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Geometrical Pumping with a Bose-Einstein Condensate

Frame from a movie demonstrating the periodic motion that drives shifts in the position of an atomic cloud. Credit: S. Kelley/NIST

Pumps have been around for millennia, but recently physicists have sought to build a different kind of pump—one that could use the rules of quantum mechanics to pump atoms or charges in a quantized way. PFC-supported researchers have created the first pump based on the geometry of quantum physics.

By periodically jostling many individual atoms, the researchers were able to shift an entire BEC without any apparent overall motion by the individual atoms. They are the first to test this predicted behavior, which arises in a geometric charge pump.

The idea of a quantum pump dates back decades and relies on the quantum phase that a system picks up when it is driven around a closed loop in parameter space. The system accumulates a different phase depending on the path it takes—a phenomenon known as Berry’s phase. The local curvature along the path leaves an imprint on the quantum system. 

In the new pump, this curvature comes from energy differences that atoms experience over time. Each atom in the BEC sees an energy landscape with a left and a right well. The repeating pattern of wells, all arranged in a line, is generated by interfering lasers at just the right frequency and power to trap atoms with the right quantum properties.

Each atom initially sits in its left well, which starts out as a deeper trough. As time increases, the left well becomes shallower and the right well gets deeper, causing the atoms to spill toward the right. To complete the cycle, the change happens in the other direction, and the atoms tip back into the left well. In the end, the density within each well is the same as when it started. Despite this, the entire cloud of atoms has been shifted by a small amount—smaller, in fact, than the distance between lattice sites.

H.-I Lu, M. Schemmer, L. M. Aycock, D. Genkina, S. Sugawa, and I. B. Spielman
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