Dissertations / Theses on the topic 'Qubits'

As correlações quânticas desempenham um papel importante na computação quântica. Em comparação com a computação clássica, muitas tarefas podem ser implementadas com uma eficiência significativamente maior quando são usadas as propriedades quânticas de um sistema. O emaranhamento quântico é considerado o principal recurso físico, responsável na melhoria da eficiência de tarefas computacionais. Neste trabalho, é destacada a existência de correlações quânticas que vão além do emaranhamento. Cada átomo de um par de átomos de dois níveis, inicialmente emaranhados, é colocado numa cavidade óptica independente. A dinâmica do emaranhamento dos estados reduzidos dos átomos exibe o fenômeno de morte súbita e renascimento do emaranhamento, que apresenta um cenário interessante para o estudo de correlações de dois corpos. Não é considerado a interação entre o sistema quântico e o ambiente. Negatividade da transposta parcial do estado de qubits atômicos é usado como medida de emaranhamento do estado de dois qubits. Foram obtidas expressões analíticas para a entropia condicional e a discordância quântica, seguido de cálculos numéricos. Para examinar as relações entre correlações presentes no estado composto e a pureza do estado atômico, foi feito também um cálculo na primeira ordem de aproximação para a discordância. Isso implicou na substituição da entropia de Von Neumann pela entropia linear na definição da discordância quântica. Foi usado um programa escrito em fortran para gerar os valores numéricos dos quantificadores de correlações clássicas e quânticas, para casos onde o quadrado de negatividade do estado atômico tem valores inicias 0,35 e 0,90. A discordância quântica, plotada como função do parâmetro de interação tem valores positivos durante o intervalo entre a morte súbita e o renascimento, isso é, o intervalo de tempo durante o qual o emaranhamento livre entre os qubits atômicos é zero, exceto onde o estado é separável. Também foi calculada a discordância quântica no contexto dos operadores de medida fraca, de maneira a estudar as correlações chamadas de super discordância quântica. Os gráficos que correspondem ao uso de operadores de medida fraca mostram um aumento nas correlações quânticas comparado com a discordância quântica usual obtida através de medidas projetivas ou de Von Neumann.
Quantum correlations play an important role in quantum computation. In comparison with classical computation, several tasks can be implemented with significantly higher efficiency when quantum properties of a system are used. Quantum entanglement is considered the main physical resource, responsible for improving the efficiency of computational tasks. In this work, the existence of quantum correlations that go beyond entanglement is highlighted. Each atom of a pair of two level atoms in entangled state, is placed in an independent optical cavity. Entanglement dynamics of the two atom reduced state exhibits the phenomenon of sudden death and rebirth of entanglement, which presents an interesting scenario to study two body correlations. Interaction of the quantum system with environment is not considered. Negativity of partial transpose of the state of atomic qubits is used as the measure of entanglement of two qubit state. Analytical expressions have been obtained for conditional entropy and quantum discord, followed by numerical calculations. To examine the relationship between the correlations present in the composite state and the purity of atomic state, calculation of first order approximation to discord have also been done. It involves replacing von Neumann entropy by linear entropy in the definition of quantum discord. A program written in fortran has been used to generate numerical values of quantifiers of classical and quantum correlations for cases where the entangled initial state of atoms has squared negativity value of 0,35 and 0,90. Quantum discord, plotted as a function of interaction parameter, is found to be positive definite during the interval between sudden death and rebirth, that is, the time interval during which the free entanglement between atomic qubits is zero, except where the state is separable. Quantum discord has also been calculated in the context of weak measurement operators, in order to study the correlations called super quantum discord. The graphs corresponding to the use of weak measurement operators show an increase in quantum correlations as compared to the usual quantum discord obtained through projective von Neumann measurement.

Thesis (Ph. D.) -- University of Maryland, College Park, 2004.
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.

Conventional computing based on classical technologies is approaching its limits. Therefore scientists are starting to consider the applications of quantum mechanics as a means for constructing more powerful computers. After proposing theoretical methods, many experimental setups have been designed to achieve quantum computing in reality. This thesis gives some background information on the subject of quantum computing. We first review the concept of quantum entanglement, which plays a key role in quantum computing, and then we discuss the physics of the SQUIDs-cavity method proposed by Yang et al., and give the definitions of quantum gates which are the elements that are needed to construct quantum circuits. Finally we give an overview of recent developments of SQUIDs-cavity systems and quantum circuits after Yang et al.'s proposal in 2003. These new developments help to take a step towards the constructions of higher levels of quantum technologies, e.g. quantum algorithms and quantum circuits.

