Physicists demonstrate how atoms can be pumped through a synthetic crystal without having to apply external periodic driving

Physicists at ETH Zurich demonstrate how atoms can be pumped through a synthetic crystal without having to apply an external periodic drive. These experiments combine several key features of many-body quantum physics in unexpected ways, opening a new avenue for understanding and creating exotic states of quantum matter.

Pumps, in a nutshell, are devices that use cyclical motion to achieve smooth transportation of certain goods. In a bicycle pump, the repeated up and down strokes of a piston create a flow of air. In an Archimedean screw pump, water is transferred between reservoirs by turning a crank. Related ideas have also been explored in quantum systems, in particular for transporting electrons one by one through solid-state materials, thereby generating a quantized current. Now a team led by Dr. Tobias Donner, a senior researcher in the group of Professor Tilman Esslinger from the Department of Physics at ETH Zurich, is adding a surprising twist to the story. Writing in Mother Nature, they report a quantum pump that requires no periodic drive from the outside – a pump that winds up without the crank.

The search for new puzzles

Esslinger and Donner’s team do not works not with electrons in solid-state materials, but rather with atoms confined in complex structures created by the intersection of laser beams. Such synthetic crystals have the advantage that the atoms and crystal lattice can be controlled with exquisite precision and great flexibility. The platform can then be exploited either to better understand known effects or to generate scenarios in which quantum systems behave in unexpected ways, ideally pointing to new phenomena in quantum physics. And that’s precisely what the team achieved in the work now reported.

A key ingredient in their experiment is an optical cavity in which the synthetic crystal is formed. The cavity serves to mediate a coupling between the atoms and the light fields involved. Additionally. over which the experimenters also have fantastic control. Such a system including dissipation is called an open quantum system. Above all, when properly controlled, dissipation can be an asset rather than a nuisance: in 2019. giving rise to a dynamic oscillating between these configurations.

Moving forward while turning in circles

The big surprise that led to the now published work was the experimental observation that atoms trapped in the synthetic crystalline composition began to move. By taking several measurements and performing numerical simulations, the researchers identified the mechanism behind the atomic movement: the synthetic crystal periodically wound itself between different constructions, so that the center of mass of the atoms was shifted in the space of a fixed amount each cycle — in an intriguing analogy to the upward chiral motion in an Archimedean pump. By carefully analyzing the light field escaping from the cavity, ETH physicists obtained detailed insights into the mechanism and characterized the interplay between cavity dissipation and quantized pumping.

Who turns the crank?

What is unique about these experiments compared to previous realizations of quantum pumps – and unlike how we imagine a pump in general – is that a stream of particles is observed without any command external periodical. What drives the current is the dissipation of the cavity, leading to “car-oscillating” pumping. In this context, it is important that the configurations of atoms between which the system oscillates are distinct at a very fundamental level, in that they possess different so-called topologies. Concretely, this means that the transport mechanism demonstrated must be stable against external perturbations and also robust with respect to the detailed shape of the pumping protocol.

These are exciting findings . Both topology and open quantum systems are very active areas of modern physics. The connection between the two promises to provide not only a testbed for quantum many-body theory, but also a practical tool for realizing exotic states of quantum matter.

2019

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