Bloch Oscillations in the Absence of a Lattice
Concept of the experiment, A: an ensemble of 1D Bose gases in tubes formed by two pairs of counterpropagating and interfering laser beams. In each tube, a single strongly interacting impurity (green sphere) is immersed in the correlated host gas (black spheres) and is accelerated by gravity (green arrow). Inset: Scattering length as for collisions between the atoms in the host gas (dashed line) and between the impurity and the host atoms (solid line) as a function of the magnetic field B.* [Reprinted by permission from AAAS ©2017.]
In the quantum world our intuition for the motion of objects is strongly challenged and may sometimes even completely fail. What about imagining a marble falling through water oscillating up and down rather than just moving straight downwards? Sounds strange. Yet, that's what experimental physicist from Innsbruck in collaboration with Prof. Eugene Demler and theorist colleagues from Technical University of Munich and Université Paris-Saclay discovered for a quantum particle. At the heart of this surprising behavior is what physicists call 'quantum interference', the fact that quantum mechanics allows particles to behave like waves, which can add up or cancel each other.
To observe the quantum particle oscillating back and forth the team had to cool a gas of Cesium atoms just above absolute zero temperature and to confine it to an arrangement of very thin tubes realized by high-power laser beams. By means of a special trick, the atoms were made to interact strongly with each other. At such extreme conditions the atoms form a quantum fluid whose motion is restricted to the direction of the tubes. The physicists then accelerated an impurity atom, which is an atom in a different spin state, through the gas. As this quantum particle moved, it was observed to scatter off the gas particles and to reflect backwards. This led to an oscillatory motion, in contrast to what a marble would do when falling in water. The experiment demonstrates that Newton's laws cannot be used in the quantum realm...
Read more at phys.org: "Breaking Newton's Law: Intriguing oscillatory back-and-forth motion of a quantum particle."
*Also read the original Report in Science: F. Meinert, M. Knap, E. Kirilov, K.a Jag-Lauber, M.B. Zvonarev, E. Demler, and H.C. Nägerl, "Bloch oscillations in the absence of a lattice," Science 356 (02 Jun 2017) DOI: 10.1126/science.aah6616.