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ligoqnd.html
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<HTML>
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<TITLE>QND in LIGO</TITLE>
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<h1 style="color:Violet;text-align:center;">Broad-band QND using optical filters</h1>
<p style="color:Tomato;">It has been known since the <a href="https://journals.aps.org/prd/abstract/10.1103/PhysRevD.23.1693">seminal work</a> of
<a href="https://en.wikipedia.org/wiki/Carlton_M._Caves">Carlton Caves</a>, that
squeezed states of light can be used to enhance precision of optical interferometry.
<a href="https://en.wikipedia.org/wiki/W._G._Unruh">Bill Unruh</a>
<a href="https://www.researchgate.net/publication/279391935_Quantum_Noise_in_the_Interferometer_Detector">realized</a>
that one can also use it to beat the Standard Quantum Limit (SQL) in gravitational-wave interferometers,
and several versions of his idea were further developed before the end of the last millenium. However, one serious issue was
that these ideas were only applicable in the narrow range of frequencies, because of the strong frequency dependence
of the mechanical impedance of the free test mass.</p>
<p style="color:Tomato;">At the end of 1990s, <a href="https://en.wikipedia.org/wiki/Kip_Thorne">Kip Thorne</a> became concerned that no practical scheme existed for the Quantum Nondemolition (QND)
measurement in LIGO (QND in the contect of
LIGO means simply beating the Standard Quantum Limit). At the time, I was Kip's PhD studnet, and I spent
part of my PhD trying fruitlessly to come up with such a scheme. There was also a group in Moscow led by
Kip's old friend <a href="https://en.wikipedia.org/wiki/Vladimir_Braginsky">Vladimir Braginsky</a>, the originator of the concepts of
SQL and QND. Despite of the nearly-complete destruction of Soviet Physics after the dissolution of the Soviet Union, Braginsky managed
to retain some talented young people in his grop, and they carried out interesting research.
Part of the group's research funding came from Caltech and NSF,
and Kip
was instrumental in securing this funding.
In 1998, Kip and I travelled to Moscow, and we had extensive discussions with
Vladimir Braginsky, Michael (Misha) Gorodetsky,
Farid Khalili, and Sergei Vyatchanin. Upon returning to Caltech, Kip started a study group, which met regularly and discussed
various quantum-physics issues of potential relevance to LIGO. The meetings were well-attended by Caltech officionados of Quantum Measurements.
During one
of the meetings, I reported on Unruh's ideas and explained the difficulties for applying them to broadband measurements.
<a href="https://en.wikipedia.org/wiki/H._Jeff_Kimble">Jeff Kimble</a>
immediately had an idea about input optical filters. Kip, together with <a
href="https://www.linkedin.com/in/andrey-matsko-69a02415/">Andrei Matsko</a> and Sergei Vyatchanin, performed detailed computations.
This was the birth of KLMTV:</p>
<p style="color:Blue;text-align:center;">
<a
href="https://scholar.google.com/citations?view_op=view_citation&hl=en&user=_zkAFbEAAAAJ&pagesize=80&citation_for_view=_zkAFbEAAAAJ:u-x6o8ySG0sC">
Conversion of conventional gravitational-wave interferometers into quantum nondemolition interferometers by modifying their input and/or output optics
</a></p>
<p style="color:Tomato;">The input optical filters have now been built and are being tested in Advanced LIGO. They will allow us to make
larger part of the Universe "visible" to LIGO, and will increase the number of events observed by the gravitational-wave astronomers.
</p>
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