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Optimal control of a quantum sensor: A fast algorithm based on an analytic solution
by S. HernándezGómez, F. Balducci, G. Fasiolo, P. Cappellaro, N. Fabbri, A. Scardicchio
Submission summary
Authors (as registered SciPost users):  Santiago Hernandez Gomez 
Submission information  

Preprint Link:  https://arxiv.org/abs/2112.14998v4 (pdf) 
Date accepted:  20240603 
Date submitted:  20240517 09:24 
Submitted by:  Hernandez Gomez, Santiago 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approaches:  Theoretical, Experimental 
Abstract
Quantum sensors can show unprecedented sensitivities, provided they are controlled in a very specific, optimal way. Here, we consider a spin sensor of timevarying fields in the presence of dephasing noise, and we show that the problem of finding the pulsed control field that optimizes the sensitivity (i.e., the smallest detectable signal) can be mapped to the determination of the ground state of a spin chain. We find an approximate but analytic solution of this problem, which provides a \emph{lower bound} for the sensitivity and a pulsed control very close to optimal, which we further use as initial guess for realizing a fast simulated annealing algorithm. We experimentally demonstrate the sensitivity improvement for a spinqubit magnetometer based on a nitrogenvacancy center in diamond.
Author indications on fulfilling journal expectations
 Provide a novel and synergetic link between different research areas.
 Open a new pathway in an existing or a new research direction, with clear potential for multipronged followup work
 Detail a groundbreaking theoretical/experimental/computational discovery
 Present a breakthrough on a previouslyidentified and longstanding research stumbling block
Author comments upon resubmission
We gratefully acknowledge that both Referees 1 and 2 support the publication of our work in SciPost Phys. We appreciate very much their opinion that ``this manuscript reports on a well thoughtout and worthwhile study, with a clear transformative elements'', and that ``the line or argument is theoretically appealing'' and ``another great asset consists in the experimental verification''.
In the List of changes, we address pointbypoint all the Referees' comments.
List of changes
Response to Referee 1
\textbf{Referee 1. Requested change 1}: {\it It must be emphasized early on that $h(t)$ must be known beforehand. Only the amplitude can be detected. Please discuss whether this is a realistic situation occurring in practice! Me, being a theorist, am a bit skeptical whether the by far more relevant goal would not be to detect $b(t)$ including its time dependence.
Similarly, please state clearly, that the power spectrum $S(\omega)$
of the noise needs to be known a priori.}
\textbf{Response}: We thank the Referee for the comment. We added a phrase in the second paragraph to point out that our optimization method requires the previous knowledge of the target field time dependence and that of the noise spectrum. On the other hand, we introduce the fact that $h(t)$ must be known beforehand when this variable is first introduced, in Section 2. We now added a phrase to stress that knowing beforehand the time dependence of the target field has applications such as sensing spins ensembles or spins in molecules, or sensing weak signals (e.g. the action potential) coming from biological samples. Moreover, our optimization method can be part of a protocol where the timedependence is first (suboptimally) determined and then then the amplitude detected optimally.
\textbf{Referee 1. Requested change 2}: {\it Emphasize early on that you are dealing with *dephasing* only, omitting other decoherence processes. Justify that ``dephasing only'' is still a relevant issue.}
\textbf{Response}: We thank the Referee for pointing this out. In the abstract, Introduction and in Section 2 we state that the noise we are dealing with is dephasing noise. The main idea of implementing DD sequences is to overcome the loss of coherence due to dephasing noise [Barry et al., Rev. Mod. Phys. 92, 015004 (2020), Degen et al., Rev. Mod. Phys. 89, 035002 (2017)]. This is extremely relevant since most of the quantum sensing techniques are based on retrieving information stored in the coherent phase of the quantum sensor. Dephasing noise is also the strongest source of noise for NVbased sensors and many others.
\textbf{Referee 1. Requested change 3}: {\it What is meant by the "overlap" between noise and target field between Eq.~(3) and (4)? Please be a bit more precise.}
\textbf{Response}: We thank the Referee for the suggestion. What we mean is that simple signals with frequencies that are well offresonance with the frequencies composing the noise can be already amplified by CarrPurcell or generalized CarrPurcell control signals. We modified the text accordingly.
\textbf{Referee 1. Requested change 4}: {\it The quantity $\eta$ is called "sensitivity", but later minimized. From the sense of the word, however, the sensitivity should be large  so I would rather call $1/\eta$ sensitivity.}
\textbf{Response}: We appreciate the Referee's suggestion. Our notation may be confusing since some times people refer to an improved sensitivity as ``high sensitivity'', however in literature it is very common to refer to the sensitivity as the minimum signal that can be measured [\href{https://doi.org/10.1103/RevModPhys.89.035002}{Degen et al., Rev. Mod. Phys. 89, 035002 (2017)}, \href{https://doi.org/10.1103/RevModPhys.92.015004}{Barry et al., Rev. Mod. Phys. 92, 015004 (2020)}, \href{https://www.frontiersin.org/articles/10.3389/fphy.2023.1212368/full}{Bai et al., Front. Phys. 11, 1212368 (2023)}]. It makes sense then to minimize it in order to improve the sensor capability to detect weak signals. In view of the above, we prefer to keep the current definition.
\textbf{Referee 1. Requested change 5}: {\it In Eq. (11) the inverse of a deltafunction is used in the continuum. Please comment on what this really means and how one can define it mathematically.}
\textbf{Response}: We thank the Referee for the question. The delta function represents the kernel of the identity operator on the space of squareintegrable functions. Its inverse is therefore another delta function. We modified the text explaining more carefully the meaning of the inverse.
