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Quantum hypothesis testing in manybody systems
by Jan de Boer, Victor Godet, Jani Kastikainen, Esko KeskiVakkuri
Submission summary
As Contributors:  Victor Godet · Jani Kastikainen 
Arxiv Link:  https://arxiv.org/abs/2007.11711v2 (pdf) 
Code repository:  https://github.com/victorgodet/quantumhypothesistesting 
Date submitted:  20200821 09:30 
Submitted by:  Godet, Victor 
Submitted to:  SciPost Physics 
Academic field:  Physics 
Specialties: 

Abstract
One of the key tasks in physics is to perform measurements in order to determine the state of a system. Often, measurements are aimed at determining the values of physical parameters, but one can also ask simpler questions, such as "is the system in state A or state B?". In quantum mechanics, the latter type of measurements can be studied and optimized using the framework of quantum hypothesis testing. In many cases one can explicitly find the optimal measurement in the limit where one has simultaneous access to a large number $n$ of identical copies of the system, and estimate the expected error as $n$ becomes large. Interestingly, error estimates turn out to involve various quantum information theoretic quantities such as relative entropy, thereby giving these quantities operational meaning. In this paper we consider the application of quantum hypothesis testing to quantum manybody systems and quantum field theory. We review some of the necessary background material, and study in some detail the situation where the two states one wants to distinguish are parametrically close. The relevant error estimates involve quantities such as the variance of relative entropy, for which we prove a new inequality. We explore the optimal measurement strategy for spin chains and twodimensional conformal field theory, focusing on the task of distinguishing reduced density matrices of subsystems. The optimal strategy turns out to be somewhat cumbersome to implement in practice, and we discuss a possible alternative strategy and the corresponding errors.
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Review "Quantum hypothesis testing in manybody systems"
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Techniques and concepts from quantum information theory are receiving increasing interest in the field of manybody physics and quantum gravity over the last years. In this submission the authors discuss the task of quantum hypothesis testing for oneparameter families of quantum states, assuming that the parameter (e.g., temperature) is perturbatively close. Their main results
are a new bound between the relative entropy variance and the relative entropy in this perturbative limit (which is important in this setting) and evaluating the respective quantities in some exemplary models of qubits, free fermionic (Gaussian) manyparticle systems and conformal field theory. In the latter two examples they apply the results to reduced density matrices of the manybody
system in question. While these calculations are interesting from a technical point of view, a clear discussion of what one actually learns from these computations and how to interpret the results physically is lacking.
Besides deriving their results, the authors give a relatively thorough and nice review of quantum hypothesis testing. This review may in particular be useful for researchers outside the field of quantum information theory as it collects various results (but in itself does not yield new results). It does, however, have the downside of making the paper rather long. To put it a bit strongly, at times one has the feeling that the authors want to report all that they have learned about hypothesis testing and quantum information theory, but it's less clear what the overall aim is.
The discussion mentions some natural open problems, which I agree should be studied, such as whether in manybody systems one can study, instead of taking many independent copies of the system, many disconnected subsystems of the single large manybody system. (Given the title of the work, it was my hope that some of them would have been tackled in this work.)
The authors also mention the connection of their work to research in quantum gravity which indeed seems like an interesting avenue for future work.
Overall I think the submission fulfills the "General acceptance criteria", but does not provide a breakthrough result. One may argue, though, that it
"opens a new pathway in an existing or a new research direction, with clear potential for multipronged followup work;"
or
"provides a novel and synergetic link between different research areas."
Hence it may be suitable for publication in scipost physics.
Remarks

 The authors study parametrically close manybody states. However, there is a subtle issue of (non)commuting limits here, namely taking the limit λ>0 and taking the thermodynamic limit. For example, even if two manybody systems have arbitrarily close temperatures (independent of system size) then in the thermodynamic limit their global states will generically be perfectly distinguishable in the single shot by a global energy measurement (their tracedistance will approach 1).
I think it would be important to discuss this issue in this paper.
 There are two different kind of expansions in this paper: One with respect to the parameter value, one with respect to the number of copies one takes in the hypothesis test. The expressions "first order" or "second order" are used differently in the two settings. It may therefore be useful to somehow distinguish these (for example by saying "leading order in n" vs. "first order in λ" or so).
 In secs. 3.2.13.2.3 the authors demonstrate that their, rigorously proven, theorem holds in particular simple examples. It's not clear to me what the use of these sections is, since the examples don't seem to deliver additional insight.
 As someone not very well versed in CFT techniques, I found the discussion of the CFT computations extremely dense and hard to follow (especially in contrast to the very explicit, detailed and pedagogical calculations in the rest of the paper). It would be very useful to be more explicit in the calculations, in particular if the aim of the paper is (as it seems to be) to bring together different communities.
Smaller points, typos etc.

 In the introduction it says that subsystem reduced density matrices of fermion chains are determined by twopoint functions. This is only true for Gaussian states (free fermions) as studied later in the paper and should be specified.
 After eq. (2.6) it says that "[...] log Q_s(ρ,σ) are the relative Rényi entropies [...]".
The quantity log Q_s(ρ,σ) is only proportional to the relative Rényi entropy of order s?!
 At the bottom of page 5 it says that Q satisfies the dataprocessing inequality. Given the pedagogical aim of the paper, it may be useful to state what the dataprocessing inequality is.
 Footnote 9 on p. 12: ΔK is not yet defined.
 What the authors call "capacity of entanglement" is known under various names in information theory, such as variance of surprisal, varentropy etc.
 After (3.49) the authors say that if "β_2 > ∞, ρ_2 reduces to the ground state, and the relative entropy variance vanishes (along with C(β_2)>0)." This is only true if the ground state of the Hamiltonian is nondegenerate.
 In sec 4.2 shouldn't the acceptance condition be  E  \tilde E >= \mathcal E? I.e. with absolute value sign? Similarly for (4.23) etc.?
 After (4.33) it should probably read "completely positive" instead of "positive".
 In (6.28) and alter ΔA is not defined.
 Why is the sandwiched Rényi relative entropy suddenly mentioned after (6.33)?
 In sec. 6.3.3. it says that the XY spin chain can be mapped to a free fermion chain in the thermodynamic limit. This is also possible for finite chains (care must be taken with regard to the boundary condition) and indeed done in (6.85).
 I didn't understand what is meant with the last sentence before Sec. 7: "It is interesting that the breaking of translation invariance leads to noncommutativity from the perspective of two fermions"
 In (7.3), L_0 and c hasn't been defined.
 In (7.9,7.10) are referred to as giving the "entanglement spectrum", but they give states
(the eigenstates not the eigenvalues of the modular Hamiltonians).
 While maybe not directly relevant for the questions studied here (and hence does not need to be cited here), the authors may be interested in the recent preprint arxiv:2009.08391, which discusses mathematical properties of the relative entropy variance from an information theoretic point of view.