A neutral atom interacting with a single mode of a high finesse cavity provides an opportunity to study uncharted quantum mechanical systems and to explore the field of quantum computing and networking. Ranging from being a deterministic single photon source to a coherent storage unit for quantum information, a strong coupling cavity QED system has proven to be a powerful tool. In this thesis, single atoms are deterministically delivered over long distances and probed in an optical cavity. Once in the cavity, a single atom is stored and continuously observed for over 15 seconds. Progress towards using atoms in the cavity to produce entangled photon pairs is presented. Dual 1D optical lattices are implemented to create a foundation for advancements in two qubit quantum operations and entanglements.

Cette thèse porte sur la réalisation d'un processeur quantique hybride, dans lequel les degrés de liberté collectifs d'un ensemble de spins sont utilisés comme une mémoire quantique multimode pour les qubits supraconducteurs. Nous concevons un protocole capable de stocker et de récupérer à la demande les états d'un grand nombre de qubits dans un ensemble de spin et nous démontrons les briques de bases des opérations mémoires avec des centres NV dans le diamant. Le protocole repose sur le couplage des spins à un résonateur à fréquence et facteur de qualité accordable. Les états quantiques sont écrits par absorption résonante d'un photon micro-ondes dans l'ensemble de spins, et lus par application d'une séquence d'impulsions aux spins. L'étape d'écriture du protocole est démontrée dans une première expérience dans laquelle sont intégrés sur la même puce un qubit supraconducteur, un résonateur à fréquence accordable, et l'ensemble de spins. Les états du qubit sont stockés dans les spins via le résonateur. Après le stockage, l'état quantique collectif qui en résulte est rapidement déphasé en raison de l'élargissement inhomogène de l'ensemble et une séquence de refocalisation doit être appliquée sur les spins pour déclencher la réémission collective comme un écho de l'état quantique initialement absorbé. Dans une seconde expérience, nous démontrons une brique de base importante de cette opération de lecture, qui consiste à récupérer de multiples impulsions micro-ondes classiques au niveau du photon unique en utilisant des techniques d’écho de Hahn. Enfin, le repompage optique des spins est implémenté afin de réinitialiser la mémoire entre deux séquences successives. / This thesis work discusses the development of a hybrid quantum processor, in which collective degrees of freedom of an ensemble of spins are used as a multimode quantum memory for superconducting qubits. We design a memory protocol able to store and retrieve on demand the state of a large number of qubits in a spin ensemble and we demonstrate building blocks of its operations with NV centers in diamond. The protocol relies on the coupling of the NV ensemble to a resonator with tunable frequency and quality factor. Incoming quantum states are written by resonant absorption of a microwave photon in the spin ensemble, and then read out of the memory by applying a sequence of control pulses to the spins and to the resonator. The write step of the protocol is demonstrated in a first experiment by integrating on the same chip a superconducting qubit, a resonator with tunable frequency, and the NV ensemble. Arbitrary qubit states are stored into the spin ensemble via the resonator. After storage, the resulting collective quantum state is rapidly dephased due to inhomogeneous broadening of the ensemble and a refocusing sequence must be applied on the spins to bring them to return in phase and to re-emit collectively the quantum state initially absorbed as an echo. In a second experiment, we demonstrate an important building block of this read-out operation, which consists in retrieving multiple classical microwave pulses down to the single photon level using Hahn echo refocusing techniques. Finally, optical repumping of the spin ensemble is implemented in order to reset the memory in-between two successive sequences.

This report provides a new theoretical measure for the nonlocality of an arbitrary three-qubit pure state system similar to the method used to describe tripartite entanglement, resulting in a concept referred to as residual nonlocality, η. This report also investigates the special cases that can be encountered when using η. This method assigns a numerical value between 0 and 1 in order to indicate the degree of nonlocality between three-qubits. It was discovered that η has the characteristic of being consistently larger or equal to the value found for the residual entanglement which can provide further insights regarding the relation between nonlocality and entanglement. / I rapporten föreslås och analyseras ett nytt teoretisk mått för ickelokalitet hos tre-kvantbitsystem på ett liknande sätt till metoden som används för tredelad sammanflätningar. Detta ger en koncept som vi har valt att benämna residual ickelokalitet η. Rapporten undersöker också specialfall som kan påträffas när man använder η. Metoden som läggs fram i rapporten ger ett numeriskt värde mellan 0 och 1 för att visa graden av ickelokalitet mellan kvantbitarna. Vår undersökning visar att η kommer under alla sammanhang vara större eller lika med den graden av tredelad sammanflätning i samma system vilket kan ge en bättre förståelse av relationen mellan sammanflätning och ickelokalitet.

