There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

This paper reviews recent experimental and theoretical progress concerning many-body phenomena in dilute, ultracold gases. It focuses on effects beyond standard weak-coupling descriptions, such as the Mott-Hubbard transition in optical lattices, strongly interacting gases in one and two dimensions, or lowest-Landau-level physics in quasi-two-dimensional gases in fast rotation. Strong correlations in fermionic gases are discussed in optical lattices or near-Feshbach resonances in the BCS-BEC crossover.

/pdf/many-body-physics-with-ultracold-gases-552s10zfdn.pdf

Many-Body Physics with Ultracold Gases

Bose-Einstein condensation (BEC) has been observed in a dilute gas of sodium atoms. A Bose-Einstein condensate consists of a macroscopic population of the ground state of the system, and is a coherent state of matter. In an ideal gas, this phase transition is purely quantum-statistical. The study of BEC in weakly interacting systems which can be controlled and observed with precision holds the promise of revealing new macroscopic quantum phenomena that can be understood from first principles.

Bose-Einstein condensation in a gas of sodium atoms

Feshbach resonances are the essential tool to control the interaction between atoms in ultracold quantum gases. They have found numerous experimental applications, opening up the way to important breakthroughs. This review broadly covers the phenomenon of Feshbach resonances in ultracold gases and their main applications. This includes the theoretical background and models for the description of Feshbach resonances, the experimental methods to find and characterize the resonances, a discussion of the main properties of resonances in various atomic species and mixed atomic species systems, and an overview of key experiments with atomic Bose-Einstein condensates, degenerate Fermi gases, and ultracold molecules.

Feshbach resonances in ultracold gases

We review recent developments in the physics of ultracold atomic and molecular gases in optical lattices. Such systems are nearly perfect realisations of various kinds of Hubbard models, and as such may very well serve to mimic condensed matter phenomena. We show how these systems may be employed as quantum simulators to answer some challenging open questions of condensed matter, and even high energy physics. After a short presentation of the models and the methods of treatment of such systems, we discuss in detail, which challenges of condensed matter physics can be addressed with (i) disordered ultracold lattice gases, (ii) frustrated ultracold gases, (iii) spinor lattice gases, (iv) lattice gases in “artificial” magnetic fields, and, last but not least, (v) quantum information processing in lattice gases. For completeness, also some recent progress related to the above topics with trapped cold gases will be discussed. Motto: There are more things in heaven and earth, Horatio, Than are dreamt of in your...

/pdf/ultracold-atomic-gases-in-optical-lattices-mimicking-tfesylj4h7.pdf

Ultracold atomic gases in optical lattices: mimicking condensed matter physics and beyond

We report on the generation of a Bose-Einstein condensate in a gas of chromium atoms, which have an exceptionally large magnetic dipole moment and therefore underlie anisotropic long-range interactions. The preparation of the chromium condensate requires novel cooling strategies that are adapted to its special electronic and magnetic properties. The final step to reach quantum degeneracy is forced evaporative cooling of 52Cr atoms within a crossed optical dipole trap. At a critical temperature of T(c) approximately 700 nK, we observe Bose-Einstein condensation by the appearance of a two-component velocity distribution. We are able to produce almost pure condensates with more than 50,000 condensed 52Cr atoms.

https://arxiv.org/pdf/cond-mat/0503044.pdf

Bose-Einstein Condensation of Chromium

We have investigated the expansion of a Bose-Einstein condensate of strongly magnetic chromium atoms. The long-range and anisotropic magnetic dipole-dipole interaction leads to an anisotropic deformation of the expanding chromium condensate which depends on the orientation of the atomic dipole moments. Our measurements are consistent with the theory of dipolar quantum gases and show that a chromium condensate is an excellent model system to study dipolar interactions in such gases.

