OP: Quantum Phases and Dynamics of Bose-Einstein Condensates with Artificial Gauge Fields

Washington State University, Pullman WA

Investigators

Abstract

This project investigates fundamental laws of nature predicted by quantum mechanics, one of the cornerstones of modern physics. The predictions of quantum mechanics are very counterintuitive and unfamiliar compared to the experiences of everyday life, but they play a dominant role when spatial dimensions get small, e.g. at length scales reached using modern nanofabrication or computer chip fabrication techniques, and on atomic scales. Conducting experiments on the nanoscopic length scale poses severe technical difficulties, but an alternative approach exists: the same laws of quantum mechanics can also be studied when a gas of atoms is cooled down to temperatures near absolute zero. At these temperatures, fundamental quantum lengths become larger and details about these ultracold quantum gases can be imaged in a custom microscope, allowing their properties to be examined in very flexible ways using tools from the realm of atomic physics. The particular predictions investigated in this project concern a property called spin-orbit coupling. In the vast majority of existing electronic devices, the transport of electrically charged particles (i.e., electrons) is exploited to perform a device function. However, electrons have an additional property called spin, which can be pictured as a fast spinning motion akin to that of a spinning top. The orientation of this rotation can also be used to perform a device function, leading to "spintronic" devices. Spin-orbit coupling denotes an interplay between this spinning motion and the linear flow of particles that constitutes a conventional current. It is the basic building block of many proposed future devices and of advanced materials with exotic properties. The underlying physics are rather complex, and the experiments of this project provide an important test bed with which new concepts are explored. Over the recent years, immersing ultracold atoms into suitably tailored laser beams has emerged as a powerful tool to investigate quantum dynamics. The different hyperfine states coupled by the lasers can be considered orientations of a "pseudo" spin, so that absorption and emission of a photon constitutes a spin flip. At the same time, due to momentum conservation, the atom's motion is changed when a photon is absorbed or emitted. As a result, the pseudospin and the motion of an atom become coupled in a laser-driven transition, in analogy to spin-orbit coupling known from condensed matter physics. A suitable dressing of atoms with laser light can also lead to artificial gauge fields and artificial magnetic and electric fields, so that charged-particle behavior can be investigated with neutral atomic quantum gases. Furthermore, the dispersion relation of the atoms can be modified to exhibit intriguing double-well structures showing roton-like minima. Over the previous grant period, such techniques have been applied to a rubidium Bose-Einstein condensate to investigate quantum phases and quantum dynamics in spin-orbit coupled condensates. Capitalizing on the insights gained and technological developments performed in the previous grant period, this project will explore new frontiers that challenge current theoretical descriptions. Aspects include novel quantum phases induced by spin-orbit coupling and by the interplay with optical lattices, as well as the dynamics of solitons in a spin-orbit coupled environment and topological structures in higher dimensions.