## Sunday, August 18, 2013

### LIGO: improving sensitivity by squeezed states

Gravitational waves could become visible next year

On Friday, SciTechDaily wrote about an interesting recent article in Nature:
Improvements to LIGO Detector Will Allow Scientists to ‘Listen’ to Black Holes Forming (SciTechDaily, Daily Galaxy)

Enhanced sensitivity of the LIGO gravitational wave detector by using squeezed states of light by J. Aasi and 24 co-authors (Nature Photonics: full PDF paper here)

LIGO.org press release
LIGO, the Laser Interferometer Gravitational-Wave Observatory, a large L-shaped instrument to detect the gravitational waves, hasn't seen anything yet but it may change soon and dramatically.

The authors of the new Nature paper – the whole LIGO collaboration – is sending special packets of light, the squeezed states, to one of the LIGO detectors and this modification is improving the sensitivity.

Sometimes it sounds like they are claiming that they are circumventing the Heisenberg uncertainty principle but I hope that they're not being completely silly. They're apparently improving a suboptimal technique that has been used so far.

With the upgrade, the facility could become able to observe black holes that are just being born. The gravitational waves could provide us with completely new "eyes" to see many phenomena in the Universe.

LIGO is now being upgraded to Advanced LIGO, scheduled to be operational in 2014. Correct me if I am wrong but I think that the current usage of the squeezed states isn't completely new but what's new is that the squeezed states are being used for the frequency range 150-300 Hz.

Also, my understanding is that this improvement should be ready when Advanced LIGO begins its operations. That's why another article in The Daily Galaxy claims that the direct detection of the gravitational waves is imminent. It sounds pretty exciting.

Just to be sure, state-of-the-art theorists don't have any realistic doubts about the existence of gravitational waves as predicted by GR. It seems impossible for GR to work in all the situations where it has been tested while failing in the case of the gravitational waves. Also, the discovery of a binary pulsar that generated the 1993 Physics Nobel Prize allowed one to verify that the celestial system is losing the same energy each second (the loss is measured from the accelerating frequency of the orbits) that is carried away by the GR-calculable gravitational waves. This consistency check has actually been repeated using independent celestial objects (which have different parameters) so it's almost certainly not a coincidence. We're only waiting for a direct detection of the waves and the applications of these new "eyes".

Hat tip: Bahamas

1. Interesting paper. It is not an Earth-shattering improvement in noise floor, only about a factor of 2. They are running into the thermal noise floor of the system (we need a bigger boat!).

2. In the second paper they say, "Notethattheuncertaintyintheorthogonal quadratureiscorrespondinglyincreased,alwayssatisfyingthe Heisenberginequality."
Thankfully no apparent quantum zealotry...

3. kashyap vasavadaAug 19, 2013, 3:29:00 AM

I am hoping this is not a silly question! How do gravitational waves look like? We know how to picture EM waves(vectors) varying E-B fields. But gravity would be tensorial. So is it possible to project in different x-y-z planes?

4. kashyap vasavadaAug 19, 2013, 4:10:00 AM

Somehow I sent the previous unfinished comment. If gravitational waves are deformations of space-time, that would also be problematic in picturing in coordinate space.

5. Copy an paste from pdf on android. Wow. It says heisenberis always satisfied...

6. I always wondered how the Japanese manage not having spaces between individual words butitturnsoutitisntsohardafterall.

7. Well, that does become a problem even in Japanese when you write in a phonetic script (hiragana or katakana) as someone like me would have to do. But most adult Japanese don't do this and when you use Kanji the lack of paces is no problem at all.

8. Interesting. When I copy-and-paste the sentence from the PDF file via the Chrome built-in PDF viewer to my clipboard, it looks like this:

Note that the uncertainty in the orthogonal

quadrature is correspondingly increased, always satisfying the

Heisenberg inequality

At any rate, thank God. Journalists love to write bullshit stories about "the laws of physics that are defeated", anyway. The SciTechDaily story says:

The new equipment has allowed the physicists to break the quantum measurement barrier, defined until recently by Heisenberg’s uncertainty principle.

9. All articles about physics written by journalists should be perreviewed by at least 3 reasonable serious physicists before being allowed to get published ... !

That would not only filter out such gross misunderstandings (or intentionally wrong comments to make the title funnier?), but other very bad things too, everybody knows what I mean ...

10. I worked on the MIT version of this in the 1980s. Squeezed states were already under discussion at the time. There's nothing mysterious about them nor do they violate QM, although they are possible because of QM. To get an idea of how they work, consider a Michelson interferometer. It's not obvious from what you learn in E&M but it has two input ports and two output ports, but all jumbled together at the beam splitter. The typical diagram that discusses interference only shows one of each, the light input and the output at 90 degrees where the interference pattern is measured. If you use a laser the fringes dither a little bit due to QM phase noise. This places a limit on how well you can measure the arm length difference of the interferometer. If you do nothing the noise at either output is identical. If you inject light of the right kind at the second input it's possible to shift the phase noise from one output to the other. One gets quieter and gives you better resolution, the other gets noisier. There's no violation of anything going on here.

The improvement they're getting is good but isn't' going to make too much difference. An improvement of 2.15 dB in the strain sensitivity translates into a factor of 1.28 better strain sensitivity. The volume of space searched goes as the cube so they will be able to scan approximately a 2x volume for sources of gravitational waves. In the distant past it was hoped to get 10x in strain with the result of 1000x in volume scanned. They have a long way to go yet.

11. The gravity waves are described by the strain tensor, h. See Weinberg's book on GR for a full discussion. They are the exact analog of the EM waves but act on two axes instead of one. A linearly polarized GW normally incident on an interferometer and with its axes aligned with the interferometer arms will lengthen one arm and shorten the other. It's an oscillatory wave, just like an EM wave, so the length changes will oscillate from one arm to the other as the wave passes through.

12. kashyap vasavadaAug 20, 2013, 3:23:00 AM

13. kashyap vasavadaAug 20, 2013, 2:22:00 PM

Paul,I am still somewhat hazy about deformation of space around the detector by gravitational waves. I will appreciate comments about that. Thanks.

14. Dear Kashyap

15. Dear Kashyap, look at the two animations in 2/3 of this page:

http://physics.stackexchange.com/questions/68824/counting-degrees-of-freedom-for-gravitational-waves-as-a-gauge-field

The left one shows one "linear" polarization of the gravitational waves. The g_{xx} component of the metric tensor behaves like 1+epsilon*cos(t), the g_{yy} component behaves like 1-epsilon*cos(t). So the vertical distances are being stretched exactly when horizontal ones are being extended, and vice versa.

The second animation is the same one rotated by 45 degrees. So it's polarized in a different direction. In this wave, g_{xx} and g_{yy} are constant, say 1, but g_{xy}, the off-diagonal one, goes like epsilon*cos(t).

In small enough region, the space is still approximately flat, but in a larger region where the wave changes, comparable to the wavelength, the curvature of the spacetime is nonzero. That's why the objects immersed in such a space feel "tidal" forces of a sort. People literally feel - if it were a really strong wave - that something is trying to squeeze them or stretch them in the indicated directions.