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Inhomogeneous quenches in the transverse field Ising chain: scaling and front dynamics
by Márton Kormos
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As Contributors:  Márton Kormos 
Arxiv Link:  http://arxiv.org/abs/1704.03744v1 (pdf) 
Date submitted:  20170420 02:00 
Submitted by:  Kormos, Márton 
Submitted to:  SciPost Physics 
Academic field:  Physics 
Specialties: 

Approach:  Theoretical 
Abstract
We investigate the nonequilibrium dynamics of the transverse field quantum Ising chain evolving from an inhomogeneous initial state obtained by joining two macroscopically different semiinfinite chains. We obtain integral expressions for all twopoint correlation functions of the JordanWigner fermions at any time and for any value of the transverse field. Using this result, we compute analytically the profiles of various physical observables in the spacetime scaling limit and show that they can be obtained from a hydrodynamic picture based on ballistically propagating quasiparticles. Going beyond the scaling limit, we analyze the approach to the nonequilibrium steady state and the fine structure of the propagating fronts. Near the front we observe the phenomenon of energy backflow where the energy locally flows from the colder to the hotter region.
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Anonymous Report 3 on 201766 (Invited Report)
 Cite as: Anonymous, Report on arXiv:1704.03744v1, delivered 20170606, doi: 10.21468/SciPost.Report.159
Strengths
1 Interesting and timely subject
2 Selfcontained work
3 The analytical predictions are checked against numerics
Weaknesses
1 Long and somehow technical paper
2 The relevance of the new results is unclear
Report
This paper studies time evolution following an inhomogeneous quench in the transversefield Ising chain (TFIC). Specifically, two open chains are prepared in equilibrium at different temperatures; then, the coupling between two spins at the boundaries is turned on, and the system is let to unitarily time evolve. As also mentioned by the author, this protocol of nonequilibrium dynamics has attracted much attention, and it was already analytically investigated in many integrable models, including the TFIC. I think it is fair to say that the new result of this paper is to show the leading finitetime corrections to the nonequilibium steady state (NESS) emerging in the limit of infinite time. To my knowledge, such finitetime corrections have never been discussed before, so I think that this work gives a substantial contribution to the field of research.
In my opinion the paper is written rather well, but it looks like a technical paper. For example, the author did not include a summary of the results, and the reader is forced to read the entire paper to become aware of all the findings. In addition, there are natural questions that remain unanswered, like: "Why are the finitetime corrections important?", "What do I learn from the new findings?". I recommend this paper for publication, however I think that the author should improve these aspects.
Requested changes
1 I suggest that the author include a section where the new results are presented and discussed.
2 It is not clear to me if the author has taken into account the contribution from the bound state when h<1.
3 At the end of section 4, the author writes: "In our approach, the NESS appears as a single special member of a continuous family that gives the spacetime profile of correlations in the semiclassical limit". The author should consider to add a reference to [Bertini and Fagotti, Phys. Rev. Lett. 117, 130402 (2016)], which, to the best of my knowledge, is the first paper stressing that point.
4 The leading finitetime corrections to the NESS display parity effects. I think that the author should comment on this aspect, especially in relation to the recent attempts to go beyond the hydrodynamic description assuming that the leading corrections can still be described within a local density approximation.
5 The expressions for the fermion correlations close to the light cone (eqs (53) and (55)) seem to approach zero in the infinitetime limit. Is this correct? Shouldn't some correlations approach a finite value?
6Do the leading finitetime corrections to the NESS depend on the details of the initial state close to the junction? Is there something universal in the leading finitetime corrections?
Anonymous Report 2 on 201765 (Invited Report)
 Cite as: Anonymous, Report on arXiv:1704.03744v1, delivered 20170605, doi: 10.21468/SciPost.Report.157
Strengths
1 The problem studied is very interesting;
2 The paper finds exact results and controlled expansions;
Weaknesses
1 Not enough discussion of the results;
2The bound state for h<1 is not considered;
Report
The paper studies nonequilibrium dynamics in the transverse field Ising chain (TFIC), focussing on inhomogeneous initial states that give rise to nontrivial transport. Specifically, the initial states considered are the junction of two homogeneous thermal states at different temperatures and the "domain wall" state.
