12.06.2018 - Wim Vassen - Experiments with ultracold quantum degenerate metastable triplet helium atoms

We apply laser cooling and trapping techniques to cool and trap helium atoms in the metastable triplet state. Temperatures below 1 microkelvin are reached where the atoms exhibit their wavelike character and Bose-Einstein condensation is observed for helium-4 and Fermi degeneracy for helium-3. I will first discuss our experiments observing bunching of helium-4 (boson) atoms and anti-bunching of helium-3 atoms (fermions). Then I will discuss our experiments exciting the 1557-nm transition between the metastable triplet state and the metastable singlet state (natural linewidth 8 Hz) in a 0.2 microkelvin cloud. We studied the line shape for both helium-4 and helium-3 observing a linewidth of 75 kHz for the fermions and a 15 kHz for the bosons at the same temperature, the latter only limited by the cold-collision mean-field of the Bose-Einstein condensate. Studying the helium-4 transition in a so-called magic wavelength optical dipole trap at 320 nm, we realized a 10 kHz linewidth and measured the transition frequency with 200 Hz accuracy, allowing stringent tests of quantum electrodynamics theory for two-electron atoms and quantum chemistry calculations of molecular potentials. The high accuracy  provides a handle on the tiny shift of the transition frequency due to the size of the helium-4 nucleus (alpha-particle). These experiments are in the framework of the proton-size puzzle, where we hope to see whether the size of the nucleus in experiments in muonic systems differs from that of normal electronic matter.

See poster.