Physicists in Germany have created spectacular, cloverleaf-shaped explosions in a gas of ultracold atoms trapped by magnetic fields. The cloverleaf shapes were formed by finely tuning the magnetic interactions between the atoms, which had formed a Bose-Einstein condensate (BEC) and so were all in the same quantum state. Although such "bosenovas" have been seen before, they have previously always been the same shape in all directions.

Bosenovas were first created about 10 years ago by adjusting the magnetic-field strength between the atoms in a BEC so that the short-range "van der Waals" forces between the atoms are attractive, rather than repulsive. This causes the BEC to collapse in on itself much like a dying star. It then explodes like a tiny supernova and throws off many of its constituent atoms.

Physicists believe that the explosion occurs when the atoms are close enough for short-range interactions to affect groups of three atoms (“three-body” interactions) rather than just pairs. Until now the attractive forces between atoms were isotropic, which meant that the explosions ejected atoms equally in all directions.

Preferred directions

Now, however, Tilman Pfau and colleagues at the University of Stuttgart have created the first bosenovas in which the attractive forces between atoms are dipolar and therefore depend on the relative orientation of the atoms. This caused clover-leaf shaped explosions, which the team says reflect the underlying symmetry of the attractive forces (Phys Rev Lett 101 080401).

The team used a BEC of chromium-52 atoms, which have large magnetic dipole moments. The magnetic forces between the atoms are attractive and are normally much weaker than the van der Waals forces. However, the team adjusted the magnetic field so that the van der Waals forces were near zero, allowing the magnetic forces between atoms to take over.

According to Pfau, this caused the BEC to contract until the atoms were close enough for three-body interactions to cause an explosion. In this process, some of the atoms are thrown out of the BEC, while the remaining BEC expands outwards with a distinctive cloverleaf pattern. Such a pattern is expected for an expanding BEC with dipolar interactions in the presence of a magnetic field, said Pfau.

A gentler kind of implosion

Because the implosion was driven by the much weaker magnetic interactions, it was much more “gentle” than implosions caused by van der Waals interactions, according to Pfau. This he said, made it easier for Masahito Ueda and colleagues at the University of Tokyo to use current theories of BECs to describe the bosenova process -- something that had proved difficult in the past.

Computer simulations by Ueda’s team suggest that the BEC collapse involves the formation of two “vortex rings” that spin in opposite directions. The physicists are now keen to see if they can create stable vortices by switching the repulsive van der Waals interactions back on before the BEC explodes.

Pfau believes that the insights gained into how to create and control chromium-52 BECs could someday be technologically relevant. Chromium is already used in a number of nanotechnologies and chromium-52 BECs could form the basis of “atom lasers” that could deposit tiny amounts of chromium to an extremely high degree of spatial precision.

Simulating magnetism

On a more fundamental level, the dipole interactions in the BEC are the same as those found in magnetic materials and Pfau believes that the system could be used as a “quantum simulator” to study magnetism.

Dave DeMille of Yale University agrees. He told physicsworld.com that such BECs could allow physicists to make “a new, deep connection between ultracold atom experiments and … many interesting magnetic systems in real-world materials”.