Physicists in Japan have observed Bose-Einstein condensation in a gas of ytterbium atoms for the first time. Ytterbium differs from most of the elements that have previously been condensed because it has two valence electrons rather than one and can be prepared in a non-magnetic state. Yoshiro Takahashi and colleagues at Kyoto University say that the novel properties of the ytterbium condensate could be exploited in atomic clocks and tests of fundamental symmetries (Y Takasu et al. 2003 Phys. Rev. Lett. 91 040404).
Bose-Einstein condensation occurs when a gas of atoms is cooled until the de Broglie wavelength of the atoms is comparable with the average distance between them. When this happens the atoms all collapse into the same quantum ground state and the entire ensemble can be described by a single wavefunction. This means that the quantum properties of the gas become evident on the macroscopic scale.
Physicists have already made Bose condensates from five alkali metals – rubidium, sodium, lithium, potassium and cesium – as well as hydrogen and helium. All of these atoms, apart from helium, have one valence electron and are therefore paramagnetic. Helium atoms have two electrons, but the helium atoms that have been condensed so far have been in a triplet state, which is also magnetic. Researchers have therefore been keen to condense other atoms with two electrons such as ytterbium and the alkali-earth elements such as calcium and strontium.
The two valence electrons in ytterbium can pair up in two different ways. If their spins point in opposite directions the result is a ‘singlet’ state with zero spin that is not influenced by an applied magnetic field – unlike all the other atoms that have been condensed so far. If the spins point in the same direction, a magnetic ‘triplet’ state is formed.
Many of the techniques employed to make Bose condensates rely on magnetic fields, so Takahashi and colleagues had to use an ‘all-optical’ method. First they first trapped about a million ytterbium atoms between two laser beams at about 180 microkelvin, and then modified the intensity of the beams. This forced the most energetic atoms to leave the trap, which in turn reduced the temperature of the remaining atoms. The condensate contained about 5000 atoms and lasted for around 500 milliseconds.
“The insensitivity of the spinless ground state to a magnetic field is a great advantage for many atom optics and atom laser experiments,” Takahashi told PhysicsWeb. The transition between the singlet and triplet states is also very narrow, he adds, and this could be used to make a very precise atomic clock or optical frequency standard. The Kyoto team also hopes to exploit the fact that there are seven different stable isotopes of ytterbium – five bosons and two fermions – in further experiments on the fundamental properties of Bose condensates and the equivalent state for Fermi gases.
