A brief overview of quantum effects in spatial structures such as nonlinear optical patterns, chains of trapped ions and atoms in optical lattices is presented. Some of the main results of the contributions to this Focus Issue are also briefly described.
Homogeneous, periodic, quasiperiodic, irregular and disordered spatial structures have been at the heart of scientific interest and research for a long time and in many disciplines as diverse as chemistry, physics, biology and morphology. Some recent, spectacular achievements in experimental quantum optics have, however, made it possible to study quantum effects in a variety of spatial structures. The main aim of this Focus Issue is to present some of the most recent theoretical, numerical and experimental progresses in the area of Quantum Structures, i.e. spatial structures that display quantum features. A short background overview of Quantum Structures is provided at the beginning of this Focus Issue, in which individual authors' works are specifically commented. In brief, we can say that the articles can be catalogued into two main areas of research interest in Quantum Structures: quantum effects in spatial structures in nonlinear optics, and in atomic physics.
We theoretically, numerically and experimentally investigate spontaneous transverse instabilities in cold atomic gases, arising from the action of dispersive light forces. Previous research focused on pattern-forming instabilities in hot gases where optical nonlinearities arise from the internal structure of the atoms and spatio-temporal structures are encoded in the populations and coherences of the medium. Dipole forces acting on the center-of-mass of laser-cooled atoms, being dependent on gradients of the optical intensity, are also nonlinear in nature: previous studies focused, for instance, on beam filamentation. Here we investigate the situation where a positive feedback loop is present in the system leading to a pattern-forming instability. We stress that the resulting spatial structures are encoded also in the spatial density distribution, effectively leading to the self-assembly of an optical atomic lattice.