Coherent Control of Vacuum Squeezing in the Gravitational-Wave Detection Band

Abstract

We propose and demonstrate a coherent control scheme for stable phase locking of squeezed vacuum fields. We focus on sideband fields at frequencies from 10 Hz to 10 kHz, which is a frequency regime of particular interest in gravitational-wave detection and for which conventional control schemes have failed so far. A vacuum field with broadband squeezing covering this entire band was produced using optical parametric oscillation and characterized with balanced homodyne detection. The system was stably controlled over long periods utilizing two coherent but frequency shifted control fields. In order to demonstrate the performance of our setup the squeezed field was used for a nonclassical sensitivity improvement of a Michelson interferometer at audio frequencies. DOI: 10.1103/PhysRevLett.97.011101 PACS numbers: 04.80.Nn, 42.50.Lc, 42.65.Yj, 95.55.Ym It was first proposed by Caves [1] that injected squeezed states may be used to improve the sensitivity of laser interferometers and might therefore contribute to the challenging effort of direct observation of gravitational waves [2]. The goal of that proposal was the reduction of the measurement's shot noise. Later Unruh [3] has found that squeezed light can be used to correlate interferometer shot noise and radiation pressure noise thereby breaking the socalled standard quantum limit and allowing for a quantum nondemolition measurement on the mirror test mass position, for an overview we refer to Ref. [4]. Harms et al. [5] have shown that advanced interferometer recycling techniques [6] that also aim for an improvement of the signalto-shot-noise ratio are fully compatible with squeezed field injection. Gravitational-wave detectors require squeezing in their detection band from about 10 Hz to 10 kHz. The majority of current squeezing experiments, however, have been performed in the MHz regime. Furthermore the orientation of the squeezing ellipse needs to be designed for every sideband frequency. The transformation from frequency independent squeezing to optimized frequency dependent squeezing can be performed by optical filter cavities as proposed in [4] and demonstrated in [7] for MHz frequencies. Also in the MHz regime, the combination of squeezed field injection and recycling techniques has been demonstrated [8,9]. Squeezing at audio frequencies has been demonstrated recently for the first time [10,11]. However, the phase of the squeezed vacuum could not be controlled by a coherent field.Controlling squeezed vacuum fields is the basic problem for squeezed field applications in gravitational wave (GW) detectors. Common control schemes rely on the injection of a weak, phase modulated seed field at the carrier frequency into the optical parametric oscillator (OPO) thereby turning the device into an optical parametric amplifier. It has been shown that even lowest carrier powers introduce large amounts of classical laser noise at audio frequencies and squeezing can no longer be achieved [10].On the other hand phase modulation sidebands are not present in a pure vac...

Quantum Up-Conversion of Squeezed Vacuum States from 1550 to 532 nm

“…An experimental demonstration of up-converted low-frequency squeezing will be achievable once a control system as used in Refs. [14,16,29] is added to the generation of the input squeezing as well as to the up-conversion process.…”

“…Spectral components of the pump's amplitude fluctuations solely result in (small) modulations of the up-conversion efficiency. Phase fluctuations of the pump lead to an averaging effect that can reduce the measured squeezing factor [28]; however, phase fluctuations of the pump field can be minimized by adopting the phase control scheme that was developed for pump field stabilization in audio-band squeezed-light generation via parametric down-conversion [29]. Since white squeezing spectra have already been demonstrated down to a sideband frequency of 1 Hz [16,17,30], we conjecture that our setup is in principle capable of producing a white squeezing spectrum down to the audio band and below.…”

Local readout enhancement for detuned signal-recycling interferometers

“…Finally, we would like to point out that this scheme should be further investigated as a candidate design for third-generation detectors, possibly in conjunction with the injection of squeezed vacuum states [23,24] into the interferometer's dark port [25,26].…”

A gravitational wave detector operating beyond the quantum shot-noise limit: Squeezed light in application

“…[9]. Important milestones were the controlled generation of squeezed states at audio-band frequencies [19][20][21], the test of compatibility of squeezedlight injection with enhancement cavities [22][23][24][25][26], and the generation of strongly squeezed states of light [27][28][29][30][31][32]. Figure 3 shows a photograph of our squeezed-light laser that was completed in 2010 at the Institute for Gravitational Physics (Albert-Einstein-Institute Hannover) and that is now part of the GEO 600 02002-p.3 Figure 3.…”