Logic and functional operations using a near-field optically coupled quantum-dot system

KIYOSHI KOBAYASHI

Suguru Sangu, Kiyoshi Kobayashi, Akira Shojiguchi +1 more

This paper investigates the exciton dynamics in a three-quantum-dot system coupled via an optical near field. The system consists of two identical quantum dots coupled coherently (the coherent operation part) and a third quantum dot with exciton sublevels (the output part). It provides certain characteristic functional operations depending on the initial excitation, as well as symmetry of the coupling strengths or the spatial arrangement. First, we analytically obtain the coupling strength between two quantum dots via an optical near field and give a numerical estimation for a CuCl quantum-cube system. Then, a resonance condition between the two parts is shown; this depends on the initial excitation in the coherent operation part. Using this condition, which can be realized by adjusting the energy level of a quantum dot in the output part, AND- and XOR-logic operations can be demonstrated in a symmetrically arranged quantum-dot system. We also discuss how the asymmetry of the system affects the energy transfer through certain coupled states in the coherent operation part that would be forbidden in a symmetrically arranged system. Although the asymmetry degrades the signal contrast for logic operations, it is expected to open up new techniques for novel device technologies where quantum entangled states are mediated in the operations.

Xin-Qi Li, Yasuhiko Arakawa

Suguru Sangu, Kiyoshi Kobayashi, Akira Shojiguchi +1 more

We investigate the population dynamics in a three quantum-dot system coupled via an optical near field, which consists of a coherent operation part and an output part. We analytically show that a resonance condition between the two parts depends on initial excitation in the coherent operation part. Using this feature, AND- and XOR-gate operations are demonstrated in the case of symmetrically arranged quantum dots. We also evaluate the effects of asymmetric arrangement on these operations, and show that the coherence plays an important role in an asymmetric system.

Mauricio Garrido

Kiyoshi Kobayashi, Suguru Sangu, Akira Shojiguchi +3 more

Using density operator formalism, we discuss interdot excitation energy transfer dynamics driven by the optical near-field and phonon bath reservoir, as well as coherent excitation dynamics of a quantum dot system. As an effective interaction between quantum dots induced by the optical near-field, the projection operator method gives a renormalized dipole interaction, which is expressed as a sum of the Yukawa functions and is used as the optical near-field coupling of quantum dots. We examine one- and two-exciton dynamics of a three-quantum dot system suggesting a nanometric photonic switch, and numerically obtain a transfer time comparable with the recent experimental results for CuCl quantum dots.