An atom-by-atom assembler of defect-free arbitrary 2d atomic arrays
  Large arrays of individually controlled atoms trapped in optical tweezers are a very promising platform for quantum engineering applications. However, to date, only disordered arrays have been demonstrated, due to the non-deterministic loading of the traps. Here, we demonstrate the preparation of fully loaded, two-dimensional arrays of up to 50 microtraps each containing a single atom, and arranged in arbitrary geometries. Starting from initially larger, half-filled matrices of randomly loaded traps, we obtain user-defined target arrays at unit filling. This is achieved with a real-time control system and a moving optical tweezers that performs a sequence of rapid atom moves depending on the initial distribution of the atoms in the arrays. These results open exciting prospects for quantum engineering with neutral atoms in tunable geometries. 
Daniel Barredo, Sylvain de L\'es\'eleuc, Vincent Lienhard, Thierry   Lahaye and Antoine Browaeys
quant-ph cond-mat.quant-gas physics.atom-ph