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Physical Review A

Ground-state coherence versus orientation: Competing mechanisms for light-induced magnetic self-organization in cold atoms

G. Labeyrie, J. G. M. Walker, Gordon R. M. Robb, Robin Kaiser, T. Ackemann

Published Feb 7, 2022• 2 citations
Publisher

Abstract

The authors study experimentally and numerically magnetic self-organized phases that appear when a cloud of cold rubidium atoms is illuminated by a retroreflected laser beam. Phase transitions between a driven-dissipative ground-state coherence phase and a square-symmetry antiferromagnetic phase are observed, providing insights into self-organization mechanisms.

DOI
10.1103/physreva.105.023505

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The authors study experimentally and numerically magnetic self-organized phases that appear when a cloud of cold rubidium atoms is illuminated by a retroreflected laser beam. Phase transitions between a driven-dissipative ground-state coherence phase and a square-symmetry antiferromagnetic phase are observed, providing insights into self-organization mechanisms.
Laser-driven cold atoms near a plane retroreflecting mirror exhibit self-organization above a pump threshold. We analyze the properties of self-organized spin patterns in the ground state of cold rubidium atoms. Antiferromagnetic patterns in zero magnetic field give way to ferrimagnetic patterns if a small longitudinal field is applied. We demonstrate how the experimental system can be modeled as spin-1 atoms diffractively coupled by the light reflected by the mirror. The roles of both dipolar and quadrupolar magnetization components in determining the threshold and symmetry variations with a weak longitudinal magnetic field are examined. Although the magnetic structures correspond dominantly to a lattice of magnetic dipoles, the symmetry breaking to ferrimagnetic structures in a finite field is mediated by the coupling to a homogenous quadrupole (alignment), which is not possible in a spin-1/2 system. Our study provides a basis for further exploration of instabilities in driven multilevel systems with feedback.
When applying a red-detuned retro-reflected laser beam to a large cloud of cold atoms, we observe the spontaneous formation of 2D structures in the transverse plane corresponding to high contrast spatial modulations of both light field and atomic spins. By applying a weak magnetic field, we explore the rich resulting phase space and identify specific phases associated with both dipolar and quadrupolar terms of the atomic magnetic moment. In particular we demonstrate spontaneous structures in optically induced ground state coherences representing magnetic quadrupoles.
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.
Laser driven cold atoms in front of a retro-reflecting mirror exhibit self-organization above pump threshold. It was recently demonstrated how this system can be used to mimic a transverse lattice of interacting spins. The spontaneously formed lattice breaks both translational and rotational symmetries of the initial homogeneous spin state. By applying a constant longitudinal magnetic field of |Bz| ~ 100 mG a switching between antiferromagnetic square and ferri-magnetic hexagonal pattern ordering can be induced. We study the coupling between this magnetic field and modulations in the atomic magnetization.
Cold atoms, driven by a laser and simultaneously coupled to the quantum field of an optical resonator, may self-organize in periodic structures. These structures are supported by the optical lattice, which emerges from the laser light they scatter into the cavity mode and form when the laser intensity exceeds a threshold value. We study theoretically the quantum ground state of these structures above the pump threshold of self-organization by mapping the atomic dynamics of the self-organized crystal to a Bose-Hubbard model. We find that the quantum ground state of the self-organized structure can be the one of a Mott insulator, depending on the pump strength of the driving laser. For very large pump strengths, where the intracavity-field intensity is maximum and one would expect a Mott-insulator state, we find intervals of parameters where the phase is compressible. These states could be realized in existing experimental setups.