Semiconductor Quantum Dot Molecule

Toshimasa Fujisawa
Physical Science Laboratory

@ Semiconductor quantum dots are often referred as artificial atoms, since variable number of electrons occupy well-defined discrete energy states. If two of these quantum dots are combined, one can consider an artificial and tunable two-level system, in which a quantum energy state in each dot can be independently controlled by external gate voltages. We have tested the quality of the two-level system in the following two experiments.
Our double quantum dot is fabricated in AlGaAs/GaAs 2DEG (see pictures in the frontispiece). The left and the right gates are used to change the energy states in the left and right dot, respectively. The central gate modifies the coupling strength between the dot, which determines whether the bonding is 'ionic' (in which electrons are localized on individual dots and are interacted with an electrostatic coupling) or 'covalent' (in which an electron is delocalized over both dots). We can control the coupling form from ionic to covalent with increasing the inter-dot coupling strength, which was demonstrated by microwave excitation measurement [1].
The other test of the two-level system is interaction to bosonic environments, which is similar to quantum optics in real atoms or molecules. We have experimentally confirmed that the Einstein relation in quantum optics is satisfied in quantum dot molecules, in which stimulated emission and absorption are related to spontaneous emission with Bose-Einstein distribution of bosons. The little but significant difference is that the boson is 'phonon' in quantum dot, whereas 'photon' in real atoms [2].
Series of experiments on quantum dot have shown successful control of particle-wave duality of a single particle state. Further dynamical studies are needed to apply to quantum logic gates.
These works are collaborated with Prof. L. P. Kouwenhoven et al. at TUDelft and Prof. S. Tarucha at Univ. Tokyo.
[1] T. H. Oosterkamp, et al., Nature 395(1998) 873.
[2] T. Fujisawa, et al., Science 282(1998) 932.

Fig. 1: Schematic diagram of double quantum dot coupled to source and drain contacts (upper part) and schematic wavefunction of the symmetric and anti-symmetric states (lower part). In the first experiment [1], coherent microwave (hn) is applied to measure the energy spacing between the symmetric and antisymmetric states. In the second [2], inelastic current spectra were measured to analyze emission and absorption from Bose-Einstein distributed noise of the Bosonic environment.