  Tunnel-coupled pairs of optically active quantum dots - quantum dot molecules
(QDMs) - offer the possibility to combine excellent optical properties such as
strong light-matter coupling with two-spin singlet-triplet ($S-T_0$) qubits
having extended coherence times. The $S-T_0$ basis formed using two spins is
inherently protected against electric and magnetic field noise. However, since
a single gate voltage is typically used to stabilize the charge occupancy of
the dots and control the inter-dot orbital couplings, operation of the $S-T_0$
qubits under optimal conditions remains challenging. Here, we present an
electric field tunable QDM that can be optically charged with one (1h) or two
holes (2h) on demand. We perform a four-phase optical and electric field
control sequence that facilitates the sequential preparation of the 2h charge
state and subsequently allows flexible control of the inter-dot coupling.
Charges are loaded via optical pumping and electron tunnel ionization. We
achieve one- and two-hole charging efficiencies of 93.5 $\pm$ 0.8 % and 80.5
$\pm$ 1.3 %, respectively. Combining efficient charge state preparation and
precise setting of inter-dot coupling allows control of few-spin qubits, as
would be required for on-demand generation of two-dimensional photonic cluster
states or quantum transduction between microwaves and photons.
