Understanding changes in materials properties as a function of size is crucial for both fundamental science development and technological applications. Size restriction results in quantum confinement effects that modify both energy level structures and electron dynamics of solid materials. This study investigates individual quantum states in a single nanocrystal. Single electron charging effects in gold and semiconductor nanocrystals are observed. Charging effects are found to be dominant in samples, where the nanocrystals are weakly coupled to the substrate. For nanocrystals strongly coupled to the substrate, nanocrystal-substrate tunneling rate is larger than tip-nanocrystal tunneling rate. Therefore, the resulting peaks in the dI/dV spectrum are attributed to tunneling through the energy levels of the nanocrystal. A newly developed nanocrystals BEES technique is used successfully to further explore quantized energy levels and electron dynamics in single gold nanocrystals. BEES samples were grown successfully by depositing $unit[10]{nm}$ thick gold on silicon substrates. Nanocrystals are chemically attached to the gold substrate using a self assembled monolayer (SAM) of xyelendithiol molecules. Immobile and single isolated nanocrystals were imaged at low temperature. A BEES turn-on voltage of $unit[0.84]{V}$ was found on nanocrystal-free region of the substrate. The BEES spectrum acquired on a single gold nanocrystal is found to be attenuated by a factor of 10 when compared with BEES acquired on the substrate. The attenuation is attributed to electron relaxation to lower energy states before tunneling out of the nanocrystal. The measured hot electron lifetimes from experimental data were found to be on the order of $unit[16]{picoseconds}$, which is a long time compared to lifetimes in bulk metals or large nanocrystals. The long measured lifetimes result from the molecular-like energy level structures of these small nanocrystals.

Although mathematical cryptography has been widely used, its security has only been proven under certain assumptions such as the computational power of opponents. As an alternative, quantum communication, in particular quantum key distribution (QKD) can get around unproven assumptions and achieve unconditional security. However, the key generation rate of practical QKD systems is limited by device imperfections, excess noise from the quantum channel, limited rate of true random-number generation, quantum entanglement preparation, and/or post-processing efficiency. This dissertation contributes to improving the performance of quantum communication systems. First, it proposes a new continuous-variable QKD (CVQKD) protocol that loosens the efficiency requirement on post-processing, a bottleneck for long-distance CVQKD systems. It also demonstrates an experimental implementation of the proposed protocol. To achieve high rates, the CVQKD experiment uses a continuous-wave local oscillator (CWLO). The excess noise caused by guided acoustic-wave Brillioun scattering (GAWBS) is avoided by a frequency-shift scheme, resulting in a 32 dB noise reduction. The statistical distribution of the GAWBS noise is characterized by quantum tomography. Measurements show Gaussian statistics upto 55 dB of dynamical range, which validates the security calculations in the proposed CVQKD protocol. True random numbers are required in quantum and classical cryptography. A second contribution of this thesis is that it experimentally demonstrates an ultrafast quantum random-number generator (QRNG) based on amplified spontaneous emission (ASE). Random numbers are produced by a multi-mode photon counting measurement on ASE light. The performance of the QRNG is analyzed with quantum information theory and verified with NIST standard random-number test. The QRNG experiment demonstrates a random-number generation rate at 20 Gbits/s. Theoretical studies show fundamental limits for such QRNGs. Quantum entanglement produced in nonlinear optical processes can help to increase quantum communication distance. A third contribution is the research on nonlinear optics of graphene, a novel 2D material with unconventional physical properties. Based on a quantum-dynamical model, optical responses of graphene are derived, showing for the first time a link between the complex linear optical conductivity and the quantum decoherence. Nonlinear optical responses, in particular four-wave mixing, is studied for the first time. The theory predicts saturation effects in graphene and relates the saturation threshold to the ultrafast quantum decoherence and carrier relaxation in graphene. For the experimental part, four-wave mixing in graphene is demonstrated. Twin-photon production in graphene is under investigation.

No description available.

The purpose of this thesis is to report a theoretical investigation on the current-current correlation and noise in the tmnsient quantum transport regime. In particular, we calculate current correlations when the bias voltage of a LDL quantum device is suddenly turned off. Namely, we consider the situation that when time t < 0 the device is in a steady-state under bias Vb, when t > 0 the bias is turned off to zero. Under such a bias, the transport current l goes from a finite steady-state value 10 at t < 0 to zero at large times. When electronic structure of the leads as well as well as the device scattering region are to be taken into account, it is a difficult problem to calculate the time dependent current-current correlation. However, for the sharp step-down bias shape, we discover that the time-dependent problem can be solved exactly for non-interacting systems.

