Periodic boundary conditions are an indispensable tool in theoretical studies, allowing researchers to not only simplify calculations but also to explore profound concepts. Imagining how to implement these boundary conditions in physical systems can be especially fruitful. A prototypical example is a 1981 thought experiment devised by the Nobel-prize-winning physicist Robert Laughlin to explain the quantization of Hall conductance [1]. This thought experiment introduced the idea of Laughlin’s charge pump, which has so far remained unrealized. Now, Aurélien Fabre, from the Kastler Brossel Laboratory, France, and his colleagues have created a “synthetic Hall cylinder” for dysprosium atoms that realizes Laughlin’s charge pump, allowing the team to experimentally observe features of this system [2].
In his thought experiment, Laughlin considered quantum Hall states on a cylinder whose surface and two ends are penetrated by finite magnetic fluxes. Laughlin argued that changing the axial magnetic flux would result in a current along the axial direction such that charges would be pumped along the cylinder’s surface. The most important characteristic of Laughlin’s charge pump is that it is topological: If the rate of change of the axial flux is slow enough for the system to be adiabatic, the details of its time dependence don’t matter—the number of pumped electrons always increases by an integer number when the axial flux changes by one quantum.
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