Dissertations / Theses on the topic 'Solid state qubit'

[Mathematical symbols can be only approximated here. For the correct display see the Abstract in the PDF files linked below] This work has demonstrated that hyperfine decoherence times sufficiently long for QIP and quantum optics applications are achievable in rare earth ion centres. Prior to this work there were several QIP proposals using rare earth hyperfine states for long term coherent storage of optical interactions [1, 2, 3]. The very long T_1 (~weeks [4]) observed for rare-earth hyperfine transitions appears promising but hyperfine T_2s were only a few ms, comparable to rare earth optical transitions and therefore the usefulness of such proposals was doubtful. ¶ This work demonstrated an increase in hyperfine T_2 by a factor of 7 × 10^4 compared to the previously reported hyperfine T_2 for Pr^[3+]:Y_2SiO_5 through the application of static and dynamic magnetic field techniques. This increase in T_2 makes previous QIP proposals useful and provides the first solid state optically active Lamda system with very long hyperfine T_2 for quantum optics applications. ¶ The first technique employed the conventional wisdom of applying a small static magnetic field to minimise the superhyperfine interaction [5, 6, 7], as studied in chapter 4. This resulted in hyperfine transition T_2 an order of magnitude larger than the T_2 of optical transitions, ranging fro 5 to 10 ms. The increase in T_2 was not sufficient and consequently other approaches were required. ¶ Development of the critical point technique during this work was crucial to achieving further gains in T_2. The critical point technique is the application of a static magnetic field such that the Zeeman shift of the hyperfine transition of interest has no first order component, thereby nulling decohering magnetic interactions to first order. This technique also represents a global minimum for back action of the Y spin bath due to a change in the Pr spin state, allowing the assumption that the Pr ion is surrounded by a thermal bath. The critical point technique resulted in a dramatic increase of the hyperfine transition T_2 from ~10 ms to 860 ms. ¶ Satisfied that the optimal static magnetic field configuration for increasing T_2 had been achieved, dynamic magnetic field techniques, driving either the system of interest or spin bath were investigated. These techniques are broadly classed as Dynamic Decoherence Control (DDC) in the QIP community. The first DDC technique investigated was driving the Pr ion using a CPMG or Bang Bang decoupling pulse sequence. This significantly extended T_2 from 0.86 s to 70 s. This decoupling strategy has been extensively discussed for correcting phase errors in quantum computers [8, 9, 10, 11, 12, 13, 14, 15], with this work being the first application to solid state systems. ¶ Magic Angle Line Narrowing was used to investigate driving the spin bath to increase T_2. This experiment resulted in T_2 increasing from 0.84 s to 1.12 s. Both dynamic techniques introduce a periodic condition on when QIP operation can be performed without the qubits participating in the operation accumulating phase errors relative to the qubits not involved in the operation. ¶ Without using the critical point technique Dynamic Decoherence Control techniques such as the Bang Bang decoupling sequence and MALN are not useful due to the sensitivity of the Pr ion to magnetic field fluctuations. Critical point and DDC techniques are mutually beneficial since the critical point is most effective at removing high frequency perturbations while DDC techniques remove the low frequency perturbations. A further benefit of using the critical point technique is it allows changing the coupling to the spin bath without changing the spin bath dynamics. This was useful for discerning whether the limits are inherent to the DDC technique or are due to experimental limitations. ¶ Solid state systems exhibiting long T_2 are typically very specialised systems, such as 29Si dopants in an isotopically pure 28Si and therefore spin free host lattice [16]. These systems rely on on the purity of their environment to achieve long T_2. Despite possessing a long T_2, the spin system remain inherently sensitive to magnetic field fluctuations. In contrast, this work has demonstrated that decoherence times, sufficiently long to rival any solid state system [16], are achievable when the spin of interest is surrounded by a concentrated spin bath. Using the critical point technique results in a hyperfine state that is inherently insensitive to small magnetic field perturbations and therefore more robust for QIP applications.

