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The lies that fueled the invention of pong

Now comes a report on a quantum gas, called a Bose-Einstein condensate, that scientists at the Massachusetts Institute of Technology first stretched into a thin rod and then spun until it broke. The result was a series of daughter vortices, each a mini-me of the mother form.

The research, published in Nature, was made by a team of scientists affiliated with the MIT-Harvard Center for Ultracold Atoms and the MIT Research Laboratory of Electronics.

The spinning quantum clouds, effectively quantum tornadoes, are reminiscent of phenomena seen in the large-scale classical world with which we are familiar. An example would be the so-called Kelvin-Helmholtz clouds, which look like jagged cartoon images of ocean waves that repeat periodically.

These wave-shaped clouds, seen over an apartment complex in Denver, exhibit what is called a Kelvin-Helmholtz instability.Rick Duffy/Wikipedia

the way to do quantum However, cloud vortices involve more laboratory equipment and less atmospheric wind shear. “We start with a Bose-Einstein condensate, 1 million sodium atoms that share the same quantum mechanical wave function,” says Martin Zwierlein, a professor of physics at MIT.

The same mechanism that confines the gas, an atom trap made of laser beams, allows researchers to squeeze it out and then spin it like a propeller. “We know which direction we’re pushing and we see the gas elongating,” he says. “The same thing would happen to a drop of water if you were to spin it in the same way: the drop would elongate as it spins.”

What they actually see is effectively the shadow cast by the sodium atoms as they fluoresce when illuminated with laser light, a technique known as absorption imaging. Successive frames of a movie can be captured with a properly placed CCD camera.

At a particular rotational speed, the gas breaks up into small clouds. “It develops these fun ripples, we call it flaky, then it gets even more extreme. We see how this gas ‘crystallizes’ into a chain of droplets; in the last image there are eight droplets”.

Why settle for one dimensional glass when you can opt for two? And, indeed, the researchers say they have done just that, in as-yet-unpublished research.

Theory had predicted that a rotating quantum gas would break up into droplets, that is, one could infer that this would happen from previous theoretical work. “We in the laboratory did not expect this, I was not aware of the paper; we just found it,” says Zwierlein. “It took us a while to figure it out.”

The crystalline form appears clearly in an enlarged portion of one of the images. You can see two connections, or bridges, in the quantum fluid, and instead of the one big hole you’d see in water, the quantum fluid has a whole train of quantized vortices. In an enlarged portion of the image, the MIT researchers found several of these tiny hole-like patterns, chained together in a regular fashion.

“It’s similar to what happens when clouds cross in the sky,” he says. “An originally homogeneous cloud begins to form successive fingers in the Kelvin-Helmholtz pattern.”

Very nice, you say, but surely there can be no practical application. Of course he can; the universe is quantum. The research at MIT is funded by DARPA, the Defense Advanced Research Projects Agency, which hopes to use a ring of quantum tornadoes as fabulously sensitive rotation sensors.

Today, if you are a submarine lying under the sea, incommunicado, you may want to use a fiber optic gyroscope to detect a slight rotational movement. Light travels both ways in the fiber, and if everything rotates, you should get an interference pattern. But if you use atoms instead of light, you should be able to do the job better, because atoms are much slower. Such a quantum tornado sensor could also measure slight changes in Earth’s rotation, perhaps to see how Earth’s core might be affecting things.

MIT researchers have gone very far down the rabbit hole, but not quite. It can be confirmed that those small tornadoes are still Bose-Einstein condensates because even the smallest ones still have around 10 atoms each. If it could be reduced to one per vortex, it would have the quantum Hall effect, which is a different state of matter. And with two atoms per vortex, you’d get a “fractional quantum Hall” fluid, with each atom “doing its own thing, not sharing a wave function,” says Zwierlein.

The quantum Hall effect is now used to define the ratio of Planck’s constant divided by the electron charge squared (h/etwo), a number called the von Klitzing constant, which is as basic as basic physics. But this effect is not yet fully understood. Most studies have focused on the behavior of electrons, and the MIT researchers are trying to use sodium atoms as stand-ins, says Zwierlein.

So while they haven’t hit the bottom of the scale yet, there’s plenty of room to discover on the way to the bottom. As Feynman might as well have said (sort of).

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Jeoffro René

I photograph general events and conferences and publish and report on these events at the European level.

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