Thesis (Ph.D.) -- McMaster University, 1998. / Includes bibliographical references. Also available via World Wide Web.

This research investigates characteristics of PT (parity-time) symmetry breaking in a system of two optically-coupled, time-delayed semiconductor lasers. A theoretical rate equation model for the lasers’ electric fields is presented and then reduced to a 2x2 Hamiltonian model, which, in the absence of time-delay, is PT-symmetric. The important parameters we control are the temporal separation of the lasers (τ), the frequency detuning (∆ω), and the coupling strength (κ). The detuning is experimentally controlled by varying the lasers’ temperatures, and intensity vs. ∆ωbehavior are examined, specifically how the PT-transition and the period and amplitude of sideband intensity oscillations change withκandτ. Experiments are compared to analytic predictions and numerical results, and all are found to be in good agreement. Eigenvalues, eigenvectors, and exceptional points of the reduced Hamiltonian model are numerically and analytically investigated, specifically how nonzero delay affects existing exceptional points.

Translational biomedical informatics is the application of computational methods to facilitate the translation of basic biomedical science to clinical relevance. An example of this is the multi-step process in which large-scale microarray-based discovery experiments are refined into reliable clinical tests. Unfortunately, the quality of microarray data is a major issue that must be addressed before microarrays can reach their full potential as a clinical molecular profiling tool for personalized and predictive medicine. A new methodology, titled caCORRECT, has been developed to replace or augment existing microarray processing technologies, in order to improve the translation of microarray data to clinical relevance. Results of validation studies show that caCORRECT is able to improve the mean accuracy of microarray gene expression by as much as 60%, depending on the magnitude and size of artifacts on the array surface. As part of a case study to demonstrate the widespread usefulness of caCORRECT, the entire pipeline of biomarker discovery has been executed for the clinical problem of classifying Renal Cell Carcinoma (RCC) specimens into appropriate subtypes. As a result, we have discovered and validated a novel two-gene RT-PCR assay, which has the ability to diagnose between the Clear Cell and Oncocytoma RCC subtypes with near perfect accuracy. As an extension to this work, progress has been made towards a quantitative quantum dot immunohistochemical assay, which is expected to be more clinically viable than a PCR-based test.

The study of nano-electronic devices is fundamental to the advancement of the semiconductor industry. As electronic devices become increasingly smaller, they will eventually move into a regime where the classical nature of the electrons no longer applies. As the quantum nature of the electrons becomes increasingly important, classical or semiclassical theories and methods will no longer serve their purpose. For example, the simplest non-classical effect that will occur is the tunnelling of electrons through the potential barriers that form wires and transistors. This results in an increase in noise and a reduction in the device?s ability to function correctly. Other quantum effects include coulomb blockade, resonant tunnelling, interference and diffraction, coulomb drag, resonant blockade and the list goes on. This thesis develops both a theoretical model and computational method to allow nanoelectronic devices to be studied in detail. Through the use of computer code and an appropriate model description, potential problems and new novel devices may be identified and studied. The model is as accurate to the physical realisation of the devices as possible to allow direct comparison with experimental outcomes. Using simple geometric shapes of varying potential heights, simple devices are readily accessible: quantum wires; quantum transistors; resonant cavities; and coupled quantum wires. Such devices will form the building blocks of future complex devices and thus need to be fully understood. Results obtained studying the connection of a quantum wire with its surroundings demonstrate non-intuitive behaviour and the importance of device geometry to electrical characteristics. The application of magnetic fields to various nano-devices produced a range of interesting phenomenon with promising novel applications. The magnetic field can be used to alter the phase of the electron, modifying the interaction between the electronic potential and the transport electrons. This thesis studies in detail the Aharonov-Bohm oscillation and impurity characterisation in quantum wires. By studying various devices considerable information can be added to the knowledge base of nano-electronic devices and provide a basis to further research. The computational algorithms developed in this thesis are highly accurate, numerically efficient and unconditionally stable, which can also be used to study many other physical phenomena in the quantum world. As an example, the computational algorithms were applied to positron-hydrogen scattering with the results indicating positronium formation.

