In 1972 P.W. Anderson wrote that "More is different", but can we actually quantify what "more" is? Forty years later, experiments at the Large Hadron Collider show that key signals of emergent hydrodynamic behavior, typically associated with the formation of quark-gluon plasma in high-energy heavy-ion collisions, persist mysteriously down to p-p interactions producing only a few dozen final-state hadrons. Could it be that p-p collisions at moderate multiplicities are already "different"?
Motivated by these questions, we look at the other end of the energy spectrum: we exploit the capabilities of the few-fermion experiment at Heidelberg University to investigate, with strongly-interacting ultra-cold atoms, how many particles are needed to witness the emergence of fluid-type behavior. Pure quantum states of fermions (6Li atoms) are prepared in optical traps where the geometry of the confining potential, the strength of the inter-atom interactions, as well as the number of these atoms can be controlled deterministically. Borrowing techniques from high-energy collisions, we release the atoms from an elliptical trap to investigate the emergence of the so-called elliptic flow, that is, the build-up of a quadrupole modulation in the momentum of the expanding cloud, followed the inversion of its initial aspect ratio in space. We observe elliptic flow with as few as 10 fermions, demonstrating the onset of an interaction-driven collective phenomenon, potentially due to the emergence of superfluidity in the few-body system. We discuss the implications of this observation for our understanding of many-body physics, highlighting the prospects for future cross-disciplinary research aimed at elucidating the experimental results.
References:
- Floerchinger et al., Phys.Rev.C 105 (2022) 4, 044908
- Brandstetter et al., arXiv:2308.09699, to appear in Nature Physics (2024)
- Floerchinger, Giacalone, Heyen, arXiv:2408.06104
- Brandstetter et al., arXiv:2409.18954
Joachim Kopp