Title: The First Measurement of the $2^{3}S_{1} \rightarrow 3^{3}P - 2^{3}P$ Tune-Out Wavelength in He*,
Abstract: The workhorse of atomic physics: quantum electrodynamics is one of the best
tested theories in physics. However recent discrepancies have shed doubt on its
accuracy for complex atomic systems. To facilitate the development of the
theory further we aim to measure transition dipole matrix elements of
metastable helium (He*) (the ideal 3 body test-bed) to the highest accuracy
thus far. We have undertaken a measurement of the `tune-out wavelength' which
occurs when the contributions to the dynamic polarizability from all atomic
transitions sum to zero; thus illuminating an atom with this wavelength of
light then produces no net energy shift. This provides a strict constraint on
the transition dipole matrix elements without the complication and inaccuracy
of other methods.
Using a novel atom-laser based technique we have made the first measurement
of the the tune-out wavelength in metastable helium between the
$3^{3}P_{1,2,3}$ and $2^{3}P_{1,2,3}$ states at 413.07(2)nm which compares well
with the predicted value\cite{Mitroy2013} of 413.02(9). We have additionally
developed many of the methods necessary to improve this measurement to the
100fm level of accuracy where it will form the most accurate determination of
transition rate information ever made in He* and provide a stringent test for
atomic QED simulations. We believe this measurement to be one of the most
sensitive ever made of an optical dipole potential, able to detect changes in
potentials of $\sim200pK$ and is widely applicable to other species and areas
of atom optics.