  Spin qubits defined in carbon nanotube quantum dots are of considerable
interest for encoding and manipulating quantum information because of the long
electron spin coherence times expected. However, before carbon nanotubes can
find applications in quantum information processing schemes, we need to
understand and control the coupling between individual electron spins and the
interaction between the electron spins and their environment. Here we make use
of spin selection rules to directly measure - and demonstrate control of - the
singlet-triplet exchange coupling between two carbon nanotube quantum dots. We
furthermore elucidate the effects of spin-orbit interaction on the electron
transitions and investigate the interaction of the quantum dot system with a
single impurity spin - the ultimate limit in spin sensitivity.