A GaAs triple quantum dot was cooled to below 100mK and electrically manipulated for two distinct experiments. The first was a new measurement technique, called enhanced charge detection, which involving optimized excited state path relaxation used to increase the charge detection signal-to-noise by a factor of four. This was illustrated with the two spin S-T+ qubit [1]. A spin funnel with 50 coherent oscillations, i.e. 100 rotations, was measured using this scheme. This technique can be used with other spin qubits and readout measurement schemes [2]. The second experiment involved realizing the theoretically predicted all exchange qubit by studying the necessary activation parameters and comparing to theory. The coherent behaviour of an interacting three spin system is complex with contributions arising from several individual qubits, however, the all-exchange qubit is predicted to be protected against some forms of global noise [3]. Magnetic eld dependence measurements were used to isolate this qubit from other qubits. These measurements were compared to theory by modeling the time evolution of spin states [4].
Un triple point quantique fait d'arséniure de gallium a été refroidi en dessous de 100mK et a été manipulé électriquement dans le but de faire deux expériences distinctes. La première expérience est une nouvelle technique de mesure, appelé détection de charge accrue, qui utilise des trajectoires de relaxation d'un état sensible pour augmenter la détection du rapport signal sur bruit de d'un facteur quatre. Cela a été démontré par le qubit de spin S-T+ [1]. Un entonnoir de spin avec environ 50 oscillations cohérentes, c-à-d 100 rotations de piradians, a été mesuré utilisant ce schéma. Cette echnique peut être utilisée avec d'autres modèles de mesures de qubit de spin [2]. La deuxième expérience consiste à la réalisation d'un qubit prédit théoriquement appelé all-exchange qubit. Les paramètres nécessaires à l'activation de ce qubit ont été étudiés et comparés à la théorie. Le comportement d'un système de trois spins en interaction est très complexe à cause des contributions provenant de plusieurs qubits, cependant, il est prédit que le all-exchange qubit devrait être protégé contre la plupart des bruits globaux [3]. Des mesures en champ magnétique sont utilisées pour isoler ce qubit des autres. Ces mesures sont comparées à la théorie en modélisant l'évolution temporelle des états de spin [4].

Rare-earth-doped crystals have been used for optical signal processing and storage applications. In this dissertation, their potential for quantum computing applications is explored. In one quantum computing scheme, information is stored in nuclear spin states and this information is then processed by using optical pulses through the coupling of these nuclear spin states to a common electronic level. To implement this scheme, nuclear spin states and coupling of these nuclear spin states to a common electronic level is required. Preliminary work in rare-earth materials like Pr3+ and Eu3+ has shown promising results regarding their suitability for quantum computing applications. One particular problem with these materials is that their transition wavelengths are only accessible with dye lasers. These lasers are inherently unstable, and currently few available systems exhibit the stability required for quantum computing applications. An alternative choice was to investigate other rare-earth ions like thulium. Thulium has a transition wavelength that can be accessed with diode lasers, which are commercially available, easy to stabilize, and compact. This dissertation is based on our investigations of Tm3+:YAG for quantum computing applications. Investigations involved a detailed characterization of the material. Nuclear spin states, in Tm3+:YAG, were obtained by applying an external magnetic field to the sample. First, interaction of an external magnetic field with the thulium ions at various sites in the crystal was analyzed. This analysis was used to measure the magnetic anisotropy in the material. These results show that it is possible, with the suitable choice of the magnetic orientation and the site in the crystal, to build a working 3-level quantum system. In the demonstration of single qubit operations in Tm3+:YAG, we first theoretically studied the effect of Gaussian spatial beam on the single qubit operations. Later on, we experimentally prepared a single isolated ensemble of ions in the inhomogeneously broadened absorption profile of the medium. This single isolated ensemble of ions was used as a test-bed to implement the single qubit operations. We also isolated two ensembles of ions in the inhomogeneous absorption profile of the medium. The interaction between these two isolated ensembles of ions was also studied.

