arXiv:1307.3443 · Science

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

A. N. Wenz, G. Zürn, Simon Murmann, Ioannis Brouzos, Thomas Lompe, Selim Jochim

Published Oct 24, 2013• 331 citations

Abstract

Knowing when a physical system has reached sufficient size for its macroscopic properties to be well described by many-body theory is difficult. We investigated 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 measured 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 increased the number of majority atoms one by one, we observed fast convergence of the normalized interaction energy toward a many-body limit calculated for a single impurity immersed in a Fermi sea of majority particles.

DOI: 10.1126/science.1240516

Related papers

From Few to Many: Ultracold Atoms in Reduced Dimensions

A. N. Wenz

This thesis reports on experimental studies exploring few and many-body physics of ultracold \nBose and Fermi gases with reduced dimensionality. These experiments illustrate the versatility \nand great amount of control over the particle number, the interaction and other degrees of \nfreedom, like the spin, that these generic quantum systems offer. In the first part of this \nthesis, we use quasi one-dimensional few-particle systems of one to ten fermionic atoms to \ninvestigate the crossover from few to many-body physics. This is achieved by measuring the \ninteraction energy between a single impurity atom in a state |down> which repulsively interacts \nwith an increasing number of majority atoms in a state |up>. We find that the system quickly \napproaches the results from the many-body theory, which describes the behavior of a single \nimpurity immersed in a Fermi sea of an infinite number of majority particles. The second \npart of this thesis presents studies of the time evolution of a bosonic F=1 spinor BEC of 87Rb \natoms. In this system, we investigate the emergence and coarsening of ferromagnetic spin \ntextures from initially unmagnetized samples. While the ferromagnetic domains grow, we \nobserve the development of a spin space anisotropy which is in agreement with the predicted \nphase-diagram. The last part of this thesis presents our first steps towards the investigation \nof phase coherence of quasi two-dimensional quantum gases in the crossover from bosonic \nmolecules to fermionic atoms.

Jan 1, 20140 citations

Few-fermion systems in one dimension

G. Zürn

This thesis reports on experiments with few-fermion systems in quasi one dimensional confining potentials with tunable interaction. \nUsing ultracold atoms we prepare these systems in a well defined quantum state with fidelities of 90% for up to 8 particles. The interparticle interaction in the 1D environment can be effectively described by a 1D contact interaction where the coupling strength can be tuned with a confinement induced resonance (CIR). \nWe investigate a system of two repulsively interacting distinguishable fermions and compare it to a system of two identical noninteracting fermions. For diverging coupling strength we show the fermionization of two distinguishable fermions, i.e. we observe the energy and the square modulus of the wavefunction of both system to be identical. We also perform radio frequency spectroscopy to measure the energy of a single minority particle interacting repulsively with a defined number of majority particles of different spin. We study the crossover from a few-particle system to a many-particle system by adding majority particles one by one. \nWe observe that already four majority particles are enough to describe the properties of the minority by that of a polaron-like particle, i.e. by a single impurity dressed by a 1D Fermi sea. \nInvestigating attractively interacting systems we observe that for increasing interaction strength the pair correlations in the system increases. This correlation leads to a strong odd-even effect of the single particle dissociation energy similar to the one observed for nuclei.

Jan 1, 20136 citations

Ultrafast many-body interferometry of impurities coupled to a Fermi sea

Marko Cetina, Michael Jag, Rianne S. Lous +8 more

The fastest possible collective response of a quantum many-body system is related to its excitations at the highest possible energy. In condensed matter systems, the time scale for such "ultrafast" processes is typically set by the Fermi energy. Taking advantage of fast and precise control of interactions between ultracold atoms, we observed nonequilibrium dynamics of impurities coupled to an atomic Fermi sea. Our interferometric measurements track the nonperturbative quantum evolution of a fermionic many-body system, revealing in real time the formation dynamics of quasi-particles and the quantum interference between attractive and repulsive states throughout the full depth of the Fermi sea. Ultrafast time-domain methods applied to strongly interacting quantum gases enable the study of the dynamics of quantum matter under extreme nonequilibrium conditions.

arXiv:1604.07423ScienceOct 6, 2016359 citations

One-dimensional Fermi gas with a single impurity in a harmonic trap: Perturbative description of the upper branch

Seyed Ebrahim Gharashi, Xiangyu Yin, Yangqian Yan +1 more

The transition from ``few to many'' has recently been probed experimentally in an ultracold harmonically confined one-dimensional lithium gas, in which a single impurity atom interacts with a background gas consisting of one, two, or more identical fermions [A. N. Wenz et al., Science 342, 457 (2013)]. For repulsive interactions between the background or majority atoms and the impurity, the interaction energy for relatively moderate system sizes was analyzed and found to converge toward the corresponding expression for an infinitely large Fermi gas. Motivated by these experimental results, we apply perturbative techniques to determine the interaction energy for weak and strong coupling strengths and derive approximate descriptions for the interaction energy for repulsive interactions with varying strength between the impurity and the majority atoms and any number of majority atoms.

arXiv:1501.04369Physical Review AJan 20, 201538 citations

Forming a Fermi sea

How big must an ensemble of particles be before the exact number of individual particles becomes inconsequential and the entire system can be described using many-body theories? This important question in condensed-matter physics has proved difficult to answer, but now researchers in Germany have observed the transition from "few" to "many" in an experiment using ultracold fermionic atoms.

Physics WorldDec 1, 20130 citations

From few- to many-body physics with ultracold atoms

Richard Schmidt

In this thesis ultracold quantum gases are studied with field theoretical methods. The quantum mechanical three-body problem is solved exactly and the recently experimentally observed universality of the three-body parameter is explained. By means of the functional renormalization group (fRG) the four-body problem is analyzed and it is shown that no additional four-body parameter has to be introduced. The second part of this thesis studies a single impurity immersed in a majority Fermi sea. We develop a new, numerical fRG technique which allows the computation of Green’s functions with full frequency and momentum dependence. Our results are in excellent agreement with recent experiments and exemplify the intricate interplay between few- and many-body physics in ultracold quantum gases.

Jan 1, 20133 citations