  The realization of large-scale fully controllable quantum systems is an
exciting frontier in modern physical science. We use atom-by-atom assembly to
implement a novel platform for the deterministic preparation of regular arrays
of individually controlled cold atoms. In our approach, a measurement and
feedback procedure eliminates the entropy associated with probabilistic trap
occupation and results in defect-free arrays of over 50 atoms in less than 400
ms. The technique is based on fast, real-time control of 100 optical tweezers,
which we use to arrange atoms in desired geometric patterns and to maintain
these configurations by replacing lost atoms with surplus atoms from a
reservoir. This bottom-up approach enables controlled engineering of scalable
many-body systems for quantum information processing, quantum simulations, and
precision measurements.