Spin properties of donor impurities in silicon have been investigated by electron spin resonance (ESR) techniques for more than sixty years. These studies gave us a contribution towards understanding some of the physics of doped semiconductor materials in general, which is the platform for much of our current technology. Despite the fact that donor electron and nuclear spins have been researched for so long, ESR studies of their properties are still giving us interesting insights. With the introduction of the concept of quantum information in the 1980s, some properties of donor spins in silicon, that were known from the fifties (such as long relaxations), have been reinterpreted for their potential application in this field. Since then, incredible experimental results have been achieved with magnetic resonance control, including manipulation and read-out of individual spins. However, some open questions are still to be answered before the realisation of a spin-based silicon quantum architecture will be achieved. Currently, ESR studies still contribute to help answering some of those questions. In this thesis, we demonstrate electrical and optical methods for donor charge state manipulation measured by ESR. Recent experiments have demonstrated that coherence time of nuclear spins may be enhanced by manipulating the state of donors from neutral to singly charged. We investigate electric field ionisation/neutralisation of arsenic donors in a silicon SOI device measured by ESR. Below ionisation threshold, we also measure the hyperfine Stark shift of arsenic donors spins in silicon. These results have, for instance, implications on how fast individual addressability of donor spins may be achieved in certain quantum computer architectures. Here, we also study optical-driven charge state manipulation of selenium impurities in silicon. Selenium has two additional electrons when it replaces an atom in the silicon crystal (i.e. double donor). The electronic properties of singly-ionised selenium make it potentially advantageous as spin qubit, compared to the more commonly studied group-V donors. For instance, we find here that the electron spin relaxation and coherence times of selenium are up to two orders of magnitude longer than phosphorus at the same temperature. Finally, we demonstrate that it is possible to bring selenium impurity in singly-charged state and subsequently re-neutralise them leaving a potential long-lived ⁷⁷Se nuclear spin.

Ce travail de thèse porte sur l’étude fondamentale de systèmes nano-électroniques,mesurés à très basse température. Nous avons réalisé des interféromètres électroniques àdeux chemins à partir d’électrons balistiques obtenus dans un gaz 2D d’électrons d’unehétéro-structure GaAs/AlGaAs. Nous montrons que la phase des électrons, et ainsileur état quantique,peut être contrôlée par des grilles électrostatiques. Ces dispositifsse révèlent être des candidats prometteurs pour la réalisation d’un qubit volant. Nousavons développé une simulation numérique évoluée d’un modèle de liaisons fortes à partirde transport quantique ballistique qui décrit toutes les découvertes expérimentales etnous apporte une connaissance approfondie sur les signatures expérimentales de cesdispositifs particuliers. Nous proposons des mesures complémentaires de ce système dequbit volants. Pour atteindre le but ultime, à savoir un qubit volant à un électron unique,nous avons assemblé la source à électron unique précédemment développée dans notreéquipe à un beam splitter électronique. Les électrons sont alors injectés depuis une boîtequantique à un train de boîte quantiques en mouvement. Ce potentiel électrostatique enmouvement est généré par des ondes acoustiques de surface créées par des transducteursinter-digités sur le substrat GaAs piézo-électrique. Nous avons étudié et optimisé chacunde ces composants fondamentaux nécessaires à la réalisation d’un beam splitter à électronunique et développé un procédé local et fiable de fabrication. Ce dispositif nous permet d’étudier les interactions électroniques pour des électrons isolés et pourra servir de basede mesure pour des expériences d’optique quantiques sur un système électronique del’état condensé. Enfin, nous avons développé un outil puissant de simulation du potentielélectrostatique à partir de la géométrie des grilles. Ceci permet d’optimiser la conceptiondes échantillons avant même leur réalisation. Nous proposons ainsi un prototype optimiséde beam splitter à électron unique
