A simple 2 W continuous-wave laser system for trapping ultracold metastable helium atoms at the 319.8 nm magic wavelength

High-precision spectroscopy on the

Comparison of spectral linewidths for quantum degenerate bosons and fermions

We observe a dramatic difference in optical line shapes of a $^4\text{He}$ Bose-Einstein condensate and a $^3\text{He}$ degenerate Fermi gas by measuring the 1557-nm $2~^3S-2~^1S$ magnetic dipole transition ($8~\text{Hz}$ natural linewidth) in an optical dipole trap. The $15~\text{kHz}$ FWHM condensate line shape is only broadened by mean field interactions, whereas the degenerate Fermi gas line shape is broadened to $75~\text{kHz}$ FWHM due to the effect of Pauli exclusion on the spatial and momentum distributions. The asymmetric optical line shapes are observed in excellent agreement with line shape models for the quantum degenerate gases. For $^4$He a triplet-singlet s-wave scattering length $a=+50(10)_{\text{stat}}(43)_{\text{syst}}~a_0$ is extracted. The high spectral resolution reveals a doublet in the absorption spectrum of the BEC, and this effect is understood by the presence of a weak optical lattice in which a degeneracy of the lattice recoil and the spectroscopy photon recoil leads to Bragg-like scattering.Comment: 5 pages, 4 figures, 5 pages supplemental informatio

Magic wavelengths for the 2 3 S→2 1 S transition in helium

We have calculated ac polarizabilities of the 23S and 21S states of both He4 and He3 in the range 318 nm to 2.5 μm and determined the magic wavelengths at which these polarizabilities are equal for either isotope. The calculations, only based on available ab initio tables of level energies and Einstein A coefficients, do not require advanced theoretical techniques. The polarizability contribution of the continuum is calculated using a simple extrapolation beyond the ionization limit, yet the results agree to better than 1% with such advanced techniques. Several promising magic wavelengths are identified around 320 nm with sufficient accuracy to design an appropriate laser system. The extension of the calculations to He3 is complicated due to the additional hyperfine structure, but we show that the magic wavelength candidates around 320 nm are predominantly shifted by the isotope shift

Ultracold metastable helium: Ramsey fringes and atom interferometry

We report on interference studies in the internal and external degrees of freedom of metastable triplet helium atoms trapped near quantum degeneracy in a 1.5μm optical dipole trap. Applying a single π/ 2 rf pulse we demonstrate that 50% of the atoms initially in the m= + 1 state can be transferred to the magnetic field insensitive m= 0 state. Two π/ 2 pulses with varying time delay allow a Ramsey-type measurement of the Zeeman shift for a high precision measurement of the 23S1–21S0 transition frequency. We show that this method also allows strong suppression of mean-field effects on the measurement of the Zeeman shift, which is necessary to reach the accuracy goal of 0.1 kHz on the absolute transition frequencies. Theoretically the feasibility of using metastable triplet helium atoms in the m= 0 state for atom interferometry is studied demonstrating favorable conditions, compared to the alkali atoms that are used traditionally, for a non-QED determination of the fine structure constant

Extreme ultraviolet interferometer using high-order harmonic generation from successive sources

We present a new interferometer technique whereby multiple extreme ultraviolet light pulses are generated at different positions within a single laser focus (i.e., from successive sources) with a highly controllable time delay. The interferometer technique is tested with two generating media to create two extreme ultraviolet light pulses with a time delay between them. The delay is found to be a consequence of the Gouy phase shift. Ultimately the apparatus is capable of accessing unprecedented time scales by allowing stable and repeatable delays as small as 100 zs. © 2012 American Physical Society