Improved spin coherence times of the singlet-triplet system in a self-assembled quantum dot molecule*

In a doubly-charged quantum dot molecule, singlet and triplet ground state energies are split by the electron-electron exchange interaction. At the sweet spot, the system is largely decoupled from both magnetic and electric fluctuations, similar to atomic-clock transitions. We directly measure the T₂* coherence time in this system by modulating the laser field with a programmable microwave sequence and perform electron spin rotation via two-photon Raman process. We show that at the sweet spot, the coherence between singlet (S) and triplet (T₀) is dramatically enhanced, reaching tens of nanoseconds, an order of magnitude higher than that of an electron spin in a single quantum dot. Long coherence times open up the feasibility of using self-assembled quantum dot molecules in quantum information application such as photonic cluster state generation.