Quantum Noise Reduction

Advanced Virgo employs the “squeezed light” technique to achieve Quantum Noise Reduction (QNR). Squeezing changes the balance between the fluctuations that produce the shot noise and the radiation pressure noise, and hence allows Advanced Virgo to gain in sensitivity. 

During the third observation run O3, Advanced Virgo routinely used squeezing to reduce the shot noise, increasing by approx 40% the sensitivity above 200 Hz: this gain at high frequencies is achieved by squeezing the phase quantum fluctuations, at the expense of the amplitude quantum fluctuations. Therefore, the shot noise is lowered while the radiation pressure noise increases; but that was not a concern during O3 the radiation pressure noise was not limiting the sensitivity . This technique is called Frequency Independent Squeezing (FIS) as it has the same effect (shot noise reduction and radiation pressure noise increase) at all frequencies, The noise reduction it provides is equivalent to increasing the input  laser power, an operation that is usually quite complex due to the thermal effects that accompany changes of power. Squeezed states of the vacuum quantum electromagnetic field are generated by a dedicated optical system kindly provided by the Albert Einstein Institute (AEI, Germany) and injected into Advanced Virgo before the Output Mode Cleaner, after crossing an optical viewport that separates the Virgo in-vacuum environment. 

The enhanced sensitivity expected for Advanced Virgo in the following runs O4 and O5 makes it important to keep under control also the radiation pressure noise. This is why Advanced Virgo will implement a Frequency Dependent Squeezing (FDS) scheme:  this allows to squeeze phase fluctuations at high frequency (while increasing the quantum radiation pressure, unimportant in this frequency band), like in FIS, but at the same time to also squeeze amplitude fluctuations at low frequencies (increasing shot noise where it is not a concern). With FDS, the sensitivity of Advanced Virgo can thus be improved at all frequencies.

To achieve FDS an additional 300 meter long, ultra low-loss, optical cavity had to be installed, parallel to the North Arm: it is called the Filter Cavity. Before entering the interferometer from the dark port of the beam splitter, the squeezed quantum states of light generated by the same system provided by AEI are reflected by the Filter Cavity: in this way, the squeezing is turned from phase noise squeezing at high frequencies to amplitude noise squeezing at low frequency. The squeezing is said to be “rotated as a function of the frequency”.