New Form of Matter Is Supersolid

Vacuum chamber used in the supersolid experiment. Image: MIT/Ketterle Group

Vacuum chamber used in the supersolid experiment. Image: MIT/Ketterle Group

An MIT research team has created a supersolid, a new form of matter that combines both form and viscosity-free flow—properties that most people consider mutually exclusive. The team formed the new material from a Bose-Einstein condensate (BEC), suspending the superfluid gas in an ultrahigh vacuum and then manipulating the sample with a combination of laser and evaporative cooling until it developed a periodic density modulation. “A lot of solids are crystalline,” says Wolfgang Ketterle, the John D. MacArthur Professor of Physics at MIT. “And the crystal means that the density of the solid is periodically modulated. By creating a Bose-Einstein condensate that is superfluid but that also has a density modulation, we have combined the properties of a superfluid and a solid.”

Ketterle received the 2001 Nobel Prize in physics in recognition of his 1995 co-discovery of BECs. His research team began work on the supersolid two years ago, motivated to explore whether scientific ideas related to supersolids were both sound and compatible with the laws of nature. “We want to map out what is possible in nature,” Ketterle says. “We do it under idealized conditions—extremely low temperature, nanokelvin, and extremely low densities, a million times more dilute than air. And ultimately, the idea is that 10 or 20 years down the road, the insight we create about new forms of matter will enable material designers to find new materials.”

This long-term vision, he hopes, may eventually lead to room temperature superconductors. But for now, the researchers are experimenting at nanokelvin temperatures. The supersolid experiment used a commercially available vacuum chamber, customized in-house with magnet coils and other parts required to create the atom traps and cooling processes necessary for the experiment. The only thing cold in the experiment was the atoms, held inside the vacuum chamber using electrical and magnetic forces from laser light and magnetic forces from the magnet coils. The cooling phase began with several sets of lasers, before the samples were taken through evaporative cooling to the nanokelvin temperatures required to form a BEC. Subsequently, several laser beams induced spin-orbit coupling.

The researchers had to run the experiment through several cycles. Each lasted around 30 seconds, creating a supersolid for less than one. “We can maintain the cold environment for longer, but since we are actively modifying the properties, the lifetime is shorter,” says Ketterle. “What matters is, does it live long enough that you can measure its properties and detect it? Often we only need tens of milliseconds to do measurements.”

Getting to the point where the researchers could take accurate measurements took longer than anticipated, according to Ketterle. “The main challenge was to keep all the balls in the air. We had so many lasers, electronics, vacuum. Everything had to work at the same time. Everything had to be precisely aligned. And, if the temperature in the room changed because the air conditioning was not properly working, we could lose a whole day or two of alignment.”

Laser alignment was “bloody difficult,” he says, “because the signal level, the amount of signal we got from the supersolid was very small. When we scattered light off the supersolid to detect its properties—every time we created it—we just collected a few photons. And so we had to learn many properties of the supersolid from just a few photons at a time.”

Ketterle’s team includes graduate students Junru Li, Boris Shteynas, Furkan Çagrı Top, and Wujie Huang; undergraduate Sean Burchesky; and postdocs Jeongwon Lee and Alan O. Jamison, all of whom are associates at MIT’s Research Laboratory of Electronics. The team described the experiment in “A stripe phase with supersolid properties in spin–orbit-coupled Bose–Einstein condensates,” an article appearing in Nature.

Ketterle says the group plans to continue exploring new forms of matter, some more related to this experiment and some less. He wants to research other methods that could lead to either supersolids or systems with that property. The initial focus will be on other approaches to realize spin orbit coupling in ultracold atoms. “If you go very, very cold, you have the potential to make new discoveries,” he says, “because at low temperature there are many new forms of matter waiting to be discovered. We need the low temperature to make certain forms of matter stable. You can only observe ice if you cool below the freezing point of water and you can only observe superfluid helium if you go below 2.2 K. So that’s why nanokelvin temperature, the ability to reach those low temperatures, is a tool to make new discoveries.” ■