Editing LIGO (section)

===A+===
Like Enhanced LIGO, certain improvements will be retrofitted to the existing Advanced LIGO instrument. These are referred to as {{em|A+}} proposals, and are planned for installation starting from 2019 until the upgraded detector is operational in 2024.<ref>{{Cite web|url=https://www.nsf.gov/news/news_summ.jsp?cntn_id=297414|title=Upgraded LIGO to search for universe's most extreme events|website=www.nsf.gov|date=14 February 2019 |language=en|access-date=2020-04-09}}</ref> The changes would almost double Advanced LIGO's sensitivity,<ref>{{Cite journal |title=Prospects for doubling the range of Advanced LIGO |journal=Physical Review D |volume=91 |issue=62005 |pages=062005 |date=16 March 2015 |first1=John |last1=Miller |first2=Lisa |last2=Barsotti |author-link2=Lisa Barsotti|first3=Salvatore |last3=Vitale |first4=Peter |last4=Fritschel |first5=Matthew |last5=Evans |first6=Daniel |last6=Sigg |doi=10.1103/PhysRevD.91.062005 |arxiv=1410.5882 |url=http://dspace.mit.edu/bitstream/handle/1721.1/96149/PhysRevD.91.062005.pdf|bibcode=2015PhRvD..91f2005M |s2cid=18460400 }}</ref><ref>{{cite conference |title=Getting an A+: Enhancing Advanced LIGO |first=Michael E. |last=Zucker |url=https://dcc.ligo.org/LIGO-G1601435/public |id=LIGO-G1601435-v3 |date=7 July 2016 |conference=LIGO–DAWN Workshop II |conference-url=https://wiki.ligo.org/LSC/LIGOworkshop2016/WebHome}}</ref> and increase the volume of space searched by a factor of seven.<ref>{{Cite web|url=https://www.popularmechanics.com/space/telescopes/a26362135/ligo-gravitational-wave-observatory-update/|title=LIGO Gravitational Wave Observatory Getting $30 Million Upgrade|last=Thompson|first=Avery|website=www.popularmechanics.com|language=en-US|access-date=17 February 2019|date=15 February 2019}}</ref> The upgrades include:
* Improvements to the mirror suspension system.<ref>{{Cite news|url=https://www.bbc.com/news/science-environment-47213202|title=Black hole detectors to get big upgrade|last=Ghosh|first=Pallab|date=15 February 2019|access-date=17 February 2019|language=en-GB}}</ref>
* Increased reflectivity of the mirrors.
* Using frequency-dependent [[squeezed light]], which would simultaneously decrease [[radiation pressure]] at low frequencies and [[shot noise]] at high frequencies, and
* Improved [[Optical coating|mirror coatings]] with lower mechanical loss.<ref>{{Cite web|url=https://dcc.ligo.org/LIGO-T1800042/public|title=LIGO-T1800042-v5: The A+ design curve|website=dcc.ligo.org|access-date=2020-04-09}}</ref>

Because the final LIGO output photodetector is sensitive to phase, and not amplitude, it is possible to squeeze the signal so there is less [[phase noise]] and more amplitude noise, without violating the [[Uncertainty principle|quantum mechanical limit]] on their product.<ref>{{Cite web|title=The Quantum Enhanced LIGO Detector Sets New Sensitivity Record |url=http://ligo.org/science/Publication-SqueezedVacuum/index.php}}</ref> This is done by injecting a "squeezed vacuum state" into the dark port (interferometer output) which is quieter, in the relevant parameter, than simple darkness. Such a squeezing upgrade was installed at both LIGO sites prior to the third observing run.<ref>{{Cite journal|last1=Tse|first1=M.|last2=Yu|first2=Haocun|last3=Kijbunchoo|first3=N.|last4=Fernandez-Galiana|first4=A.|last5=Dupej|first5=P.|last6=Barsotti|first6=L.|author-link6=Lisa Barsotti|last7=Blair|first7=C. D.|last8=Brown|first8=D. D.|last9=Dwyer|first9=S. E.|last10=Effler|first10=A.|last11=Evans|first11=M.|date=2019-12-05|title=Quantum-Enhanced Advanced LIGO Detectors in the Era of Gravitational-Wave Astronomy|journal=Physical Review Letters|volume=123|issue=23|pages=231107|doi=10.1103/PhysRevLett.123.231107|pmid=31868462|bibcode=2019PhRvL.123w1107T|doi-access=free|hdl=1721.1/136579.2|hdl-access=free}}</ref> The A+ improvement will see the installation of an additional [[optical cavity]] that acts to rotate the squeezing quadrature from phase-squeezed at high frequencies (above 50&nbsp;Hz) to amplitude-squeezed at low frequencies, thereby also mitigating low-frequency [[radiation pressure]] noise.