Wednesday, January 18, 2017

Scientists Make One Extremely Cold Drum

I’m on my second Minnesota winter and it’s cold. On really cold days, your eyelashes can freeze and baby wipes become a useless block of ice if you leave them in the car. It’s pretty extreme, in my mind. All of this is put in perspective though, by new research published in Nature last week. A team of scientists at the National Institute for Standards and Technology (NIST) cooled a tiny aluminum drum down to a temperature so cold that most scientists thought it was unreachable.

NIST researchers applied a special form of microwave light to cool a microscopic aluminum drum to below the generally accepted quantum limit.

From the tiniest building blocks of atoms to the expansive universe, life is balanced somewhere between the extremes. The extremes have a lot to teach us, though. You can’t fully understand your car, sound system, or yourself until the limits are tested. Similarly, we can’t fully understand the matter and interactions that shape the universe until we test their limits—including how they behave at extreme temperatures.

When it comes to cold, the most extreme temperature is absolute zero, the point where atoms stop moving. In meteorological terms this is - 459.67 °F, while in the temperature scales more commonly used in research, absolute zero is 0 K and -273 °C.

It’s kind of absurd to think about absolute zero in terms of how that would feel to us, and that’s not how physicists define the word. Temperature is associated with movement. Imagine that you could see the atoms and molecules of a gas in a closed room. They would be zipping around in all different directions, bouncing off of each other and the walls. The temperature of the gas in the room is defined as the average kinetic energy, the energy of motion, of the particles.

So, how do you stop all motion?

A few years ago, Physics Central released a poster (which you can download for free) that posed this question: How do lasers help create both the coldest and hottest spots on earth? You can read the long answer here, but the short answer is energy. In a nutshell, lasers are beams of light where everything lines up just right. They are really powerful because you can direct very precise amounts of energy into a very small space. Scientists have been able to cool atoms very close to absolute zero through a technique called laser cooling. Laser cooling is worthy of its own article, and has one here, but suffice it to say that it involves using carefully tuned lasers to counteract the motion of atoms, bringing them to a standstill.

About five years ago, NIST scientists used a modified version of laser cooling to cool an aluminum drum way down, close to absolute zero. With a diameter of 0.02 millimeters and a thickness of 0.0001 millimeters, the drum is small by most standards, but much bigger than an atom—and therefore harder to cool down.

The drum was part of an electrical circuit driven by a microwave laser. The experiment was designed so that each beat of the drum produced a photon, a particle of light, that was coaxed into carrying energy away from the drum. Using this method, they were able to cool the drum down to the lowest temperature allowed by conventional methods. The conventional low temperature limit is based on quantum mechanical fluctuations in the intensity of the light produced by the laser. These fluctuations generate noise that adds heat to the system.

To the surprise of many, the team just demonstrated how to bypass this limit and cooled the drum down even closer to absolute zero. They did so by using something called “squeezed light” to drive the circuit. It’s a quantum mechanics thing, and it’s really not the light itself that is squeezed, it’s the quantum fluctuations—the noise—that is squeezed.

Imagine you are holding a small balloon in your palm. If you squeeze the balloon with your fingers spaced apart, air-filled pockets will pop up between your fingers. You’ve moved the air in the balloon from one place in space to another. When you squeeze light, you move these quantum fluctuations from one place to another. Not from one physical, 3-dimensional place to another, but from one type of noise fluctuation to another. You decrease the kind of noise that generates heat, and increase another kind of noise that doesn’t add heat to the system.

By reducing the quantum fluctuations in the experiment, the scientists were able to reach a significantly lower temperature. In fact, their mathematical model predicts that the more you can “squeeze” the light, the closer to absolute zero you can get. Not only does this mean more sensitive sensors and experiments that can explore matter under more extreme conditions, but it also bridges the mechanical and quantum worlds in new ways.
Now, can we get a picture of that drum in a tiny scarf?

Kendra Redmond

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