Improvements in both theory and frequency metrology of fewelectron systems such as hydrogen and helium have enabled increasingly sensitive tests of quantum electrodynamics (QED), as well as ever more accurate determinations of fundamental constants and the size of the nucleus. At the same time advances in cooling and trapping of neutral atoms have revolutionized the development of increasingly accurate atomic clocks. Here, we combine these fields to reach the highest precision on an optical tranistion in the helium atom to date by employing a Bose-Einstein condensate confined in a magic wavelength optical dipole trap. The measured transition accurately connects the ortho-and parastates of helium and constitutes a stringent test of QED theory. In addition we test polarizability calculations and ultracold scattering properties of the helium atom. Finally, our measurement probes the size of the nucleus at a level exceeding the projected accuracy of muonic helium measurements currently being performed in the context of the proton radius puzzle. 1 2 R.J. RENGELINK ET AL.In the past decades, high-precision spectroscopy measurements in atomic physics scale systems have pushed precision tests of quantum electrodynamics (QED), one of the cornerstones of the standard model of physics, ever further [1, 2] and have led to accurate determinations of fundamental constants [3][4][5][6]. Recently however, measurements of transition frequencies in muonic hydrogen (µH) have revealed a discrepancy of six standard deviations [7, 8] with respect to the accepted CODATA value for the proton charge radius. This discrepancy, which has become known as the "proton radius puzzle", has stimulated strong interest in the field, as its confirmation implies the violation of lepton universality, one of the pillars of the standard model. New experiments in atomic hydrogen [9, 10], and muonic deuterium [11] have only deepened the puzzle, prompting research into other elements such as muonic helium (µ 3,4 He + ) [12]. From these measurements the charge radii of the alpha-particle and the helion (1.68 fm resp. 1.97 fm) are projected to be determined with sub-attometer accuracy, which should be compared to high-precision experiments in electronic helium atoms or ions.QED theory of the helium atom, with two electrons more complicated than hydrogen, has seen impressive improvements in recent years, with QED corrections up to order mα 6 now evaluated [2]. Recent experiments are in good agreement [13][14][15][16][17][18][19][20] and may allow a competitive value for the fine structure constant in the near future [21][22][23][24]. The anticipated evaluation of the next highest order corrections (mα 7 ) [2] would allow the determination of the 4 He nuclear charge radius with an accuracy better than 1%. At present nuclear charge radii can already be determined differentially, i.e. with respect to 4 He, due to cancellation of higher-order terms in the isotope shift. Using this approach the radii of the exotic halo nuclei 6 He and 8 He [25,26], as well as t...

The Pauli exclusion principle in quantum mechanics has a profound influence on the structure of matter and on interactions between fermions. Almost 30 years ago it was predicted that the Pauli exclusion principle could lead to a suppression of spontaneous emission, and only recently several experiments confirmed this phenomenon. Here we report that this so-called Pauli blockade not only affects incoherent processes but also, more generally, coherently driven systems. It manifests itself as an intriguing sub-Doppler narrowing of a doubly-forbidden transition profile in an optically trapped Fermi gas of 3He. By actively pumping atoms out of the excited state, we break the coherence of the excitation and lift the narrowing effect, confirming the influence of Pauli blockade on the transition profile. This insight into the interplay between quantum statistics and coherent driving is a promising development for future applications involving fermionic systems.

The Pauli exclusion principle in quantum mechanics has a profound influence on the structure of matter and on interactions between fermions. Almost 30 years ago it was predicted that the Pauli exclusion principle could lead to a suppression of spontaneous emission, and only recently several experiments 1-3 confirmed this phenomenon. Here we report that this so-called Pauli blockade not only affects incoherent processes but also, more generally, coherently driven systems. It manifests itself as an intriguing sub-Doppler narrowing of a doubly-forbidden transition profile in an optically trapped Fermi gas of 3 He. By actively pumping atoms out of the excited state, we break the coherence of the excitation and lift the narrowing effect, confirming the influence of Pauli blockade on the transition profile. This new insight into the interplay between quantum statistics and coherent driving is a promising development for future applications involving fermionic systems.The Pauli exclusion principle arises from the simple requirement of antisymmetrization of two-fermion wavefunctions under particle exchange 4 , but it has a remarkable impact on how our world is shaped. It determines the electronic structure of atoms, which dictates their chemical properties, and it gives rise to the Fermi energy of electron ensembles in solid state materials, lead-1