Quantum computation has been a major focus of research in the past two decades, with recent experiments demonstrating basic algorithms on small numbers of qubits. A large-scale universal quantum computer would have a profound impact on science and technology, providing a solution to several problems intractable for classical computers. To realise such a machine, today's small experiments must be scaled up, and a system must be built which provides control and measurement of many hundreds of qubits. A device of this scale is challenging: qubits are highly sensitive to their environment, and sophisticated isolation techniques are required to preserve the qubits' fragile states. Solid-state qubits require deep-cryogenic cooling to suppress thermal excitations. Yet current state-of-the-art experiments use room-temperature electronics which are electrically connected to the qubits. This thesis investigates various scalable technologies and techniques which can be used to control quantum systems. With the requirements for semiconductor spin-qubits in mind, several custom electronic systems, to provide quantum control from deep cryogenic temperatures, are designed and measured. A system architecture is proposed for quantum control, providing a scalable approach to executing quantum algorithms on a large number of qubits. Control of a gallium arsenide qubit is demonstrated using a cryogenically operated FPGA driving custom gallium arsenide switches. The cryogenic performance of a commercial FPGA is measured, as the main logic processor in a cryogenic quantum control system, and digital-to-analog converters are analysed during cryogenic operation. Recent work towards a 100-qubit cryogenic control system is shown, including the design of interconnect solutions and multiplexing circuitry. With qubit fidelity over the fault-tolerant threshold for certain error correcting codes, accompanying control platforms will play a key role in the development of a scalable quantum machine.

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Financiadora de Estudos e Projetos
In this dissertation we approached the tomography of discrete systems, understood as the representation and the reconstruction of states. For the tomography we used distribution functions of symmetrical pseudo-probabilities. The coefficients of the distribution function of "probabilities" are "joint probabilities", that eventually can be negative, associated to incompatible observables. The "negative probabilities" contains not only information about the measurements of counts, but also on the quantum state of the systems. We present the argument of Scully, Walther and Schleich that uses double slit interference to give a meaning to the "negative probabilities" and we propose one alternative example using beam splitters and an single photon. Like this, moreover we define distribution functions based in generalized quantization axes for any directions, we present the physical interpretation of the resulting "negative probabilities". We showed the reason because all explanation usually done to justify "negative probabilities" seems to be contradictory and are not convincing. Is the interpretation of the "negative probabilities" that retain the heart, not only of the present work, but also, of the whole Quantum Mechanics, its only mystery, as Feynman says
Nesta dissertação abordamos a tomografia de sistemas discretos, entendida como a representação e a reconstrução de estados. Para a tomografia utilizamos funções distribuição de pseudo-probabilidades simétricas. Os coeficientes dessa função distribuição de "probabilidades" são "probabilidades conjuntas", que eventualmente podem ser negativas, associadas a observáveis incompatíveis. As "probabilidades negativas" contém não só informação sobre as medições de contagens, mas também sobre o estado quântico dos sistemas. Apresentamos o argumento de Scully, Walther e Schleich que utiliza interferência na dupla-fenda para dar um significado às "probabilidades negativas" e propomos um exemplo alternativo utilizando divisores de feixe e um único fóton. Assim, além de definir funções de distribuição baseadas em eixos de quantização generalizados para direções quaisquer, apresentamos a interpretação física das "probabilidades negativas" decorrentes. Mostramos porque toda explicação que possa ser feita para justificar "probabilidade negativa" parece ser contraditória e não é convincente. É na interpretação das pseudo-probabilidades onde está o coração não só do presente trabalho, mas também, de toda a Mecânica Quântica, o seu único mistério, como diz Feynman

Thesis (Ph. D.) -- 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.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 85-87).
Experiments in quantum optics have long been implemented with atoms in 3D free space or with atoms interacting with cavities. Over the past decade, the field of microwave quantum optics using superconducting circuits has gained a tremendous amount of attention. In particular, the confinement of photonic modes to 1D enables a new parameter regime of strong interactions between qubits and open waveguides. In these setups, known as waveguide quantum electrodynamics (WQED), superconducting qubits interact with a continuum of propagating photonic modes. In this thesis, we will explore the physics of WQED devices that consist of multiple qubits and their potential application to quantum information and simulation.
by Bharath Kannan.
S.M.

A full-scale quantum computer requires physical qubits that can be controlled with high precision and accuracy. Unfortunately, few state-of-the-art qubits can perform all their elementary operations (preparation, measurement, single-qubit gates and two-qubit gates) with sufficient fidelity. In this thesis, we investigate alternative schemes for such operations in solid-state qubits. Specifically, two-qubit gates and measurements, which are often the noisiest of the four. We first provide a preliminary introduction to quantum computing, and describe how quantum information can be encoded and manipulated in quantum systems. We include background information for the three different solid-state qubit architectures that feature in this thesis: spin qubits, superconducting qubits and Majorana qubits. Following this, we investigate a scheme for mediating a two-qubit interaction between spin qubits via a multielectron quantum dot. We study a multielectron dot in detail, and characterise its exchange interaction with a single spin. With the aid of a theoretical model, we show that the multielectron dot possesses an irregular triplet-preferring ground state, analogous to Hund's rule from atomic physics. Using these findings, we then demonstrate that the multielectron dot can be used to mediate a fast, long-range exchange interaction between two spin qubits. Subsequently, we examine two resonator-based measurement schemes for Majorana qubits. We first propose a readout technique based on a longitudinal qubit-resonator interaction. This leads to a measurement that is fast, high-fidelity and quantum non-demolition (QND). We then investigate a more conventional dispersive readout scheme. Not only does this yield a high quality measurement, but it can also offers a more protected readout mechanism in comparison to the dispersive readout of conventional superconducting qubits.

