Quantum computation with two-electron spins in semi-conductor quantum dots

Thumbnail Image

Files

isbn9789526062006.pdf (1.26 MB)

URL

Journal Title

Journal ISSN

Volume Title

School of Science | Doctoral thesis (article-based) | Defence date: 2015-06-05
Checking the digitized thesis and permission for publishing
Instructions for the author
Unless otherwise stated, all rights belong to the author. You may download, display and print this publication for Your own personal use. Commercial use is prohibited.

Authors

Date

2015

Department

Teknillisen fysiikan laitos
Department of Applied Physics

Major/Subject

Mcode

Degree programme

Language

en

Pages

117

Series

Aalto University publication series DOCTORAL DISSERTATIONS, 66/2015

Abstract

A quantum computer would exploit the phenomena of quantum superposition and entanglement in its functioning and with them offer pathways to solving problems that are too hard or complex to even the best classical computers built today. The implementation of a large-scale working quantum computer could bring about a change in our society rivaling the one started by the digital computer. However, the field is still in its infancy and there are many theoretical and practical issues needing to be solved before large-scale quantum computing can become reality.  In digital computers, data is stored in bits. The quantum equivalent is called a qubit (quantum bit) and it is basically a quantum mechanical two-level system that can be in a superposition of its two basis states. There are many different proposals for implementing qubits, but one of the most promising ones is to encode the qubit using electron spins trapped in semiconductor quantum dots. Singlet-triplet qubits are spin qubits where the two-electron spin eigenstates are used as the qubit's basis. The required one and two-qubit operations have already been demonstrated experimentally by several research groups around the world in this qubit architecture. The most severe factor limiting the implementation of larger systems of qubits is decoherence. The qubits are not isolated systems, they interact with their environment, which can lead to the loss of quantum information.  Few-electron systems can be simulated accurately using first principle methods that become too taxing when the particle number increases. The topic of this thesis is the simulation of quantum dot singlet-triplet qubit systems using accurate exact diagonalization based methods. The emphasis is on the realistic description of qubit operations, both single-qubit ones and those involving the interaction between neighboring qubits. The decoherence effects are also discussed alongside with certain proposals to alleviate their effects.

Kvanttitietokone hyödyntäisi kvantti-ilmiöitä, kuten superpositiota ja lomittumista ja mahdollistaisi tiettyjen nykyisille klassisille tietokoneille vaikeiden ongelmien ratkaisemisen. Suuren mittakaavan kvanttilaskenta saattaisi muuttaa yhteiskuntaamme yhtä perustavalla tavalla kuin digitaalisen tietokoneen keksiminen 1900-luvulla. Toimivaan ison mittakaavan kvanttitietokoneeseen on kuitenkin yhä matkaa, ja on useita teoreettisia ja käytännöllisiä ongelmia, jotka täytyy ratkaista ennen kuin tästä tulee todellisuutta.  Klassisessa tietokoneessa data tallennetaan bitteihin. Bitin kvanttivastinetta kutsutaan kubitiksi. Se on kvanttimekaaninen kaksitasosysteemi, joka voi olla ominaistilojensa superpositiossa. Kubitti voidaan realisoida monessa erilaisessa systeemissä. Yksi lupaavimmista ehdotuksista on käyttää puolijohdekvanttipisteisiin vangittujen elektronien spinejä kubitin pohjana. Jos kubitin tilat muodostuvat kahden elektronin spin-ominaistiloista, kubittia kutsutaan singletti-tripletti kubitiksi. Universaaliin kvanttilaskentaan tarvittavat yksi- ja kaksikubittioperaatiot on jo toteutettu kokeellisesti singletti-tripletti kubiteilla. Suuren mittakaavan kubittisysteemien implementoimista estävät tällä hetkellä ennen kaikkea dekoherenssi-ilmiöt. Kubitit eivät ole eristettyjä systeemejä; ne vuorovaikuttavat ympäristönsä kanssa, mikä johtaa kvantti-informaation katoamiseen.  Muutaman elektronin systeemejä voidaan mallintaa tarkoilla yksittäisten elektronien tasolla toimivilla malleilla. Monet tällaiset mallit ovat liian kompleksisia laskennallisesti suurempien systeemien simuloimiseen, mutta soveltuvat hyvin esimerkiksi muutaman singletti-tripletti kubitin tapaukseen. Tämän väitöskirjan aihe kvanttipiste singletti-tripletti kubittien mallintaminen käyttäen tarkkoja ns. exact diagonalization-metodeita. Painopiste on yksi- ja monikubittioperaatioiden tarkassa kuvaamisessa. Myös dekoherenssi-ilmiöitä ja tiettyjä ratkaisuja niiden helpottamiseksi käsitellään.

Description

Supervising professor

Nieminen, Risto, Aalto Distinguished Prof., Aalto University, Department of Applied Physics, Finland

Thesis advisor

Harju, Ari, Adjunct Prof., Aalto University, Department of Applied Physics, Finland

Keywords

quantum computer, spin qubit, singlet-triplet qubit, entanglement, decoherence, configuration interaction, Hubbard model, kvanttilaskenta, spin-kubitti, lomittuminen, dekoherenssi, CI-metodi, Hubbardin malli

Other note

Parts

  • [Publication 1]: Tuukka Hiltunen and Ari Harju. Y-junction splitting spin states of moving quantum dot. Physical Review B (Rapid Communications), 86, 12, 121301(R), September 2012.
    DOI: 10.1103/PhysRevB.86.121301. View at publisher
  • [Publication 2]: Tuukka Hiltunen, Juha Ritala, Topi Siro, and Ari Harju. Non-adiabatic charge state transitions in singlet-triplet qubits. New Journal of Physics, 15, 10, 103015, October 2013.
    DOI: 10.1088/1367-2630/15/10/103015. View at publisher
  • [Publication 3]: Tuukka Hiltunen and Ari Harju. Maximal tripartite entanglement between singlet-triplet qubits in quantum dots. Physical Review B, 89, 11, 115322, March 2014.
    DOI: 10.1103/PhysRevB.89.115322. View at publisher
  • [Publication 4]: Tuukka Hiltunen and Ari Harju. Capacitative coupling of singlet-triplet qubits in different interqubit geometries. Physical Review B, 90, 12, 125303, September 2014.
    DOI: 10.1103/PhysRevB.90.125303. View at publisher
  • [Publication 5]: Tuukka Hiltunen, Hendrik Bluhm, Sebastian Mehl, and Ari Harju. Charge-noise tolerant exchange gates of singlet-triplet qubits in asymmetric double quantum dots. Physical Review B, 91, 7, 075301, February 2015.
    DOI: 10.1103/PhysRevB.91.075301. View at publisher
  • [Publication 6]: Michiel A. Bakker, Sebastian Mehl, Tuukka Hiltunen, Ari Harju, David P. DiVincenzo. Validity of the single-particle description and charge noise resilience for multi-electron quantum dots. Physical Review B, 91, 15, 155425, April 2015.
    DOI: 10.1103/PhysRevB.91.155425. View at publisher

Citation

Permanent link to this item

https://urn.fi/URN:ISBN:978-952-60-6200-6

Collections

[diss] Perustieteiden korkeakoulu / SCI