Frequency metrology in quantum degenerate helium
LaserLaB VU University, Department of Physics and Astronomy, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands
We have measured the absolute frequency of the 1557-nm doubly forbidden transition between the two metastable states of helium, 2 3S1 (lifetime 8000 s) and 2 1S0 (lifetime 20 ms), with 1 kHz precision. With an Einstein coefficient of 10−7 s−1 this is one of weakest optical transitions ever measured. The measurement was performed in a Bose-Einstein condensate of 4He* as well as in a Degenerate Fermi Gas of 3He*, trapped in a crossed dipole trap. From the isotope shift we deduced the nuclear charge radius difference between the α-particle and the helion. Our value differs by 4σ with a very recent result obtained on the 2 3S → 2 3P transition.
© Owned by the authors, published by EDP Sciences, 2013
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