Physicists have found that a one-dimensional Mott insulator can still show clear interference peaks — a wave-like signal usually taken as a sign of superfluidity. The team loaded strongly interacting ultracold atoms into a shallow optical lattice and measured momentum patterns after release. Even deep in the insulating regime, pronounced side peaks appeared, and these peaks grew stronger as the fraction of the sample in the Mott state increased.

The experiment began with a Bose–Einstein condensate of about 1.2×10^5 cesium atoms. The researchers made a two-dimensional array of independent one-dimensional tubes by keeping the lattice deep in the transverse directions and changing the longitudinal lattice depth Vx from 0 to 20 recoil energies (Er). They tuned the atom–atom interactions using a Feshbach resonance to reach a dimensionless interaction parameter γ ≈ 2.8, a regime where theory predicts a lattice pinning transition near 0.7 Er. After holding the atoms at each lattice depth, they suddenly switched off the traps and took time-of-flight images to record the momentum distribution n(k). At intermediate lattice depth (for example Vx = 5 Er), n(k) showed narrow side peaks at ±2kL in addition to the central peak.

To check that the system was really a gapped Mott insulator and not simply a leftover superfluid, the team did lattice-modulation spectroscopy. For weak interactions the heating response was nearly linear with modulation frequency, as expected for a gapless superfluid. For the strong-interaction case (γ = 2.8) the heating stayed weak below a threshold and rose sharply above it. From these data they extracted an excitation gap of about 0.52(8) kHz at one lattice setting. In parallel, quantum Monte Carlo (QMC) simulations — both for a homogeneous one-dimensional box near zero temperature and for trapped systems including finite temperature — reproduced interference peaks even for an almost pure Mott state.