Ultracold atoms trapped in appropriately prepared optical traps can arrange themselves in complex, hitherto unobserved structures, according to scientists from the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN). In accordance with their recent work, the scientists said, matter in optical lattices should form tensile and inhomogeneous quantum rings in a controlled manner. When two laser beams are matched properly, it is possible to create an optical lattice with well-known properties, the scientists said. By overlapping electromagnetic waves, the minima of potential can be obtained, whose arrangement enables the simulation of systems and models from solid-state physics. The researchers said it is relatively simple to modify the positions of these minima. In practice, this would make it possible to prepare various types of lattices. “If we introduce appropriately selected atoms into an area of space that has been prepared in this way, they will congregate in the locations of potential minima. However, there is an important condition — the atoms must be cooled to ultralow temperatures. Only then will their energy be small enough not to break out of the subtle prepared trap,” professor Andrzej Ptok said. Ultracold atoms caught in an optical trap form surprisingly complex structures. Dependent on mutual interactions between particles with opposite spins, phases with various properties can be created locally. Courtesy of IFJ PAN. For the experiments, groups of fermions — atoms with a spin of 1/2 — were trapped in optical lattices. In each site where fermion groups were placed, a certain number of the atoms had the spin oriented in one direction (up), while the rest were oriented in the opposite direction (down). Modifying the interaction between atoms in such a way led to the creation of pairs of atoms with opposite spins in the same lattice site. “The parameters of the optical lattice can be used to influence the interaction between atoms of different spin trapped in individual sites,” professor Konrad J. Kapcia said. “Moreover, in such [a] way a state can be prepared which mimics applied external magnetic fields on the system. It is given by control[ling] the proportions between the numbers of atoms of different spin.” Kapcia said that systems prepared in this way can reproduce the effects of relatively large magnetic fields without needing to actually use magnetic fields. According to the predictions of the research team, a phase separation should take place in systems prepared in this manner. As a result, a core-shell structure formed by matter trapped in an optical lattice — a core of paired atoms of one phase, surrounded by a shell of paired atoms of the second phase — should automatically form. Ptok used a plate with two types of food as an example: “Imagine a plate of rice with a thick sauce. By proper preparation of the plate, we can affect the relative position between the rice and the sauce. For example, we can prepare [the] system in such way that the rice will be in the center, while the sauce forms a ring around it. From the same ingredients we can also construct the reverse system — in the middle of the plate there will be the sauce surrounded by a ring of the rice. In our case, the plate is the optical trap with atoms and their pairs, and the rice and sauce are the two phases, grouping different types of atom pairs.” Structures formed by atoms (or groups of atoms) trapped in an optical lattice resemble crystals. Depending on the configuration of the laser beams, these structures can be one-, two-, or three-dimensional. Unlike crystals, they are defect-free. While in crystals the possibility of modifying the structure of the lattice is negligible, optical lattices are easy to configure. All that is needed is to change the properties of the laser light or the cutting angles of the beams. These features make optical lattices popular as quantum simulators. They can be used to reproduce various spatial configurations of atoms or groups of atoms, including configurations that do not exist in nature. Although the work is theoretical, the researchers said that systems of ultracold atoms in optical traps can be quickly verified in laboratory experiments. The IFJ PAN team predicted that ultracold atoms trapped in optical lattices could form quantum rings with an inhomogeneous structure. The research was published in the Journal of Physics Communications (www.doi.org/10.1088/2399-6528/ab8f02) and in Scientific Reports (www.doi.org/10.1038/s41598-019-42044-w).