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.

A quantum computer is an information processing machine, much like an ordinary classical computer, but its function is based on quantum mechanical principles. To be able to construct such a machine would be a fantastic accomplishment---to have total control over a quantum system is a dream for both physicists and science-fiction enthusiasts. The basic information unit in a quantum computer is the quantum bit, or qubit for short. A quantum computer consists of many coupled qubits. To get a single qubit to work properly, would be a major step towards building this machine.

Here we study two different qubit ideas. The central element in both setups is the superconducting tunnel junction---the Josephson junction. By connecting the Josephson junctions to standard electronics in a clever way, a qubit can be realised. With these constructions it is in principle very easy to manipulate and read out the quantum probabilities, by varying voltages and currents in time. However, this ease of manipulation has a cost: strong interactions with uncontrolled degrees of freedom of the environment transfer information from the qubit. For superconducting qubits this decoherence is typically very fast.

There are ways to deal with the decoherence. One way is to tune the circuit parameters so that the decoherence becomes minimal. Another way is to engineer the qubits so fast so that the effect of decoherence becomes small. In this thesis, we will apply both these strategies. Specifically, the measurement speed of the second qubit we study, turns out to be very sensitive to the topology of the phase space of the detector variables.

Most neutral atom quantum computing experiments rely on destructive state detection techniques that eject the detected qubits from the trap. These techniques limit the repetition rate of these experiments due to the necessity of reloading a new quantum register for each operation. We address this problem by developing reusable neutral atom qubits. Individual Rubidium 87 atoms are trapped in an optical lattice and are held for upwards of 300 s. Each atom is prepared in an initial quantum state and then the state is subsequently detected with 95% fidelity with less than a 1% probability of losing it from the trap. This combination of long storage times and nondestructive state detection will facilitate the development of faster and more complex quantum systems that will enable future advancements in the field of quantum information.

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.

Thesis: S.M. in Computer Science and Engineering, 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 91-95).
Most quantum control and quantum error-correction protocols assume that the noise causing decoherence is described by Gaussian statistics. However, the Gaussianity assumption breaks down when the quantum system is strongly coupled to a sparse environment or has a non-linear response to external degrees of freedom. Here, we experimentally validate an open-loop quantum control protocol that reconstructs the higher-order spectrum of a non-Gaussian dephasing process using a superconducting qubit as a noise spectrometer. This experimental demonstration of non-Gaussian noise spectroscopy protocol represents a major step towards the goal of demonstrating a complete noise spectral characterization of quantum devices.
by Youngkyu Sung.
S.M. in Computer Science and Engineering

Dans cette thèse, nous développons plusieurs outils pour la Correction d’Erreur Quantique (CEQ) autonome avec les qubits supraconducteurs.Nous proposons un schéma de CEQ autonome qui repose sur la technique du « reservoir engineering », dans lequel trois qubits de type transmon sont couplés à un ou plusieurs modes dissipatifs. Grâce à la mise au point d’une interaction effective entre les systèmes, l’entropie créée par les éventuelles erreurs est évacuée à travers les modes dissipatifs.La deuxième partie de ce travail porte sur un type de code récemment développé, le code des chats, à travers lequel l’information logique est encodée dans le vaste espace de Hilbert d’un oscillateur harmonique. Nous proposons un protocole pour réaliser des mesures continues et non-perturbatrices de la parité du nombre de photons dans une cavité micro-onde, ce qui correspond au syndrome d’erreur pour le code des chats. Enfin, en utilisant les résultats précédents, nous présentons plusieurs protocoles de CEQ continus et/ou autonomes basés sur le code des chats. Ces protocoles offrent une protection robuste contre les canaux d’erreur dominants en présence de dissipation stimulée à plusieurs photons
In this thesis, we develop several tools in the direction of autonomous Quantum Error Correction (QEC) with superconducting qubits. We design an autonomous QEC scheme based on quantum reservoir engineering, in which transmon qubits are coupled to lossy modes. Through an engineered interaction between these systems, the entropy created by eventual errors is evacuated via the dissipative modes.The second part of this work focus on the recently developed cat codes, through which the logical information is encoded in the large Hilbert space of a harmonic oscillator. We propose a scheme to perform continuous and quantum non-demolition measurements of photon-number parity in a microwave cavity, which corresponds to the error syndrome in the cat code. In our design, we exploit the strongly nonlinear Hamiltonian of a highimpedance Josephson circuit, coupling ahigh-Q cavity storage cavity mode to a low-Q readout one. Last, as a follow up of the above results, we present several continuous and/or autonomous QEC schemes using the cat code. These schemes provide a robust protection against dominant error channels in the presence of multi-photon driven dissipation

