Abstract
High precision spectroscopy on the $2 \ ^3 S \rightarrow 2 \ ^1 S$ transition is possible in ultracold optically trapped helium but the accuracy is limited by the ac-Stark shift induced by the optical dipole trap. To overcome this problem, we have built a trapping laser system at the predicted magic wavelength of 319.8 nm. Our system is based on frequency conversion using commercially available components and produces over 2 W of power at this wavelength. With this system, we show trapping of ultracold atoms, both thermal ($\sim0.2 \ \mathrm{\mu K}$) and in a Bose-Einstein condensate, with a trap lifetime of several seconds, mainly limited by off-resonant scattering.
Highlights
The helium atom has proven to be a productive testing ground for fundamental physics
The former experiment resolved the 2 3S → 2 3P transition to within one thousandth of the 1.6-MHz natural linewidth and is not expected to be improved upon in the near future. The latter experiment was performed on the doubly forbidden 2 3S → 2 1S transition whose 8 Hz natural linewidth is not a limiting factor but has a very low excitation rate and requires a long interaction time. To achieve this He∗ atoms were cooled to quantum degeneracy (a Bose– Einstein condensate (BEC) of 4He∗, and in a degenerate Fermi gas of 3He∗) and trapped in an optical dipole trap (ODT)
The system can be divided in a sum frequency generation (SFG) part, which generates 639.6 nm light from two infrared lasers, and a second harmonic generation (SHG) part which frequency doubles the SFG light to 319.8 nm
Summary
The helium atom has proven to be a productive testing ground for fundamental physics. The two most accurate measurements of the 4He −3 He differential nuclear charge radius have achieved accuracies of 3 [9] and 11 am2 [10], roughly an order of magnitude less precise than the projection of the μHe experiment, but disagree by 4σ The former experiment resolved the 2 3S → 2 3P transition to within one thousandth of the 1.6-MHz natural linewidth and is not expected to be improved upon in the near future. The latter experiment was performed on the doubly forbidden 2 3S → 2 1S transition whose 8 Hz natural linewidth is not a limiting factor but has a very low excitation rate and requires a long interaction time To achieve this He∗ atoms were cooled to quantum degeneracy (a Bose– Einstein condensate (BEC) of 4He∗, and in a degenerate Fermi gas of 3He∗) and trapped in an optical dipole trap (ODT).
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