A strange quantum object called a "domain wall" has been created in a laboratory for the first time. This “wall” between high- and low-density atoms in a quantum state can behave as if it was an object independent of either, even though there is nothing actually there. Moreover, the dividing line’s responses to external forces can be different to that of the atoms it separates.
When groups of people separate themselves on either side of an imaginary line, we may come to see the boundary as a thing in itself, an “iron curtain” or “color bar” perhaps, even without a physical barrier. In the classical world, this is an illusion of the human mind, possibly akin to pareidolia. In quantum mechanics, however, it can be much more real.
Domain walls – boundaries between segregated atoms that become like independent quantum objects – have proven hard to produce consistently. Now, however, a team at the University of Chicago have announced in Nature they can create domain walls at will, and have already found some surprises in how the creations behave.
“It’s kind of like a sand dune in the desert – it’s made up of sand, but the dune acts like an object that behaves differently from individual grains of sand,” PhD student and first author of the study Kai-Xuan Yao said in a statement.
Dunes, at least, are real things, even if their behavior blurs the boundaries between solid and wave. We wouldn’t say the same of the interface between the dune and the air around it.
However, when Yao, Professor Cheng Chin, and co-authors looked at the domain wall between two types of atoms they found it has a mind of its own.
“We know if you push atoms to the right, they will move right. But here, if you push the domain wall to the right, it moves left.” Chin said. Just to make things even stranger, the reaction is not a mirror image of what might be expected. Instead, the paper reports: “We find that the domain walls respond to synthetic electric field with a charge-to-mass ratio larger than and opposite to that of the bare atoms.”
This makes the domain wall an emergent phenomenon, something where multiple particles appear to follow different physical laws when they are together than when they are acting individually. Yet, here, the emergent phenomenon contains no actual atoms, just the line between them.
The authors created the domain wall within a Bose-Einstein condensate (BEC), a phenomenon where large amounts of atoms collectively show quantum behavior like a single subatomic particle. Forty thousand cesium atoms were supercooled to become a BEC. The BEC was then forced to separate into high- and low-density regions, creating electromagnetic fields, which in turn produced the unexpected effects on the domain wall the team observed. In over 90 percent of samples, domain walls formed between these regions, one in parallel to the field, the other perpendicular to it, proving the reliability of their method. Vortices formed along the parallel wall.
The fact the walls form so reliably, allowed the researchers to investigate behavior under varying conditions, such as differing numbers of atoms and strength of externally applied forcing.
“There may be applications for this phenomenon,” Chin said. “It can be used to create a more robust way to store quantum information or enable new functions in materials. But before we can find that out, the first step is to understand how to control them.”