“Any sufficiently advanced technology is indistinguishable from magic,” Arthur C. Clarke
I think about this quote everytime I look into the quantum vacuum squeezing system used in LIGO to increase its sensitivity!
So what is this quantum squeezing technology used in LIGO? It’s a way to trick Heisenberg’s uncertainty principle, and make sure that the omnipresent quantum fluctuations are smaller for the laser degree of freedom that we need to measure precisely to detect gravitational waves. But the uncertainty principle dictates that if you reduce the fluctuations in one degree of freedom (squeezing the quantum noise) you have to pay the price of large fluctuations in the conjugate degree of freedom. So we have to carefully tune this squeezing to ensure it acts in the correct quadrature of the laser signal at each frequency.
More technically, we achieve this goal by using non-linear optical crystals: the non-linearity can convert a photon at higher energy (with a wavelength of 532 nm) into correlated pairs of photos at lower energy (1064 nm). The correlation can be engineered to provide squeezing of the quantum fluctuations in the phase quadrature of the laser field. But the trick is that the fluctuations that are relevant to us, because they produce noise in the main gravitational-wave signal, are coming from the quantum vacuum photons entering the interferometer from the detection port. So we need to generate squeezed quantum vacuum photons, and inject them from the interferometer output port in place of the normal vacuum photons.
Sounds complicated? It’s magic!
Stay tuned for more on the optical techniques used in LIGO! I post every week simple explanations of the science and engineering behind the discovery and detection of gravitational waves. Follow and connect!
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