Spontaneous light-mediated magnetism in cold atoms

Abstract

Cold atom setups are now commonly employed in simulations of condensed matter phenomena. We present an approach to induce strong magnetic interactions between atoms on a self-organized lattice using diffraction of light. Diffractive propagation of structured light fields leads to an exchange between phase and amplitude modulated planes which can be used to couple atomic degrees of freedom via optical pumping nonlinearities. In the experiment a cold cloud of Rb atoms placed near a retro-reflecting mirror is driven by a detuned pump laser. We demonstrate spontaneous magnetic ordering in the Zeeman sublevels of the atomic ground state: anti-ferromagnetic structures on a square lattice and ferrimagnetic structures on a hexagonal lattice in zero and a weak longitudinal magnetic field, respectively. The ordered state is destroyed by a transverse magnetic field via coherent dynamics. A connection to the transverse (quantum) Ising model is drawn.

Inversion-symmetry breaking in spin patterns by a weak magnetic field

“…In this paper, we study the phenomenon of self-organization arising in an optically nonlinear sample due to diffractive coupling via single mirror feedback [20,21]. While initial observations of these structures were performed in liquid crystals and warm atomic vapors [22,23], the scheme was recently extended to thermal cold gases, where the nonlinearity can be of optomechanical, electronic saturable, or magnetic origin, with corresponding structuring of atomic density, optical coherence, and magnetization, respectively [5,6,[24][25][26]. These systems are interesting as they have a single pump axis and hence allow for spontaneous symmetry breaking of the translational and rotational degrees of freedom in the plane transverse to this axis, whereas in systems with multiple distinguished axes (e.g., a cavity axis and a pump axis), the potential symmetries and realizations are constrained.…”

“…As in Refs. [25,26], in this article the relevant degrees of freedom are populations and coherences in the ground-state Zeeman sublevels and the optical nonlinearity is provided by optical pumping, i.e., is magnetic optical in origin. The resulting instability creates both transverse spatial modulations of the atomic spins and the polarization profile of the laser beam.…”

“…In Ref. [25], an analogy was established between this system and the Ising model. Diffractive ripples in the feedback field caused by a local perturbation of the atomic magnetization lead to optical pumping in the same direction one lattice period away and opposite direction half a lattice period away, leading to the antiferromagnetic coupling.…”

“…In Refs. [25,26], it was hypothesized that the symmetry breaking at large |B z | is due to the linear and the incoherent nonlinear Faraday effect, but that at small |B z | the coupling of the dipole states to higher multipoles via coherent effects is important. In this article, we provide a systematic study of the variation of pattern properties with the longitudinal magnetic field B z , outlined in Ref.…”

Cavity-Quantum-Electrodynamical Toolbox for Quantum Magnetism

“…This is a remarkable feature of cavity-mediated long-range spin interactions, similar to dipolar interactions between polar molecules [35]. The first step in this direction has been taken very recently by the experimental realization of density and spin self-ordering with ultracold bosonic atoms inside an optical cavity [44,45], and with thermal atoms near a retroreflecting mirror [46]. These experiments have basically realized a long-range Heisenberg model, with an emergent domain-wall AFM order.…”