Ghost imaging with atoms

Ghost imaging is a counter-intuitive phenomenon-first realized in quantum optics(1,2)-that enables the image of a two-dimensional object (mask) to be reconstructed using the spatio-temporal properties of a beam of particles with which it never interacts. Typically, two beams of correlated photons are used: one passes through the mask to a single-pixel (bucket) detector while the spatial profile of the other is measured by a high-resolution (multi-pixel) detector. The second beam never interacts with the mask. Neither detector can reconstruct the mask independently, but temporal cross-correlation between the two beams can be used to recover a 'ghost' image. Here we report the realization of ghost imaging using massive particles instead of photons. In our experiment, the two beams are formed by correlated pairs of ultracold, metastable helium atoms(3), which originate from s-wave scattering of two colliding Bose-Einstein condensates(4,5). We use higher-order Kapitza-Dirac scattering(6-8) to generate a large number of correlated atom pairs, enabling the creation of a clear ghost image with submillimetre resolution. Future extensions of our technique could lead to the realization of ghost interference(9), and enable tests of Einstein-Podolsky-Rosen entanglement(9) and Bell's inequalities(10) with atoms.