  Self-assembled optically active quantum dot molecules (QDMs) allow the
creation of protected qubits via singlet-triplet spin states. The qubit energy
splitting of these states is defined by the tunnel coupling strength and is,
therefore, determined by the potential landscape and thus fixed during growth.
Applying an in-plane magnetic field increases the confinement of the hybridized
wave functions within the quantum dots, leading to a decrease of the tunnel
coupling strength. We achieve a tuning of the coupling strength by
$(53.4\pm1.7)$ %. The ability to fine-tune this coupling is essential for
quantum network and computing applications that require quantum systems with
near identical performance.
