Researchers at the Institute of Physics Zagreb and the Faculty of Science have experimentally demonstrated discrete one-dimensional solitons in an optically induced photonic lattice within warm rubidium vapor. Published in Laser & Photonics Reviews, the work shows how adjusting beam power stabilizes light beams against diffraction, creating new prospects for tunable quantum optics.
Scientists from the Quantum Technologies Group at the Institute of Physics Zagreb observed discrete one-dimensional solitons inside a warm rubidium vapor cell. They worked alongside a colleague from the Physics Department at the Faculty of Science (PMF), according to Phys. Publishing their findings in Laser & Photonics Reviews, the researchers used a tunable atomic medium shaped by intersecting laser beams.
Photonic lattices are periodic structures through which light travels. This mimics how electrons move through crystals, a principle studied in solid-state waveguide arrays and photorefractive crystals. Conventional implementations rely on fixed physical structures like semiconductors or lithium niobate. The Zagreb team utilized an atomic vapor cell as a dynamic, tunable environment.
Beams Shape the Rubidium Vapor Lattice
The experimental setup relies on two coupling laser beams. They intersect at a small angle inside a glass cell filled with 87Rb atoms and heated to 100 degrees Celsius, according to Arxiv. This interference pattern creates alternating bright and dark fringes with a small lattice period of less than 50 micrometers. Atoms travel ballistically through this grid, leading to transit time broadening. The broadening rate is approximately 2 pi times 1.3 MHz for a 37-micrometer lattice at 100 degrees Celsius, as detailed in Arxiv.
Phys reports that an additional probe laser beam is focused onto a single lattice site to monitor propagation. At low intensity, the probe light spreads into neighboring sites through discrete diffraction. To model this atomic response, the research team developed a theoretical framework. It couples the paraxial wave equation with optical Bloch equations that account for the full hyperfine structure of the rubidium atoms under electromagnetically induced transparency conditions.
Solitons Form at Six Milliwatts
As the power of the probe beam increases, the response of the rubidium vapor transitions into a nonlinear regime. The light self-focuses and counterbalances the spreading effects of discrete diffraction, according to Phys. Arxiv notes that both the coupling and probe fields are detuned by approximately 1 GHz from the single-photon resonance.
As probe power scales upward from 0.5 mW to 6.0 mW, distinct self-focusing emerges. At approximately 6.0 milliwatts, the team observed a stable 1D discrete soliton. This is a localized beam whose transverse profile remains nearly unchanged across its propagation through the 50-millimeter vapor cell, Phys states.
Platform Potential for Quantum Systems
Optical lattice parameters can be modified by altering beam frequencies, intensities, and other experimental parameters. Warm atomic vapor offers a flexible platform for studying resonant light-matter interactions. Phys notes that this flexibility opens new research avenues in non-Hermitian nonlinear dynamics, parity-time symmetry, dissipative solitons, and quantum fluids of light. In the longer term, these insights may inform the development of quantum memories, atomic clocks, and chip-scale optical devices.