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Time-dependence of correlation functions following a quantum quench

Statistical Mechanics · Submitted 11 Jan 2006 · 14 references

Pith's verdict: A genuinely foundational Letter: the quench-to-boundary-CFT mapping is the real contribution, the equal-time formulas hold up, and the soft spots are all in the deferred detail rather than the load-bearing claims.

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.

  1. T0 AI review An AI read the paper and answered a fixed list of questions.
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  3. T2 Compute The paper's code or calculations re-run with the same results.
  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/0601225 (version 2)
Pith IDSRZLBDGG, Pith's permanent record of this paper and its review
CitationJohn Cardy, Pasquale Calabrese. "Time-dependence of correlation functions following a quantum quench." arXiv:cond-mat/0601225 (2006). https://pith.science/paper/SRZLBDGG
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Referee report
The paper

The paper claims

A quench's aftermath can be read off from a boundary critical problem in one higher dimension, giving explicit decay laws and a light-cone time for correlations.

The reading

Take a quantum system in its ground state, then suddenly change the hamiltonian so it evolves under a new one. The paper argues that the resulting time-dependent correlation functions are not a model-by-model mystery: they are the correlation functions of a classical critical system in one extra dimension, living in a slab whose two faces are set by the initial state. When the new hamiltonian sits at a quantum critical point with linear quasiparticle dispersion, the slab problem becomes a boundary conformal field theory, and everything can be computed. The payoff is explicit formulas: one-point functions decay exponentially at a rate proportional to the operator's scaling dimension, and two-point correlations stay frozen until a sharp time set by separation divided by twice the quasiparticle speed, then saturate to a separation-dependent value. The authors check these against exactly solvable lattice models and explain them with a picture of entangled quasiparticles flying apart at a classical speed.

What carries the argument

The load-bearing device is the identification of the real-time expectation value with a euclidean path integral on a slab of width $2t_0$, where $t_0$ is the extrapolation length of boundary critical phenomena — the distance the actual initial state sits from the renormalisation-group-invariant boundary state. For $d=1$ critical quenches the slab is mapped to the upper half-plane by $w = (2t_0/1000000000)1000000000 z$, turning the problem into standard boundary conformal field theory: one-point functions come from the half-plane amplitude $A_b (2,1000000000 z)^{-x}$, two-point functions from the universal half-plane form with its undetermined function $F(1000000000)$, and the limits of $F$ are fixed by bulk-boundary short-distance expansions. The second device is the quasiparticle picture, which explains the formulas and generalises past CFT: quasiparticles entangled over the initial correlation length fly apart at the group velocity, so correlations at separation $r$ change only once $t$ reaches $r/2v$.

Extended reading notes

Core claim

The central claim is that a quench — preparation in the ground state of $H_0$, then unitary evolution under a different $H$ — maps onto a $d+1$-dimensional slab geometry whose boundary conditions encode the initial state, so that the renormalisation-group theory of boundary critical phenomena controls the late-time, long-distance behaviour. In $d=1$ with $H$ at a critical point described by a conformal field theory, the slab correlators follow from the conformal map $w=(2t_0/1000000000...)$ — concretely, from $w = (2t_0/1000000000)$-style strip-to-half-plane mapping with an extrapolation length $t_0$ of order the inverse gap of $H_0$. The resulting predictions are that a primary scalar of bulk dimension $x$ decays as $[1000000000/(4t_0)/1000000000(1000000000 t/2t_0)]^x$, that the two-point function is essentially unchanged for $t<r/2v$ and saturates to $e^{-1000000000 x r/2t_0}$ for $t>r/2v$, and that the local energy density does not decay at all because it is conserved. The authors present these as universal in their exponents and in the light-cone time, with the overall scale $t_0$ non-universal but cancelling in ratios.

Load-bearing premise

The authors state it themselves: the whole real-time prediction rests on the assumption that the euclidean boundary-conformal-field-theory answer, valid at large imaginary times, can simply be analytically continued to large real times.

