Department of Quantum Science and Technology, ANU

Ultracold helium atoms trapped in the first excited state (termed metastable helium, or He*) form an ideal testbed for some of the most fundamental theories of quantum physics. At temperatures ~1µK above absolute zero, He* atoms condense into a single coherent quantum state - a Bose-Einstein condensate (BEC). The internal energy stored in the metastable state then allows single atom detection with 3D resolution. These properties allow the basic quantum principle of entanglement, and whether quantum mechanics is a non-local theory, to be probed via Bell tests. In this talk I will describe our recent experiments, where we demonstrated Bell correlations between entangled atomic spin states, as well as our current progress towards a Bell test with momentum entangled states. This is a promising system for the investigation of the interaction of gravity with quantum entanglement.

Additionally, I will report on our work testing another fundamental cornerstone of quantum physics, quantum electro dynamics (QED). This theory describes the energy level structure of atoms at an extremely precise level that modern experiments are now able to challenge. Such experiments can cause revisions and extensions to QED theory to be considered, such as the recent proton radius puzzle. The simple atomic structure of helium makes it an ideal test bed for QED theory, and I will report on the energy measurements we have performed recently. The most important of these involves a measurement of a tuneout wavelength, where the dipole shift from all different energy levels cancels out. This makes a tuneout measurement a highly precise probe of QED, as it simultaneously tests contributions from all atomic energy levels. Our result challenges the current level of QED theory.**Dr Sean Hodgman** was awarded his PhD in Physics in 2011 from ANU under the supervision of Prof Andrew Truscott, working with ultracold metastable helium atoms. From 2012-2015 he worked as a postdoc in the group of Prof Immanuel Bloch, at the Max Planck Institute for Quantum Optics in Munich, using degenerate quantum gases in optical lattices as experimental quantum emulators for solid state systems. In 2015 he returned to RSPE on a DECRA fellowship, where he has since been working on quantum gases of ultracold helium, establishing a new lab to focus on many body physics, and more recently starting a new project using cold atoms as a target for positron scattering.

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