Electron spins confined in semiconductor quantum dots are emerging as a promising system to study quantum information science and to perform sensitive metrology. Their weak interaction with the environment leads to long coherence times and robust storage for quantum information, and the intrinsic tunability of semiconductors allows for controllable operations, initialization, and readout of their quantum state. These spin qubits are also promising candidates for the building block for a scalable quantum information processor due to their prospects for scalability and miniaturization. owever, several obstacles limit the performance of quantum information experiments in these systems. For example, the weak coupling to the environment makes inter-qubit operations challenging, and a fluctuating nuclear magnetic field limits the performance of single-qubit operations. The focus of this thesis will be several experiments which address some of the outstanding problems in semiconductor spin qubits, in particular, singlet-triplet (S-T0) qubits. We use these qubits to probe both the electric field and magnetic field noise that limit the performance of these qubits. The magnetic noise bath is probed with high bandwidth and precision using novel techniques borrowed from the field of Hamiltonian learning, which are effective due to the rapid control and readout available in S-T0 qubits. These findings allow us to effectively undo the undesired effects of the fluctuating nuclear magnetic field by tracking them in real-time, and we demonstrate a 30-fold improvement in the coherence time T2*. We probe the voltage noise environment of the qubit using coherent qubit oscillations, which is partially enabled by control of the nuclear magnetic field. We find that the voltage noise bath is frequency- dependent, even at frequencies as high as 1MHz, and it shows surprising and, as of yet, unexplained temperature dependence. We leverage this knowledge of the voltage noise environment, the nuclear magnetic field control, as well as new techniques for calibrated measurement of the density matrix in a singlet-triplet qubit to entangle two adjacent single-triplet qubits. We fully characterize the generated entangled states and prove that they are, indeed, entangled. This work opens new opportunities to use qubits as sensors for improved metrological capabilities, as well as for improved quantum information processing. The singlet-triplet qubit is unique in that it can be used to probe two fundamentally different noise baths, which are important for a large variety of solid state qubits. More specifically, this work establishes the singlet-triplet qubit as a viable candidate for the building block of a scalable quantum information processor.
Physics

This thesis presents a series of quantum dot studies, performed with an eye towards improved conventional and topological qubits. Chapters 1-3 focus on improved conventional (spin) qubits; Chapters 4-6 focus on the topological Majorana qubits. Chapter 1 presents the first investigation of Coulomb peak height distributions in a spin-orbit coupled quantum dot, realized in a Ge/Si nanowire. Strong spin-orbit coupling in this hole-gas system leads to antilocalization of Coulomb blockade peaks, consistent with theory. In particular, the peak height distribution has its maximum away from zero at zero magnetic field, with an average that decreases with increasing field. Magnetoconductance in the open-wire regime places a bound on the spin-orbit length (lso < 20 nm), consistent with values extracted in the Coulomb blockade regime (lso < 25 nm). Chapters 2 & 3 demonstrate operation of improved spin qubits. Chapter 2 continues the investigation of Ge/Si nanowires, demonstrating a qubit with tenfold-improved dephasing time compared to the standard GaAs case. e combination of long dephasing time and strong spin-orbit coupling suggests that Ge/Si nanowires are promising for a spin-orbit qubit. In Chap. 3, multi-electron spin qubits are operated in GaAs, and improved resilience to charge noise is found compared to the single-electron case. Chapters 4 & 5, present a series of studies on composite superconductor/semiconductor Al/InAs quantum dots. Detailed study of transport cycles and Coulomb blockade peak spacings in zero magnetic field are presented in Chap. 4, and the parity lifetime of a bound state in the nanowire is inferred to exceed 10 milliseconds. Next, in Chap. 5, finite magnetic field behavior is investigated while varying quantum dot length. Coulomb peak spacings are consistent with the emergence of Majorana modes in the quantum dot. The robustness of Majorana modes to magnetic-field perturbations is measured, and is found to be exponential with increasing nanowire length. Coulomb peak heights are also investigated, and show signatures of electron teleportation by Majorana fermions. Finally, Chap. 6 outlines some schemes to create topological Majorana qubits. Using experimental techniques similar to those in Chap.’s 2 & 3, it may be possible to demonstrate Majorana initialization, readout, and fusion rules.
Physics

Superconducting qubits and resonators with quality factors exceeding 10⁷ are of great interest for quantum information processing applications. The improvement of present devices necessarily involves the consideration of participation ratios, which budget the influence of each physical component in the total energy decay rate. Experiments on compact resonators in which participation ratios were varied has demonstrated the validity of this method, yielding a two-fold improvement in quality factor. Similar experiments on compact transmon qubit devices led to a three-fold improvement over previous transmons, validating the method of participation ratios for qubits as well. Through the use of a 3D cavity, a further minimization of the participation of surface components combined with the removal of unnecessary components, produced an additional ten-fold increase in coherence times. Finally, the fluxonium qubit was redesigned in a similar minimalist environment with an improved superinductance, thus combining the advantages of the 3D architecture with the natural insensitivity to dissipation of the fluxonium, resulting in another tenfold increase in relaxation times. This large increase in relaxation and coherence times enables experiments that were previously impossible, thus preparing the field of quantum information to advance on other fronts.