This thesis contains the fundamental study of nano-electronic systems at cryogenictemperatures. We made use of ballistic electrons in a two-dimensional electron gasin a GaAs/AlGaAs heterostructure to form a real two-path electronic interferometerand showed how the phase of the electrons and hence their quantum state can becontrolled by means of electrostatic gates. The device represents a promising candidateof a flying qubit. We developed a sophisticated numerical tight-binding model based onballistic quantum transport, which reproduces all experimental findings and allows togain profound knowledge about the subtle experimental features of this particular device.We proposed further measurements with this flying qubit system. With the ultimate goalof building a single electron flying qubit, we combined the single electron source that hasbeen developed in our lab prior to this manuscript with an electronic beam splitter. Theelectrons are injected from static quantum dots into a train of moving quantum dots.This moving potential landscape is induced in the piezoelectric substrate of GaAs bysurface acoustic waves from interdigial transducers. We studied and optimized all keycomponents, which are necessary to build a single electron beam splitter and built up areliable local fabrication process. The device is capable of studying electron interactionson the single electron level and can serve as a measurement platform for quantum opticsexperiments in electronic solid state systems. Finally, we developed a powerful toolcapable of calculating the potential landscapes of any surface gate geometry, which canbe used as a fast feedback optimization tool for device design and proposed an optimizedprototype for the single electron beam splitter

Optical quantum memories (QMs) are one of the fundamental building blocks in quantum information science (QIS). They might find important use in quantum communication and computation applications. Rare-earth ions (REIs) have been investigated for decades for their optical properties. They exhibit excellent coherence properties when cooled down to cryogenic temperatures. Not surprisingly, they emerged as a promising candidate for use in QIS as QMs. In this thesis, we investigated the quantum storage of photonic qubits in a Pr3+ :Y2SiO5 (PrYSO) crystal for potential use in quantum communication and networking applications. We started by constructing the experimental setup and the laser system from scratch as our research group had just been established at the beginning of this PhD study. First experiments included spectroscopy of the PrYSO system in order to identify the electronic transitions that are suitable for the QM experiments. We used the atomic frequency comb (AFC) memory protocol in all the experiments presented in this thesis. We also developed complex pulse sequences that are necessary for the optical preparation of an AFC. As a first experiment, we demonstrated the storage of photonic polarization qubits encoded in weak coherent states in the excited states of Pr3+ ions for a predetermined storage time of 500 ns. This had not been achieved previously due to the polarization dependent absorption of the material. We achieved average storage fidelities of ~95% which surpass the best achievable value with a measure and prepare strategy, thus proving the quantum character of our interface. Nevertheless, in order to be implemented in realistic quantum networking architectures, a QM should have the capability of on-demand retrieval of the stored information. As a first step towards this goal, our next experiment concerned the transfer of the input pulses to and from the long-lived hyperfine ground levels of Pr3+ ions, albeit with bright pulses. Furthermore, by performing time-bin interference experiments, we demonstrated that the coherence is preserved during the storage, transfer and retrieval processes. Temporal multimode storage in the spin-states up to 5 modes was also shown. Finally, in the last part of this thesis we demonstrated a solid-state spinwave quantum memory, with qubits encoded in weak coherent states at the single photon level. Storing and retrieving single-photon level fields in the ground levels of the PrYSO system is challenging as the strong control pulses and the weak input pulse to be stored in the memory are separated by only 10:2 MHz. The control pulses create noise, mostly as free-induction decay, fluorescence and scattering off the optical surfaces. In order to circumvent this problem we employed narrow-band spectral, temporal and spatial filtering. By using spectral-hole burning based narrow band filter created in a second PrYSO crystal, we could achieve signal-to-noise ratio (SNR) > 10 for input pulses with mean photon number of around 1. The high SNR we achieved allowed us to store and recall time-bin qubits with conditional fidelities again higher than that is possible with a measure and prepare strategy. This experiments also represents the first demonstration of a quantum memory for time-bin qubits with on demand read-out of the stored quantum information. The results presented in this thesis fill an important gap in the field of solid-state quantum memories and open the way for the long-lived storage of non-classical states of light. They further strengthen the position of REI based systems in QIS, specifically as nodes in scalable quantum network architectures.