The author maps the transverse field Ising chain in a free fermionic model and derives exact double integral representations for the two point fermionic correlators, with fermions at generic positions $n$ and $m$. Expanding this result for large $n, m$ and time he finds that the leading contribution gives the so called ''semiclassical" or "hydrodynamic" limit, which can be obtained by using the semiclassical picture of moving quasiparticles. He also determines the first finite time corrections to the NESS fermionic correlations (i.e. he considers the limit where the positions of the fermionic operators are fixed and the time goes to infinity), finding that they are $O(1/t)$. Finally, he determines the leading contribution to the correlations at the edge of the propagating front ($n\sim m \sim v_{\rm max} t$, where $v_{\rm max}$ is the maximal propagation velocity of quasiparticles), finding that they are described by the Airy kernel, he also determines the first corrections. He uses these results to compute profiles of observables (such as density and current of energy and magnetization) and correlation functions.
I think that the paper is interesting. Exact results and controlled expansions in free models are very important to test the validity of general ideas, such as the hydrodynamic approach. Moreover, the author also finds explicitly some of the finite time corrections, showing for example how the non equilibrium steady state is approached. Even if the paper is quite technical, the author made a substantial effort to make it more readable, moving most of the technical parts to the appendices. I, therefore, recommend the publication of this paper in Scipost.
I have two main points that I think should be addressed before publication, one more general and the other more technical.
1) I think that the only weak point of the paper is the too hasty discussion of the results obtained. As I detail in the specific points later in a number of cases the author could examine a little bit more the results obtained, discussing their importance and their dependence on the parameters, the relation with other works, and why one should expect those results on based on some general principles.
2) After Eq (11) the author writes that for $\frac{L}{L+1}<h<1$ there is a complex solution for the quantization conditions, but it does not affect the results in the thermodynamic limit. This is incorrect, as it follows from what correctly reported in Appendix A, the complex solution appears for $h<\frac{L}{L+1}$. This means that the complex solution remains in the thermodynamic limit for $h<1$. Since the bound state is localised at $x=0$ its contribution will not affect the results in the scaling limit for finte $u$. In the $u=0$ case it could in principle affect observables close to the origin. I think that the author should analyse the contribution of this bound state.
Requested changes
I have the following comments
 In the introduction the author writes that integrable model have " an extensive number of locally conserved quantities". I think that this sentence should read as "an extensive number of local conserved quantities". It is indeed believed that also local defects can break the integrability.
 Even if, as the author says, a thorough analysis of the partitioning protocol in the TFIC was missing (the work: Perfetto and Gambassi arXiv:1704.03437 is contemporary to the current one), a tightly related problem was considered last year in: Bertini and Fagotti, Physical Review Letters 117, 130402 (2016). This paper presents the leading order expansion for large $n$, $m$ and time of all the fermionic correlators in a quench from a generic gaussian translationally invariant state to the TFIC with open boundary conditions. Also in that case, it was found that the result is described by the semiclassical approximation. I think that the author should compare to this work.
 In the introduction, when talking about the TFIC the author refers to [18], saying that in there the energy current in the NESS and its fluctuations are computed. This is probably a typo as the model studied in there is not the TFIC.
 Can the author say something about the corrections away from the NESS? Does he expect also those to be $O(1/t)$?
 Is there a reason why one expects the Airy kernel to describe the scaling near the front in general?
 After Eq. (56) the author could note that what he finds is in agreement with LiebRobinson bounds.
 In Figures 1 and 2 the energy density is called $u_n$ while in the text $h_n$.
 Does the author have a physical interpretation for purely quantum the energy backflow phenomenon observed?
 I am surprised by how large are the corrections to the leading order result (68) (note that the equation referenced in the caption is incorrect) in Fig. 4 (b) and how well agree with the numerics. When the corrections are larger than the result I would expect the time to be too short for the expansion (53) and (58) to apply (this seems not to be the case given the agreement with the numerics).
 In Equation (73) would be useful to remind the reader of the definition of $u$, as that quantity is defined much before in the text.
