Dipolar physics: a review of experiments with magnetic quantum gases

Abstract

Since the achievement of quantum degeneracy in gases of chromium atoms in 2004, the experimental investigation of ultracold gases made of highly magnetic atoms has blossomed. The field has yielded the observation of many unprecedented phenomena, in particular those in which long-range and anisotropic dipole-dipole interactions play a crucial role. In this review, we aim to present the aspects of the magnetic quantum-gas platform that make it unique for exploring ultracold and quantum physics as well as to give a thorough overview of experimental achievements. Highly magnetic atoms distinguish themselves by the fact that their electronic ground-state configuration possesses a large spin (as well as a large $g$ factor). This results in a large magnetic moment and a rich electronic transition spectrum. Such transitions are useful for cooling, trapping, and manipulating these atoms. The complex atomic structure and large dipolar moments of these atoms also lead to a dense spectrum of resonances in their two-body scattering behaviour. These resonances can be used to control the interatomic interactions and, in particular, the relative importance of contact over dipolar interactions. These features provide exquisite control knobs for exploring the few- and many-body physics of dipolar quantum gases. The study of dipolar effects in magnetic quantum gases has covered various few-body phenomena that are based on elastic and inelastic anisotropic scattering. Various many-body effects have also been demonstrated. These affect both the shape, stability, dynamics, and excitations of fully polarised repulsive Bose or Fermi gases. Beyond the mean-field instability, strong dipolar interactions competing with slightly weaker contact interactions between magnetic bosons yield new quantum-stabilised states, among which are self-bound droplets, droplet assemblies, and supersolids. Dipolar interactions also deeply affect the physics of atomic gases with an internal degree of freedom as these interactions intrinsically couple spin and atomic motion. Finally, long-range dipolar interactions can stabilise strongly correlated excited states of 1D gases and also impact the physics of lattice-confined systems, both at the spin-polarised level (Hubbard models with off-site interactions) and at the spinful level (XYZ models). In the present manuscript, we aim to provide an extensive overview of the various related experimental achievements up to the present.

Spin squeezing from bilinear spin-spin interactions: Two simple theorems

“…While rotor and spin waves should eventually come to thermalize with each other thanks to their residual coupling, the time scales over which such thermalization occurs are currently unknown to us. Our findings suggest that effective rotor/spin-wave decoupling represents the mechanism by which a very large class of power-law interacting Hamiltonians implemented by quantum simulators -including Rydberg atoms [8], magnetic atoms [36], trapped ions [11], superconducting circuits [22] -can evade standard thermalization at low energy. And, by virtue of this mechanism, they can act as entangling resources producing scalable multipartite entanglement of interest for fundamental studies, as well as for potential metrological applications.…”

“…We then apply the RSW approach to the relevant case of the dipolar S = 1/2 XX model, corresponding to ∆ = 0 and J ij = J|r i − r j | −3 , in the case of a square lattice. This model is literally realized in Rydberg-atom arrays with resonant interactions [8,34], but many more platforms are described by XXZ dipolar models [35,36]. Fig.…”

Field-linked resonances of polar molecules

“…The anisotropic nature of such a dipolar superfluid gives rise to new quantum phenomena such as gapless superfluidity 47 and topological p x + ip y symmetry 48 . In a Bose-Einstein condensate (BEC), independent control over the s-wave scattering length and dipolar length has led to the observation of self-bound droplets and the formation of supersolids in magnetic atoms 49 . Making use of FL resonance with bosonic polar molecules, the dipolar lengths of which are orders of magnitude larger than magnetic atoms, would enable the study of such exotic phenomena in entirely unexplored regimes 16 .…”

Globally Accurate Gaussian Process Potential Energy Surface and Quantum Dynamics Studies on the Li(2S) + Na2 → LiNa + Na Reaction at Low Collision Energies

“…In recent years, preparing cold and ultracold molecules have received increasingly intensive attention owing to their important applications in quantum simulation, quantum many-body physics, quantum information processing and other cutting-edge fields [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 ]. In the ultracold regime, the reactive collisions between molecules are mainly controlled by quantum effects such as resonances, tunneling and quantum interference, since the translational de Broglie wavelength is much larger than the range of interaction potential.…”