Les memòries quàntiques òptiques (MQs) son un dels elements fonamentals en la ciència de la informació quàntica (CIQ). El seu ús podria ser important en aplicacions relacionades amb la comunicació i la computació quàntiques. Els ions de terres rares (ITRs) han sigut investigats durant dècades per les seves propietats òptiques. Exhibeixen excel·lents propietats de coherència quan es refreden a temperatures criogèniques. Per tant, no es sorprenent que hagin emergit com a candidats per ser usats en la CIQ com a MQs. En aquesta tesis, hem investigat l'emmagatzematge quàntic de qubits fotònics en un cristall de Pr3+:Y2SiO5 (PrYSO) per al seu possible ús en aplicacions relacionades amb xarxes d'informació quàntiques. Vam començar construint el dispositiu experimental i sistemes làser des de zero, ja que el nostre grup de recerca acabava de néixer. Els primers experiments van incloure espectroscòpia del sistema de PrYSO per identificar les transicions electròniques més apropiades per als següents experiments de MQs. En tots els experiments vam utilitzar el protocol de memòria basat en una pinta de freqüències atòmiques (PFA). També vam desenvolupar complexes seqüències de polsos, necessàries per a la preparació òptica d'una PFA. En el primer experiment vam demostrar l'emmagatzematge de qubits fotònics de polarització codificats en estats coherents febles. Aquest emmagatzematge es va dur a terme en els estats excitats dels ions Pr3+ durant un temps d'emmagatzematge predeterminat de 500 ns. Aquesta fita no s'havia assolit abans degut a que l'absorció òptica del material depèn de la polarització llum. Vam aconseguir fidelitats d'emmagatzematge d'un 95% de mitjana les quals sobrepassen el millor valor que es pot aconseguir amb una estratègia de mesura i preparació provant per tant el caràcter quàntic de la nostra interfície. Per poder-se implementar de manera realista en xarxes quàntiques, una MQ hauria de tenir la capacitat de recuperar la informació en-demanda (en el moment que es desitgi). Com a primer pas, el nostre següent experiment va involucrar la transferència dels polsos d'entrada cap a i des de els nivells fonamentals hiperfins i longeus dels ions Pr3+, mitjançant polsos brillants. A més, duent a terme experiments d'interferència, vam demostrar que la coherència es preserva durant els processos d'emmagatzematge, transferència i recuperació. També vam demostrar l'emmagatzematge temporalment multimodal en els estats d'espín, de fins a 5 modes. En l'última part d'aquesta tesis vam demostrar una memòria quàntica d'estat sòlid basada en ones d'espín, amb qubits codificats en estats coherents febles al nivell d'intensitat de fotons individuals. Emmagatzemar i recuperar camps òptics al nivell de fotons individuals en estats fonamentals del sistema PrYSO és exigent perquè els potents polsos de control i el polsos dèbils d'entrada que s'emmagatzemen a la memòria estan separats per només 10.2 MHz. Els polsos de control creen soroll, la majoria consistent en decaïment de lliure inducció, fluorescència i dispersió en les superfícies òptiques. Per resoldre aquest problema vam utilitzar filtratge estret de banda en freqüència i també filtratges temporal i espacial. Utilitzant un filtre estret de banda basat el la crema de forats espectrals en un segon cristall de PrYSO, vam poder aconseguir una relació senyal soroll (RSS) > 10 per a polsos d'entrada amb un número mitjà de fotons al voltant de 1. L'alta RSS que vam aconseguir ens va permetre emmagatzemar i recuperar qubits de inteval-de-temps amb fidelitats condicionals més altes una altra vegada que el que és possible amb l'estratègia de mesura i preparació. Els resultats presentats omplen un buit important en el camp de les memòries quàntiques d'estat sòlid i obren la porta a l'emmagatzematge de llarga durada d'estats de llum no-clàssics. A més, enforteixen la posició dels sistemes de IQ basats en ITR, específicament com a nodes en arquitectures de xarxes quàntiques.

Thesis (PhD) -- University of Maryland, College Park, 2007.
Thesis research directed by: Physics. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

Thesis: S.M., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2018.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 67-74).
A central goal in quantum information science is to establish entanglement across multiple quantum memories in a manner that allows individual control and readout of each constituent qubit. In the area of solid state quantum optics, a leading system is the negatively charged nitrogen vacancy center in diamond, which allows access to a spin center that can be entangled to multiple nuclear spins. Scaling these systems will require the entanglement of multiple NV centers, together with their nuclear spins, in a manner that allows for individual control and readout. Here we demonstrate a technique that allows us to prepare and measure individual centers within an ensemble, well below the diffraction limit. The technique relies on optical addressing of spin-dependent transitions, and makes use of the built-in inhomogeneous distribution of emitters resulting from strain splitting to measure individual spins in a manner that is non-destructive to the quantum state of other nearby centers. We demonstrate the ability to resolve individual NV centers with subnanometer spatial resolution. Furthermore, we demonstrate crosstalk-free individual readout of spin populations within a diffraction limited spot by performing resonant readout of one NV during a spectroscopic sequence of another. This method opens the door to multi-qubit coupled spin systems in solids, with individual spin manipulation and readout.
by Eric Bersin.
