MAGIS-100: Towards atom interferometry over a 100m baseline*

The upcoming MAGIS-100 experiment is under construction in the MINOS access shaft at Fermilab.  We plan to exploit its 100m baseline to search for ultralight dark matter candidates; it will also be a pathfinder for future gravitational wave detectors operating in the mid-band.  This frequency range, from 10 mHz to 3 Hz, spans the gap in sensitivity between LIGO and LISA, and is the optimal frequency range for sky localization to support multimessenger astronomy.  Furthermore, MAGIS-100 aims to search for ultralight dark matter with both scalar and vector couplings over a mass range spanning several decades.  Its long baseline enables tests of macroscopic quantum mechanics at new scales, including wavepacket separation over a meter and with an interferometer duration up to 9 seconds.  We will describe the design of the detector, focusing on how it will enable the pursuit of these science signals.