REVIEW
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
Why Pith says this
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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
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
- 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.
The abstract, sentence by sentence
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
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| 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 was examined, and what was not
- arXiv listing abstract v2
- full text
- figures and tables
- references and citing sentences
- relations to other papers
- P abstract
- S sections
- T body sentences
- C citations
- R cross-paper relations
References
How later papers used it
331 later papers cite it
1 paper by the same authors is listed below: 1 used its method or tool.
Uses its method or tool 1 of 12 papers with this label · 1 by the same authors
By the same authors
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Used the path integral approach in imaginary time developed in this paper to map quantities to correlation functions in a strip geometry with boundary conditions corresponding to conformally invariant boundary states.
Quenches in initially coupled Tomonaga-Luttinger Liquids: a conformal field theory approach (2021) (same authors) · 1.00 · paraphrased: the licence does not allow quoting
How the literature uses it
119 arXiv papers cite this in 177 sentences
Papers citing it · 89 sentences without a confident label
Contests it 1 paper
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In the limit when the duration of the quench $δt→0$ and $d2≤Δ<d$ the response of the system becomes singular, which is at odds with the predictions of sudden quench approximation [ref] .
Universality of fast quenches from the conformal perturbation theory · Introduction · contest 0.82
Builds on it 1 paper
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To obtain the conformal dynamics on the strip, we generalize the Calabrese–Cardy ansatz for critical initial states to the non-Hermitian setting [ref] .
Biorthogonal Conformal Dynamics in Non-Hermitian Quantum Quenches · Complex extrapolation parameters · 0.99
Uses its method or tool 5 papers
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Within CFT we use the Calabrese-Cardy representation of such a state as [ref] | Ψ_0 ⟩∝e^- τ_0 H_0 |B ⟩⟩, where $|B ⟩⟩$ is a (unnormalized) conformally invariant boundary state.
Entanglement entropy growth in stochastic conformal field theory and the KPZ class · Introduction · 0.90
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Following the setup of [ref] , the global quench is effectively modeled as a BCFT calculation.
Entanglement Tsunami in (1+1)-Dimensions · Global Quench · 0.86
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The strategy of [ref] to compute correlators after this process is to treat them as path integrals on a bounded complex surface in complex spacetime (real space times complex time), conformally map this surface to the complex upper half plane (UHP) and compute the correlators in the UHP using boundary conformal field theory (BCFT), also known as surface critical behavior [ref] .
Mutual information after a local quench in conformal field theory · Introduction · 0.93
Uses its data or values 1 paper
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From Ref. [ref] one has $Δ_b=0$.
Entanglement entropy growth in stochastic conformal field theory and the KPZ class · Introduction · 0.93
Compares with it 5 papers
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[In [ref] , it was shown that UV modes of a $1+1$ dimensional CFT thermalize faster than the IR modes irrespective of the strength of the coupling constant.] While a standard perturbation technique in QCD predicts that the thermalization process should be bottom-up [ref] , these papers get an opposite behavior.
Holographic Thermalization with Weyl Corrections · Introduction · 0.80
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This is contrasted with the finite behavior in the case of an instantaneous quench [ref] .
On fast quenches and spinning correlators · Introduction · 1.00
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The physics is similar to that seen in homogenous quantum quenches in quantum field theories [ref] , but the explicit entanglement between subsystems brings in new features.
Quenched coupling, entangled equilibria, and correlated composite operators: a tale of two O(N) models · Introduction · 0.89
Cites it as support 1 paper
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As argued by the authors of [ref] , the EE after quench is well approximated by the universal parts.
Quantum Local Quench, AdS/BCFT and Yo-Yo String · Entanglement entropy after the quench · 0.96
Cites it as background 65 papers
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In the quantum field theory context, the conformal symmetry is strong enough to fix the dynamics of entanglement for certain subregions in two dimensional conformal field theories (CFT) [ref] .
On the Time Scaling of Entanglement in Integrable Scale-Invariant Theories · Introduction · 0.90
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Up to now, only few results [ref] are known beyond non-interacting Hamiltonians.
Quenching the magnetic flux in 1d fermionic ring: Loschmidt echo and edge singularity · Introduction · 0.89
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Recall the representation of gapped states with small coherence length introduced by Calabrese and Cardy [ref] .
Entanglement entropy growth in stochastic conformal field theory and the KPZ class · Ic- Twist operators & entanglement entropy in stochastic CFT · 0.95
Discussion
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