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The workhorse of atomic physics, quantum electrodynamics, is one of thebest-tested theories in physics. However recent discrepancies have shed doubton its accuracy for complex atomic systems. To facilitate the development ofthe theory further we aim to measure transition dipole matrix elements ofmetastable helium (He*) (the ideal 3 body test-bed) to the highest accuracythus far. We have undertaken a measurement of the `tune-out wavelength' whichoccurs when the contributions to the dynamic polarizability from all atomictransitions sum to zero; thus illuminating an atom with this wavelength oflight then produces no net energy shift. This provides a strict constraint onthe transition dipole matrix elements without the complication and inaccuracyof other methods.  Using a novel atom-laser based technique we have made the first measurementof 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 thepredicted value [PhysRevA.88.052515] of 413.02(9) nm. We have additionallydeveloped many of the methods necessary to improve this measurement to the 100fm level of accuracy where it will form the most accurate determination oftransition rate information ever made in He* and provide a stringent test foratomic QED simulations. We believe this measurement to be one of the mostsensitive ever made of an optical dipole potential, able to detect changes inpotentials of $\sim$200 pK and is widely applicable to other species and areasof atom optics.

The First Measurement of the $2^{3\!}S_1 - 2^{3\!}P / 3^{3\!}P$ Tune-Out Wavelength in He*