Quantum entanglement — a phenomenon where the states of two or more particles become interconnected regardless of the distance between them — usually links intrinsic properties such as spin or polarization. However, a new experiment has for the first time realized entanglement of the positions of two atoms in space. Imagine a pair of dancers: one can move only if the partner freezes — but who takes the step is not predetermined due to fundamental quantum uncertainty. This analogy simplifies the essence: in reality, it is a superposition of states where the atoms share a single quantum state, yet their individual positions remain undefined until measurement.

The key to the experiment is the use of so-called giant atoms. Using precision laser spectroscopy — a method that allows precise control of energy transitions in atoms — an electron is promoted to a distant orbit, and the atom “inflates,” increasing in size thousands of times. Such Rydberg atoms experience strong mutual repulsion, so they cannot both be excited simultaneously: if one becomes a giant, the other remains in the ground state. In the experiment, a laser pulse traveling at the speed of light — the ultimate speed of signal transmission in vacuum nudged one of the atoms. If it transitioned to the giant state, the other froze, and vice versa. This created entanglement of positions: one atom is displaced in space, the other is not, but which one remains uncertain until measurement.

The distance between possible atomic positions is hundreds of microns — on atomic scales, this is equivalent to separating two people by kilometers. Such spatial entanglement opens prospects for hybrid quantum systems: it could serve as a bridge between stationary quantum elements, such as atoms or ions, and moving particles of light — photons — which is crucial for future quantum networks.