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Japanese Journal of Applied Physics

Optical Control of 2-Qubit Exciton States in a Coupled Quantum Dot

Keishiro Goshima, Kazuhiro Komori, Takeyoshi Sugaya

Published Apr 1, 2008• 2 citations
Publisher

Abstract

We investigated the formation dynamics of an artificial exciton molecule state, which consists of two different excitons confined in each quantum dot of a pair of quantum dots, namely, a coupled quantum dot (CQD). By the selective optical excitation of the inherent energy states of two different excitons using two-color pulsed excitation, we created an artificial exciton molecule state and investigated the transient dynamics, and monitored the photoluminescence, namely, the population of the artificial exciton molecule states. By employing the power dependence of the two colors, we tried the two-qubit logic gate operation and succeeded in controlling the population of an artificial exciton molecule (pulse area θ<π).

DOI
10.1143/jjap.47.3111

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We investigated the formation dynamics of an artificial exciton molecule state, which consists of two different excitons confined in each quantum dot of a pair of quantum dots, namely, a coupled quantum dot (CQD). By the selective optical excitation of the inherent energy states of two different excitons using two-color pulsed excitation, we created an artificial exciton molecule state and investigated the transient dynamics, and monitored the photoluminescence, namely, the population of the artificial exciton molecule states. By employing the power dependence of the two colors, we tried the two-qubit logic gate operation and succeeded in controlling the population of an artificial exciton molecule (pulse area θ<π).
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We present models of quantum gates based on coupled quantum dots in which a qubit is regarded as the superposition of ground states in each dot. Coherent control on the qubit is performed by a monochromatic light pulse illuminated on the quantum dots. Considering particular types of artificial molecules, we show that the qubit is easily manipulated with the polarization and frequency of a light pulse and, especially, the controlled-NOT operation can be possible exploiting the electron-electron interaction. To examine the decoherence of quantum states, we discuss electronic relaxation contributed mainly by LA phonon processes.
We demonstrate coherent nonlinear-optical control of excitons in a pair of quantum dots (QDs) coupled via dipolar interaction. The single-exciton population in the first QD is controlled by resonant picosecond excitation, giving rise to Rabi oscillations. As a result, the exciton transition in the second QD is spectrally shifted and concomitant Rabi oscillations are observed. We identify coupling between permanent excitonic dipole moments as the dominant interaction mechanism, whereas quasiresonant (Förster) energy transfer is weak. Such control schemes based on dipolar interaction are a prerequisite for realizing scalable quantum logic gates.
Currently there is strong interest in realizing implementations of quantum computation and quantum cryptography in a solid state environment. One of the systems that are actively studied are semiconductor quantum dots (QDs). Due to their discrete energy level structure, they are often called artificial atoms, and they attract immediately interest of quantum information science since they allow to mimic the design developed for atomic physics systems such as ions in traps or atoms in cavities. However, despite of the similarities, one has to keep in main that any elementary excitation in a QD has a generic many-body character. An essential building block of a quantum processor is a quantum gate which entangles the states of two quantum bits. Recently it has been proposed that a pair of vertically aligned QDs could be used as an optically driven quantum gate: The quantum bits are individual carriers either on dot zero or dot one. The different dot indices play the same role as a "spin," therefore we term them "isospin." Quantum mechanical tunneling between the dots rotates the "isospin" and leads to superposition of two quantum dot states. The quantum gate is built when two different particles, an electron and a hole, are created optically. The two particles form entangled isospin states. The entanglement can be controlled by application of an electric field along the heterostructure growth direction. Here we present spectroscopic studies on single quantum dot molecules (QDMs) with different vertical separation between the dots that support the feasibility of this proposal. The comparison of the evolution of the excitonic recombination spectrum with the results of calculations allows us to demonstrate coherent tunneling of electrons and holes across the separating barrier and the formation of entangled exciton states. For a given barrier width, we find only small variations of the tunneling induced splitting between the entangled states demonstrating a good homogeneity within the obtained QDM ensembles.