/pdf/observation-of-dipole-dipole-interaction-in-a-degenerate-22xez9s1ya.pdf

Observation of Dipole-Dipole Interaction in a Degenerate Quantum Gas

A paper in this issue addresses a hot topic in the physics of ultracold atoms — quantum ferrofluids, which are superfluid quantum gases that consist of polarized dipoles, either electric or magnetic. The symmetry-breaking interaction between such dipoles is expected to lead to new physical phenomena. Now a group from Stuttgart University has succeeded in producing a chromium Bose–Einstein condensate with strong anisotropic magnetic dipole–dipole interaction between the atoms, which induces a pronounced change of the aspect ratio of the cloud. This much-sought form of matter should enable fundamental studies of physical systems in previously unexplored regimes. This paper reports the realization of a chromium Bose-Einstein condensate (BEC) with strong anisotropic magnetic dipole–dipole interaction between the atoms, which induces a pronounced change of the aspect ratio of the cloud. The experiment opens the way for exploration of the unique properties of quantum ferrofluids. Symmetry-breaking interactions have a crucial role in many areas of physics, ranging from classical ferrofluids to superfluid 3He and d-wave superconductivity. For superfluid quantum gases, a variety of new physical phenomena arising from the symmetry-breaking interaction between electric or magnetic dipoles are expected1. Novel quantum phases in optical lattices, such as chequerboard or supersolid phases, are predicted for dipolar bosons2,3. Dipolar interactions can also enrich considerably the physics of quantum gases with internal degrees of freedom4,5,6. Arrays of dipolar particles could be used for efficient quantum information processing7. Here we report the realization of a chromium Bose–Einstein condensate with strong dipolar interactions. By using a Feshbach resonance, we reduce the usual isotropic contact interaction, such that the anisotropic magnetic dipole–dipole interaction between 52Cr atoms becomes comparable in strength. This induces a change of the aspect ratio of the atom cloud; for strong dipolar interactions, the inversion of ellipticity during expansion (the usual ‘smoking gun’ evidence for a Bose–Einstein condensate) can be suppressed. These effects are accounted for by taking into account the dipolar interaction in the superfluid hydrodynamic equations governing the dynamics of the gas, in the same way as classical ferrofluids can be described by including dipolar terms in the classical hydrodynamic equations. Our results are a first step in the exploration of the unique properties of quantum ferrofluids.

Strong dipolar effects in a quantum ferrofluid

For the first time, a purely dipolar quantum gas has been prepared experimentally. Different regimes have been explored; in some, the gas is stable, whereas in others it collapses due to the strong dipole–dipole interaction between the constituent atoms. Although the phenomenon of Bose–Einstein condensation1 is a purely statistical effect that also appears in an ideal gas, the physics of Bose–Einstein condensates (BECs) of dilute gases is considerably enriched by the presence of interactions among the atoms. In usual experiments with BECs, the only relevant interaction is the isotropic and short-range contact interaction, which is described by a single parameter, the scattering length a. In contrast, the dipole–dipole interaction between particles possessing an electric or magnetic dipole moment is of long-range character and anisotropic, which gives rise to new phenomena2,3. Most prominently, the stability of a dipolar BEC depends not only on the value of the scattering length, a, but also strongly on the geometry of the external trapping potential4,5,6,7. Here, we report on the experimental investigation of the stability of a dipolar BEC of 52Cr as a function of the scattering length and the trap aspect ratio. We find good agreement with a universal stability threshold arising from a simple theoretical model. Using a pancake-shaped trap with the dipoles oriented along the short axis of the trap, we are able to tune the scattering length to zero, stabilizing a purely dipolar quantum gas.

/pdf/stabilization-of-a-purely-dipolar-quantum-gas-against-1uipakxlrc.pdf

Stabilization of a purely dipolar quantum gas against collapse

We report on the realization of a Chromium Bose-Einstein condensate (BEC) with strong dipolar interaction. By using a Feshbach resonance, we reduce the usual isotropic contact interaction, such that the anisotropic magnetic dipole-dipole interaction between 52Cr atoms becomes comparable in strength. This induces a change of the aspect ratio of the cloud, and, for strong dipolar interaction, the inversion of ellipticity during expansion - the usual "smoking gun" evidence for BEC - can even be suppressed. These effects are accounted for by taking into account the dipolar interaction in the superfluid hydrodynamic equations governing the dynamics of the gas, in the same way as classical ferrofluids can be described by including dipolar terms in the classical hydrodynamic equations. Our results are a first step in the exploration of the unique properties of quantum ferrofluids.

Axel Griesmaier

Papers

Bose-Einstein Condensation of Chromium

Observation of Dipole-Dipole Interaction in a Degenerate Quantum Gas

Strong dipolar effects in a quantum ferrofluid

Stabilization of a purely dipolar quantum gas against collapse

A quantum ferrofluid