Optical centers and quantum entangled states of Nd3+ ions in doped fluoride crystals

Abstract

Haruldaste muldmetallide ioonidega (HMI) aktiveeritud fluoriidkristallid ja nende nanoosakesed pakuvad suurt huvi nii fundamentaal- kui ka rakendusteaduse jaoks. Viimaste kümnenditega on esile kerkinud mitmeid uusi optika, lasertehnoloogia ja fotoonika rakendusi. Uudne ja paljulubav valdkond nende materjalide jaoks on ülikiirete optiliste kvantarvutite arendamine, kus HMI baasil optilised tsentrid toimivad kvantbittidena. Selles kontekstis pakub suurt huvi erinevat tüüpi HMI optiliste tsentrite tekkimise ja omaduste uurimine. Käesoleva väitekirja raames uuriti Nd3+ ioonidega dopeeritud erinevat tüüpi fluoriidkristalle vedela heeliumi temperatuuridel, kasutades aeglahutusega kohtselektiivset laserspektroskoopiat. Fluoriit-struktuuriga kristallides, kus Nd3+ lisandioonide lähedal paiknevad laengut kompenseerivad F− ioonid, tekivad väga erinevad, unikaalsete omadustega optilised tsentrid. Uuriti detailselt mitmesuguste optiliste tsentrite spektraalseid ja kineetilisi omadusi. Identifitseeriti ja kirjeldati mitu uut, eelnevalt tundmatut optilist tsentrit. Kahevärvuselist laserergastusskeemi kasutades kahe lähestikku paikneva HMI vastasmõju. Näidati, et sellist laadi ioonpaarist koosnev optiline tsenter on võimalik viia nn kooperatiivsesse kvantpõimitud seisundisse ja seeläbi kasutada ehituskivina tuleviku kvantarvutites. Seega väitekirja tulemused annavad väärtusliku panuse sellesse aktuaalsesse ja kiiresti arenevasse moodsa teaduse ja tehnoloogia valdkonda.

Fluoride crystals and their nanoparticles activated by rare-earth ions (REIs) are of great interest for both fundamental and applied science, where several applications in optics, laser technologies, and photonics have emerged during last decades. A novel and very promising area for these materials is the development of ultra-fast optical quantum computers where the REIs-based optical centers can serve as basic qubits. In this context, the study of formation and properties of different types of REI optical centers is of great interest. In this thesis, different types of fluoride crystals doped with Nd3+ ions were studied using site-selective time-resolved laser spectroscopy at liquid helium temperatures. It was shown that there is a complex set of optical centers depending on the crystal host. In the crystals of fluorite structure where the Nd3+ ions and nearby charge compensating F- ions are located, very different optical centers with unique properties are formed. Spectral and kinetic properties of various optical centers were studied in detail. Several new previously undiscovered types of optical centers were identified and described. A two-color laser excitation scheme was used to study the interacting optical centers consisting of two closely-spaced REIs. It was shown that for such pair centers it is possible to create so called quantum entangled cooperative states, which could be used as building blocks for future quantum computers. Thus, the results of the dissertation make a valuable contribution to this topical and rapidly developing field of modern science and technology.