S.M.

Information processors that harness quantum mechanics may be able to outperform their classical counterparts at certain tasks. Quantum information processing (QIP) can utilize the quantum mechanical phenomenon of entanglement to implement quantum algorithms. Endohedral fullerenes, where atoms, ions or clusters are trapped in a carbon cage, are a class of nanomaterials that show great promise as the basis for a solid-state QIP architecture. Some endohedral fullerenes are spin–active, and offer the potential to encode information in their spin-states. This thesis addresses the challenges of how to engineer the components of a scalable QIP architecture based on endohedral fullerenes. It focuses on the synthesis and characterization of molecules which may, in the future, permit the demonstration of entanglement; the optical read-out of quantum states; and the creation of quasi-one-dimensional molecular arrays. Due to its long spin decoherence time, N@C₆₀ is the selected as the basic molecular unit for ‘coupled’ fullerene pairs, molecular systems for which it may be possible to demonstrate entanglement. To this end, isolated fullerene pairs, in the form of spin-bearing fullerene dimers, are created. This begins with the processing of N@C₆₀ at the macroscale and leads towards the synthesis of ¹⁵N@C₆₀-¹⁵N@C₆₀ dimers at the microscale. High throughput processing is introduced as the most efficient technique to obtain high purity N@C₆₀ on a reasonable timescale. A scheme to produce symmetric and asymmetric fullerene dimers is also demonstrated. EPR spectroscopy of the dimers in the solid-state confirms derivatization, whilst permitting the modelling of spin–spin interactions for 'coupled' fullerene pairs. This suggests that the optimum inter–spin separation for which to observe spin–spin coupling in powders is circa 3 nm. Motivated by the properties of the trivalent erbium ion for the optical detection of quantum states, optically–active erbium–doped fullerenes are also investigated. These erbium metallofullerenes are synthesized and isolated as individual isomers. They are characterized by low temperature photoluminescence spectroscopy, emitting in the infra- red at a wavelength of 1.5 μm. The luminescence is markedly different where a C₂ cluster is trapped alongside the erbium ions in the fullerene cage. Er₂C₂@C₈₂ (isomer I) exhibits emission linewidths that are comparable to those observed for Er³⁺ in crystals. Finally, the discovery of a novel praseodymium-doped fullerene is reported. The balance of evidence favours the structure being assigned as Pr₂@C₇₂. This novel endohedral fullerene forms quasi-one-dimensional arrays in carbon nanotubes, which is a useful proof-of-principle of how a scaled fullerene-based architecture may be achieved.

Dans la course à l’ordinateur quantique, le silicium est devenu ces dernières années un matériau de choix pour l'implémentation des qubits de spin. De tels dispositifs sont fabriqués au CEA en utilisant les technologies CMOS, afin de faciliter leur intégration à grande échelle. Cette thèse porte sur la modélisation de ces qubits, et en particulier sur la manipulation de l’état de spin par un champ électrique. Pour cela nous utilisons un ensemble de techniques numériques avancées pour calculer le potentiel et la structure électronique des qubits (notamment les méthodes de liaisons fortes et k.p), afin d’être le plus proche possible des dispositifs expérimentaux. Ces simulations nous ont permis d’étudier deux résultats expérimentaux d’importance : l’observation de la manipulation par champ électrique du spin d’un électron d’une part, et la caractérisation de l’anisotropie de la fréquence de Rabi d’un qubit de trou d’autre part. Le premier résultat était plutôt inattendu, étant donné; le très faible couplage spin-orbite dans la bande de conduction du silicium. Nous développons un modèle, validé par les simulations et certains résultats expérimentaux, qui met en évidence le rôle essentiel du couplage spin-orbite inter-vallée, exacerbé par la faible symétrie du système. Nous utilisons ces résultats pour proposer et tester numériquement un schéma de manipulation électrique consistant à passer réversiblement d’un qubit de spin à un qubit de vallée. Concernant les qubits de trous, le couplage spin-orbite relativement élevé autorise la manipulation du spin par champ électrique, toutefois les mesures expérimentales d’anisotropie donnent à voir une physique complexe, insuffisamment bien décrite par les modèles actuels. Nous développons donc un formalisme permettant de caractériser simplement la fréquence de Rabi en fonction du champ magnétique, et qui peut s’appliquer à d’autre type de qubit spin-orbite. Les simulations permettent de reproduire les résultats expérimentaux, et de souligner le rôle important de la contrainte
