Frequency-Dependent Squeezed Vacuum Source for the Advanced Virgo Gravitational-Wave Detector

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Frequency-Dependent Squeezed Vacuum Source for the Advanced Virgo Gravitational-Wave Detector

F. Acernese et al. (Virgo Collaboration)

Phys. Rev. Lett. 131, 041403 – Published 25 July 2023

Abstract

In this Letter, we present the design and performance of the frequency-dependent squeezed vacuum source that will be used for the broadband quantum noise reduction of the Advanced Virgo Plus gravitational-wave detector in the upcoming observation run. The frequency-dependent squeezed field is generated by a phase rotation of a frequency-independent squeezed state through a 285 m long, high-finesse, near-detuned optical resonator. With about 8.5 dB of generated squeezing, up to 5.6 dB of quantum noise suppression has been measured at high frequency while close to the filter cavity resonance frequency, the intracavity losses limit this value to about 2 dB. Frequency-dependent squeezing is produced with a rotation frequency stability of about 6 Hz rms, which is maintained over the long term. The achieved results fulfill the frequency dependent squeezed vacuum source requirements for Advanced Virgo Plus. With the current squeezing source, considering also the estimated squeezing degradation induced by the interferometer, we expect a reduction of the quantum shot noise and radiation pressure noise of up to 4.5 dB and 2 dB, respectively.

Received 12 February 2023
Revised 24 April 2023
Accepted 9 May 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.041403

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Gravitation Gravitational waves

Gravitational wave detectors

Gravitation, Cosmology & Astrophysics

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Article Text

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Issue

Vol. 131, Iss. 4 — 28 July 2023

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Images

Figure 1
Experimental setup (not to scale) for the FDS generation installed at the Virgo site. The interferometer arms and the filter cavity are 3 km and 285 m long, respectively, while the SQB1-SQB2 and SQB2-FCIM vacuum tanks are 10 m and 40 m apart, respectively. The expanded area in the green frame contains the simplified optical layout, the description of which can be found in the text. The measurements reported in Figs. 2 and 3 are made with the half-wave plate HWP1, setting the IR beam polarization axis in order to maximize the polarizing beam splitter PS1 reflection toward the M5 mirror, and from there to the diagnostic homodyne detector. Prior to the measurements, the subcarrier beam is used to align the IR fields to the FC. It is subsequently blocked by the motorized shutter SH. This procedure is automatically repeated after each FC unlock. Alternatively, by rotating with HWP1 the beam’s polarization by 90°, the IR beams are directed toward the interferometer for squeezing injection, which is part of the currently ongoing second phase of the project. The figure shows the beam paths for the two mutually exclusive cases. The relevant acronyms are as follows: AOM, acousto-optic modulator; HWP, half-wave plate; M, mirror; DM, dichroic mirror; SHG, second harmonic generator; OPA, optical parametric amplifier; HD, homodyne detector; QPD, rf quadrant photodiode; PD, photodiode; PLL, phase locked loop; SH, beam shutter; PS, polarizing beam splitter; DET, detection bench; FCIM and FCEM, filter cavity input and end mirror, respectively.
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Figure 2
Relative squeezing level to shot noise in dB (positive is antisqueezing, negative is squeezing) measured at the homodyne detector, for different values of the coherent control phase, and for (44–55) Hz detuning of the FC resonance. The experimental data (solid lines) are fitted using an analytic model of squeezing degradation (dashed lines). The squeezing source generates (8.0–8.5) dB of squeezing. Note that below 20 Hz, all experimental curves increase in noise due to scattered light coupled to low-frequency seismic motion.
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Figure 3
Power spectral density (PSD) of the diagnostic homodyne detector output normalized to the shot-noise level versus the time of data acquisition. Every 3 min a power spectral density similar to that of Fig. 2 (blue line) with a homodyne angle of 90° is estimated. In correspondence with the acquisition time, the PSD is drawn in the plot with the frequency in the ordinate and the amplitude with a color scale, the calibration of which in dB of squeezing, is represented by the vertical bar on the right. The homodyne angle was chosen to maximize the contrast with which the detuning frequency is observed. The detuning frequency, initially set to be equal to 80 Hz, where the quantum noise suppression is at its maximum, is represented by the yellow curve, which fluctuates in time by about 10 Hz peak to peak. The sudden increase in the antisqueezing level occurring after about 6 h results from an optimization of the infrared alignment.
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Figure 4
Filter cavity detuning frequency versus the end mirror ring heater temperature. Each point corresponds to a pair of values measured every 3 min within 24 h. The maximum correlation is obtained assuming that the temperature of the mirror lags behind that of the ring heater by about 60 min.
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