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Fireflies flash in a dark forest: you see each flash, but not the flight. A quantum gas microscope can show individual cold atoms, like those fireflies. But until now, it couldn't tell you how fast they're moving.

Now scientists have found a way to fix that. They propose illuminating the atoms in a special way so that they cast a kind of 'shadow' on neighboring auxiliary particles. From that shadow, you can reconstruct not only the atom's position but also its momentum: that is, the direction and speed of its motion. It's like reading the ripples on water to figure out where a fish swam and how quickly.

The authors propose two operating modes for such a microscope. In the first, 'soft' mode, the instrument measures both position and momentum at once, but with limited accuracy: the uncertainty is split evenly between them. In the second, 'averaged' mode, the instrument shows very precisely how atoms move on average at different points in space, but it doesn't track each atom individually.

This approach opens up new possibilities for studying quantum states of cold atoms. For example, you could see how vortices are born and die in a superfluid gas, or how density waves propagate. It's a step toward understanding the most subtle properties of matter at ultra-low temperatures.

Original abstract

as written by the authors arXiv:2603.29568

Quantum gas microscopes offer unprecedented insights into quantum many-body states of cold atomic gases. Here we introduce concrete protocols for extending quantum gas microscopes to measure in phase space, by mapping momentum onto auxiliary degrees of freedom and using positive operator-valued measures. We distinguish between two distinct operational modes. In the Husimi-Q phase space microscope, position and momentum are jointly measured; in this mode the fundamental quantum noise is distributed between position and momentum. Conversely, the averaged-mode phase space microscope extracts the spatial dependence of averages of the momentum density (and its moments); these averages can be retrieved with arbitrary spatial resolution. We illustrate the utility of these techniques in diverse physical settings.