · randomized assignment 0.01 · states quantitative result 0.06 · compares with prior results 0.15 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v2. Results and the main claim are underlined; .

We derive quantum analogues of Jarzynski's relations, and discuss two applications, namely, a derivation of the law of entropy increase for general compound systems, and a preliminary analysis of heat transfer between two quantum systems at different temperatures. We believe that the derivation of the law of entropy increase is new and of importance.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 We derive quantum analogues of Jarzynski's relations, and discuss two applications, namely, a derivation of the law of entropy increase for general compound systems, and a preliminary analysis of heat transfer between two quantum systems at different temperatures. contribution 1.00 0.96 0.50 asserted 1.00
s1 We believe that the derivation of the law of entropy increase is new and of importance. interpretation 0.78 0.71 0.22 hedged possibility 0.35

What kind of paper the classifier reports this is: theoretical result 1.00 · evidence from proof or derivation 1.00.

What was examined, and what was not

sources read
  • arXiv listing abstract v2
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

References
How later papers used it

281 later papers cite it

Evidence: substantial. 280 of the citing papers share no author with it, and 39 of those carry a confident label from Pith's citation classifier.

Pith read 38 of the 500 citing sentences, choosing only those whose citation markers point at this paper alone, and wrote the paraphrases from them. The summary, the counts, the evidence level, the links and the quotes come from code; the summary counts the papers listed below. A later paper is listed under a label when Pith's citation classifier gives one of its sentences that label at probability 0.8 or more (0.95 for support), Pith's reading finds that the citing paper itself did it, and a second check confirms the paraphrase against the sentence. This section lists uses only. It does not list criticism or disagreement, so their absence here says nothing about how the paper has been received. Papers that share an author with this one are listed apart, under "By the same authors". A quote is the citing authors' own sentence, shown when their paper's licence is CC BY, CC BY-SA or CC0. JSON

Builds on it 1 of 8 papers with this label
Uses its method or tool 3 of 18 papers with this label
How the literature uses it

280 arXiv papers cite this in 495 sentences

Each citing sentence is taken from the citing paper's LaTeX source and typed by Pith's classifier. A label appears at probability 0.8 or more (0.95 for support), a contest at 0.7 or more; the quoted words are the citing authors'. How each label was checked · JSON

Papers citing it · 275 sentences without a confident label

Contests it 1 paper
  • This result demands us to change the derivation of the quantum Jarzynski equality from the previous ones [ref] based on the conventional approach, which employed the internal unitary time evolution and regarded the measured value of the energy loss of the internal system as the “work”, whereas the energy gain of the external system is the actual work that we can use.

    Quantum Jarzynski equality of measurement-based work extraction · Introduction · contest 0.81

Builds on it 7 papers
Uses its method or tool 18 papers
Compares with it 10 papers
  • Using notation ([ref]) for a function of two eigenvalues, we then obtain [equation] This result was recently derived in Ref. [ref] and earlier by Tasaki [ref] under a weaker assumption of a unitary evolution.

    Jarzynski equality for quantum stochastic maps · Jarzynski equality for bistochastic maps · 0.84

  • The two central results in the paper are: the observation of intermittency between dynamical phases of distinct activity, itself a consequence of the reducibility of the dynamics in an appropriate limit due to the collective jump operators; and the existence of a Gallavotti-Cohen symmetry in LD functions associated to the time-asymmetric order parameter, analogous to that found in driven classical systems [ref] , which gives rise to a fluctuation theorem [ref] relating to the quantum jump rate.

    Dynamical symmetries and crossovers in a three-spin system with collective dissipation · Introduction · 0.84

  • Hence, we obtain the same form as the previous derivations [ref] of the Jarzynski equality under the cyclic process.

    Quantum Jarzynski equality of measurement-based work extraction · Cyclic process · 0.95

Cites it as support 3 papers
Cites it as background 147 papers

Discussion

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