Scientific Method / Science & Exploration

Putting the squeeze on gravity waves

Researchers will make use of the quantum properties of light to make the …

Gravity wave detectors have been operating for quite a while now, with little success in actually detecting a gravity wave. Each detector has been capable of detecting gravity waves—provided the wave was sufficiently intense. Essentially, rotating neutron stars, black holes, and supernovas are all expected to produce gravity waves. However, the amplitude of these waves gets smaller as the square of the distance.  Although that's good in the sense that gravity wave sources are sufficiently far away that we were still alive, it does make them harder to detect.

Nevertheless, the sensitivity of gravity wave detectors has been steadily getting better, and there is some chance that the Laser Interferometer Gravitational-Wave Observatory (LIGO) could detect some. Realistically, though, hopes are being pinned on Advanced LIGO, which will make use of the quantum properties of light to give it more sensitivity than its older brother.

To give you an idea of the challenge, gravity waves stretch and contract space as they move. The way to detect this is to repeatedly measure the distance between two locations and observe it change. LIGO does this using interferometry. Light from a laser is split into two beams that travel at right angles to each other. Both reflect off mirrors and return to the point where they were split, where they are recombined.

When the light beams are added together, the observed brightness depends on the relative distance the two have traveled. If the distance is different by an odd multiple of half a wavelength, then the two light beams cancel each other out, and darkness results. As the distances along the two paths are changed by gravity waves, the brightness will increase and decrease repeatedly. In this way, LIGO can measure a change of a few attometers (10-18m) over light paths of a kilometer—yet this is not sensitive enough.


At LIGO's current sensitivity, the limiting factor is now the quantum nature of light itself. In quantum mechanics, knowledge of one part of the system can limit knowledge of other parts. Typically, these properties come in pairs, like position and momentum, or, for light, phase and amplitude. This is expressed by the Heisenberg uncertainty principle, where the absolute limit of knowledge is defined by a product of the two properties.

Using a good laser, a symmetric state is achieved, where both the amplitude and phase are at their common minimum. However, the limiting factor is the product of the two states so, if you need a highly phase-stable beam of light, you can increase the phase stability at the expense of increased amplitude fluctuations—called a squeezed state. We can make use of this property by using a normal laser beam and squeezed light in our interferometer. Now the relative phase shift between the normal laser and the squeezed state is measured. Since the laser is much more intense than the squeezed light, the amplitude noise is dominated by the laser, which is already at the symmetric minimum. The phase, however, is contributed in equal amounts by both light sources, meaning that the noise in the phase measurement is reduced because the phase of one of the light beams is much better defined than in the other. Since the phase is now known to better precision, the distance between the mirrors can be obtained to higher precision as well.

This has now been tested on a small-scale prototype of the advanced LIGO, where it was found to work pretty well. The signal-to-noise ratio increased by about 44 percent over the entire range. Looking at the graphs, it appears that they will be able to measure displacements on the order of 10-20m.

While I was reading this paper, an interesting question occurred to me. If an atom is about one nanometer in diameter (including the electron cloud), and LIGO measures displacements on the order of the attometer, can we still consider the mirrors a smooth surface? Do we have to take into account the statistical movements of the electrons in the surface layer? The obvious answer is no (otherwise LIGO would not work), but it took me a little while to see why. The measurements take place on time scales that are much much slower than the movements of the electrons in the mirrors, therefore they are effectively averaged to zero. Which leads to me to another question: if we used laser pulses with a duration of a few femtoseconds, would we be able to see the statistical fluctuations in the electron cloud?

Answers in the form of a grant application please.

Nature Physics, 2008, DOI: 10.1038/nphys920

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