Spin coherence measurement of the singlet-triplet system in a self-assembled quantum dot molecule*

InGaAs self-assembled quantum dots (QDs) are an excellent source of single photons with many favorable characteristics: high indistinguishability, fast emission rate, and high single photon purity. A quantum dot molecule (QDM) – two vertically stacked QDs, offers further advantages in terms of tunability, spin coherence, and spin readout. With one electron in each dot, the singlet and triplet ground states energies are split by the electron-electron exchange interaction without the need to use a high magnetic field. The antiparallel orientation of the spins protects the system from the nuclear spin bath. As a function of applied bias there also exists a “sweet spot” where the exchange interaction is insensitive to bias. At this point the spin is ideally decoupled from both electric and magnetic fluctuations, enhancing the spin coherence time T^*₂ compared to a single QD spin. We examine the spin coherence of a system of two QDs separated by a 9 nm tunnel barrier, with an exchange splitting of about 12 GHz. We perform Ramsey interferometry to measure T^*₂ as a function of bias near the sweet spot. Our results show promise for improved spin coherence times in QDs.