Circuit Complexity through phase transitions: consequences in quantum state preparation

The manuscript presents a study of circuit (and geometric) complexity of various quantum algorithms and unitary evolutions where the initial and target states are potentially separated by a quantum phase transition. The purpose of the manuscript is to show that the crossing of the quantum critical point leads to complexity increasing with system size. While such divergence is studied systematically for the Fubini-Study complexity where the scaling (dominated but the neighborhood of the QCP) is essentially connected to that of the fidelity susceptibility, for the Nielsen complexity the computation is done numerically for adiabatic algorithms and for VQE. The qualitative statement obtained is that whenever the algorithm is “aware” of a QCP the associated complexity is seen to increase and diverge with system size.

I think the subject matter of this paper quite interesting though at present the study seems more a collection of semi-qualitative observations rather than a systematic study of the relation between complexity and QCP. The introduction, section 1.A, is very nice though it would help to expand the explanation a bit, in particular between Eq.(11) and (14) where it is explained a connection between the two concepts. In particular I think it would be better not to confuse the reader and maybe use more space but state clearly the problem at all stages: the Nielsen complexity is related to transformations between two fixed initial and target state, the FS is a distance between states belonging to a given parametric manifold, the relation is given by such and such.

In the part concerning FS Complexity is not clear to me what new quantitative elements are brought in in this paper as compared to what was already know about fidelity susceptibilities though QCP (see e.g. the works of De Grandi, Gritsev and Polkovnikov a few years back). It would be useful to state it clearly.

In turn, the part concerning Nielsen complexity, which to my knowledge is novel, is extremely qualitative. Certainly it gives a hint towards interesting phenomenology but It would have been useful to have a study (even numerical) of scaling with systems size, how this is affected (or not) by the properties of the QCP, etc.

Overall I think that the paper with significant improvements in the presentation, in discussing the relation with previous literature and more quantitative statements in the final part could be published in SciPost Physics.

1- revise the section on various concepts of complexity improving clarity 2- discuss relation to fidelity susceptibility literature 3- make more quantitative statements in Nielsen complexity

The authors present an analysis of computational complexity in quantum algorithms and its behavior upon crossing a quantum phase transition in well-known quantum many-body systems. The paper is well written, the figures are clear, and the authors give a clear discussion and interpretation of their results.

An interesting result from the authors is the impact of the energy gap on computational complexity. It would be interesting to extend the present study to the case where the Hamiltonian is driven in a way that takes into account the spectral structure of the system. (see arXiv:2208.09271).

I recommend the publication of the manuscript after taking into account the requested changes

1) In the "Complexity through QPTs" subsection on page 7, there is a typo in the second line. I think the authors are referring to Figure 1 instead of Figure 6.

2) The authors should stick to one notation for the chain size, either $N$ or $L$, throughout the whole paper