In this thesis, the entangled mixed states of two qubits are considered. In the case where the matrix rank of the corresponding density matrix is 2, such a state can be purified to a pure state of 3 qubits. By utilizing this representation, the classification of such states of two qubits by stochastic local operations assisted by classical communication (SLOCC) is obtained. Also for such states, the optimal ensemble that appears in the computation of the concurrence and entanglement of formation is obtained.

I discuss the design, fabrication and measurement at millikelvin-temperatures of Al/AlO_x/Al Josephson junction-based transmon qubits coupled to superconducting thin-film lumped element microwave resonators made of aluminum on sapphire. The resonators had a center frequency of around 6GHz, and a total quality factor ranging from 15,000 to 70,000 for the various devices. The area of the transmon junctions was about 150 nm × 150 nm and with Josephson energy E_J such that 10GHz ≤ E_J ≤ 30 GHz. The charging energy of the transmons arising mostly from the large interdigital shunt capacitance, was E_c/h ≈ 300MHz.

I present microwave spectroscopy of the devices in the strongly dispersive regime of circuit quantum electrodynamics. In this limit the ac Stark shift due to a single photon in the resonator is greater than the linewidth of the qubit transition. When the resonator is driven coherently using a coupler tone, the transmon spectrum reveals individual "photon number'' peaks, each corresponding to a single additional photon in the resonator. Using a weighted average of the peak heights in the qubit spectrum, I calculated the average number of photons n¯ in the resonator. I also observed a nonlinear variation of n¯ with the applied power of the coupler tone P_rf. I studied this nonlinearity using numerical simulations and found good qualitative agreement with data.

In the absence of a coherent drive on the resonator, a thermal population of 5.474 GHz photons in the resonator, at an effective temperature of 120 mK resulted in a weak n = 1 thermal photon peak in the qubit spectrum. In the presence of independent coupler and probe tones, the n = 1 thermal photon peak revealed an Autler-Townes splitting. The observed effect was explained accurately using the four lowest levels of the dispersively dressed Jaynes-Cummings transmon-resonator system, and numerical simulations of the steady-state master equation for the coupled system.

I also present time-domain measurements on transmons coupled to lumped-element resonators. From T₁ and Rabi oscillation measurements, I found that my early transmon devices (called design LEv5) had lifetimes (T₁ ∼ 1 μs) limited by strong coupling to the 50 Ω transmission line. This coupling was characterized by the the rate of change of the Rabi oscillation frequency with the change in the drive voltage (df_Rabi /dV) – also termed the Rabi coupling to the drive. I studied the design of the transmon-resonator system using circuit analysis and microwave simulations with the aim being to reduce the Rabi coupling to the drive. By increasing the resonance frequency of the resonator ω_r/2π from 5.4 GHz to 7.2 GHz, lowering the coupling of the resonator to the transmission line and thereby increasing the external quality factor Q_e from 20,000 to 70,000, and reducing the transmon-resonator coupling g/2π from 70 MHz to 40 MHz, I reduced the Rabi coupling to the drive by an order of magnitude (∼ factor of 20). The T ₁ ∼ 4 μs of devices in the new design (LEv6) was longer than that of the early devices, but still much shorter than the lifetimes predicted from Rabi coupling, suggesting the presence of alternative sources of noise causing qubit relaxation. Microwave simulations and circuit analysis in the presence of a dielectric loss tangent tan δ ≃ 5 × 10 ^-6 agree reasonably well with the measured T ₁ values, suggesting that surface dielectric loss may be causing relaxation of transmons in the new designs.

This thesis presents research on the initialization, control, and readout of electron spin states in gate defined GaAs quantum dots. The first three experiments were performed with Singlet-Triplet spin qubits in double quantum dots, while the remaining two experiments were performed with an Exchange-Only spin qubit in a triple quantum dot.
Physics

This thesis describes the development of an entanglement experiment for ytterbium ions making use of a new entanglement method utilizing microwaves and a static magnetic field gradient. This thesis will begin by modelling the populations of the main levels in ytterbium using rate equations to find the optimum parameters required for the preparation and detection of qubit states. Coherent manipulation of these qubit states will be shown and coherence times of the states measured. Additionally a highly stable double resonance frequency locking setup for the ytterbium cooling lasers is built. This thesis will go on to give an overview of the main entanglement schemes and will give a justification as to why microwaves combined with a magnetic field gradient is the most suitable method. The magnetic field gradient creates an effective Lamb-Dicke parameter which allows microwave fields to couple to the motional states of magnetic field sensitive qubit states. The use of magnetic field sensitive states can however make the qubit highly susceptible to decoherence from magnetic field fluctuations. A method to decrease this decoherence by two orders of magnitude using a microwave dressed state qubit will be demonstrated and optimised and a new coherent manipulation method of the dressed state qubit will be presented which allows for arbitrary Bloch sphere rotations. The production of the highest recorded magnetic field gradient of 24Tm⁻¹ at the position of the trapped ion using in-vacuum permanent magnets is shown and used to provide individual addressing of ions. Static gradient microwave entanglement of a single ion's internal and motional states within the bare qubit states is then demonstrated (Schrodinger cat states). Furthermore, the first ever observation of motional coupling of the microwave dressed state qubit is shown and progress towards a two ion entanglement gate with microwave dressed state qubits is reported.

