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Quantum GrossPitaevskii Equation
by Jutho Haegeman, Damian Draxler, Vid Stojevic, J. Ignacio Cirac, Tobias J. Osborne, Frank Verstraete
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Submission summary
Authors (as registered SciPost users):  Jutho Haegeman · Frank Verstraete 
Submission information  

Preprint Link:  http://arxiv.org/abs/1501.06575v4 (pdf) 
Date submitted:  20170309 01:00 
Submitted by:  Haegeman, Jutho 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approach:  Theoretical 
Abstract
We introduce a noncommutative generalization of the GrossPitaevskii equation for onedimensional quantum gasses and quantum liquids. This generalization is obtained by applying the timedependent variational principle to the variational manifold of continuous matrix product states. This allows for a full quantum description of many body system including entanglement and correlations and thus extends significantly beyond the usual meanfield description of the GrossPitaevskii equation, which is known to fail for (quasi) onedimensional systems. By linearizing around a stationary solution, we furthermore derive an associated generalization of the Bogoliubov  de Gennes equations. This framework is applied to compute the steady state response amplitude to a periodic perturbation of the potential.
List of changes
* We now already discuss the quantum Bogoliubov de Gennes equations (and its application) in the abstract and introduction.
* We have expanded our discussion regarding the comparison of cMPS results with results from phase space methods for dynamical simulations.
* We have adapted Figure 1 and the surrounding discussion.
Current status:
Reports on this Submission
Report #2 by Anonymous (Referee 4) on 201741 (Invited Report)
 Cite as: Anonymous, Report on arXiv:1501.06575v4, delivered 20170401, doi: 10.21468/SciPost.Report.103
Strengths
1  Clear and interesting message
2  Concisely written
Weaknesses
1  Very technical
Report
The article demonstrates how a continuous matrix product state ansatz in combination with the timedependent variational principle gives rise to the GrossPitaevskii equation (GPE) for a matrix dimension of $D=1$. For $D>1$ this ansatz provides quantum corrections and results in a generalized quantum GPE. Furthermore, this quantum GPE can be linearized into a quantum version of Bogoliubov  de Gennes equations. This is a nice and interesting result and is certainly worth publishing.
I disagree with the second referee on the issues of “length” and “scope” of the paper. More details from the supplementary material in the main text would only obscure the message and make it even harder to read. Furthermore, the paper clearly avoids standard MPS language and is sufficiently general for readers without a tensor network background.
However, a clear disadvantage of the paper is that it is indeed very technical and some parts are hard to follow. For example, I don’t think that phrases like “… since any complex submanifold of Hilbert space is automatically Kähler …” speak to many people. The paper falls short in addressing readers without the appropriate mathematical background. Unfortunately, the last part of the paper that presents a specific example calculation (and that would be accessible to a broader audience) seems unmotivated.
Ideally, the authors would simply show a comparison to an exact calculation as suggested by the referee 2. Then for a general reader the ideas would not seem “insufficiently verified” as referee 2 remarked. At least, however, the authors should explain the physics of Fig. 1 in more detail. Since the authors write in the abstract that their method includes “entanglement and correlations”, they should also explain how this can be seen from Fig. 1. At the moment they only refer to [45] and write that “This response cannot be explained using standard GPE, as interactions are a key ingredient for exciting Type II excitations.” Why is this peak at $2k_F$? What are these excitations? Why are they not described in the GPE and why are they present in the exact solution?
I recommend publication, if the authors add an explanation of the physics.
Requested changes
1  Add explanation of the physics of Fig. 1
Report #1 by Anonymous (Referee 3) on 2017330 (Invited Report)
 Cite as: Anonymous, Report on arXiv:1501.06575v4, delivered 20170330, doi: 10.21468/SciPost.Report.102
Strengths
N/A
Weaknesses
N/A
Report
The authors have made appropriate changes with respect to my previous comments. Specifically, they have added more focus on the application to the QBdG equations, commented in more detail on comparisons to preexisting techniques and made miscellaneous corrections.
With respect to the correspondence of the second referee, I am supportive of the authors response. I believe the paper is suitably written as to outline the method as a generalization of the conventional GPE. Whilst it is true that some of the more technical details will only be fully appreciated by an MPSfocused audience, the authors do do write the manuscript at a sufficient level to allow the reader to appreciate their conclusions. Lastly, I disagree with the referees assertion that the paper is too short. Much of the calculation details are indeed attached in the supplemental material, and I believe this is the appropriate place for them. I believe the paper in it's current form is suitably concise for the subject matter. Inclusion of further technical details would only serve to obfuscate the message of the paper.
Requested changes
N/A