SciPost Submission Page
Bose polaron in a quantum fluid of light
by Amit Vashisht, Maxime Richard, Anna Minguzzi
Submission summary
As Contributors:  Maxime Richard · Amit Vashisht 
Preprint link:  scipost_202107_00019v1 
Date submitted:  20210712 16:29 
Submitted by:  Vashisht, Amit 
Submitted to:  SciPost Physics 
Academic field:  Physics 
Specialties: 

Approach:  Theoretical 
Abstract
We study the Bosepolaron problem in a nonequilibrium setting, by considering an impurity embedded in a quantum fluid of light realized by excitonpolaritons in a microcavity, subject to a coherent drive and dissipation on account of pump and cavity losses. We obtain the polaron effective mass, the drag force acting on the impurity, and determine polaron trajectories at a semiclassical level. We find different dynamical regimes, originating from the unique features of the excitation spectrum of drivendissipative polariton fluids, in particular a nontrivial regime of motion against the flow. Our work promotes the study of impurity dynamics as an alternative testbed for probing superfluidity in quantum fluids of light.
Current status:
Submission & Refereeing History
You are currently on this page
Reports on this Submission
Anonymous Report 2 on 2021915 (Invited Report)
Strengths
1Detailed theoretical description
2Effective graphics
3Well written
Weaknesses
1Solution methods are not stated
2Lacks some discussion
Report
This work reports a study of an impurity in a nonequilibrium excitonpolariton condensate, with the dynamics of both the quantum fluid and the impurity taken into account. A peculiar regime of negative drag on the impurity is identified and characterized. The paper is well written and the theoretical description is clearly presented. The results are interesting, seem sound and are nicely illustrated by the figures. I think however that some additions are needed to make the paper clearer and more relevant.
First of all I think that the means (numerical, analytical...) with which the results are obtained should be stated more clearly, for example at the beginning of Section 4.
Another point that in my opinion is important to discuss is the experimental relevance of these calculations. In particular: how would a moving impurity be realized in an excitonpolariton condensate?
Moreover I think that the results of Ref. [22] are not quoted with sufficient clarity, since some of the results obtained here regarding the negative drag were anticipated in that paper, albeit with a much simpler model of a fixed impurity. A discussion of those results should be included in the Introduction.
Finally the exposition may benefit from adding some explanations to the theoretical part of Section 3, as I indicate in the "Requested changes", where I also indicate some minor points.
Even if I think that (after these changes) this work is well worth publishing, I am not sure that it meets one of the very selective expectations of SciPost Physics. While the paper contains interesting results obtained in an original way, I must admit that I do not see a groundbreaking discovery, a novel link between different fields or much followup work. For these reasons in my opinion the paper would fit better in SciPost Physics Core.
Requested changes
In order of appearance
1I think the predictions of Ref.[22] should be reported in the introduction
2A discussion of the experimental relevance of the calculations should be included
3Operators $\hat{a}_\mathbf{k}$ in Eq. (2) and $\hat{\alpha}_\mathbf{k}$ in Eq. (4) are not defined
4Maybe the $\hat{p}$ operator in equation (3) (and also in (10)) should be bold
5I think the density n appearing after equation (8) is not defined
6At the end of Sec. 2.1 and at the beginning of Sec. 2.2 it would be nice to have references for the BogoliubovFrolich Hamiltonian and the Lee Low Pines transformation
7When $\Pi$ is an operator it should have the hat
8After Eq. (22) a coherent state of excitations is considered. When is this a valid procedure? I think some comment is worth adding.
9At the end Sec. 3.2 I think the definition of the effective mass and of $\eta$ and how they follow from Eq. (26) should be explained in more detail
10Also the definition of the drag force in Eq. (28) would benefit from some more details. Also I think $n_{imp}$ is not defined.
11Eq. (28) and (29) should have only one number
12In the line after (29) ie should be i.e.
13A discussion of the solution methods should be added in Section 4
14At the end of the second paragraph of Sec. 4.1 the value $\Delta/gn=2$ is used to distinguish the different regimes, but in Fig. 2 $\Delta/gn=1$ appears as a threshold
15I could not find a definition of the $\Gamma$ independent of $\mathbf{k}$ that is used in Section 4
16Is "decreasing $\Gamma$" in at the end of Sec. 4 supposed to be "increasing $\Gamma$"
17In the conclusions it is stated that for $\Delta>0$ the impurity is subjected to negative drag force, but this does not always happen for $\Delta>0$
18In the caption of Fig. 6, in the list of values of $\Delta$, there is a cursive "and"
Anonymous Report 1 on 2021825 (Invited Report)
Report
The manuscript by A. Vashisht et al. discusses the properties of a single impurity in a drivendissipative polariton bath. The paper is very well written, the problem is experimentally interesting, the formalism is clearly explained, and the equations and the results appear correct. As such, I can surely recommend publication of this manuscript in SciPost.
Upon resubmission, I think the Authors may extend the discussion on the key result presented here, i.e., the presence of a region of negative effective mass. In particular, is there an intuitive understanding of why such effect appears when the Bogolubov dispersion becomes complex? If I understood correctly, such effect was already noticed in earlier works on static impurities (Refs. [22,23]). In this case, this discussion may be placed already in the Introduction.
Here below follow further minor remarks and suggestions, presented in chronological order, that the authors may want to take into account.
# caption of Fig. 1: "Central panel as a" > "Central panel: as a " (":" missing). Moreover, define what is an "LP fluid".
# Sec. 2, line 3: "by its coupling" > "by their coupling"
# one line below Eq. (1): explain the adjective "lower" used in front of "polariton"
# Eqs. (2) and (4): define the operators $a_k$ and $\alpha_k$
# Eq. (5): in the moving reference frame, shouldn't the two operators $a_{k_p}$ and $a^\dagger_{k_p}$ in the last line of the equation actually read $a_0$ and $a^\dagger_0$?
# one line below Eq. (9): "all the the" > "all the". Further, the discussion in this paragraph is unclear. It is first said that $u_k = u^*_k$, but then it is specified that when $\Delta>0$ this is not the case. To avoid ambiguities, I suggest then to say "if $\Delta>0$ then $u_k = u^*_k$. Else, ..."
# to simplify notation, I suggest to introduce the operator $\Pi$ already around Eq. (13).
# Eqs. (17) and (20): a factor $1/M$ is missing in the term $\hbar k \cdot p$
# two lines above Eq. (22): $2\pi/\Gamma_k$ > $2\pi/\Gamma_{\bf k}$
# Eq. (28): $\langle \int$ > $\int \langle$. Moreover, I have the impression that $n_{imp}$ is not defined.
# line following Eq. (29): "ie" > "i.e."
# Eq. (34): I have the impression that $\Gamma$ (without pedix k) is not defined, please check. It is also unclear how such quantity emerges from the previous equations. A few more lines of explanation may be spent to explain this key equation.
# end of page 9: The two sentences "In all the above cases... . This is possible ..." contain important information, which I think deserves further clarification for the reader unfamiliar with polaritons. The authors may want to extend slightly the discussion here.
# Fig.2b: inside the various panels, $\Delta = ...$ should be $\Delta/gn = ...$
# Fig. 3: what is the dashed part of the red line?
# last paragraph of Sec. 4: "the region ... disappear" > "the region ... disappearS"