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Getting rid of quantum noise

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One characteristic of quantum physics is being used to defeat another, with the aim of making more sensitive gravity wave detectors, in an international project with contributions from the University of Western Australia, the Australian National University, and the GEO600 Gravitational Wave Observatory in Germany.

A problem with trying to detect gravity waves is that they’re very, very weak, and there’s lots of sources of noise to spoil the experiment. One of those noise sources turns out to be the strangeness of the quantum world.

Heisenberg’s Uncertainty Principle – your inability to perfectly measure velocity and direction** at the same time – leads to very strange outcomes in the quantum world. If any piece of vacuum were truly a vacuum all the time, it would be amenable to an absolute measurement, since there’s nothing there. Hence the notion arises of vacuum fluctuation, in which “virtual particles” appear and disappear in otherwise empty space.

This is one of those concepts that most people file under “too strange to be true”, and ignore, but it turns out to be a real problem for those trying to detect gravity waves.

A typical gravity wave detector uses interferometry: watching the interference patterns where two laser beams intersect (or one laser beam that’s been split). In theory, if a gravity wave disturbs the interference pattern, then you’ll see it. In practice, however, there’s a fly in this ointment: so far, nobody has built a sufficiently-sensitive detector, partly because of vacuum fluctuations.

Winthrop Professor David Blair, the Director of the Australian International Gravitational Research Centre, explained to El Reg that the detector is usually the “dark” port of the interferometer – in other words, you’re trying to arrange the instruments so that the interferometer only sees the interference pattern between the intersecting lasers.

That port is so dark that vacuum fluctuations are sufficient to be picked up: “When you try to analyse the data, what you find is that you’re detecting quantum fluctuations – those ‘half photons’ are disturbing the measurement.”

So some way is needed to clean up the signal: to eliminate the noise generated by vacuum fluctuations, while retaining the hoped-for signal generated by passing gravity waves.

Here, Professor Blair explained, the key work came from the head of the Department of Quantum Science at the Australian National University, Professor David McClelland, who devised a way to use phase-sensitive amplifiers to address the problem.

He said that when the quadratures (short version: the amplitude-phase plot) of a light wave are analysed, they have an interesting characteristic: the quantum uncertainty is conjugate: increasing uncertainty in the amplitude plane results in decreasing uncertainty in the phase plane, and vice-versa. The total uncertainty remains the same.

Put it another, and probably over-simplified*, way: if you can transmute some of the phase uncertainty into amplitude uncertainty, you have the same sum of uncertainty, but greater certainty in one characteristic.

“If you pass that noise into a phase-sensitive amplifier … you can make [the noise] in one direction get smaller and the other direction bigger.”

This turned out to be key to improving the noise-sensitivity of the gravity wave detector, because the noise detected by the interferometer can be handled by the phase-sensitive amplifier, just as the light generated by the instrument’s laser. That means (I think) that noise can be decreased both in the instrument’s lasers and in the vacuum fluctuations that otherwise get in the way of the detector.

Handling light in this way is referred to as “squeezing”: “You can make a ‘squeezer’ squeeze the vacuum fluctuations,” Professor Blair said, “and that means you can make it so that the noise of your detector is less.”

He said Professor McClelland’s particular contribution was to be the first to build a squeezer that would operate in the frequency band of gravity wave detectors. This work, he said, was the basis for the further development in Germany being deployed into the Laser Interferometer Gravitational Wave Observatory. ®

*Author’s Note: discussing quantum mechanics without maths places a very heavy metaphoric burden on the English language. I can only do my best, and I ask readers not to attribute my errors to Professor David Blair of the University of Western Australia, who devoted a very patient interview to explaining this stuff to me. If I misunderstood him, the fault is mine. ®

**All right, all right. Replace "velocity and direction" with "momentum and position" as the perfect knowledge precluded by Heisenberg, as a reader pointed out. If Heisenberg didn't also say something about science-writing, he should have. ®

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