MIT physicists have created a superfluid gas, the so-called Bose-Einstein condensate, for the first time in an extremely high magnetic field.
The magnetic field is a synthetic magnetic field, generated using laser beams, and is 100 times stronger than that of the world’s strongest magnets. Within this magnetic field, the researchers could keep a gas superfluid for a tenth of a second — just long enough for the team to observe it. The researchers report their results this week in the journal Nature Physics.
A superfluid is a phase of matter that only certain liquids or gases can assume, if they are cooled to extremely low temperatures. At temperatures approaching absolute zero, atoms cease their individual, energetic trajectories, and start to move collectively as one wave.
Superfluids are thought to flow endlessly, without losing energy, similar to electrons in a superconductor. Observing the behavior of superfluids therefore may help scientists improve the quality of superconducting magnets and sensors, and develop energy-efficient methods for transporting electricity.
But superfluids are temperamental, and can disappear in a flash if atoms cannot be kept cold or confined. The MIT team combined several techniques in generating ultracold temperatures, to create and maintain a superfluid gas long enough to observe it at ultrahigh synthetic magnetic fields.
The team first used a combination of laser cooling and evaporative cooling methods, to cool atoms of rubidium to nanokelvin temperatures. Atoms of rubidium are known as bosons, for their even number of nucleons and electrons. When cooled to near absolute zero, bosons form what’s called a Bose-Einstein condensate — a superfluid state.After cooling the atoms, the researchers used a set of lasers to create a crystalline array of atoms, or optical lattice. The electric field of the laser beams creates what’s known as a periodic potential landscape, similar to an egg carton, which mimics the regular arrangement of particles in real crystalline materials.
When charged particles are exposed to magnetic fields, their trajectories are bent into circular orbits, causing them to loop around and around. The higher the magnetic field, the tighter a particle’s orbit becomes. However, to confine electrons to the microscopic scale of a crystalline material, a magnetic field 100 times stronger than that of the strongest magnets in the world would be required.
The MIT group came up with a technique to generate a synthetic, ultrahigh magnetic field, using laser beams to push atoms around in tiny orbits, similar to the orbits of electrons under a real magnetic field.
After developing the tilting technique to simulate a high magnetic field, the group worked for a year and a half to optimize the lasers and electronic controls to avoid any extraneous pushing of the atoms, which could make them lose their superfluid properties.
In the end, the researchers were able to keep the superfluid gas stable for a tenth of a second. During that time, the team took time-of-flight pictures of the distribution of atoms to capture the topology, or shape, of the superfluid. Those images also reveal the structure of the magnetic field — something that’s been known, but never directly visualized until now.