This thesis describes work on numerical modelling of the 43Ca+ ion in a Paul trap using the optical Bloch equations. This is a challenging system to study, with many states involved in the internal dynamics. A major outcome is the development of a cooling scheme for the 146.09 gauss atomic clock transitions which makes use of a dark resonance. It is much more effective than methods that avoid coherent effects. The scheme is realised in experiment. Complicated fluorescence data is modelled very well, and predictions for the ion's motional temperature show good agreement with measured values. Data and fits for an ion that has been Doppler cooled below the Doppler limit are presented. I describe GLOBES, a set of routines that simulates an arbitrary ion in the presence of an arbitrary system of laser beams using the optical Bloch equations. Techniques used to efficiently calculate the steady state, analyse fluorescence scans and solve time-dependent problems for small and large times are discussed. A new routine SILVER IMPER that leapfrogs over the initial dynamics to model the approach to the steady state is introduced. Doppler cooling in 40Ca+ is analysed and two extensions made to the basic theory. The 'excursion method' of calculation takes account of the non-linear variation with velocity of the scattering rate. The 'dynamic method' allows for the fact that the ion may not be in equilibrium with the incident radiation during its oscillations, a necessity as the timescale of the external motion is of order the characteristic timescale of the internal motion for standard secular frequencies. This 'dynamic effect' is a general property of trapped ion systems and is also observed in a two-state system. A two-variable fluorescence scan taken from a four-laser, five-level system in 40Ca+ is analysed. Techniques to fit large data sets and automatically resolve resonant features are demonstrated. A general treatment of resonant behaviour in three, four and five level pump/probe systems is used to describe the data. This is verified by a second, tailor-made set of scans.

