From Few to Many: Observing the Formation of a Fermi Sea One Atom at a Time

Knowing when a physical system has reached sufficient size for its macroscopic properties to be well described by many-body theory is difficult. We investigate the crossover from few to many-body physics by studying quasi one-dimensional systems of ultracold atoms consisting of a single impurity interacting with an increasing number of identical fermions. We measure the interaction energy of such a system as a function of the number of majority atoms for different strengths of the interparticle interaction. As we increase the number of majority atoms one by one we observe the fast convergence of the normalized interaction energy towards a many-body limit calculated for a single impurity immersed in a Fermi sea of majority particles.Comment: 9 pages, 5 figure

Pairing in few-fermion systems with attractive interactions

We have studied quasi one-dimensional few-particle systems consisting of one to six ultracold fermionic atoms in two different spin states with attractive interactions. We probe the system by deforming the trapping potential and by observing the tunneling of particles out of the trap. For even particle numbers we observe a tunneling behavior which deviates from uncorrelated single-particle tunneling indicating the existence of pair correlations in the system. From the tunneling timescales we infer the differences in interaction energies of systems with different number of particles which show a strong odd-even effect, similar to the one observed for neutron separation experiments in nuclei.Comment: 9 pages, 6 figure

Radio Frequency Association of Efimov Trimers

The quantum-mechanical three-body problem is one of the fundamental challenges of few-body physics. When the two-body interactions become resonant, an infinite series of universal three-body bound states is predicted to occur, whose properties are determined by the strength of the two-body interactions. We report on the association and direct observation of a trimer state consisting of three distinguishable fermions using radio-frequency (RF) spectroscopy. The measurements of its binding energy are consistent with theoretical predictions which include non-universal corrections.Comment: 12 pages, 6 figure

Coherent molecule formation in anharmonic potentials near confinement-induced resonances

We perform a theoretical and experimental study of a system of two ultracold atoms with tunable interaction in an elongated trapping potential. We show that the coupling of center-of-mass and relative motion due to an anharmonicity of the trapping potential leads to a coherent coupling of a state of an unbound atom pair and a molecule with a center of mass excitation. By performing the experiment with exactly two particles we exclude three-body losses and can therefore directly observe coherent molecule formation. We find quantitative agreement between our theory of inelastic confinement-induced resonances and the experimental results. This shows that the effects of center-of-mass to relative motion coupling can have a significant impact on the physics of quasi-1D quantum systems.Comment: 7 pages, 4 figure

Antiferromagnetic Heisenberg Spin Chain of a Few Cold Atoms in a One-Dimensional Trap

We report on the deterministic preparation of antiferromagnetic Heisenberg spin chains consisting of up to four fermionic atoms in a one-dimensional trap. These chains are stabilized by strong repulsive interactions between the two spin components without the need for an external periodic potential. We independently characterize the spin configuration of the chains by measuring the spin orientation of the outermost particle in the trap and by projecting the spatial wave function of one spin component on single-particle trap levels. Our results are in good agreement with a spin-chain model for fermionized particles and with numerically exact diagonalizations of the full few-fermion system

Fermionization of two distinguishable fermions

In this work we study a system of two distinguishable fermions in a 1D harmonic potential. This system has the exceptional property that there is an analytic solution for arbitrary values of the interparticle interaction. We tune the interaction strength via a magnetic offset field and compare the measured properties of the system to the theoretical prediction. At the point where the interaction strength diverges, the energy and square of the wave function for two distinguishable particles are the same as for a system of two identical fermions. This is referred to as fermionization. We have observed this phenomenon by directly comparing two distinguishable fermions with diverging interaction strength with two identical fermions in the same potential. We observe good agreement between experiment and theory. By adding one or more particles our system can be used as a quantum simulator for more complex few-body systems where no theoretical solution is available

Collisional stability of a three-component degenerate Fermi gas

We report on the creation of a degenerate Fermi gas consisting of a balanced mixture of atoms in three different hyperfine states of $^6$Li. This new system consists of three distinguishable Fermions with different and tunable interparticle scattering lengths $a_{12}$, $a_{13}$ and $a_{23}$. We are able to prepare samples containing $5 \cdot 10^4$ atoms in each state at a temperature of about $215$nK, which corresponds to $T/T_F \approx 0.37$. We investigated the collisional stability of the gas for magnetic fields between 0 and 600 G and found a prominent loss feature at 130 G. From lifetime measurements we determined three-body loss coefficients, which vary over nearly three orders of magnitude