Quantum noise is one of the fundamental limitations to the sensitivity of GW detectors that
appear as quantum shot noise, caused by statistical fluctuations in the arrival time of photons
at the interferometer output and quantum radiation pressure noise, due to quantum fluctuations
in the photon flux impinging on the interferometer mirrors. A key technology to improve the
GW detector sensitivity beyond this quantum noise limit is the injection of squeezed vacuum
states of light into the dark port of the interferometer. Since up to now the quantum radiation
pressure noise is just below the residual technical noise sources at low GW detection frequencies,
a broadband sensitivity improvement can be achieved by reducing the shot noise contribution via
a moderate injection of frequency-independent squeezed vacuum states, whose fluctuations are
reduced in the quadrature of the light aligned with the gravitational-wave signal. In this case,
the effect of squeezing injection is equivalent to an increase of the circulating optical power in
the detector, without the drawback of an increase of the thermal effects inside the interferometer
optics due to their residual absorption and consequent thermal aberrations. In the recent years,
Advanced LIGO and Advanced Virgo have operated with injected squeezed light. Advanced Virgo
has obtaining up to 3 dB of sensitivity improvement (increasing the binary neutron star sensitivity
range by 5-8%) [33] while Advanced LIGO , has achieved 3 dB of sensitivity improvement but
operating a dual recycling configuration [34]. Injected squeezed state of light allow to increase
the sensitivity without increasing the laser power and thus limiting the thermal effects that are
hard to control.
All these enhancements from a simple Michelson interferometer makes Advanced LIGO and
Advanced Virgo able to measure phase shift corresponding to mirror displacement much smaller
than ∼ 10−18 m.

### Language ###
English

### Assessment ###
The query could focus on the specific sensitivity improvement achieved by Advanced Virgo using squeezed light.

### Query ###
How much did Advanced Virgo improve its sensitivity by using squeezed light?
How much did Advanced Virgo improve its sensitivity by using squeezed light?