Laser Intensity Noise Suppression for Preparing Audio-Frequency Squeezed Vacuum State of Light

Round 1

Reviewer 1 Report

The problem of squeezed state observation is the problem with long history. Authors make good contribution to this problem. However, I would suggest to discuss some resoult beyond Eq. (1) that  describes linatization approach.  Also it is necessary to represent expressions for thevariables and coefficients represented in (1). I think, some fitting of theoretical predictions with experimental results will be useful for Fig.2,3 

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Reviewer 2 Report

The authors have obtained experimental results but it is not clear the novelty. Two feedback loops have been developed by using acousto-optical modulators (AOM) to stabilize the intensity of 795-nm near infrared (NIR) fundamental laser and 397.5-nm ultraviolet (UV) laser generated by cavity-enhanced frequency doubling. They have suppressed the 795-nm NIR and 397.5-nm UV intensity fluctuation of the laser involved in the preparation of squeezed vacuum state of light. In particular, the peak-to-peak laser intensity fluctuation is reduced to 1/100. The authors before publication should clearly describe with more details the novelty of their work and its impact on the industrial application or scientific panorama. The exact collocation of their work in literature should be better underlined in the introduction, avoiding to anticipate detailed description of the experiment which should be given in the following sections.

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Reviewer 3 Report

Your article entitled "Laser Intensity Noise Suppression for Preparing 2 Audio-frequency Squeezed Vacuum State of Light" is interesting as it explores new findings on audio-frequency squeezed vacuum state of light.  They are not so many things to clarify but two major and one minor comments need to be addressed by author prior the publication in Applied Science.

Figures 2b,d and 3b are the time-domain peak-to-peak fluctuations spectra with feedback on or off. For me, it is quite strange to see that time domain spectra with feedback on have very small fluctuations in comparison with the real noise. From Fig. 2a,c and 3a seem that the noise is reduced only less than 10%. Can you put the time-domain peak-to-peak fluctuations spectra in log scale? In this presentation, we can not really observe the fluctuations when the feedback is on. The claim in conclusion page 6, line 219 about 1/100 of the original laser intensity fluctuations need to be confirmed. The author contribution in page 7 line 229. Can you more elaborate what experiments and what data analysis that have been completed by each author? Also ". And J. W.'s contributions" is wrong.  

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Reviewer 4 Report

The authors present a study of a stabilization loop aiming at stabilization of a squeezed vacuum state of the light field in the low-frequency regime. Here, the authors focus on the audio-band range of frequencies in the low kHz-range, which is relevant e.g. for the demonstration experiments trying to include squeezed light interferometry into gravitational wave detection. To this end, the authors introduce an AOM-based feedback loop that uses a feedback signal from a photodetector to stabilize the zero-order beam. The feedback loop is applied to both light fields used to generate the squeezed vacuum - both the fundamental light field and its second harmonic. This stabilization results in higher squeezing of the squeezed vacuum state.

The authors demonstrate a peak enhanced squeezing level of about -4 dB in the audio frequency range when using the stabilization feedback loop in contrast to a squeezing level of -3.3 dB when not using the feedback loop. This enhancement is demonstrated approximately between 3.5 kHz and 9 kHz. The authors also investigate the performance of their system for varying parameters, e.g. for different feedback bandwidths of the photodiodes used to generate the feedback signal. In principle, the results presented here are of interest to the community working on using squeezed light for precision sensing of low-frequency signals, e.g. in gravitational wave detection. However, at current the manuscript is lacking relevant information that the reader would certainly need to know in order to make full use of the results presented here. For example, the standard paper on squeezing in the audio-frequency regime (reference [31] by Mckenzie et al.) already reported a squeezing level of 5.5 dB without additional stabilization. Is the difference related to the different operation wavelengths and lasers used (795 nm here compared to 1064 nm in reference [31]) or are there some other fundamental differences? Also, the role of the feedback bandwidth is not really discussed in a way that makes it easy for the reader to follow it. It is not really clear what determines the effective noise characteristics and what limits the stabilization circuit. Obviously, the bandwidth of photodiode 1 has a major influence on the stabilization procedure. This is somewhat expected as PD1 will be more susceptible to noise at larger bandwidths. However, in order for the reader to really be able to judge the results, the authors should present the properties of the other components as well. For example when considering the results in figure 2, the bandwidth of photodiode 2 will be very important as well. Is it set to a fixed value? Or is this bandwidth changed corresponding to how the bandwidth of photodiode 1 is changed? Also, I assume that the highest possible operation frequency of the AOM is much larger than the typical frequency ranges considered here, but this is not made clear. Accordingly, some specifications of the AOM, such as the highest possible frequency or its rise time might be helpful for the reader. Accordingly, I suggest that the manuscript should be revised significantly and a revised manuscript that enables the reader to design a similar stabilization circuit tailored to his own needs may be publishable in Applied Sciences.

Some minor points that should be taken into account:
- The terms RBW and VBW used in figure 4 are not defined in the manuscript.
- In the caption of figure 4, the authors state that a squeezing level of -4.1 dB may be achieved when the finite quantum efficiency of the homodyne detector and the finite transmission efficiency are not taken into account. By how much do these values improve when they are taken into account properly?

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