MICROWAVE SPECTROSCOPY OF A QUANTUM-DOT MOLECULE

Quantum dots are small conductive regions in a semiconductor, containi ng a variable number of electrons (from one to a thousand) that occupy well-defined, discrete quantum states-for which reason they are often referred to as artificial atoms(1). Connecting them to current and vo ltage contacts allows the discrete energy spectra to be probed by char ge-transport measurements. Two quantum dots can be connected to form a n 'artificial molecule'. Depending on the strength of the inter-dot co upling (which supports quantum-mechanical tunnelling of electrons betw een the dots), the two dots can form 'ionic' (refs 2-6) or 'covalent' bonds. In the former case, the electrons are localized on individual d ots, while in the latter, the electrons are delocalized over both dots . The covalent binding leads to bonding and antibonding states, whose energy difference is proportional to the degree of tunnelling. Here we report a transition from ionic bonding to covalent bonding in a quant um-dot 'artificial molecule' that is probed by microwave excitations(5 -8). Our results demonstrate controllable quantum coherence in single- electron devices, an essential requirement for practical applications of quantum-dot circuitry.