In the race for quantum computing, these last years silicon has become a material of choice for the implementation of spin qubits. Such devices are fabricated in CEA using CMOS technologies, in order to facilitate their large-scale integration. This thesis covers the modeling of these qubits andin particular the manipulation of the spin state with an electric field. To that end, we use a set numerical tools to compute the potential and electronic structure in the qubits (in particular tightbinding and k.p methods), in order to be as close as possible to the experimental devices. These simulations allowed us to study two important experimental results: on one hand the observation of the electrical manipulation of an electron spin, and on the other hand the characterization of the anisotropy of the Rabi frequency of a hole spin qubit. The first one was rather unexpected, since the spin-orbit coupling is very low in the silicon conduction band. We develop a model, confirmed by thesimulations and some experimental results, that highlights the essential role of the intervalley spinorbit coupling, enhanced by the low symmetry of the system. We use these results to propose and test numerically a scheme for electrical manipulation which consists in switching reversibly betweena spin qubit and a valley qubit. Concerning the hole qubits, the relatively large spin-orbit coupling allows for electrical spin manipulation. However the experimental measurements of Rabi frequency anisotropy show a complex physics, insufficiently described by the usual models. Therefore we developa formalism which allows to characterize simply the Rabi frequency as a function of the magnetic field, and that can be applied to other types of spin-orbit qubits. The simulations reproduce the experimental features, underline the important role of strain

This work has demonstrated that hyperfine decoherence times sufficiently long for QIP and quantum optics applications are achievable in rare earth ion centres. Prior to this work there were several QIP proposals using rare earth hyperfine states for long term coherent storage of optical interactions. The very long T_1 (~weeks ) observed for rare-earth hyperfine transitions appears promising but hyperfine T_2s were only a few ms, comparable to rare earth optical transitions and therefore the usefulness of such proposals was doubtful. ¶ This work demonstrated an increase in hyperfine T_2 by a factor of 7 × 10^4 compared to the previously reported hyperfine T_2 for Pr^[3+]:Y_2SiO_5 through the application of static and dynamic magnetic field techniques. This increase in T_2 makes previous QIP proposals useful and provides the first solid state optically active Lamda system with very long hyperfine T_2 for quantum optics applications. ¶ ...

Superconducting qubits have in recent years become a promising candidate for the implementation of a quantum computer due to their design flexibility, good protection from decohering elementary excitations, and availability of well developed fabrication and measurement techniques. Superconducting flux qubits, for which the effect of offset charge noise is reduced due to the fact that the Josephson energy dominates over the charging energy, correspond to one of the proposed means of designing a superconducting qubit. A nonlinear dispersive readout scheme of flux qubits involving a DC SQUID magnetometer that avoids the effects of on-chip dissipation can be readily implemented, yielding high contrast output for single qubit readout. Coupling schemes via nonlinear Josephson elements have also been realized. On the other hand, while the means of isolating superconducting qubits from external noise sources has been found, the mechanisms by which they undergo relaxation and decoherence due to intrinsic noise sources in the junctions themselves are not very well understood, and the question of how to deal with these noise sources remains unanswered in the general case. Other questions deal with the problem of experimentally observing entanglement in an array of coupled superconducting qubits, and finding the means by which the existence of entanglement in a typical laboratory setup may to some extent be verified by measurements on a global scale. Following a brief introductory review, we will first investigate the influence of a Two-Level Fluctuator on a DC SQUID driven by a finite current bias. Then we introduce a directly measurable signature of multiqubit entanglement for a large system of qubits and show that it is compatible to a recently introduced measure of global entanglement.
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