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Physicists 'Squeeze' the Uncertainty Out of the Uncertainty Principle

Thank absolute zero temperatures and something called 'quantum squirting.'

by Michael Byrne
Aug 29 2015, 10:30am

Image: John Hoey/Flickr

Life in the quantum realm means never sitting still. A photon doesn't rest (or "stop") because it turns out that achieving zero energy and, thus, complete predictability requires a bit more effort than it does to just buzz around in a probabilistic cloud. This inherent uncertainty of a quantum system requires it to fluctuate even at what should be dead zero. This is incredibly, fundamentally weird.

It's weird because as large-scale entities, we seem to observe rest all of the time everywhere. The unfathomably vast collections of constituent particles behind these large-scale entities might be unpredictable and constantly in motion, but we get to experience them on average, which means that all of these variations just kind of cancel out and instead we're able to imagine zero motion.

I say "imagine" because that's not quite it. The noise of quantum fluctuations doesn't just disappear, so much as appear to disappear. Forces like friction and gravity easily overcome the zero-point energy of particles, but it's still down there. Moreover, as described in a paper this week in Science, researchers at Caltech have come up with a way of observing and even controlling this quantum motion by cooling a small (but not quantum-small) device to a temperature of almost absolute zero (or as absolute zero as it gets), the point at which the only remaining forces come from quantum fluctuations.

The effect is that a trillion atoms suddenly seem to behave as a single particle. This allows for the possibility of manipulating and reducing that noise through a process known as quantum squeezing. "Although the zero-point fluctuations are unavoidable, they can be manipulated," the Caltech group writes.

Image: Chan Lei and Keith Schwab/Caltech

The physicists took a device consisting of an aluminum plate on top of a silicon substrate. The plate is a natural mechanical resonator and so it just vibrates there at 3.5 million times per second. Harmonic oscillators feature a natural frequency, which is where they vibrate more or less indefinitely in the absence of either a driving or damping (the opposite of driving) force. Usually, thermal energy will keep things moving while far overpowering any quantum mechanical effects, at least until we remove it.

Which is just another way of saying "until we make the system really, really cold." This is where thermal effects are suppressed in favor of quantum fluctuations, which we can now observe in our resonator.

But how can we control these fluctuations? That's the second part of the Caltech work.

"There are two main variables that describe the noise or movement," offers Keith Schwab, a Caltech physics professor and co-author of the current study, in a statement. "We showed that we can actually make the fluctuations of one of the variables smaller—at the expense of making the quantum fluctuations of the other variable larger. That is what's called a quantum squeezed state; we squeezed the noise down in one place, but because of the squeezing, the noise has to squirt out in other places. But as long as those more noisy places aren't where you're obtaining a measurement, it doesn't matter."

So, yes: quantum squirting. The work isn't merely making a point about quantum mechanics, but could have implications for experiments requiring very high precision measurements in which quantum mechanical noise begins to interfere at unacceptable levels. Gravitational wave detection is one example.

"We've been thinking a lot about using these methods to detect gravitational waves from pulsars—incredibly dense stars that are the mass of our sun compressed into a 10 km radius and spin at 10 to 100 times a second," Schwab explains. "In the 1970s, Kip Thorne and others wrote papers saying that these pulsars should be emitting gravity waves that are nearly perfectly periodic, so we're thinking hard about how to use these techniques on a gram-scale object to reduce quantum noise in detectors, thus increasing the sensitivity to pick up on those gravity waves."