Caesium atoms are the basis of atomic clocks – on which global time standards depend – and they also play a key role in several metrological applications, including measurements of the fine-structure constant and the electric dipole moment of the electron. Physicists are therefore keen to know more about the quantum behaviour of caesium atoms.

In a Bose-Einstein condensate, certain quantum properties of the constituent atoms are evident on a macroscopic scale. This happens because all of the atoms occupy the same quantum state and can therefore be described by a single wavefunction. Although physicists can condense alkali atoms such as sodium and rubidium with relative ease, the unusual quantum properties of caesium atoms have defeated all previous attempts at condensation.

To tackle this problem, the Innsbruck team first trapped around 20 million atoms in the focus of two lasers. These lasers induced a dipole moment in the atoms, which attracted them to the intense electric field of the focus. By reducing the power of the lasers, the most energetic atoms were allowed to escape, which reduced the temperature of the remaining atoms. This technique is known as evaporative cooling.

But when Grimm and co-workers tried to chill the atoms further, they found that this cooling technique only worked when they also applied a carefully chosen magnetic field. When the temperature of the atoms fell to 160 nK, the researchers switched off the lasers to reveal a condensate that contained around 20 000 atoms. It lasted for 15 seconds.

By adjusting the applied magnetic field, the researchers found that they could sensitively control the attractive and repulsive interactions between the caesium atoms. Grimm and colleagues now suggest that their technique could be used to create molecular condensates, and that single atoms extracted from a caesium condensate could form the basis of a new, super-accurate atomic clock. They also hope that the high sensitivity of caesium atoms to magnetic fields could allow physicists to probe the properties of Bose-Einstein condensates in the greatest detail yet.