authors contributed equally to this work.Systems consisting of few interacting fermions are the building blocks of matter, with atoms and nuclei being the most prominent examples. We have created a few-body quantum system with complete control over its quantum state using ultracold fermionic atoms in an optical dipole trap. Ground-state systems consisting of 1 to 10 particles were prepared with fidelities of ∼ 90%. We can tune the interparticle interactions to arbitrary values using a Feshbach resonance and observed the interaction-induced energy shift for a pair of repulsively interacting atoms. This work is expected to enable quantum simulation of strongly correlated few-body systems.The exploration of naturally occurring few-body quantum systems such as atoms and nuclei has been extremely successful, largely because they could be prepared in well defined quantum states. Because these systems have limited tunability, researchers created quantum dots-"artificial atoms"-in which properties such as particle number, interaction strength and confining potential can be tuned (1,2). However, quantum dots are generally strongly coupled to their environment which hindered the deterministic preparation of well-defined quantum states. In contrast, ultracold gases provide tunable systems in a highly isolated environment (3, 4). They have been proposed as a tool for quantum simulation (5, 6) which has been realized experimentally for various many-body systems (7-10). Achieving quantum simulation of fewbody systems is more challenging because it requires complete control over all degrees of freedom: the particle number, the internal and motional states of the particles, and the strength of the inter-particle interactions. One possible approach to this goal is using a Mott insulator state of atoms in an optical lattice as a starting point. In this way, systems with up to four bosons per lattice site have been prepared in their ground state (11,12). Recently, single lattice sites have been addressed individually (13). In single isolated trapping geometries, researchers could suppress atom number fluctuations by loading bosonic atoms into small-volume optical dipole traps (14-18). However, these experiments were not able to gain control over the system's quantum state. We prepare few-body systems consisting of 1 to 10 fermionic atoms in a well-defined quantum state making use of Pauli's principle, which states that each singleparticle state cannot be occupied by more than one identical fermion. Therefore, the occupation probability of the lowest energy states approaches unity for a degenerate Fermi gas, and we can control the number of particles by controlling the number of available singleparticle states. We realize this by deforming the confining potential such that quantum states above a well defined energy become unbound. This approach requires a highly degenerate Fermi gas in a trap whose depth can be controlled with a precision much higher than the separation of its energy levels. To fulfill these requirements, we use a ...
Just as in clay moulding or glass blowing, physically sculpting biological structures requires the constituent material to locally flow like a fluid while maintaining overall mechanical integrity like a solid. Disordered soft materials, such as foams, emulsions and colloidal suspensions, switch from fluid-like to solid-like behaviours at a jamming transition. Similarly, cell collectives have been shown to display glassy dynamics in 2D and 3D and jamming in cultured epithelial monolayers, behaviours recently predicted theoretically and proposed to influence asthma pathobiology and tumour progression. However, little is known about whether these seemingly universal behaviours occur in vivo and, specifically, whether they play any functional part during embryonic morphogenesis. Here, by combining direct in vivo measurements of tissue mechanics with analysis of cellular dynamics, we show that during vertebrate body axis elongation, posterior tissues undergo a jamming transition from a fluid-like behaviour at the extending end, the mesodermal progenitor zone, to a solid-like behaviour in the presomitic mesoderm. We uncover an anteroposterior, N-cadherin-dependent gradient in yield stress that provides increasing mechanical integrity to the presomitic mesoderm, consistent with the tissue transiting from a wetter to a dryer foam-like architecture. Our results show that cell-scale stresses fluctuate rapidly (within about 1 min), enabling cell rearrangements and effectively 'melting' the tissue at the growing end. Persistent (more than 0.5 h) stresses at supracellular scales, rather than cell-scale stresses, guide morphogenetic flows in fluid-like tissue regions. Unidirectional axis extension is sustained by the reported rigidification of the presomitic mesoderm, which mechanically supports posterior, fluid-like tissues during remodelling before their maturation. The spatiotemporal control of fluid-like and solid-like tissue states may represent a generic physical mechanism of embryonic morphogenesis.
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