Virgo and the gravitational interferometry

. The Advanced Virgo detector is a long scale enhanced Michelson interferometer placed in Italy, close to Pisa, with the aim of detecting gravitational waves from astronomical sources. The Advanced Virgo interferometer has detected, together with the LIGO interferometers located in USA [1], an impressive collection of gravitational waves emissions in the last observation runs O2 and O3. During the last observation run (O3), which lasted about one year of data taking from April 2019 to March 2020, were detected about 80 events. When the run ended, the detector has been upgraded toward the Advanced Virgo + configuration, in order to enhance its sensitivity, which it is currently in the commissioning phase. After an introduction on gravitational wave detection, the paper will focus on giving an overview of the evolution of the Virgo antenna in the past decades.


Introduction and gravitational wave detection principle
The gravitational waves (GWs) have been predicted in 1916 by Albert Einstein [2] as quadrupole emissions creating ripples in the space-time due to extremely violent astrophysics events (e.g. supernova explosions, collisions of neutron stars and black holes, etc...). Due to the weak interaction between the gravitational waves and the matter, their direct detection occurred only one century after the prediction. The technological advancements allowed the LIGO-Virgo collaboration to detect the GW emission from two coalescing black holes on the 14 th of September 2015 (GW150914) [3]. As it has been mentioned before, the GW modifies the space-time metric in a quadrupole way, which means that modifies the distance measured between free-falling masses in a differential way for orthogonal directions. For this reason the most promising detection principle results to be the use of a Michelson interferometer, detecting the GW passage as a deviation from the interference condition [4]. The change of differential length due to a GW signal of strain amplitude h measured by a Michelson interferometer having orthogonal arms with length L is equal to:: In order to have an idea of the order of magnitude of the observable displacement, it can be considered that for an arm length of 1 km the GW having a strain amplitude of 10 −22 the generated displacement is very small of the order of ∆L = 10 −19 m. For this reason the optical configuration of the GW detector is much more complex than a simple Michelson interferometer. Additional cavities are added to enhance the detector response to GW signals, as shown in Figure 1: -the Fabry-Perot cavities in the long arms. The addition of a high reflecting mirror in the long arm allows to have the laser beam to bounce between the two mirrors increasing the laser power stored in the cavity, to reduce the Shot-Noise limit of the sensitivity, and to increase the optical path length of the arm.
-the Power Recycling mirror (PR). The addition of the PR between the laser and the interferometer increases the recycled light to improve the Shot-Noise limit.
-the Signal Recycling mirror (SR). The SR mirror is placed between the Beam-Splitter mirror (BS) and the detection port to recycle the GW signal enhancing the detector response.
The addition of all these optical resonators allowed to improve the detector sensitivity up to the first detection of GW emission in 2015, but on the other hand the increase of complexity of the detector has strongly increased the difficulties in its commissioning. The main reason is that the detector provides the best sensitivity only if all the optical cavities are actively controlled to maintain precise resonant conditions 1 [5] [6]. Moreover, the sensitivity of the detector is not limited only by the Quantum-Noise (Radiation pressure and Shot Noises), but also other fundamental noises contribute to the sensitivity, such as the thermal noise 2 and the seismic noise. The technological enhancements pushed these limits towards unprecedented levels.

The Virgo detector and its evolution
The Virgo experiment has been started in 1997 thanks to a French-Italian collaboration [7]. The Virgo collaboration has grown in time, being currently a collaboration formed by 800 1 In order to have an idea of the accuracy requirements of the control on the position of the main optical component it can be said that the differential displacement of the two long arms has to be lower than 10 −16 m. 2 The thermal noise is due to the thermal excitation of the mechanical modes of the test masses and the suspensions, resulting in a spurious signal at the detection port Unfortunately the sensitivity reached in the VSR* runs was not enough to allow a GW detection.
After the end of the VSR3 observing run, the detector has been upgraded toward the Advanced Virgo experiment [11]. The long arm cavity geometry has been strongly changed, from plano-concave to quasi-confocal configuration, to reduce the test mass thermal noise. Moreover to improve the Shot-noise and to increase the optical path length the arm finesse has been strongly increased of a factor 9 respect to the Virgo configuration, see second column of Table. The strong increase of the power stored in the arm cavities, and the marginality of the recycling cavity stability, has lead to strong effort in developing and implementing a Thermal Compensation system (TCS) to cope with thermal transients and defects [12]. The Advanced Virgo experiment allowed to detect on the 14 th August 2017 the first triple coincidence LIGO/Virgo, BH-BH merging GW170814. Only few days later, on the 17 th of August, the first detection of binary neutron star system occurred. The detection has lead to an intense observing campaign in the electromagnetic follow-up [13]. The O2 and O3 runs contributed to detect more than 80 GW events. The O3 observing run was concluded in March 2020, at the beginning of the pandemic emergency, the scientific data taking period was stopped to start the installation of the Advanced Virgo+ experiment. The main differences between the Advanced Virgo+ and the Advanced Virgo detector [14] are (see third column of Table 1): -addition of the Signal recycling mirror, see Figure 1, to improve the detector response -addition of frequency dependent squeezing [15], to improve the broadband quantum noise The installation of the Advanced Virgo+ has been concluded in December 2020, after that the detector entered in the commissioning phase to put in operation the aforementioned upgrades. The interferometer has been commissioned for almost two years, reaching a stable operability of the detector. Now it is currently in the phase of optimization and noise hunting to achieve the design sensitivity to start the O4 joint run, in the spring of 2023. The main detector parameters for the three Virgo configurations are summarized in Table 1.

Conclusions
The Virgo detector, together with the LIGO ones, has evolved in time reaching unprecedented sensitivities allowed to open a new window on the universe by detecting a impressive large amount of GW emissions. The detector is currently in the Advanced Virgo+ configuration which is meant to start the scientific data taking, in a joint run, in the next spring.