Quantum Backaction on kg-Scale Mirrors: Observation of Radiation Pressure Noise in the Advanced Virgo Detector

The quantum radiation pressure and the quantum shot noise in laser-interferometric gravitational wave detectors constitute a macroscopic manifestation of the Heisenberg inequality. If quantum shot noise can be easily observed, the observation of quantum radiation pressure noise has been elusive, so far, due to the technical noise competing with quantum effects. Here, we discuss the evidence of quantum radiation pressure noise in the Advanced Virgo gravitational wave detector. In our experiment, we inject squeezed vacuum states of light into the interferometer in order to manipulate the quantum backaction on the 42 kg mirrors and observe the corresponding quantum noise driven displacement at frequencies between 30 and 70 Hz. The experimental data, obtained in various interferometer configurations, is tested against the Advanced Virgo detector quantum noise model which confirmed the measured magnitude of quantum radiation pressure noise.

Figure 1

Simplified layout of the quantum manipulated Advanced Virgo detector as operated during the observational run O3. Advanced Virgo is a power recycled Michelson interferometer using 3 km long Fabry-Perot arm cavities. The interferometer output field is spatially filtered by two output mode cleaning cavities (OMCs), for simplicity drawn as a single stage cavity in the figure and split in two beams before detection. The sum of the two signals from the photodetectors is used to derive the interferometer strain signal $h(t)$. The interferometer main optics and the injection or detection benches are suspended and operated inside a vacuum envelope. Squeezed vacuum states of light are prepared on an external optical bench and are injected into the interferometer after the transmission through a sequence of Faraday isolators.

Figure 2

Displacement sensitivities $\sqrt{S_L}$ as measured in February 2019 and the corresponding quantum noise models $\sqrt{S_q}$ (dashed lines). Black trace: without squeezed vacuum injection. Red trace: with phase-squeezed light injection ($\theta =0$, $x=0.66$). Blue trace: with amplitude-squeezed light injection ($\theta =\pi /2$, $x=0.66$). The gray trace illustrates the total nonquantum noise $\sqrt{S_{\mathrm{nq}}}$. Experimental spectra are calculated from FFT averaged over 300 s time.

Figure 3

Power spectral density (PSD) $S_{dq}$ in the presence of phase-squeezed light (red trace) and of amplitude-squeezed light (blue trace) with respect to nonsqueezing; data are calculated from the same data set of Fig. 2. Error bars, not shown on the plot, are about 30% of the $S_L$ value. Black dotted lines represent the quantum noise model, i.e., Eq. (4), estimated from high-frequency phase squeezing and amplitude squeezing; the black dashed-dotted line represents the two-parameters least-squares fit of experimental data with phase squeezing against Eq. (4).

Figure 4

QRPN antisqueezing factor $A_{\pi /2}$, measured from low-frequency data with phase squeezing, versus QSN antisqueezing factor measured from high-frequency data with amplitude squeezing. Error bars represent statistical uncertainty, i.e., the standard error in the least-squares fit of experimental spectral data with quantum noise model. Horizontal error bars are not visible on the scale of the plot. The black shaded area indicates the confidence interval for the model, derived from uncertainties presented in Table 1. Orange line and shaded area are a linear fit to data and the corresponding $1\sigma$ confidence interval. The yellow point corresponds to data presented Fig. 3.