 I think that would be better to study the critical regime for $T_L=0$ and $T_R=\infty$ separately, not in the "Tranverse Magnetization" Section (note that in the inline equation after 74 the author uses $u_n$ for the energy density).
 In Section 6.5 I think that the author should at least include the footnote 6 in the main text, i.e. should explicitly write that he restricts to $h<1$.
 The typo "naiv" appears two times, once in Section 6.5 and once in the Conclusion.
 In the last line of Eq. (174) $u$ is replaced by $x$.
Author: Márton Kormos on 20170728 [id 160]
(in reply to Report 2 on 20170605)
I would like to thank the Referee for the very careful, critical reading of the manuscript and for the insightful comments and questions. I must say that the paper has definitely improved upon implementing the suggested modifications.
"1) I think that the only weak point of the paper is the too hasty discussion of the results obtained. As I detail in the specific points later in a number of cases the author could examine a little bit more the results obtained, discussing their importance and their dependence on the parameters, the relation with other works, and why one should expect those results on based on some general principles."
In the revised version I have made an effort to improve this aspect of the manuscript. There are more discussions of the results throughout the paper following the comments of all the referees. There is a new subsection 5.2.2 on the edge behavior in the critical case. I have extended the conclusions section. Following the comments of Referee 3, I have also included comments on the lattice effect shown by the leading finite time corrections. Please find my answers to the specific questions below.
"2) After Eq (11) the author writes that for $h<1$ there is a complex solution for the quantization conditions, but it does not affect the results in the thermodynamic limit. This is incorrect, as it follows from what correctly reported in Appendix A, the complex solution appears for $h<1$. This means that the complex solution remains in the thermodynamic limit for $h<1$. Since the bound state is localised at $x=0$ its contribution will not affect the results in the scaling limit for finite $u$. In the $u=0$ case it could in principle affect observables close to the origin. I think that the author should analyse the contribution of this bound state."
All the numerical results shown in the plots were obtained using Eqs. (19), (25), and (26), where in the summation over the modes the boundary bound states were also included for $h<1.$ Their effect is however negligible, as demonstrated by the excellent agreement between the numerical and analytic results shown in several plots. Even the leading finite time corrections to the NESS are seen to agree with the numerics (see Figs. 1, 3, and 7).
To exclude the logical possibility that there is an error in the formulas or in their numerical implementation, I have compared the short time behavior of correlations near the junction with exact diagonalization results and found perfect agreement.
The reason for the unimportance of the boundary state is that it can only affect the dynamics through the initial correlations. The $AA$ and $BB$ type correlations give a Kronecker delta in the presence or absence of the boundary state, while the contribution of the boundary state to $\langle A_jB_l \rangle_0$ is negligible. This follows from Eqs. (25b,c) in which $\phi$ and $\psi$ are localized at opposite edges of the chain so their overlap is exponentially small.
Let me turn to the list of requested changes.
I have corrected the typos and implemented the small changes requested in comments number 1, 3, 6, 7, 10, 11, 12, 13.
" Even if, as the author says, a thorough analysis of the partitioning protocol in the TFIC was missing (the work: Perfetto and Gambassi arXiv:1704.03437 is contemporary to the current one), a tightly related problem was considered last year in: Bertini and Fagotti, Physical Review Letters 117, 130402 (2016). This paper presents the leading order expansion for large $n$, $m$ and time of all the fermionic correlators in a quench from a generic gaussian translationally invariant state to the TFIC with open boundary conditions. Also in that case, it was found that the result is described by the semiclassical approximation. I think that the author should compare to this work."
I thank the Referee for reminding me of this nice work. I have inserted references to this paper at several places, including the Introduction and Sec. 4 (in the first paragraph and below Eq. (37) and below Eq. (39)).
" Can the author say something about the corrections away from the NESS? Does he expect also those to be $O(1/t)$?"
A derivation very similar to that in Appendix D could also be performed at a ray corresponding to a finite $n/t\approx m/t$ value. The stationary point would be shifted in this case, of course. The natural expectation is that the leading order corrections are also $O(1/t),$ which is consistent with the numerical checks I have performed. I have added a remark about this at the end of Sec. 5.1.
" Is there a reason why one expects the Airy kernel to describe the scaling near the front in general?"