\textbf{Referee 1. Requested change 6}: {\it After Eq. (15) please state that you take lambda to be constant. The notation $\lambda(t)=\lambda$ is not unambiguous.}
\textbf{Response}: We thank the Referee for the suggestion. We modified the text as requested.
\textbf{Referee 1. Requested change 7}: {\it Fig. 2 should be rendered much larger, including the fonts, to reach a decent readability.}
\textbf{Response}: We thank the Referee for the suggestion. We modified the figure as requested.
\textbf{Referee 1. Requested change 8}: {\it An important point needs to be elucidated in the experimental part: How can the "true" value of b read off and with which error? This must be elucidated, at best by a figure.}
\textbf{Response}: We agree with the referee that this point was not explicitly stated in our manuscript. For a fixed value of $T$, the value of $b$ is obtained by measuring the probability $P(T,b)$, i.e. the probability for the NV spin to stay in the initial state, estimated from the frequency of the $\left0\right\rangle$ result in repeated measurements. The value of $b$ is then estimated from the measurement and the (known) functional dependence of $P(T,b)$ on $b$. The error of this estimate is bounded by the sensitivity. The sensitivity reported in Fig. 5a is optimized assuming to implement ``slope detection'' [see Degen et al., Quantum sensing, Rev. Mod. Phys., 89, 3 (2017)  Sec. IV.E]. Therefore, to achieve this sensitivity it is necessary to be in the maximum slope section of the curves 5b. Therefore, to minimize the sensitivity, it is desirable to have the slope as steep as possible for the first half period of the oscillation shown in Fig.5b. In that range, there is an approximate linear relation between the variable to be measured $b$ and the output of our experiments $P(T,b)$, thus simplifying its estimate. In this sense, an improved sensitivity comes at the cost of a smaller dynamic range. Notice though that measuring fields higher than the ones in the first half period can be achieved by changing the range of the sensor (e.g.\ by changing the final observablephase of the final $\pi/2$ pulse, a global phase will be added to the signal $P(T,b)$). A similar explanation was added in a new paragraph at the very end of Section 4. We hope that this explanation is enough, we believe that a figure is not necessary, since it won't be different from others in literature [see e.g., Fig. 3 in Ref. Degen et al., Quantum sensing, Rev. Mod. Phys., 89, 3 (2017)].
\textbf{Referee 1. Requested change 9}: {\it Fig. 6, panel b) the tick labels on the yaxis should start at 0.}
\textbf{Response}: We have rescaled the figure to include 0 in the yaxis
\textbf{Referee 1. Requested change 10}: {\it Why does the rightmost panel of Fig. 7 not contain data for Sph.?}
\textbf{Response}: The case Sph. was not included in the right most panel of Fig. 7 because we didn't perform an experiment for that specific case. We could, however, include the simulated value (square) of the inverse sensitivity. A new version of the figure was implemented.
\textbf{Referee 1. Requested change 11}: {\it Finally, a careful reading by an English native speaker can improve the manuscript by leading to more idiomatic sentences at several occasions. "A minima" should read "A minimum", on page 9.}
\textbf{Response}: We thank the Referee for the suggestion. We improved the text following these suggestions and thoroughly spellchecked the manuscript.
Response to Referee 2
\textbf{Referee 2. Requested change 1}: {\it The overall clarity of the manuscript would be much increased if the abstract already specified the usage of the word "optimal", with a sentence to the effect of "in the sense that it optimises the sensitivity, i.e., the smallest detectable signal". It would also be beneficial to explicitly state in the introduction that the minimisation of the sensitivity is analogous to the classical Hamiltonian minimisation (upon time discretisation).}
\textbf{Response}: We appreciate the Referee for their suggestions. We have added a phrase in the abstract and at the beginning of the Introduction to improve the clarity of the manuscript. And we explicitly state in the introduction and in Fig 1 the connection between minimizing the sensitivity and finding the ground state of the classical Hamiltonian.
\textbf{Referee 2. Requested change 2}: {\it In deriving (A.4), why is it that C can be set to 1?}
\textbf{Response}: We thank the Referee for the question. In Eq.~(A.4) the constant $C$ can be set to 1 since it represents an overall scale factor in the detection efficiency, fixed by the experimental architecture~[Degen et al, Rev.\ Mod.\ Phys.\ 89, 035002 (2017)]. As such, it affects all the frequencies in the same fashion, and does not modify the optimization. We modified the text to highlight this fact.
\textbf{Referee 2. Requested change 3}: {\it In Eqs.(2,4), I take it Phi(T)=Phi(T,b)/b ? (Since Phi(T,b) is proportional to b?) This should be clarified.}
\textbf{Response}: We thank the Referee for the comment. Our notation was not clear, since $\varphi(T,b)$ was sometimes written as $\varphi(T)$. We purged the text from these inconsistencies.
\textbf{Referee 2. Requested change 4}: {\it Eq.(4): absolutely explain the operational significance of the sensitivity}
\textbf{Response}: We thank the referee for pointing out this. The sensitivity is the smallest detectable signal (variation) above the noise level. Operationally, in a magnetometer, $\eta$ is the minimum variation of the target field $b$ that can be measured in a slope detection protocol. We have now modified the paragraph around Eq.~(4) to include more details about the sensitivity in our experiments.
Current status:
Editorial decision:
For Journal SciPost Physics: Publish
(status: Editorial decision fixed and (if required) accepted by authors)
Reports on this Submission
Report
I found the replies of the authors to my questions and remarks persuasive. The modifications applied have improved the clarity of the manuscript so that I am now in favor of publication without reservation.
Recommendation
Publish (surpasses expectations and criteria for this Journal; among top 10%)
Report
The authors have taken all of our comments on board and accordingly improved the manuscript, which is worthwhile as per the previous report. I am therefore happy to recommend its publication in current form.
Recommendation
Publish (meets expectations and criteria for this Journal)