Researchers in the US have made ultracold molecules from a gas of fermionic atoms for the first time. Deborah Jin of the National Institute of Standards and Technology in Boulder and colleagues at the JILA laboratory, also in Boulder, have created more than a quarter of a million molecules in their experiment. The result is an important step towards making molecular Bose-Einstein condensates and an exotic fermionic superfluid (C Regal et al. 2003 Nature 424 47).
All atoms are either bosons or fermions depending on whether they possess integer or half-integer spin, and the difference between the two types of atoms becomes clear when they are cooled to near absolute zero. Bosonic atoms can all collapse into the same quantum ground state to form a Bose-Einstein condensate. Fermionic atoms, on the other hand, obey the Pauli exclusion principle and cannot form such a condensate. However, if a gas of fermionic atoms is cooled to a low enough temperature the atoms will occupy all the available quantum states to form a so-called quantum degenerate Fermi gas. Both Bose condensates and quantum degenerate Fermi gases display many novel physical properties.
In 1995 physicists observed Bose-Einstein condensation in a gas of atoms for the first time, and four years later Jin and co-workers made the first degenerate Fermi gas. Since then physicists have been trying to do the same with molecules but this is difficult because the techniques used to cool atoms do not necessarily work for molecules.
Jin and co-workers started with a quantum gas of fermionic potassium-40 atoms and applied a magnetic field to produce a weakly bound state known as a ‘Fesbach resonance’. By carefully tuning the value of the magnetic field the resonance energy can be made equal to the energy of the atoms, which leads to the formation of molecules. The Boulder team was able to confirm that the binding energy of the molecules agreed with theoretical predictions by using a radio-frequency field to split them apart
The team now hopes to observe fermionic superfluidity – in which fermions pair up to form bosons which then undergo Bose-Einstein condensation, as happens in low-temperature superconductivity – in such a system. The experiment could also help in the search for an electron dipole moment or the creation of entangled states for quantum computing.