PhD Defense by Valerii Novikov

Entangled states of light and atomic spin oscillators for quantum noise reduction in Gravitational Wave Detectors

This thesis is dedicated to the methods of the sensitivity improvement of laser interferometric Gravitational Wave Detectors (GWDs), whose performance is limited by quantum noise. In particular, we investigate the scheme, where the quantum noise reduction in GWDs can be achieved by using an auxiliary harmonic oscillator that is the ensemble of spin polarized cesium atoms.

The dynamics of a spin oscillator, suitable for broadband sensitivity enhancement of GWDs, should be predominantly driven by quantum noise at acoustic spectral frequencies. Moreover, in order to set the link between the GWD and the atomic ensemble, an Einstein–Podolsky–Rosen (EPR)-entangled state of light resilient to low-frequency technical noise is needed.

In this thesis, we report the preparation and characterization of each subsystem, performed separately. First, we present the entanglement source of two optical modes, having the wavelengths of 852 nm and 1064 nm that match the frequencies of lasers used to probe the atomic spin ensemble and the GWD, respectively.

We observe EPR correlations spanning down to audioband (Ω ≳ 10 kHz). The entanglement is verified by EPR-steering and Duan criteria. The recorded level of two mode squeezing (−7.1 dB) is one of the highest (to our knowledge) to date for such large wavelength separation.

Secondly, we prove the strong contribution of quantum back action (QBA) noise to the dynamics of the spin oscillator, using ponderomotive squeezing as a benchmark. In particular, we measure −3 dB and −0.7 dB of light noise reduction below shot noise level caused by interaction with the atomic ensemble, whose resonance frequency is set to 20 kHz and 6 kHz, respectively.

The factors compromising the spin oscillator in quantum regime at near-DC spectral frequencies are explored and discussed. It has also been shown that QBA-dominated motion can be obtained for the atomic ensemble initialized in a state with an effective negative mass, which is a key feature for quantum noise cancellation in the joint measurement. The dissertation is complemented by theoretical analysis of the scheme under realistic experimental conditions.