  The Liouvillian skin effect describes the boundary affinity of Liouvillian
eignemodes that originates from the intrinsic non-Hermiticity of the
Liouvillian superoperators. Dynamically, it manifests as directional flow in
the transient dynamics, and the accumulation of population near open boundaries
at long times. Intriguingly, similar dynamic phenomena exist in the well-known
process of optical pumping, where the system is driven into a desired state (or
a dark-state subspace) through the interplay of dissipation and optical drive.
In this work, we show that typical optical pumping processes can indeed be
understood in terms of the Liouvillian skin effect. By studying the Liouvillian
spectra under different boundary conditions, we reveal that the Liouvillian
spectra of the driven-dissipative pumping process sensitively depend on the
boundary conditions in the state space, a signature that lies at the origin of
the Liouvillian skin effect. Such a connection provides insights and practical
means for designing efficient optical-pumping schemes through engineering
Liouvillian gaps under the open-boundary condition. Based on these
understandings, we show that the efficiency of a typical side-band cooling
scheme for trapped ions can be dramatically enhanced by introducing
counterintuitive dissipative channels. Our results provide a useful perspective
for optical pumping, with interesting implications for state preparation and
cooling.
