ALICE ITS 3: the first truly cylindrical inner tracker

Abstract: The high integration density of MAPS, with silicon sensor and readout electronics implemented in the same device, allows very thin structures with a greatly reduced material budget. Thicknesses of $\mathcal{O}$(50~$\mu$m), values at which silicon chips become flexible, are readily used in many applications. In addition, MAPS can be produced in sensors of wafer size by a process known as stitching. This in turn allows to build detector elements that are large enough to cover full tracker half-layers with single bent sensors. The ALICE ITS~3 project is planning to build a new vertex tracker based on truly cylindrical wafer-scale sensors, with <0.05\% X/X$_{0}$ per layer and located as close as 18 mm to the interaction point. R&D on all project aspects (including mechanics for bent wafer-scale devices, test beams of bent MAPS, design of stitched sensors) is rapidly progressing with the aim for installation during LHC long shutdown 3 (2025--2027). This contribution summarises the project motivation, its R&D schedule, and will show selected highlights of recently accomplished project milestones, including full-scale engineering prototypes with dummy chips and small-scale, fully functional assemblies of functional, bent MAPS.