German researchers have created a new type of matter by trapping globules of Bose-Einstein condensate in a regular array of indentations. Bose-Einstein condensates are quantum liquids that exist only at very low temperatures where the thermal motion of atoms is small enough to reveal their wave-like properties. By varying the size of the indentations, the team induced a phase transition.
The researchers started with a vapour of Bose-Einstein condensate consisting of about 100 000 rubidium atoms confined to a small volume by a magnetic field and cooled to a few billionths of a degree above absolute zero. All of the atoms in a Bose-Einstein condensate are coherent, acting like a single giant particle.Consequently, the vapour becomes a superfluid. If it moves, it does so en masse. Superfluids have no viscosity because one atom cannot be slowed down without slowing down all of them.
The superfluid was then placed in an optical lattice made up of the interference pattern between several laser beams. The atoms moved easily around the lattice's 150 000 valleys. However, as the lattice was made more intense - with deeper valleys and higher peaks - there came a point at which the fluid became stuck - it ceased to be a superfluid and the localized blobs of rubidium vapour were no longer coherent with one another. This trapped state is called a Mott insulator after the British physicist Neville Mott who first observed a similar effect in semiconductors in the 1960s. Reducing the strength of the optical lattice led to the reappearance of the Bose-Einstein condensate, demonstrating that the transition is reversible.
The sharp change between a coherent condensate and a non-coherent Mott insulator state is loosely analogous to the way a magnet can become non-magnetic when heated, but heat plays no part in the change in the quantum fluid. That is driven by Heisenberg's uncertainty principle, which stipulates that when atoms get trapped in particular valleys, they must lose their coherence. Their wave-like quantum state is no longer like the rippling surface of a single ocean. Instead it is like many independently rippling little lakes. The ability to switch between quantum states in this way is a key ingredient of many proposals for quantum computers.