Journal
NATURE
Volume 587, Issue 7835, Pages 583-+Publisher
NATURE PORTFOLIO
DOI: 10.1038/s41586-020-2936-y
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Funding
- ERC [725636]
- Heidelberg Center for Quantum Dynamics
- DFG Collaborative Research Centre SFB 1225 (ISOQUANT)
- European Union [817482 PASQuanS]
- Landesgraduiertenforderung Baden-Wurttemberg
- Daimler and Benz Foundation
- Swedish Research Council
- Knut and Alice Wallenberg Foundation
- NanoLund
- Independent Research Fund Denmark-Natural Sciences [DFF-8021-00233B]
- Danish National Research Foundation through the Center of Excellence CCQ [DNRF156]
- European Research Council (ERC) [725636] Funding Source: European Research Council (ERC)
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Many-body physics describes phenomena that cannot be understood by looking only at the constituents of a system(1). Striking examples are broken symmetry, phase transitions and collective excitations(2). 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 decades(3-6). 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 mode(7). 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.
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