It takes just 6 ...

Six pairs of atoms in the focus of a laser beam. Jonas Ahlstedt / 

Lund University Bioimaging Centre (LBIC)

It takes just 6 atoms to enable any system to exhibit a phase transition akin to that of the surface of a pond to ice over whenever the temperature goes below 32 degrees Fahrenheit. To find out the absolute lower limit of an environment transiting from quantum to classical behavior required scientific creativity of the highest order plus building exquisitely delicate tech touching the very fabric of reality.

An ensemble of just six atoms displays all the signatures of a phase transition expected for a many-particle system. This is the finding of a team led by researchers at the University of Heidelberg, Germany, who used a quantum simulator to investigate how collective behaviour emerges in a microscopic structure. The new work advances our understanding of many-body physics, which describes phenomena that cannot be understood simply by studying the behaviour of individual particles.

The tech ...

Quasi-two-dimensional quantum simulator

A team led by Selim Jochim of the University of Heidelberg’s Institute of Physics tackled this problem by trapping up to 12 ultracold lithium-6 (6Li) atoms, assembled in two internal hyperfine states, at the focus of a laser beam. The trap’s geometry is such that the atoms can move in just two spatial directions, meaning that the system is effectively two-dimensional. The researchers then applied a special cooling technique that brought the system very close to its motional ground state, at a temperature just above absolute zero. This set-up also allowed the researchers to continuously tune the strength of the interactions between the atoms via an applied magnetic field, using so-called Feshbach resonances.

In their experiments, Jochim and colleagues configured their applied magnetic field so that the atoms attracted one another. If the attraction was strong enough, the atom formed pairs that could subsequently undergo a phase transition to a superfluid (a state in which the particles flow without friction). The researchers then observed how the atoms formed pairs as a function of their interaction strength and their number by measuring the binding energy of the atom pairs. To their surprise, they found that their atoms behaved like a many-body system even with only six atoms present.

How cool is that. :)