Precision Measurement for Metastable Helium Atoms of the 413 nm Tune-Out Wavelength at Which the Atomic Polarizability Vanishes

We present the first measurement for helium atoms of the tune-out wavelength at which the atomic polarizability vanishes. We utilize a novel, highly sensitive technique for precisely measuring the effect of variations in the trapping potential of confined metastable ($2{}^3{S}_1$) helium atoms illuminated by a perturbing laser light field. The measured tune-out wavelength of $413.0938(9_{\text{stat}})(20_{\text{syst}})\mathrm{nm}$ compares well with the value predicted by a theoretical calculation [413.02(9) nm] which is sensitive to finite nuclear mass, relativistic, and quantum electrodynamic effects. This provides motivation for more detailed theoretical investigations to test quantum electrodynamics.

Figure 1

Helium polarizability spectrum (solid curves) as a function of energy (a.u.). Triplet transition manifold positions are shown by the dotted vertical lines.

Figure 2

Schematic of the He* one-dimensional atomic density profile in the limit of an adiabatically varied laser potential, with rf outcoupling positions indicated by shaded regions. Black solid line: A purely magnetically trapped Bose-Einstein condensate. Red dot-dashed line: With an additional attractive laser potential increasing the outcoupling rate. Blue dashed line: With an additional repulsive laser potential decreasing the outcoupling rate.

Figure 3

TOF DLD signal for the atoms output coupled from the magnetic trap by the swept rf field in the presence of a modulated perturbing laser field. Inset: Fourier analysis of the TOF signal in the frequency range near the 491 Hz acousto-optic modulator modulation frequency.

Figure 4

Experimental results. Wavelength dependence of the phase (top) and amplitude (bottom) for the modulation signal as a function of wavelength. The solid lines indicate a fit to the data, and the tune-out wavelength thus determined is indicated.