The dance floor is packed: everyone moves on their own, no one holds hands. Suddenly the music gets louder and more rhythmic — neighbors start grabbing each other, forming pairs that spin in sync. That's roughly what the experiment looked like in the lab, where a cloud of lithium atoms, cooled to near absolute zero, was made to suddenly strengthen its mutual attraction.

These atoms behaved like electrons in certain materials that can become superconductors. Before the jolt, they were free and chaotic — physicists call this state a Fermi liquid (an ordinary fluid of particles obeying Fermi statistics). After the sudden increase in coupling, pairs formed, but not like those in a true superconductor: they hadn't yet merged into a single quantum ensemble, but resembled dancers who had just joined hands and were finding a common rhythm. This is the so-called pseudogap phase — a state with pre-formed pairs but no superfluidity.

To spot these pairs, the researchers used a clever trick: they made the pairs merge into denser clumps — doublons (two atoms on one site). These doublons, like markers, revealed where the centers of mass of the original pairs were. Detectors capable of resolving individual atoms recorded the emergence of charge-density waves in the gas — alternating regions of higher and lower particle concentration. Previously these waves were barely visible, but after the jolt their visibility sharply increased, especially in the region where the attraction was strong and the pairs were nonlocal, meaning the atoms in a pair were not at the same spot but separated by a distance.

This method opens the way to observing more exotic forms of pairing order, for example in systems where atomic spins are imbalanced (one species of atoms outnumbers the other). Under such conditions, unusual types of superfluidity may arise that are hard to achieve in solids. Moreover, a similar approach could be applied to another model — the repulsive Hubbard model (where particles repel rather than attract) — to see stripe-like structures reminiscent of charge-density waves in high-temperature superconductors.