The mathematical origin of the Airy function near the front is that at the stationary point of the phase of the integrand the second derivative of the phase vanishes. If this happens in the double integral whose integrand has a pole when both integration variables approach this degenerate critical point, the Airy kernel appears. This is thus a rather generic phenomenon in fermionic systems. I have added a paragraph with references below Eq. (55).
However, not all correlations are described by the Airy kernel. In the revised version I point out that the correlator $\langle A_nB_n\rangle$ related to the the transverse magnetization and Majorana fermion density does not follow the Airy kernel behavior in the ferromagnetic phase. Moreover, in the critical case the derivative of the Airy kernel describes all correlations which does not show the typical staircase structure.
" Does the author have a physical interpretation for purely quantum the energy backflow phenomenon observed?"
As Fig. 4b shows, the backflow phenomenon is captured by the correction at the front described by the Airy kernel and its derivatives. Based on this it would be interesting to make a connection with the Airy process or some generalization of it related to the motion of the rightmost particle, but this was beyond the scope of the paper.
" I am surprised by how large are the corrections to the leading order result (68) (note that the equation referenced in the caption is incorrect) in Fig. 4 (b) and how well agree with the numerics. When the corrections are larger than the result I would expect the time to be too short for the expansion (53) and (58) to apply (this seems not to be the case given the agreement with the numerics)."
I completely agree with the Referee and I have to admit that I do not have a good explanation for this. However, as also noted by the Referee, the numerics fully support the analytic calculations.
" I think that would be better to study the critical regime for $T_L=0$ and $T_R=\infty$ separately, not in the "Tranverse Magnetization" Section (note that in the inline equation after 74 the author uses u_n for the energy density)."
I agree that it is not optimal to quote results for the energy density and current in the transverse magnetization section. However, as the discussion of this very special case is hardly longer than a paragraph, it would not be logical to devote a whole (sub)section to it, this is why I decided to include it in another section.
" In the last line of Eq. (174) $u$ is replaced by $x$."
$u$ was correctly replaced by $x/t,$ but this is an unnecessary complication so I decided to keep simply $u$.
Anonymous Report 1 on 201754 (Invited Report)
 Cite as: Anonymous, Report on arXiv:1704.03744v1, delivered 20170504, doi: 10.21468/SciPost.Report.127
Strengths
1timely topic (quantum quenches starting from inhomogeneous initial state)
2very detailed derivations and comparison between numerical evaluation of exact expressions and semiclassical approximations
Weaknesses
1maybe one could add a link to the general results of Ref. 20
Report
The article studies quantum quenches starting from an inhomogeneous initial state in the transversefield Ising chain. All relevant observables are analysed in terms of the exact solution as well as a semiclassical approximate solution. The presentation is very detailed and well written.
Requested changes
1At the beginning of Sec. 4 the semiclassical limit is defined as $t\to\infty$ while $n/t,m/t$ is kept fixed and $(nm)/t\to 0$. I do not see how $(nm)/t\to 0$ if $n/t,m/t$ are fixed. Please clarify this.
2In Sec. 6.2 a discussion of the relation to the general results of Ref. 20 could be added.
Author: Márton Kormos on 20170728 [id 158]
(in reply to Report 1 on 20170504)
I thank the Referee for reading the manuscript, for their very positive evaluation, and for drawing my attention to the following issues.
"1At the beginning of Sec. 4 the semiclassical limit is defined as
$t\to\infty$ while n/t,m/t is kept fixed and $(nm)/t\to0$. I do not see how (nm)/t\to0 if $n/t,m/t$ are fixed. Please clarify this."
I have replaced the quoted sentence by
"This semiclassical or hydrodynamic limit corresponds to the case when $n,m,t\to\infty$ with $\lim n/t=\lim m/t$ fixed implying $(nm)/t\to0$."
"2In Sec. 6.2 a discussion of the relation to the general results of Ref. 20 could be added."
Ref. [20] focuses on the full statistics of the energy current in the NESS in conformal field theories. It has already been shown in Ref. [16] that the critical Ising spin chain reproduces the general CFT results of Ref. [20]. I have added a reference to [20] below Eq. (67).
