A double quantum dot as an artificial two-level system

We study an artificial two-level system formed by a double quantum dot. Microwave spectroscopy experiments are discussed, in which we show that the inter-dot coupling can be tuned from ionic-like to covalent-like. The current through the double dot system is dependent on the applied microwave power and, in case of strong inter-dot coupling, the separation between the bonding and anti-bonding state is power dependent as well. We measure the elastic and inelastic transitions between the two levels. The inelastic transition rates are well described by Bose-Einstein statistics. Acoustic phonons are the most effectively coupled bosons in the solid-state environment of the two-level system.