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Physical Review B

Formation and control of a correlated exciton two-qubit system in a coupled quantum dot

Keishiro Goshima, Kazuhiro Komori, Takeyoshi Sugaya, T. Takagahara

Published May 14, 2009• 8 citations
Publisher

Abstract

We have proposed the basic device structures for an all-optical controlled two-qubit quantum logic gate using two excitons confined in InAs/GaAs coupled quantum dots (CQDs). A two-qubit quantum logic gate is constructed using a correlated two-exciton system. This simplest two-qubit system involves four states such as the crystal ground states, two distinguishable exciton states, and a correlated-exciton-molecule state consisting of two excitons. The formation of this correlated-exciton-molecule state, which consists of two different excitons in a CQD, is the most important and difficult technology. In this study, we demonstrate the formation of the four states involving this correlated-exciton-molecule state and confirm two-qubit exciton system in CQDs. Moreover, we show that a correlated exciton molecule is created by employing a cascade process.

DOI
10.1103/physrevb.79.205313

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We have proposed the basic device structures for an all-optical controlled two-qubit quantum logic gate using two excitons confined in InAs/GaAs coupled quantum dots (CQDs). A two-qubit quantum logic gate is constructed using a correlated two-exciton system. This simplest two-qubit system involves four states such as the crystal ground states, two distinguishable exciton states, and a correlated-exciton-molecule state consisting of two excitons. The formation of this correlated-exciton-molecule state, which consists of two different excitons in a CQD, is the most important and difficult technology. In this study, we demonstrate the formation of the four states involving this correlated-exciton-molecule state and confirm two-qubit exciton system in CQDs. Moreover, we show that a correlated exciton molecule is created by employing a cascade process.
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).
We report coherent optical control of a biexciton (two electron-hole pairs), confined in a single quantum dot, that shows coherent oscillations similar to the excited-state Rabi flopping in an isolated atom. The pulse control of the biexciton dynamics, combined with previously demonstrated control of the single-exciton Rabi rotation, serves as the physical basis for a two-bit conditional quantum logic gate. The truth table of the gate shows the features of an all-optical quantum gate with interacting yet distinguishable excitons as qubits. Evaluation of the fidelity yields a value of 0.7 for the gate operation. Such experimental capability is essential to a scheme for scalable quantum computation by means of the optical control of spin qubits in dots.
The spin states of electrons and holes confined in InAs quantum dot molecules have recently come to fore as a promising system for the storage or manipulation of quantum information. We describe here a feasible scheme for complete quantum optical control of two electron spin qubits in two vertically-stacked singly-charged InAs quantum dots coupled by coherent electron tunneling. With an applied magnetic field transverse to the growth direction, we construct a universal set of gates that corresponds to the possible Raman transitions between the spin states. We detail the procedure to decompose a given two-qubit unitary operation, so as to realize it with a successive application of up to 8 of these gates. We give the pulse shapes for the laser pulses used to implement this universal set of gates and demonstrate the realization of the two-qubit quantum Fourier transform with fidelity of 0.881 and duration of 414 ps. Our proposal therefore offers an accessible path to universal computation in quantum dot molecules and points to the advantages of using pulse shaping incoherent manipulation of optically active quantum dots to mitigate the negative effects of unintended dynamics and spontaneous emission.
Recently a pair of vertically aligned quantum dots has been suggested as optically driven quantum gate. The later is built when two different particles, an electron and a hole, are created optically. The two particles form entangled states. We study theoretically the dynamics of a single heavy hole exciton confined in a small vertically coupled quantum dot. We use harmonic potentials to model the confining of electrons and holes and calculate the exciton's energy spectrum using the Hund-Mulliken approach, and including the Coulomb interaction. The entanglement may be controlled by the application of external electric field. At certain times of dynamical evolution one can generate a maximally entangled coherent exciton state by initially preparing the quantum dots in a superposition of coherent state. In ideal situations, entangled quantum states would not decohere during processing and transmission of quantum information. However, a real quantum system is inevitably influenced by its environment. Since, we study the decoherence effect caused by the interaction of the confined carriers with the acoustic phonon effects. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)