Author: Márton Kormos on 20170728 [id 159]
(in reply to Report 3 on 20170606)I would like to thank the Referee for the very careful, critical reading of the manuscript and for the insightful comments and questions. I must say that the paper has definitely improved upon implementing the suggested modifications.
"I think it is fair to say that the new result of this paper is to show the leading finitetime corrections to the nonequilibium steady state (NESS) emerging in the limit of infinite time."
The finite time corrections go beyond the semiclassical or hydrodynamical approximation so they are hard to analyze in general. The Ising model gives us the rare opportunity to derive the leading corrections analytically. For example, this can provide hints about the applicability of hydrodynamic approaches to describe these corrections.
I consider the derivation of the fine structure of the front equally important. On the one hand it reveals that the Airy kernel also shows up in the Ising spin chain, on the other hand I found that, perhaps surprisingly, the fermion density (transverse magnetization) is not described by the Airy kernel. Moreover, in the critical case none of the correlations follow the Airy kernel.
Finally, the extensive numerical simulations also revealed some interesting details about the dynamics and showed that a naive application of the Szeg\H{o} lemma describes the longitudinal spinspin correlations remarkably well.
"1 I suggest that the author include a section where the new results are presented and discussed.
"Why are the finitetime corrections important?", "What do I learn from the new findings?"
I seriously considered this suggestion. However, in the end I decided not to add another section but to extend the Conclusions, adding references to the relevant figures and equations. Besides this, there are more discussions of the results throughout the paper following the comments of all the referees. I have added a new subsection about the front structure in the critical case.
"2 It is not clear to me if the author has taken into account the contribution from the bound state when $h<1$. "
All the numerical results shown in the plots were obtained using Eqs. (19), (23), and (26), where in the summation over the modes the boundary bound states were also included for $h<1.$ Their effect is however negligible, as demonstrated by the excellent agreement between the numerical and analytic results shown in several plots.
For a more detailed explanation see my answer to the first question of Referee 1.
"3 At the end of section 4, the author writes: "In our approach, the NESS appears as a single special member of a continuous family that gives the spacetime profile of correlations in the semiclassical limit". The author should consider to add a reference to [Bertini and Fagotti, Phys. Rev. Lett. 117, 130402 (2016)], which, to the best of my knowledge, is the first paper stressing that point."
I thank the Referee for reminding me of this nice piece of work. I have inserted references to this paper at several places, including the Introduction and Sec. 4 (in the first paragraph and below Eq. (37) and below Eq. (39)).
"4 The leading finitetime corrections to the NESS display parity effects. I think that the author should comment on this aspect, especially in relation to the recent attempts to go beyond the hydrodynamic description assuming that the leading corrections can still be described within a local density approximation."
I would like to thank the Referee for bringing up this important point. I have included a paragraph at the end of Sec. 5.1 about this observation, and mentioned it in the Abstract and the Conclusions.
"5 The expressions for the fermion correlations close to the light cone (eqs (53) and (55)) seem to approach zero in the infinitetime limit. Is this correct? Shouldn't some correlations approach a finite value?"
This comment of the Referee was very valuable as it led to an improved discussion of the calculation of the near edge behavior of correlation functions. For correlations that are nonzero to the right of the front, one first has to subtract this finite value. It turns out that this expectation value is related to the residue of the corresponding double integral, so subtracting it amounts to changing the integration contour in the integral (i.e. to changing the sign of the infinitesimal shift $i\delta$.) The resulting double integral then yields the Airy kernel that decays to zero outside the front.
Let me note that in the literature usually the left contributions are analyzed only which decay to zero, and to the best of my knowledge the subtraction step has not been discussed.
"6Do the leading finitetime corrections to the NESS depend on the details of the initial state close to the junction? Is there something universal in the leading finitetime corrections?"
The leading finite time corrections to the NESS depend on the initial state only through the function $g(p)$ and its derivatives at momenta $p=0$ and $p=\pi.$ The function $g(p)$ is determined by the initial distribution $f(p)$. So different distributions that give the same values $g^{(n)}(p)$ ($n=0,1,2$) will give rise to the same leading corrections, but I cannot make a better or deeper statement at present.