Quantum computers have the potential to solve certain problems in minutes that would otherwise take classical computers thousands of years due to the exponential speed-up certain quantum algorithms have over classical algorithms. In order to leverage such quantum algorithms, it is necessary for them to run on quantum devices. Examples of such devices include, but are not limited to, semiconductor and superconducting qubits, and semiconductor single and entangled photon emitters. The conventional method of constructing a semiconductor qubit is to apply gates on a semiconductor surface to localize electrons, where the electronic spin states are mapped to a qubit basis. Examples of this include the spin qubit where the spin-1/2 states of a single electron is the qubit basis and the gated singlet-triplet qubit where the states of two coupled electrons are mapped to a qubit basis. In general, gated semiconductor spin qubits are subject to decoherence from the environment which alters the electronic wavefunction by entanglement with the nuclear spins and phonons in the lattice compromising the stability of the qubit. Semiconductor nanostructures can also be designed as photon emitters. Self-assembled quantum dots are an example of such nanostructures and have been shown to emit single photons through exciton recombination and entangled photons through biexciton-exciton cascade. The difficulty in designing photon sources using self-assembled quantum dots is that the size and shape varies from dot to dot, implying that the electronic and magnetic properties also vary. In this thesis, I present the design of a single photon emitter using an InAsP quantum dot embedded in an InP nanowire and the design of a singlet-triplet qubit that is topologically protected from decoherence using an array of such quantum dots embedded in an InP nanowire. The advantage of using quantum dot nanowires over self-assembled quantum dots as photon emitters is that the quantum dot thickness, radius and composition can be controlled deterministically using a technique known as vapour-liquid-solid epitaxy which allows the emission spectrum to be engineered. Using a microscopic model, I simulated an InAsP quantum dot embedded in a nanowire with upwards of millions of atoms and showed that the emission spectrum came in agreement with the actual InAsP/InP quantum dot nanowires that were fabricated at the National Research Council of Canada. Moreover, I showed that altering the distribution of As atoms in the quantum dot can cause dramatic change in the emission spectrum. For the design of the topologically protected singlet-triplet qubit, I demonstrated that the ground state of an array of such quantum dots embedded in an InP nanowire, with four electrons in each dot, is four-fold degenerate and is topologically protected from higher energy states, making the ground state robust against perturbations. This state is known as the Haldane phase and can be understood in terms of two spin-1/2 quasiparticles at each edge of the array. Though the spectral gap in my simulation was of the order of 1 meV, this work provides insight into the potential design of a room temperature operating Haldane qubit where the spectral gap is of the order of room temperature.

<p dir="ltr">This work is the first report of optically addressable spin qubits in a semi-1D material, Boron Nitride Nanotubes (BNNTs). We perform the characterization of these spin defects and utilize their properties to do omnidirectional magnetic field sensing. We transfer these BNNTs with spin defects onto an AFM cantilever and perform scanning probe magnetometry of a 2D Nickel pattern on a gold waveguide. </p>

This thesis describes theoretical and experimental work whose main aim is the development of techniques for using trapped ⁴³Ca⁺ ions for quantum information processing. I present a rate equations model of ⁴³Ca⁺, and compare it with experimental data. The model is then used to investigate and optimise an electron-shelving readout method from a ground-level hyperfine qubit. The process is robust against common experimental imperfections. A shelving fidelity of up to 99.97% is theoretically possible, taking 100 μs. The laser pulse sequence can be greatly simplified for only a small reduction in the fidelity. The simplified method is tested experimentally with fidelities up to 99.8%. The shelving procedure could be applied to other commonly-used species of ion qubit. An entangling two-qubit quantum controlled-phase gate was attempted between a ⁴⁰Ca⁺ and a ⁴³Ca⁺ ion. The experiment did not succeed due to frequent decrystallisation of the ion pair, and strong motional decoherence. The source of the problems was never identified despite significant experimental effort, and the decision was made to suspend the experiments and continue them in an improved ion trap which is under construction. A sequence of pi-pulses, inspired by the Hahn spin-echo, was derived that is capable of greatly reducing dephasing of any qubit. If the qubit precession frequency varies with time as an nth-order polynomial, an (n+1) pulse sequence is theoretically capable of perfectly cancelling the resulting phase error. The sequence is used on a 43Ca+ magnetic-field-sensitive hyperfine qubit, with 20 pulses increasing the coherence time by a factor of 75 compared to an experiment without any spin-echo. In our ambient noise environment the well-known Carr-Purcell-Meiboom-Gill dynamic-decoupling method was found to be comparably effective.

