Phase coherence in the cotunneling regime of a coupled double quantum dot

Low-temperature transport measurements were performed on a two-terminal Aharonov–Bohm interferometer with one Coulomb-blockaded quantum dot (QD) embedded in each of the two interfering paths. Layered local anodic oxidation of the Ga[Al]As heterostructure and a thin titanium top gate give full tunability of the inter-dot tunnel coupling from purely capacitive to the QD molecule regime and of the tunnel coupling of the dots to the Aharonov–Bohm ring leads. Aharonov–Bohm oscillations are observed in the cotunneling regime far away from conductance resonances. This indicates that transport in the cotunneling regime is at least partially phase coherent. The phase of the Aharonov–Bohm oscillations is monitored as one crosses conductance resonances around a degeneracy point in the charge stability diagram. The phase in this two-terminal device is constant in the cotunneling regime between conductance resonances.