Neutral atoms in optical tweezer arrays have recently emerged as one of the most versatile platforms for quantum many-body physics, quantum simulation and computation, where high quality qubits, fast quantum logic gates, quantum entanglement have been demonstrated. So far most of the experimental efforts focus on creating fully occupied arrays with a single atom per tweezer by exploiting light-assisted inelastic collisions and rearrangement to fill empty sites. However, achievable array sizes are limited to ~100 occupied sites due to high power requirements per tweezer, finite trap lifetime, and the increasing complexity associated with the rearrangement process for larger arrays.
Here we report a method to overcome this scalability challenge which involves transferring ultracold atoms from a 2D pancake shaped reservoir trap into a large array of optical tweezers produced by a digital micromirror device (DMD) in a single step. We successfully demonstrate, for the first time, fully filled atomic arrays of over 400 tweezers, where each tweezer contains an atomic ensemble (as opposed to single atoms) with microscopic dimensions, controllable atom number and surprisingly small atom fluctuation. We show such a system is well suited for studying quantum spin models, dynamics in novel geometries and realizing collective qubits with enhanced atom-light interactions, or as an starting point for more deterministic single atom preparation schemes.