We report the detection of 21 cm emission at an average redshift zˉ=2.3 in the cross-correlation of data from the Canadian Hydrogen Intensity
Mapping Experiment (CHIME) with measurements of the Lyman-α forest from
eBOSS. Data collected by CHIME over 88 days in the 400−500~MHz frequency band
(1.8<z<2.5) are formed into maps of the sky and high-pass delay filtered
to suppress the foreground power, corresponding to removing cosmological scales
with k∥≲0.13Mpc−1 at the average redshift.
Line-of-sight spectra to the eBOSS background quasar locations are extracted
from the CHIME maps and combined with the Lyman-α forest flux
transmission spectra to estimate the 21 cm-Lyman-α cross-correlation
function. Fitting a simulation-derived template function to this measurement
results in a 9σ detection significance. The coherent accumulation of the
signal through cross-correlation is sufficient to enable a detection despite
excess variance from foreground residuals ∼6−10 times brighter than the
expected thermal noise level in the correlation function. These results are the
highest-redshift measurement of \tcm emission to date, and set the stage for
future 21 cm intensity mapping analyses at z>1.8