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Quantum Noise Reduction in the LIGO Gravitational Wave Interferometer with Squeezed States of Light

L. Barsotti

Published Jan 1, 2014• 1 citation
Publisher

Abstract

The quantum nature of light imposes a fundamental limit on the sensitivity of ground based gravitational wave detectors. Squeezed states of light, produced via parametric downconversion in a non linear medium, have recently been employed to beat the quantum limit and extend the astrophysical reach of the most sensitive gravitational wave detector ever built.

DOI
10.1364/cleo_at.2014.aw3p.4

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The quantum nature of light imposes a fundamental limit on the sensitivity of ground based gravitational wave detectors. Squeezed states of light, produced via parametric downconversion in a non linear medium, have recently been employed to beat the quantum limit and extend the astrophysical reach of the most sensitive gravitational wave detector ever built.
Nearly a century after Einstein first predicted the existence of gravitational waves, a global network of Earth-based gravitational wave observatories 1-4 is seeking to directly detect this faint radiation using precision laser interferometry. Photon shot noise, due to the quantum nature of light, imposes a fundamental limit on the attometre-level sensitivity of the kilometre-scale Michelson interferometers deployed for this task. Here, we inject squeezed states to improve the performance of one of the detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) beyond the quantum noise limit, most notably in the frequency region down to 150 Hz, critically important for several astrophysical sources, with no deterioration of performance observed at any frequency. With the injection of squeezed states, this LIGO detector demonstrated the best broadband sensitivity to gravitational waves ever achieved, with important implications for observing the gravitational-wave Universe with unprecedented sensitivity.
Direct detection of gravitational waves will require earth based detectors to measure strains of the order 10−21, at frequencies of 100 Hz, a sensitivity that has been accomplished with the initial generation of LIGO interferometric gravitational wave detectors. A new generation of detectors currently under construction is designed improve on the sensitivity of the initial detectors by about a factor of 10. The quantum nature of light will limit the sensitivity of these Advanced LIGO interferometers at most frequencies; new approaches to reducing the quantum noise will be needed to improve the sensitivity further. This quantum noise originates from the vacuum fluctuations that enter the unused port of the interferometer and interfere with the laser light. Vacuum fluctuations have the minimum noise allowed by Heisenberg’s uncertainty principle, ∆X1∆X2 ≥ 1, where the two quadratures X1 and X2 are non-commuting observables responsible for the two forms of quantum noise, shot noise and radiation pressure noise. By replacing the vacuum fluctuations entering the interferometer with squeezed states, which have lower noise in one quadrature than the vacuum state, we have reduced the shot noise of a LIGO interferometer. The sensitivity to gravitational waves measured during this experiment represents the best sensitivity achieved to date at frequencies above 200 Hz, and possibly the first time that squeezing has been measured in an interferometer at frequencies below 700 Hz. The possibility that injection of squeezed states could introduce environmental noise couplings that would degrade the crucial but fragile low frequency sensitivity of a LIGO interferometer has been a major concern in planning to implement squeezing as part of baseline interferometer operations. These results demonstrate that squeezing is compatible with the low frequency sensitivity of a full scale gravitational wave interferometer. We also investigated the limits to the level of squeezing observed, including optical losses and fluctuations of the squeezing angle. The lessons learned should allow for responsible planning to implement squeezing in Advanced LIGO, either as an alternative to high power operation or an early upgrade to improve the sensitivity. This thesis is available at DSpace@MIT and has LIGO document number P1300006. Thesis Supervisor: Nergis Mavalvala Title: Professor of Physics
Direct detection of gravitational waves will require earth based detectors to measure strains of the order 10-21, at frequencies of 100 Hz, a sensitivity that has been accomplished with the initial generation of LIGO interferometric gravitational wave detectors. A new generation of detectors currently under construction is designed improve on the sensitivity of the initial detectors by about a factor of 10. The quantum nature of light will limit the sensitivity of these Advanced LIGO interferometers at most frequencies; new approaches to reducing the quantum noise will be needed to improve the sensitivity further. This quantum noise originates from the vacuum fluctuations that enter the unused port of the interferometer and interfere with the laser light. Vacuum fluctuations have the minimum noise allowed by Heisenberg's uncertainty principle, [Delta]X1 [Delta]X2 >/=1, where the two quadratures X1 and X2 are non-commuting observables responsible for the two forms of quantum noise, shot noise and radiation pressure noise. By replacing the vacuum fluctuations entering the interferometer with squeezed states, which have lower noise in one quadrature than the vacuum state, we have reduced the shot noise of a LIGO interferometer. The sensitivity to gravitational waves measured during this experiment represents the best sensitivity achieved to date at frequencies above 200 Hz, and possibly the first time that squeezing has been measured in an interferometer at frequencies below 700 Hz. The possibility that injection of squeezed states could introduce environmental noise couplings that would degrade the crucial but fragile low frequency sensitivity of a LIGO interferometer has been a major concern in planning to implement squeezing as part of baseline interferometer operations. These results demonstrate that squeezing is compatible with the low frequency sensitivity of a full scale gravitational wave interferometer. We also investigated the limits to the level of squeezing observed, including optical losses and fluctuations of the squeezing angle. The lessons learned should allow for responsible planning to implement squeezing in Advanced LIGO, either as an alternative to high power operation or an early upgrade to improve the sensitivity. This thesis is available at DSpace@MIT and has LIGO document number P1300006.
Since 2015, the gravitational-wave detectors LIGO and Virgo have opened a new window on the universe, detecting hundreds of signals and launching a new era of astronomy with profound impact on relativity, astrophysics, and cosmology. To listen deeper into the cosmos, detectors must become increasingly sensitive. One of the main limitations is quantum noise, ultimately arising from vacuum fluctuations entering the instrument. By “squeezing” this vacuum, i.e. manipulating its noise properties, LIGO and Virgo have already extended their reach by up to 65%, revealing events that would otherwise remain hidden. This article provides an overview of squeezing techniques for gravitational-wave detectors, from their origins to the most recent advances.
Squeezed vacuum states of light show a reduction in quantum noise along one quadrature compared to the vacuum state. This characteristic is the key to improve interferometric measurements beyond their quantum noise limitation. In particular for ground-based gravitational wave detectors [1] the application of squeezed light led to a significant sensitivity improvement of up to 6 dB. Future gravitational wave detectors, like the Einstein Telescope [2] or Cosmic Explorer [3], aim for an even higher enhancement in the range of 10 dB with the injection of squeezed states of light into the dark port of their interferometers.