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

The logic Lq, introduced within this thesis, is a logic for quantum information. The purpose was, in fact, to describe logically the qubit structure (that is, the intrinsic quantum superposition of a two-level quantum state) and the maximal quantum entanglement of two qubits. The logic Lq is obtained via Sambin’s reflection principle of Basic logic, by which the metalinguistic links among assertions reflect (by solving a definitional equation) into logical connectives among propositions. However, while in Basic logic the metalanguage is classical, in our case it is quantum. In the quantum metalanguage each atomic assertion carries along an assertion degree, a complex number, which is interpreted as a probability amplitude. It is just the presence of assertion degrees that allows the introduction of the connective “quantum superposition” in Lq. This connective is a generalization of the logical conjunction “and”. It is labelled by complex numbers indicating the weight by which each proposition contributes to the compound proposition. The truth-values (or truth-degrees) are the squared modules of the assertion degrees, and their range is the real interval [0,1]. Then, the logic Lq is many-valued. Differently from fuzzy logics, however, the truth-degrees are interpreted here as quantum-mechanical probabilities. The logic Lq keeps the three main properties of Basic logic, namely symmetry, reflection and visibility. This choice has been dictated by the following considerations: 1) The no-cloning and no-erase theorems of quantum information do not allow the corresponding logic to have the structural rules of weakening and contraction, with which they disagree. This fact rules out, in the search of a logic for quantum information, every kind of structural logic. 2) Choosing Basic logic instead of Linear logic (the other main sub-structural logic) was due to the fact that without visibility the connective “quantum entanglement” cannot be introduced. Furthermore, we looked for a logic of quantum information which was endowed with a deductive calculus, (in particular a sequent calculus). The logic Lq appears, in so far, as the only one which can take into account all the above desiderata. The interpretation of the assertions of the quantum metalanguage is given in terms of quantum states (the quantum metalanguage “is” the Hilbert space). The interpretation of the propositions of Lq is given in terms of (non-hermitian) operators which are weak measurements. Then, the interpretation of Lq is based on a generalization of the concepts already proposed by Birkhoff and von Neumann in “orthodox” quantum logic. The difference stands in the fact that the interpretation of Lq is not given in terms of projectors, but in terms of weak measurements, which do not give rise to an abrupt collapse of quantum wave functions. This allows a logical description of quantum superposition, because the latter is not destroyed. The possibility of interpreting propositions as weak measurements is due to the fact that we introduced a quantum metalanguage. In fact, in the interpretation of propositions, the complex factors multiplying the projection operators are nothing else than the assertion degrees. Some results of this thesis are: a) The adoption of a new kind of metalanguage, the quantum metalanguage, where the metalinguistic links are quantum correlations, and assertions have a complex assertion-degree. b) The introduction, through the reflection principle, of new (quantum) connectives, like “quantum superposition”, and “quantum entanglement”. c) The introduction of a new dual operation (which is a generalization of Sambin-Girard logical duality) to take into account the dual Hilbert space occurring in the interpretation. d) A quantum cut rule, which is interpreted as a quantum projective measurement. As the cut is a meta-rule, it follows that a quantum machine cannot perform a self-measurement and destroy itself. e) A new meta-rule, not equivalent to the cut, named “EPR rule” (to remind the Einstein-Podolsky-Rosen paradox). This rule allows to prove simultaneously two entangled theorems. f) The formulation of the “qubit theorem”, which is the logical description of the preparation of the optical qubit state. g) The lattice of propositions of Lq is, in the case of two qubits, orthomodular and non-distributive. Then, Lq is a quantum logic. It should be noticed that Lq is the first logic which is sub-structural, many-valued and quantum at the same time.
La logica introdotta in questa tesi, detta Lq, è una logica dell’ informazione quantistica. Lo scopo, infatti, era quello di descrivere logicamente la struttura del qubit (cioè, la sovrapposizione quantistica intrinseca di uno stato quantico a due livelli) e l’intreccio (entanglement) quantistico massimale di due qubits. La logica Lq è ottenuta tramite il principio di riflessione di Sambin della logica di Base, secondo il quale i legami metalinguistici tra asserzioni si riflettono (risolvendo un’ equazione definitoria) in connettivi logici tra proposizioni. Comunque, mentre nella logica di Base il metalinguaggio è classico, nel nostro caso è quantistico. Nel metalinguaggio quantistico, ciascuna asserzione atomica è dotata di un grado di asserzione, un numero complesso che viene interpretato come un’ ampiezza di probabilità. E’ proprio la presenza dei gradi di asserzione che permette l’introduzione del connettivo logico di “sovrapposizione quantistica” in Lq. Quest’ultimo è una generalizzazione del connettivo di congiunzione “and” dotato di indici complessi indicanti con quale “peso” ciascuna proposizione contribuisce alla formazione della proposizione composta. I valori (o gradi) di verità sono i moduli quadrati dei gradi di asserzione, con un range che è l’intervallo reale [0,1]. Pertanto, la logica Lq è polivalente. I gradi di verità, differentemente dalle logiche fuzzy, sono qui interpretati come probabilità quantistiche. Nella logica Lq si mantengono le tre importanti proprietà della logica di Base, cioè simmetria, riflessione e visibilità. Questa scelta è stata dettata dalle seguenti considerazioni: 1) I teoremi di no-cloning e no-erase dell’informazione quantistica non permettono di avere, nella logica corrispondente, le regole strutturali di indebolimento e contrazione, che sono in antitesi con i suddetti teoremi. Pertanto, nella ricerca di una logica dell’ informazione quantistica, ogni logica strutturale deve essere esclusa a priori. 2) La scelta tra le due più importanti logiche sub-strutturali, cioè la logica di Base e la logica Lineare, in favore della prima, è dovuta al fatto che, in assenza di visibilità, il connettivo logico “quantum entanglement” non può essere introdotto. Inoltre, si è cercata una logica dell’ informazione quantistica che avesse un calcolo deduttivo (in particolare il calcolo dei sequenti). La logica Lq sembra essere, finora, l’ unica logica dell’ informazione quantistica che possa soddisfare questi desiderata. L’ interpretazione delle asserzioni del metalinguaggio quantistico è data in termini di stati quantistici (il metalinguaggio quantistico “è” lo spazio di Hilbert). L’ interpretazione delle proposizioni di Lq è data in termini di operatori non-hermitiani, che sono misure deboli. L’ interpretazione di Lq si basa su una generalizzazione dei concetti già proposti da Birkhoff e von Neumann nella logica quantistica “ortodossa”, dove le proposizioni sono interpretate come operatori di proiezione. La differenza consiste nel fatto che in Lq le proposizioni sono interpretate invece come misure deboli, che, diversamente dalle misure proiettive, non danno luogo ad un brusco collasso della funzione d’onda. Questo permette una descrizione logica della sovrapposizione quantistica, perché essa non viene distrutta. La possibilità di interpretare le proposizioni come misure deboli, è dovuta al fatto che abbiamo introdotto un metalinguaggio quantistico. Infatti, il grado di asserzione si riflette, nell’ interpretazione delle proposizioni, con la presenza un fattore moltiplicativo complesso sui proiettori. Alcuni risultati di questa tesi sono: a) L’ adozione di un nuovo tipo di metalinguaggio, il metalinguaggio quantistico, dove i legami metalinguistici sono correlazioni quantistiche, e le asserzioni hanno un grado di asserzione complesso. b) L’ introduzione, tramite il principio di riflessione, di nuovi connettivi logici “quantistici”, quali la “sovrapposizione quantistica” e l’ “entanglement”. c) L’ introduzione di una nuova operazione duale, che è una generalizzazione della dualità logica di Sambin-Girard, che tiene conto, nell’ interpretazione, dello spazio duale di Hilbert. d) Una regola del taglio quantistica, che viene interpretata come misura quantistica proiettiva. Poiché il taglio è una meta-regola, ne consegue che una macchina quantistica non può effettuare una auto-misura e quindi auto-distruggersi. e) Una nuova meta-regola, non equivalente al taglio, detta regola EPR (rifacentesi al paradosso di Einstein-Podolsky-Rosen). Questa regola permette di dimostrare simultaneamente due teoremi entanglati. f) La formulazione del “teorema del qubit”, che è la descrizione logica della preparazione dello stato quantistico del qubit ottico. g) Il fatto che il reticolo delle proposizioni di Lq nel caso di due qubits è orto-modulare non-distributivo. Quindi Lq è una logica quantistica. E’ da notare il fatto che Lq è la prima logica ad essere contemporaneamente sub-strutturale, a molti valori di verità, e quantistica.

