  The South Pole Telescope Summertime Line Intensity Mapper (SPT-SLIM) is a
pathfinder experiment that will demonstrate the use of on-chip filter-bank
spectrometers for mm-wave line intensity mapping (LIM). The SPT-SLIM focal
plane consists of 18 dual-polarization R=300 filter-bank spectrometers covering
120-180 GHz, coupled to aluminum kinetic inductance detectors. A compact
cryostat holds the detectors at 100 mK and performs observations without
removing the SPT-3G receiver. SPT-SLIM will be deployed to the 10-m South Pole
Telescope for observations during the 2023-24 austral summer. We discuss the
overall instrument design, expected detector performance and sensitivity to the
LIM signal from CO at 0.5 < z < 2. The technology and observational techniques
demonstrated by SPT-SLIM will enable next-generation LIM experiments that
constrain cosmology beyond the redshift reach of galaxy surveys.
