For several decades, superfluid helium has provided an almost unparalleled frontier of quantum physics research, especially in topics related to macroscopic quantum phenomena: traditionally both superfluid helium-4 and helium-3 have provided a natural but versatile window to many-body quantum physics. In this presentation I show how the versatility of superfluid helium-3 translates into a variety of emergent phenomena, touching seemingly distant fields such as cosmology and high-energy physics.
In our experiments we have used a rotating ultra-low-temperature refrigerator, the superfluid sample being cooled down by a nuclear demagnetization stage and probed using nuclear magnetic resonance spectroscopy (NMR). One particularly useful NMR instrument can be constructed by trapping a Bose-Einstein condensate (BEC) of magnon quasiparticles within the superfluid. We have used such condensates in probing a variety of delicate phenomena such as other spin wave modes, including Higgs modes, and Majorana bound states. We have also studied propagation of self-trapped Q-ball solitons formed of magnons. Q-balls, if observed in the Universe, could shed light on mysteries such as the dark matter.
As perhaps the most publicized topic of this presentation, I explain how we discovered the elusive half-quantum vortices in superfluid helium-3. The very name of quantum physics refers to the observation that fundamental concepts such as energy and momentum are quantised in the microscopic world. Therefore finding vortices carrying only half-a-quantum of circulation — seemingly breaking that rule — is not only intriguing, but manifests deep understanding of the underlying physics and quantum physics in general.
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