Physicists at MIT have succeeded in causing “quantum tornadoes” to form in clouds of cold atoms, according to a recent paper published in the journal Nature. This is the first direct, in situ documentation of how a rapidly rotating quantum gas evolves, and according to the authors, the process is similar to how the Earth’s rotational effects can give rise to large-scale weather patterns.
MIT researchers were interested in studying so-called quantum Hall fluids. First discovered in the 1980s, quantum Hall fluids are composed of clouds of electrons floating in magnetic fields. In a classical system, the electrons would repel each other and form a crystal. But in quantum Hall fluids, electrons mimic the behavior of their neighbors – evidence of quantum correlation.
“People discovered all sorts of amazing properties, and the reason was, in a magnetic field, that electrons (classically) are frozen in place – all their kinetic energy is turned off, and what’s left are pure interactions,” said co-author Richard Fletcher, a physicist at MIT. “So this whole world appeared. But it was extremely difficult to observe and understand.”
But the motion of electrons in a magnetic field is extremely small and difficult to observe. So Fletcher and his co-authors thought they might be able to simulate this unusual behavior of electrons using clouds of ultra-cold quantum gases. Known as Bose-Einstein condensates (BECs), these gases are named in honor of Albert Einstein and the Indian physicist Satyendra Bose. In the 1920s, Bose and Einstein predicted the possibility that the wave-like nature of atoms could allow the atoms to disperse and overlap if packed tightly enough together.
At normal temperatures, atoms act as billiard balls and bounce off each other. Lowering the temperature reduces their speed. If the temperature becomes low enough (billionths of a degree above absolute zero), and the atoms are tightly packed enough, the various matter waves will be able to “sense” each other and coordinate themselves as if they were one large “superatom” .
The first BECs were set up in 1995, and within a few years, more than three dozen teams had replicated the experiment. The Nobel Prize-winning discovery launched a whole new branch of physics. BECs allow scientists to study the strange, small world of quantum physics as if they were looking at it through a magnifying glass, because a BEC “amplifies” atoms in the same way that lasers amplify photons.
Ultra-cold atomic gases are good at simulating electrons in solids, but they lack charge. That neutrality can make simulating phenomena like the quantum Hall effect a challenge. Putting a spin on such a neutral system is one way to overcome this obstacle.
“We thought, let’s make these cold atoms behave as if they were electrons in a magnetic field, but that we could control precisely,” said co-author Martin Zwierlein, also a physicist at MIT. “Then we can visualize what individual atoms do and see if they obey the same quantum mechanical physics.”
Using a laser trap, MIT scientists cooled about 1 million sodium gas atoms; the cooled atoms were held in place by a magnetic field. The second step is evaporative cooling, where a network of magnetic fields conspires to kick out the hottest atoms so that the cooler atoms can move closer together. The process works in much the same way as evaporative cooling is done with a cup of hot coffee: the hotter atoms rise to the top of the magnetic trap and “spring out” like steam.
The same magnetic fields can also cause the atoms in the trap to rotate at about 100 revolutions per second. This motion was captured on CCD camera, thanks to the way sodium atoms fluoresce in response to laser light. The atoms cast a shadow, which can then be observed using a technique called absorption imaging.
Within 100 milliseconds, the atoms spun into a long, thin structure resembling a needle. Unlike a classic liquid (like cigarette smoke) that just keeps getting thinner, a quantum liquid has a limit to how thin it can get. MIT researchers found that the needle-like structures formed in their ultra-cold gases hit this thinness limit. The researchers described their rotating quantum gas and related results last year in Science.
This latest article takes the MIT experiment a step further by investigating how the needle-like fluid can evolve under conditions of pure rotation and atomic interactions. The result: a quantum instability occurred, which caused the liquid needle to wobble and then corkscrew. Eventually, the liquid crystallized into a series of rotating blobs that resembled tornadoes – a quantum crystal formed solely by atomic interactions in the rotating gas. The development is similar to striking formations called Kelvin-Helmholtz clouds, in which a homogeneous cloud begins to form successive fingers due to a speed difference (velocity and direction) between two wind currents in the atmosphere.
“This development is linked to the idea of how a butterfly in China can create a storm here due to instability that triggers turbulence,” Zwierlein said. “Here we have quantum weather: The liquid, simply from its quantum instability, fragments into this crystalline structure of smaller clouds and vortices. And it is a breakthrough to be able to see these quantum effects directly.”
Apparently, this behavior had been predicted in an earlier paper by other physicists that the MIT team only just discovered. And there are some potential practical applications for this research, especially as very sensitive rotation sensors for submarine navigation. Submarines rely on fiber optic gyroscopes to detect rotational motions when submerged, producing a revealing interference pattern. Atoms move more slowly than light, so a quantum tornado sensor would be much more sensitive – possibly even sensitive enough to measure small changes in the Earth’s rotation.
DOI: Nature, 2022. 10.1038 / s41586-021-04170-2 (About DOIs).