This thesis describes experimental demonstrations of high-fidelity readout of trapped ion quantum bits ("qubits") for quantum information processing. We present direct single-shot measurement of an "optical" qubit stored in a single calcium-40 ion by the process of resonance fluorescence with a fidelity of 99.991(1)% (surpassing the level necessary for fault-tolerant quantum computation). A time-resolved maximum likelihood method is used to discriminate efficiently between the two qubit states based on photon-counting information, even in the presence of qubit decay from one state to the other. It also screens out errors due to cosmic ray events in the detector, a phenomenon investigated in this work. An adaptive method allows the 99.99% level to be reached in 145us average detection time. The readout fidelity is asymmetric: 99.9998% is possible for the "bright" qubit state, while retaining 99.98% for the "dark" state. This asymmetry could be exploited in quantum error correction (by encoding the "no-error" syndrome of the ancilla qubits in the "bright" state), as could the likelihood values computed (which quantify confidence in the measurement outcome). We then extend the work to parallel readout of a four-ion string using a CCD camera and achieve the same 99.99% net fidelity, limited by qubit decay in the 400us exposure time. The behaviour of the camera is characterised by fitting experimental data with a model. The additional readout error due to cross-talk between ion images on the CCD is measured in an experiment designed to remove the effect of qubit decay; a spatial maximum likelihood technique is used to reduce this error to only 0.2(1)x10^{-4} per qubit, despite the presence of ~4% optical cross-talk between neighbouring qubits. Studies of the cross-talk indicate that the readout method would scale with negligible loss of fidelity to parallel readout of ~10,000 qubits with a readout time of ~3us per qubit. Monte-Carlo simulations of the readout process are presented for comparison with experimental data; these are also used to explore the parameter space associated with fluorescence detection and to optimise experimental and analysis parameters. Applications of the analysis methods to readout of other atomic and solid-state qubits are discussed.

Cette thèse porte sur la dynamique quantique dans un dcSQUID inductif. Ce dispositif est une boucle supraconductrice interrompue par deux jonctions Josephson. Sa dynamique est analogue à celle d'une particule massive évoluant dans un potentiel bidimensionnel. Dans la limite quantique, le dcSQUID se comporte comme un atome artificiel à deux degrés de liberté, contrôlé par le courant et le flux de polarisation. Dans la limite où l'inductance de la boucle est petite devant celle des jonctions, celles-ci sont fortement couplées. La dynamique du circuit est alors celle d'un oscillateur anharmonique quantique unidimensionnel. Dans la limite des deux premiers niveaux d'énergie, ce circuit est un qubit de phase. Jusqu'alors la décohérence dans ce circuit était dominée par le bruit en courant. Nous montrons, par des mesures de spectroscopie et d'oscillations cohérentes, que l'effet du bruit en courant s'annule à courant de polarisation nul, permettant une augmentation des temps de cohérence. Dans la limite où l'inductance de la boucle est grande devant celle des jonctions, la dynamique devient bidimensionnelle. Le circuit exhibe alors un spectre d'énergie riche qui peut être décrit comme celui de deux oscillateurs anharmoniques couplés, correspondant aux modes d'oscillations symétrique et antisymétrique des phases des deux jonctions. Nous mettons en évidence ce spectre par des mesures de spectroscopie et nous démontrons la manipulation cohérente des états quantiques de chaque mode. En particulier nous mettons en évidence un couplage non-linéaire entre les deux modes, dans une limite de couplage fort. Ce couplage nous permet alors d'observer des oscillations cohérentes entre les deux modes internes de cet atome artificiel. De plus, dans ce manuscrit, nous présentons une technique innovante de fabrication de jonctions métalliques par évaporations sous angles qui n'a pas recours à un pont de résine suspendu. Finalement nous proposons un modèle simple basé sur les effets de chauffage qui explique pour la première fois une anomalie récurrente observée dans les caractéristiques courant-tension des dcSQUID. / This thesis focuses on the quantum dynamics in inductive dcSQUID. This device is a superconducting loop interrupted by two Josephson junctions. Its dynamics can be described as a massive fictitious particle in a two dimensional potential. A dcSQUID behaves as an artificial atom with two degrees of freedom, controlled by current and flux bias. When the loop inductance is smaller than the Josephson inductance, the junctions are strongly coupled. The device is then described as a one dimensional quantum anharmonic oscillator. In the limit of the two lowest energy levels, a dcSQUID is a phase qubit. Until now decoherence was dominated by the current noise. We show by spectroscopic measurement and coherent oscillations measurement that the effect of the current noise vanishes at zero current bias, enabling longer coherence times. When the loop inductance is larger than the Josephson inductance, the dynamics becomes two dimensional. The device exhibits a rich energy spectrum which can be describe as the one of two coupled anharmonic oscillators, corresponding to symmetric and antisymmetric oscillations modes of the phases across each junctions. We present spectroscopic measurement of this spectrum. We demonstrate the coherent manipulation of the quantum states of each mode. We show evidence of non linear coupling between the modes, in the strong coupling regime. This coupling enables the measurement of coherent oscillations between the internal modes of this artificial atom. In addition we present a novel fabrication technique that allows metallic junction fabrication by angle evaporation without the use of suspended bridge of resist. We propose also a simple model based on heating effects that explain for the first time a frequent anomaly in the IV characteristic of dcSQUID.