Editorial extensions

If this is right

  • Ratios of relaxation rates for different local observables are universal and equal ratios of their bulk scaling dimensions, giving a parameter-free prediction for quench experiments near a critical point.
  • Connected correlations after a critical quench do not decay to zero but saturate at an exponentially small value in the separation, because the excess energy does not relax.
  • Locally conserved densities, notably the energy density, are exceptions: they approach a constant with an oscillatory power-law correction set by the slowest quasiparticles at the zone boundary.
  • For gapped or lattice dispersion relations the sharp light cone is smeared, the first signal arrives at $r/2v_m$ with $v_m$ the maximum group velocity, and spatial oscillations appear when that velocity occurs at nonzero momentum.
  • In $d=3$ above the upper critical dimension, mean-field slab results give $r^{-1}e^{-1000000000(t-r/2v)/t_0}$ before saturation.

Reading between the lines

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

  • Because only the exponent $x$ and the light-cone time enter, cold-atom experiments that measure two observables at once could test the theory without knowing $t_0$ at all, by checking that decay rates scale as the ratio of the two scaling dimensions.
  • The construction suggests a practical diagnostic: any quench experiment whose one-point relaxation is not exponential, or whose correlation front is not at $r/2v$, is either away from the critical point or dominated by a conserved quantity rather than a primary field.
  • The same slab logic should apply to Renyi and entanglement entropies of subsystems after a quench, since those are also boundary-condition-dominated quantities, which would tie the observed linear entanglement growth to the same quasiparticle flight time.
  • If the analytic continuation from imaginary to real time fails for non-primary operators, the natural place to look for the failure is a quench to a theory with a dangerously irrelevant coupling, where the euclidean boundary fixed point is reached only asymptotically.
Original abstract

We show that the time-dependence of correlation functions in an extended quantum system in d dimensions, which is prepared in the ground state of some hamiltonian and then evolves without dissipation according to some other hamiltonian, may be extracted using methods of boundary critical phenomena in d+1 dimensions. For d=1 particularly powerful results are available using conformal field theory. These are checked against those available from solvable models. They may be explained in terms of a picture, valid more generally, whereby quasiparticles, entangled over regions of the order of the correlation length in the initial state, then propagate classically through the system.

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, 3,629 input tokens, $0.0002 reported.

What the abstract itself states, as probabilities: claims first 0.03 · states limitation 0.17 · human participants 0.01 · controlled comparison 0.06 · randomized assignment 0.01 · states quantitative result 0.05 · compares with prior results 0.86 · mentions code or data availability 0.02.

The abstract, sentence by sentence

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

We show that the time-dependence of correlation functions in an extended quantum system in d dimensions, which is prepared in the ground state of some hamiltonian and then evolves without dissipation according to some other hamiltonian, may be extracted using methods of boundary critical phenomena in d+1 dimensions. For d=1 particularly powerful results are available using conformal field theory. These are checked against those available from solvable models. They may be explained in terms of a picture, valid more generally, whereby quasiparticles, entangled over regions of the order of the correlation length in the initial state, then propagate classically through the system.

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 show that the time-dependence of correlation functions in an extended quantum system in d dimensions, which is prepared in the ground state of some hamiltonian and then evolves without dissipation according to some other hamiltonian, may be extracted using methods of boundary critical phenomena in d+1 dimensions. contribution 0.86 0.88 0.86 hedged possibility 0.70
s1 For d=1 particularly powerful results are available using conformal field theory. background 0.58 0.13 0.05 asserted 0.93
s2 These are checked against those available from solvable models. method 0.79 0.74 0.05 asserted 0.91
s3 They may be explained in terms of a picture, valid more generally, whereby quasiparticles, entangled over regions of the order of the correlation length in the initial state, then propagate classically through the system. interpretation 1.00 0.46 0.21 hedged possibility 0.98

What kind of paper the classifier reports this is: theoretical result 0.55 · evidence from proof or derivation 0.91.

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

331 later papers cite it

Evidence: substantial. 319 of the citing papers share no author with it, and 25 of those carry a confident label from Pith's citation classifier.

Pith read 40 of the 478 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

Uses its method or tool 1 of 12 papers with this label · 1 by the same authors

By the same authors

How the literature uses it

119 arXiv papers cite this in 177 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 · 89 sentences without a confident label

Contests it 1 paper
Builds on it 1 paper
Uses its method or tool 5 papers
Uses its data or values 1 paper
Compares with it 5 papers
Cites it as support 1 paper
Cites it as background 65 papers

Discussion

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