  We engineer a system of two strongly confined quantum dots to gain
reproducible electrostatic control of the spin at zero magnetic field. Coupling
the dots in a tight ring-shaped potential with two tunnel barriers, we
demonstrate that an electric field can switch the electron ground state between
a singlet and a triplet configuration. Comparing our experimental co-tunneling
spectroscopy data to a full many-body treatment of interacting electrons in a
double-barrier quantum ring, we find excellent agreement in the evolution of
many-body states with electric and magnetic fields. The calculations show that
the singlet-triplet energy crossover, not found in conventionally coupled
quantum dots, is made possible by the ring-shaped geometry of the confining
potential.