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.

A quantum computer has the potential to efficiently solve problems that are intractable for classical computers. Constructing a large-scale quantum processor, however, is challenging due to errors and noise inherent in real-world quantum systems. One approach to this challenge is to utilize modularity—a pervasive strategy found throughout nature and engineering—to build complex systems robustly. Such an approach manages complexity and uncertainty by assembling small, specialized components into a larger architecture. These considerations motivate the development of a quantum modular architecture, where separate quantum systems are combined via communication channels into a quantum network. In this architecture, an essential tool for universal quantum computation is the teleportation of an entangling quantum gate, a technique originally proposed in 1999 which, until now, has not been realized deterministically, Using the circuit quantum electrodynamics platform, this thesis reports on the experimental demonstration of a teleported controlled-NOT operation made deterministic by utilizing real-time adaptive control. Additionally, we take a crucial step towards implementing robust, error-correctable modules by enacting the gate between logical qubits, encoding quantum information redundantly in the states of superconducting cavities. Such teleported operations have significant implications for fault-tolerant quantum computation, and when realized within a network can have broad applications in quantum communication, metrology, and simulations. Our results illustrate a compelling approach for implementing multi-qubit operations on logical qubits within an error-protected quantum modular architecture.

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.

A major problem facing the realisation of scalable solid-state quantum computing is that of overcoming decoherence - the process whereby phase information encoded in a quantum bit ('qubit') is lost as the qubit interacts with its environment. Due to the vast number of environmental degrees of freedom, it is challenging to accurately calculate decoherence times T2, especially when the qubit and environment are highly correlated. Hybrid or mixed electron-nuclear spin qubits, such as donors in silicon, are amenable to fast quantum control with pulsed magnetic resonance. They also possess 'optimal working points' (OWPs) which are sweet-spots for reduced decoherence in magnetic fields. Analysis of sharp variations of T2 near OWPs was previously based on insensitivity to classical noise, even though hybrid qubits are situated in highly correlated quantum environments, such as the nuclear spin bath environment of 29Si impurities. This presented limited understanding of the underlying decoherence mechanism and gave unreliable predictions for T2. In this thesis, I present quantum many-body calculations of the qubit-bath dynamics, which (i) yield T2 for hybrid qubits in excellent agreement with experiments in multiple regimes, (ii) elucidate the many-body nature of the nuclear spin bath and (iii) expose significant differences between quantum-bath and classical-field decoherence. To achieve these results, the cluster correlation expansion was adapted to include electron-nuclear state mixing. In addition, an analysis supported by experiment was carried out to characterise the nuclear spin bath for a bismuth donor as the hybrid qubit, a simple analytical formula for T2 was derived with predictions in agreement with experiment, and the established method of dynamical decoupling was combined with operating near OWPs in order to maximise T2. Finally, the decoherence of a 29Si spin in proximity to the hybrid qubit was studied, in order to establish the feasibility for its use as a quantum register.

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

Quantum physics applied to computing is predicted to lead to revolutionary enhancements in computational speed and power. The interest in the implementation of an impurity spin based qubit in silicon for quantum computation is motivated by exceedingly long coherence times of the order of seconds, an advantage of silicon's low spin orbit coupling and its ability to be isotopically enriched to the nuclear spin zero form. In addition, the donor spin in silicon is tunable, its nuclear spin is available to be employed as a quantum memory, and there are major advantages to working with silicon in terms of infrastructure and scalability. In contrast, lithographically patterned artificial atoms called quantum dots have the complementary advantages of fast electrical operations and tunability. Here I present our attempts to develop a scalable quantum computation architecture in silicon, based on a coupled quantum dot and dopant system. I explore industry-compatible as well as industrial foundry-fabricated devices in silicon as hosts for few-electron quantum dots and utilise a high-sensitivity readout and charge sensing technique, gate-based radiofrequency reflectometry, for this purpose. I show few-electron quantum dot measurements in this device architecture, leading to a charge qubit with a novel multi-regime Landau-Zener interferometry signature, with possible applications for readout sensitivity. I also present spin-to-charge conversion measurements of a chalcogen donor atom in silicon. Lastly, I perform measurements on a foundry-fabricated silicon device showing a coupling between a donor atom and a quantum dot. I probe the relevant charge dynamics of the charge qubit, as well as observe Pauli spin blockade in the hybrid spin system, opening up the possibility to operate this coupled double quantum dot as a singlet-triplet qubit or to transfer a coherent spin state between the quantum dot and the donor electron and nucleus.