We report the detection of 21 cm emission at an average redshift $\bar{z}=2.3$ in the cross-correlation of data from the Canadian Hydrogen Intensity Mapping Experiment (CHIME) with measurements of the Ly 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 _∥ ≲ 0.13 Mpc ^−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 Ly α forest flux transmission spectra to estimate the 21 cm–Ly α cross-correlation function. Fitting a simulation-derived template function to this measurement results in a 9 σ 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 21 cm emission to date, and they set the stage for future 21 cm intensity mapping analyses at z > 1.8.
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