Noninvasive detection of molecular bonds in quantum dots

Abstract

We performed charge detection on a lateral triple quantum dot with starlike geometry. The setup allows us to interpret the results in terms of two double dots with one common dot. One double dot features weak tunnel coupling and can be understood with atomlike electronic states, the other one is strongly coupled forming moleculelike states. In nonlinear measurements we identified patterns that can be analyzed in terms of the symmetry of tunneling rates. Those patterns strongly depend on the strength of interdot tunnel coupling and are completely different for atomiclike or moleculelike coupled quantum dots allowing the noninvasive detection of molecular bonds. DOI: 10.1103/PhysRevB.78.153310 PACS number͑s͒: 73.21.La, 73.23.Hk, 73.63.Kv Quantum dots are often called artificial atoms 1 due to their discrete electronic level spectrum. When several quantum dots are connected, they start to interact. 2 If the tunneling rate between the dots is small, the electronic wave functions are still constricted to the single-quantum dots and the interaction is dominated by electrostatics with sequential interdot tunneling. In contrast, for large tunneling rates electronic states can be found extended over several dots. These extended states introduce covalent bonding as in real molecules. 3 Whether or not the interdot tunneling rates are sufficient to form coherent moleculelike states is a crucial information in order to properly describe a quantum dot system. Especially for quantum computing purposes 4 coherent states are necessary to form and couple qubits and to implement SWAP gates for qubit manipulation. 5,6 With time resolved charge detection tunneling rates and coupling strengths can be observed directly ͑e.g. Ref. 7͒. With dc-transport experiments, however, there are only a few methods that can give hints for molecular bonds. The width of anticrossings visible in charging diagrams is a measure, 8 although anticrossings appear for capacitively coupled dots as well. In addition the curvature of the lines forming an anticrossing can be used 9 and also the visibility of lines in nonparallel quantum dots. 10 Another alternative is to study excited states. Strongly coupled quantum dots form bonding and antibonding states that are visible in nonlinear measurements. 11 We have studied the impact of the coupling strength on the mean charge of multiple quantum dot systems coupled in series. Using a quantum point contact 12 we analyzed the mean charge in stability diagrams and interpret the results in terms of resonant tunneling. We found that depending on the symmetry of tunneling rates and on the coupling strength, characteristic patterns are formed in nonlinear measurements. This allows us to noninvasively detect the symmetry of tunneling rates and the quality of the interdot coupling.The measurements were performed on a device containing three quantum dots A, B, and C ͑see inset of Fig. 1͒. The device was produced using local anodic oxidation on a GaAs/AlGaAs heterostructure. 13,14 The three dots are positioned in a starl...

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