  Qubits based on the singlet (S) and the triplet (T0, T+) states in double
quantum dots have been demonstrated in separate experiments. It has been
recently proposed theoretically that under certain conditions a quantum
interference could occur from the interplay between these two qubit species.
Here we report experiments and modeling which confirm these theoretical
predictions and identify the conditions under which this interference occurs.
Density matrix calculations show that the interference pattern manifests
primarily via the occupation of the common singlet state. The S/T0 qubit is
found to have a much longer coherence time as compared to the S/T+ qubit.
