First Demonstration of 6 dB Quantum Noise Reduction in a Kilometer Scale Gravitational Wave Observatory

Photon shot noise, arising from the quantum-mechanical nature of the light, currently limits the sensitivity of all the gravitational wave observatories at frequencies above one kilohertz. We report a successful application of squeezed vacuum states of light at the GEO 600 observatory and demonstrate for the first time a reduction of quantum noise up to $6.03\pm 0.02\mathrm{dB}$ in a kilometer scale interferometer. This is equivalent at high frequencies to increasing the laser power circulating in the interferometer by a factor of 4. Achieving this milestone, a key goal for the upgrades of the advanced detectors required a better understanding of the noise sources and losses and implementation of robust control schemes to mitigate their contributions. In particular, we address the optical losses from beam propagation, phase noise from the squeezing ellipse, and backscattered light from the squeezed light source. The expertise gained from this work carried out at GEO 600 provides insight toward the implementation of 10 dB of squeezing envisioned for third-generation gravitational wave detectors.

Figure 1

The layout of the squeezing application to the GEO 600 detector. The three Faraday isolators used for squeezed light injection are depicted. The squeezed light is injected such that it overlaps the main beam with the correct polarization at the MSR. The photodiodes shown at the in-vacuum Faraday isolator are used for polarization diagnostics.

Figure 2

With the application of squeezing, we observe a factor of 2 improvement in sensitivity at high frequencies. (a) The top plot compares 15 minute power spectral density (PSD) estimates of the detector noise with (red) and without (blue) squeezing. Towards higher frequencies, the increase in shot noise proportional to $f$ is due to calibration compensation of the filtering effect of the SRC. The bottom plot shows the squeezing ratio as a function of frequency. The squeezing value of $6.03\pm 0.02\mathrm{dB}$ is calculated from the ratio between median noise floor estimates with and without squeezing averaged between 6.3 and 6.5 kHz and referenced against periods where the squeezing was not applied during valid science time over a two month duration. Toward lower frequencies, the squeezing is increasingly limited by technical noise. Plot (b) shows a zoomed-in section of the same data (dots). Noise floor estimates in the top plot and their ratio in the bottom plot are depicted as solid lines.

Figure 3

Detected squeezing level as a function of measured antisqueezing measured at 5.2 kHz. The blue and red traces represent fits to the respective data points. The fitting procedure fits a common parameter for optical efficiency, $\eta$. The two solid traces represent estimates of the detectable squeezing level if a factor of 2 reduction of optical loss (green) and phase noise (yellow) could be realized. The dashed black trace is a factor of 2 improvement in both. This shows that we are still primarily limited by optical loss.