  Two-dimensional hole gases in semiconductor quantum wells are promising
platforms for spintronics and quantum computation but suffer from the lack of
the $\bf{k}$-linear term in the Rashba spin-orbit coupling (SOC), which is
essential for spin manipulations without magnetism and commonly believed to be
a $\bf{k}$-cubic term as the lowest order. Here, contrary to conventional
wisdom, we uncover a strong and tunable $\bf{k}$-linear Rashba SOC in
two-dimensional hole gases (2DHG) of semiconductor quantum wells by performing
atomistic pseudopotential calculations combined with an effective Hamiltonian
for a model system of Ge/Si quantum wells. Its maximal strength exceeds 120
meV{\AA}, comparable to the highest values reported in narrow bandgap III-V
semiconductor 2D electron gases, which suffers from short spin lifetime due to
the presence of nuclear spin. We also illustrate that this emergent
$\bf{k}$-linear Rashba SOC is a first-order direct Rashba effect, originating
from a combination of heavy-hole-light-hole mixing and direct dipolar
intersubband coupling to the external electric field. These findings confirm
Ge-based 2DHG to be an excellent platform towards large-scale quantum
computation.
