Most people are familiar with the fundamental, common states of matter – solid, liquid, gas and plasma – but other states exist under extreme circumstances, and yet more have been theorized. Now one such state, first proposed almost 50 years ago, has been created in experiments for the first time. Say hello to the supersolid, a state where atoms simultaneously exhibit a crystalline structure but still flow like a frictionless fluid.
The concept of a supersolid arose from the Nobel Prize-winning discovery in the 1970s of a superfluid, a liquid that has zero viscosity, meaning it flows with no resistance or "thickness." At the time, British physicist David Thouless theorized that a state of matter could exist where atoms are both free flowing like a superfluid, but also arranged in a crystalline structure, making it a supersolid.
Earlier attempts to produce this state used helium, the element that first exhibited superfluidity, but it was never brought to fruition. Now, two simultaneous – but independent – studies, one from ETH Zurich and one from MIT, have produced supersolids from Bose-Einstein condensates, using two different techniques.
The ETH Zurich team first contained rubidium gas in a vacuum chamber, before cooling it down to just a few billionths of a Kelvin above absolute zero. This created a state of matter called a Bose-Einstein condensate, which behaves like a superfluid. Then, the condensate was placed in a specially-designed device with two chambers that each contained two mirrors, facing each other.
By exposing the condensate to laser light resonating through both chambers, the atoms arranged themselves into a crystalline structure, but were still able to flow of their own accord. They could only move in one direction, but that still isn't normally possible in a conventional solid.
The MIT study also used a Bose-Einstein condensate and laser light, but in this case, the team used the light to spin half of the condensate's atoms into a different orbit, which basically forms two different condensates, a phenomenon called "spin orbit coupling." Then, using other lasers, the researchers were able to create a "spin flip" by transferring atoms between the two condensates. The resulting spin-orbit coupled Bose-Einstein condensate forms, according to the MIT team, a supersolid.
"The simultaneous realization by two groups shows how big the interest is in this new form of matter," says Wolfgang Ketterle, team leader of the MIT study.