Multiparticle quantum interference achieved with twelve neutral atoms

When two identical photons meet at a beam splitter, they exit together through the same port. This phenomenon, known as the Hong–Ou–Mandel effect, is a fundamental demonstration of particle indistinguishability. Until recently, extending this collective bunching behavior beyond a few particles had been largely limited to photonic systems, which suffer from loss and noise as the number of particles increases. Physicists have now achieved this collective interference using up to 12 rubidium atoms, providing a clean, two-mode demonstration that scales beyond the limitations of light.

To prepare the experiment, the team at the German Aerospace Center utilized a Bose-Einstein condensate of 250 rubidium atoms. Through spin-changing collisions, they generated a twin-Fock state consisting of equal numbers of atoms in two distinct spin states. Microwave pulses acted as the beam splitter, coherently coupling these states. The challenge in such atomic systems has historically been the detection phase; typical noise levels often exceed the signal of a single atom, rendering exact counts impossible.

Execution required a novel counting method using optical molasses—a geometric arrangement of six laser beams that trap atoms in a high-viscosity field of light. This setup allowed the researchers to slow the atoms and collect enough fluorescence to reach a counting resolution of 0.2 atoms. By and accurately identifying the exact integer number of atoms at the output, the team confirmed that the particles occupied only even-numbered ports, characteristic of genuine many-body interference. This level of control brings the platform to the Heisenberg limit, the absolute floor of uncertainty allowed by nature in precision metrology.