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International Conference on Numerical Simulation of Optoelectronic Devices (NUSOD)

Type-II quantum dot nanowire structures with large oscillator strengths for optical quantum gating applications

Masoomeh Taherkhani, Niels Gregersen, Morten Willatzen, Jesper Mørk

Published Jul 1, 2017• 0 citations
Publisher

Abstract

The exciton oscillator strength (OS) in type-II quantum dot (QD) nanowires is calculated by using a fast and efficient method. We propose a new structure in Double-Well QD (DWQD) nanowire that considerably increases OS of type-II QDs which is a key parameter in optical quantum gating in the stimulated Raman adiabatic passage (STIRAP) process for implementing quantum gates.

DOI
10.1109/nusod.2017.8009964

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The exciton oscillator strength (OS) in type-II quantum dot (QD) nanowires is calculated by using a fast and efficient method. We propose a new structure in Double-Well QD (DWQD) nanowire that considerably increases OS of type-II QDs which is a key parameter in optical quantum gating in the stimulated Raman adiabatic passage (STIRAP) process for implementing quantum gates.
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A numerical tool is developed to calculate the exciton energy and oscillator strength in newly emerged type-II nanowire quantum-dots. For a singlequantum-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 chargedexciton 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.
We propose an optical gating scheme for quantum computing based on crystal-phase type II double quantum dots in an InP nanowire. The qubit is encoded on the electron spin and the gate operations are performed using stimulated Raman adiabatic passage (STIRAP), using the orbital 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 ground state, and we demonstrate that this can be achieved using a charged exciton state. Crucially, by using type II dots, the hole is localized between the two spatially separated electrons in the charged exciton complex, thereby efficiently coupling the electron ground-state orbitals. By taking advantage of the high-fidelity state transfer by means of STIRAP in type II double quantum dots, we propose a protocol for coherently manipulating the spin-orbital quantum state of confined electrons in a quantum dot chain of an InP nanowire. We subsequently exploit the protocol to realize single- and two-qubit gates with fidelity above 0.99.
Hole spin qubits in semiconductor quantum dots (QDs) are promising candidates for quantum information processing due to their weak hyperfine coupling to nuclear spins, and to the strong spin-orbit coupling, which allows for rapid operation time. We propose a coherent control on two heavy-hole spin qubits in a double QD by a fast adiabatic driving protocol, which helps to achieve higher fidelities than other experimentally commonly used protocols as linear ramping, $\ensuremath{\pi}$ pulses or Landau-Zener passages. Using fast quasiadiabatic driving via spin-orbit coupling, it is possible to reduce charge noise significantly for qubit manipulation and achieve high robustness for the qubit initialization. We also implement one- and two-qubit gates, in particular, not, cnot, and swaplike gates, of hole spins in a double QD achieving fidelities above $99\mathrm{%}$, exhibiting the capability of hole spins to implement universal gates for quantum computing.
This paper discusses the potential application of elementary excitation in semiconductor quantum dots to quantum computation. We propose a scalable hardware design and all-optical implementation of logical gates that exploit the discrete nature of electron and hole states and their well-concentrated oscillator strength for ultrafast gate operations. We also propose a multiple-bit design based on the nearest neighbor dipole-dipole coupling. Rabi population oscillation of an excitonic two-level system in an isolated single In xGa1-xAs quantum dot is manifested by quantum wave function interferometry in the time domain, demonstrating a long lived coherence of zero-dimensional excitonic states and revealing the coherent population flopping under strong optical field. Phase-sensitive coherent gate operations on a single-dot exciton qubit are demonstrated.
An exciton in a quantum dot (QD) strongly couples to light, allowing the ultrafast manipulation of the quantum states on the order of femtoseconds [1]. Since a QD exciton can be regarded as a two-level system, the excitonic state can be manipulated coherently via Rabi oscillations depending on the electric field of the excitation pulse and the resonant frequency of the QD exciton. However, uniformly manipulating the exciton ensemble in a large number of QDs using ultrashort pulses is quite difficult due to the inhomogeneous distributions of the excitonic resonant frequency and the laser intensity for each QD. Adiabatic rapid passage (ARP) has been demonstrated to overcome this problem because it provides a quantum manipulation technique robust over the inhomogeneities [2], [3] by using chirped pulses (CP) instead of using Fourier transform-limited pulses (TLP). While ARP has been studied extensively in two-level systems with inhomogeneous linewidths up to GHz, it has not been rarely applied to two-level systems with THz inhomogeneous linewidths such as QDs. In this work, we experimentally demonstrate the ultrafast coherent transfer of the exciton population via ARP in two kinds of QD samples; 150-layer stacked InAs QDs and 50-layer stacked InAs QDs embedded in a resonator (Fig. 1(a)).