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. http://www.brl.ntt.co.jp/people/fujisawa/qdm/
[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.
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