PhD defence by Masoomeh Taherkhani

Title: Modelling of optical gates for quantum information processing

Principal supervisor: Assoc. Prof. Niels Gregersen, DTU Fotonik
Co-supervisor: Professor Jesper Mørk, DTU Fotonik
Co-supervisor: Dara McCutcheon, University of Bristol

Evaluation Board:
Professor Antti-Pekka Jauho, DTU Nanotech, Denmark
Postdoctoral researcher Oliver Marquardt, Weierstraß-Institut, Germany
Assoc. Prof. Nikolaj Thomas Zinner, Aarhus University, Department of Physics and Astronomy, Denmark

Master of the Ceremony:
Assoc. Prof. Nika Akopian

A numerical tool is developed to calculate the exciton energy and oscillator strength in newly emerged type-II nanowire quantum-dots. For a single-quantum-dot, the poor overlap of the electron part and the weakly confined hole part of the exciton wavefunction leads to a small oscillator strength compared to type-I systems. To increase the oscillator strength, we propose a double-quantum-dot structure featuring a strongly localized exciton wavefunction and a corresponding four-fold relative enhancement of the oscillator strength, paving the way towards efficient optically controlled quantum gate applications in the type-II nanowire system.

Next, an optical gating scheme for quantum computing based on type-II double-quantum-dots is proposed. The qubit is encoded on the electron spin and the gate operations are performed by stimulated Raman adiabatic passage (STIRAP) using the position degree of freedom in double-quantum-dots to form an auxiliary ground-state. Successful STIRAP gating processes require an efficient coupling of both qubit ground-states of the double-quantum-dot to the gating auxiliary state and we demonstrate that this can be achieved using a charged exciton state. Crucially, by using type-II quantum-dots, the hole is localized between the two spatially separated electrons in the charged-exciton complex, thereby efficiently coupling the electron states orbitals. We subsequently exploit the scheme to realize single- and two-qubit gates for quantum computation. The conditional operation is performed by using Coulomb coupling to induce a shift of the STIRAP transition frequencies leading to a conditional violation of the STIRAP two-photon resonance. We calculate the fidelity of gates and show their performance is robust against the spin and charge noises.


man 20 aug 18
13:30 - 16:30



Lyngby Campus
Building 341, aud. 22