Enhanced sensitivity of the LIGO gravitational wave detector by using squeezed states of light
Summary (1 min read)
Introduction
- In the past, these processes have made it difficult for gravitational wave detectors to reach a shot noise limited sensitivity in their most sensitive band near 150 Hz.
- Randomly scattered light reflecting back into the interferometer has to be managed at the level of 10−18 W. Past experience has shown that measured sensitivities at higher frequencies are difficult to extrapolate to lower frequencies [2].
- The measured improvement due to squeezing is well explained given the amount of squeezing injected into the interferometer and the total measured losses in the squeezed beam path, as the authors will detail later.
- With the squeezed vacuum source employed in the H1 experiment, the authors could expect to reduce the shot noise by at least a factor of 2, improving the high frequency sensitivity of Advanced LIGO.
Injection of squeezed vacuum
- A schematic of the squeezed vacuum source is shown in the grey box of Fig.
- The optical parametric oscillator (OPO) is resonant for both 1064 nm and 532 nm light.
- The interferometer reflects both fields back towards the output mode cleaner (OMC).
- The beat between the 29 MHz frequency shifted coherent beam and the interferometer beam provides an error signal which is used to control the phase of the squeezed vacuum field relative to the interferometer field.
Optical Losses
- The optical losses measured in the path from the squeezed vacuum source to the “output photodiode” are 56%.
- Slusher, R.E. et al. Observation of Squeezed States Generated by Four-Wave Mixing in an Optical Cavity.
AUTHOR CONTRIBUTION
- The activities of the LIGO Scientific Collaboration (LSC) cover modeling astrophysical sources of gravitational waves, setting sensitivity requirements for observatories, designing, building and running observatories, carrying out research and development of new techniques to increase the sensitivity of these observatories, and performing searches for astrophysical signals contained in the data.
- S. Dwyer, S. Chua, L. Barsotti and D. Sigg were the leading scientists on this experiment, but a number of LSC members contributed directly to its success.
- K. Kawabe supervised the integration of the squeezed vacuum source into the LIGO interferometer, with invaluable support from M. Landry and the LIGO Hanford Observatory staff.
- The LSC review of the manuscript was organized by S. Whitcomb.
- The authors declare that they have no competing financial interests.
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Citations
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...The first-generation LIGO instruments were decommissioned shortly following the end of the science run (although immediately after S6 shot noise reduction was demonstrated in the H1 interferometer by using squeezed states of light [57]), and installation and early testing of aLIGO systems is now under way [23]....
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...Between the second observing run (O2) and O3, several improvements were made to increase the detectors’ (Aasi et al. 2013; Acernese et al. 2015) sensitivity....
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References
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Additional excerpts
...to the quantum fluctuations of the electromagnetic vacuum field, or vacuum fluctuations, that enter the interferometer [5, 6]....
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...[6] Caves, C....
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...Caves [5, 6] showed that replacing coherent vacuum fluctuations entering the antisymmetric port with correctly phased squeezed vacuum states decreases the “inphase” quadrature uncertainty, and thus the shot noise,...
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...work of earth-based gravitational wave observatories [1–4] is seeking to directly detect this faint radiation using precision laser interferometry....
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...field amplitudes that oscillate out of phase with each other by 90◦, labeled as “in-phase” and “quadrature phase” [7]....
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...have been necessary, but this power increase would be accompanied by the significant limitations of high power operation [15, 20]....
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...A significant upgrade, known as Advanced LIGO [15], is currently underway with the goal of increasing the strain sensitivity of the LIGO detectors by a factor of 10....
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...An important motivation for the experiment we present here was to extend the frequency range down to 150 Hz while testing squeezing at a noise level close to that required for Advanced LIGO [15]....
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Frequently Asked Questions (11)
Q2. What is the phase of the squeezed vacuum?
The beat between the 29 MHz frequency shifted coherent beam and the interferometer beam provides an error signal which is used to control the phase of the squeezed vacuum field relative to the interferometer field.
Q3. What is the recent research on the physics of aqueous quantum states?
Search for gravitational waves from low mass compact binary coalescence in LIGO’s sixth science run and Virgo’s science runs 2 and 3.
Q4. What is the optical loss in the vacuum?
The 1064 nm ‘control laser” is phase-locked to the “pump laser” to generate a 29 MHz frequency shifted coherent beam which co-propagates with the squeezed vacuum beam, entering the interferometer through the “output Faraday isolator”.
Q5. How much light is pumped to the vacuum source?
It is typically pumped with about 40 mW of 532 nm light, where the thresh-old for spontaneous sub-harmonic generation is near 95 mW.
Q6. Who was the lead scientist on the LIGO experiment?
N. SmithLefebvre, M. Evans, R. Schofield and C. Vorvick kept the LIGO interferometer at its peak sensitivity and supported the integration of the squeezed vacuum source, with contributions from G. Meadors and D. Gustafson.
Q7. What is the squeezing angle of the vacuum laser?
1. The 1064 nm “pump laser” is phase-locked to the “H1 laser” and it drives the second harmonic generator (SHG) to produce light at 532 nm.
Q8. What is the name of the paper?
New J. Phys. 11 073032 (2009)The authors gratefully acknowledge the support of the United States National Science Foundation for the construction and operation of the LIGO Laboratory and the Science and Technology Facilities Council of the United Kingdom, the Max-Planck-Society, and the State of Niedersachsen/Germany for support of the construction and operation of the GEO600 detector.
Q9. What is the cause of the mode mismatch?
The mode mismatch between the squeezed beam and the output mode cleaner (OMC) is mainly caused by a complicated optical train in the vacuum envelope, which precluded improving the mode matching on a time scale compatible with this experiment.
Q10. what is the reason for the loss in the LIGO H1 detector?
Most of these losses are due to the fact that the LIGO H1 detector was not initially designed for injection of squeezed states, and the squeezing injection path was retrofitted within the original LIGO optical layout.
Q11. What is the reason for the large optical losses in the OMC?
The losses in the OMC itself are also larger than expected, and they are believed to be due to scatter and absorption inside the mode cleaner cavity.