  We report an experimental demonstration of optical 2DCS in cold atoms. The
experiment integrates a collinear 2DCS setup with a magneto-optical trap (MOT),
in which cold rubidium (Rb) atoms are prepared at a temperature of about 200
$\mu$K and a number density of $10^{10}$ cm$^{-3}$. With a sequence of
femtosecond laser pulses, we first obtained one-dimensional second- and
fourth-order nonlinear signals and then acquired both one-quantum and
zero-quantum 2D spectra of cold Rb atoms. The capability of performing optical
2DCS in cold atoms is an important step toward optical 2DCS study of many-body
physics in cold atoms and ultimately in atom arrays and trapped ions. Optical
2DCS in cold atoms/molecules can also be a new avenue to probe chemical
reaction dynamics in cold molecules.