Submitted by Luciclea Silva (luci@ufpa.br) on 2018-11-01T17:20:15Z No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Tese_EstadosligadosMajorana.pdf: 5157889 bytes, checksum: a697f1a4428dbb52f1b3046f7b793221 (MD5) / Approved for entry into archive by Luciclea Silva (luci@ufpa.br) on 2018-11-01T17:20:40Z (GMT) No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Tese_EstadosligadosMajorana.pdf: 5157889 bytes, checksum: a697f1a4428dbb52f1b3046f7b793221 (MD5) / Made available in DSpace on 2018-11-01T17:20:40Z (GMT). No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Tese_EstadosligadosMajorana.pdf: 5157889 bytes, checksum: a697f1a4428dbb52f1b3046f7b793221 (MD5) Previous issue date: 2018-10-11 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / A pesquisa em física da matéria condensada com isolantes e supercondutores topológicos tem contribuído muito para a caracterização das propriedades de superfície e de modos zero em nanofios. Investigamos teoricamente o transporte de elétrons através do ponto quântico em forma de T (PQ) com um único nível e spinless, conectado a uma cadeia em zigue zague e acoplado a um supercondutor p-wave, utilizando o método analítico recursivo de funções de green no equilíbrio. Este modelo é uma extensão da cadeia de Kitaev para uma rede triangular de tamanho finito com três, quatro e cinco sitios. Observamos que modos zero de Majorana podem ser sintonizados através dos parâmetros de acoplamento do dispositivo e a condutância linear mostra ambos os Estados Ligados de Majorana (MBS) na fase topológica e na fase topológica geral maximamente robusta. Este modelo mais realista permite a detecção de MBS através do controle dos parâmetros que governam o tunelamento eletrônico e pode ser útil para experimentos relevantes. / The research in condensed matter physics with insulators and superconductors topological has contributed greatly to the characterization of the surface properties and modes zero in nanowires. We investigate theoretically, through the recursive Green’s function approach, the electron transport through the T-shaped quantum dot (PQ) with a single level and spinless, connected to a zigue zague chain and coupled to a p-wave superconductor. This model is an extension of the Kitaev chain for a network triangular of finite-size with for three, four, and five sites. We find that the Majorana zero modes can be tuned through the coupling parameters of the device and the linear conductance show both the Majorana Bound States (MBS) in topological phase and in the general topological phase maximally robust. This more realistic model allows the detection of MBS through of the control of the parameters governing the electronic tunneling and can be helpful for relevant experiments. Keywords: Majorana Fermions, recursive Green’s function approach, Kitaev chain, class of electronic nanodevices, Quantum Dot, T-shaped, Qubit, Majorana Bound States (MBS).

During this thesis, we have used superconducting circuits with Josephson junctions, derived from the Cooper pair box, in order to implement quantum bits (qubits). <br />To implement two-qubit gates, we have developed a new circuit, the quantroswap, which consists in two capacitively coupled Cooper pair box, each of them being manipulated and read separately. We have demonstrated coherent exchange of energy between them, but we have also observed a problem of qubit instability.<br />In order to avoid this spurious effect, we have implemented another circuit based on a charge insensitive split Cooper pair box coupled to a non-linear resonator for readout-out purpose. We have measured large coherence time, and obtained large readout fidelity (90%) using the bifurcation phenomenon. <br />For metrological purpose, microwave reflectometry measurement on a quantronium also allowed us to relate an applied current I to the frequency f=I/2e of induced Bloch oscillations.