The transport spectrum of a strongly tunnel-coupled one-electron double quantum dot electrostatically defined in a GaAs/AlGaAs heterostructure is studied. At finite source-drain-voltage we demonstrate the unambiguous identification of the symmetric ground state and the antisymmetric excited state of the double well potential by means of differential conductance measurements. A sizable magnetic field, perpendicular to the two-dimensional electron gas, reduces the extent of the electronic wave-function and thereby decreases the tunnel coupling. A perpendicular magnetic field also modulates the orbital excitation energies in each individual dot. By additionally tuning the asymmetry of the double well potential we can align the chemical potentials of an excited state of one of the quantum dots and the ground state of the other quantum dot. This results in a second anticrossing with a much larger tunnel splitting than the anticrossing involving the two electronic ground states. Key words: double quantum dot, single electron tunneling, delocalization, molecular states PACS: 73.21.La, 73.23.Hk, 73.20.Jc Electrostatically defined semiconductor double quantum dots, where electrons are confined in a double potential well, have recently attracted considerable attention . The interest in these artificial molecules is largely due to the proposed use of quantum dots as spin or charge qubits, the building blocks of the hypothetical quantum computer [2,3]. Recent works have shown spectacular advancements in reducing the number of electrons trapped in a double quantum dot (DQD) down to N = 1 [4,5,6,7]. Here we study the transport spectrum of a strongly tunnel-coupled DQD with N ≤ 1 at finite source-drain voltage USD. We ob
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