Observing the emergence of a quantum phase transition shell by shell

Luca Bayha (), Marvin Holten (), Ralf Klemt, Keerthan Subramanian, Johannes Bjerlin, Stephanie M. Reimann, Georg M. Bruun, Philipp M. Preiss and Selim Jochim
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Luca Bayha: Physikalisches Institut der Universität Heidelberg
Marvin Holten: Physikalisches Institut der Universität Heidelberg
Ralf Klemt: Physikalisches Institut der Universität Heidelberg
Keerthan Subramanian: Physikalisches Institut der Universität Heidelberg
Johannes Bjerlin: Lund University
Stephanie M. Reimann: Lund University
Georg M. Bruun: Aarhus University
Philipp M. Preiss: Physikalisches Institut der Universität Heidelberg
Selim Jochim: Physikalisches Institut der Universität Heidelberg

Nature, 2020, vol. 587, issue 7835, 583-587

Abstract: Abstract Many-body physics describes phenomena that cannot be understood by looking only at the constituents of a system1. Striking examples are broken symmetry, phase transitions and collective excitations2. To understand how such collective behaviour emerges as a system is gradually assembled from individual particles has been a goal in atomic, nuclear and solid-state physics for decades3–6. Here we observe the few-body precursor of a quantum phase transition from a normal to a superfluid phase. The transition is signalled by the softening of the mode associated with amplitude vibrations of the order parameter, usually referred to as a Higgs mode7. We achieve fine control over ultracold fermions confined to two-dimensional harmonic potentials and prepare closed-shell configurations of 2, 6 and 12 fermionic atoms in the ground state with high fidelity. Spectroscopy is then performed on our mesoscopic system while tuning the pair energy from zero to a value larger than the shell spacing. Using full atom counting statistics, we find the lowest resonance to consist of coherently excited pairs only. The distinct non-monotonic interaction dependence of this many-body excitation, combined with comparison with numerical calculations allows us to identify it as the precursor of the Higgs mode. Our atomic simulator provides a way to study the emergence of collective phenomena and the thermodynamic limit, particle by particle.

Date: 2020
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DOI: 10.1038/s41586-020-2936-y

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