La réalisation d’un ordinateur quantique est l’un des objectifs scientifiques les plus ambitieux et prometteurs de ce début de siècle.La force du calcul quantique réside dans sa capacité à se placer dans une superposition d’états et à utiliser les interférences entre eux pour dépasser la limite intrinsèque des ordinateurs classiques, qui est la description discrète des phénomènes physiques pourtant continus. Cela leur permettrait théoriquement de simplifier et de résoudre des problèmes insolubles pour les ordinateurs classiques.La première étape dans la réalisation d’un ordinateur quantique est sa brique de base : le bit-quantique, ou qubit. Il s’agit de l’analogue quantique du bit classique, qui permet de stocker l’information sous la forme de 0 ou de 1. Dans le cas quantique, l’information est formée par la superposition de ces deux états, en un nombre infini de possibilités. Si cette étape a été réalisée à de nombreuses reprises par la communauté, en utilisant des qubits de différentes natures, le couplage entre plusieurs d’entre eux reste difficile et limité en nombre. En effet, le système quantique ainsi formé a tendance à perdre sa cohérence ; ou dit autrement, à se détruire.Parmi les nombreuses possibilités de qubit existant, j’utilise le spin nucléaire. Ils ont l’avantage d’être relativement bien découplés de leur environnement, ce qui permet de les protéger des sources extérieures de décohérence et ainsi d’avoir un temps de vie supérieur aux spins électroniques.Cet avantage a un prix : il est plus difficile d’accéder à leur lecture.Pour ce faire, j’ai fabriqué un transistor moléculaire afin de connecter une molécule unique à deux centres magnétiques, le Tb2Pc3, aux électrodes de source et drain. L’aimant monomoléculaire utilisé possède deux centres magnétiques (les ions Tb3+) dont les spins électroniques J=6 sont couplés entre eux via une interaction dipolaire. De plus, chacun d’entre eux est couplé à son spin nucléaire I=3/2 via l’interaction hyperfine. On a ainsi un couple de deux qudits (d=4), ce qui porte la dimension de l’espace de Hilbert à 16, et ce à l’intérieur d’une unique molécule.Dans un premier temps, j’ai élaboré le diagramme Zeeman de la molécule, qui est sa réponse énergétique à un champ magnétique extérieur. Je détaille ensuite la fabrication des échantillons, et notamment l’utilisation de la technique d’électromigration. Je présente ensuite les mesures en transport électrique, aux très basses températures (milliKelvins) et sous champ magnétique, qui permettent de détecter le retournement du couple de spins électroniques, dont la position est dépendante de l’état du couple de spins nucléaires : c’est ainsi qu’est réalisée la lecture des états du couple de qudits.Une étude de la dynamique du système est alors réalisée par des mesures de corrélations entre la position des retournements des spins électroniques entre deux balayages consécutifs. On obtient ainsi, à la fois une meilleure visualisation des états du système, mais aussi de sa relaxation entre deux balayages en champ magnétique.Enfin, j’ai pu extraire sa température effective à l’aide d’une distribution de Maxwell-Boltzmann. De l’ordre de 300 mT, elle est cohérente avec la littérature, ainsi qu’avec celles extraites sur deux autres transistors moléculaires obtenus à d’autres moments de ma thèse.En résumé, cette thèse montre pour la première fois l’utilisation d’un transistor à molécule unique pour accéder à lecture d’un couple de qudits. Le grand nombre de molécules existantes, et le grand nombre de qubits ou qudits qui pourrait y être couplé, fait de la spintronique moléculaire une voie très prometteuse vers de possibles futurs ordinateurs quantiques moléculaires.La prochaine étape sera d’opérer la manipulation cohérente d’un tel système, notamment via l’utilisation de l’effet Stark, comme cela a déjà été réalisé à l’aide d’une molécule ne comportant qu’un centre magnétique. / The realization of a quantum computer is one of the most ambitious and promising scientific objectives of the beginning of this century.The strength of quantum computing lies in its ability to use a superposition of states and the interferences between them to overcome the intrinsic limit of classical computers, which is the discrete description of the continuous physical phenomena. This would theoretically allow them to simplify and solve impossible problems for conventional computers.The first step in the realization of a quantum computer, is its basic block: the quantum-bit, or qubit. It is the quantum analogue of the classical bit, which stores information in the form of 0 or 1. In the quantum case, information is formed by the superposition of these two states, leading to an infinity of possibilities. If this step has been done many times by the community, using qubits of different natures, the coupling between several of them remains difficult and limited in number. Indeed, the quantum systems thus formed tend to lose their coherence; or said otherwise, to destroy itself.Among the many possibilities of existing qubit, I have used the nuclear spin. They have the advantage of being relatively well decoupled from their environment, which makes it possible to protect them from external sources of decoherence, and thus to have a longer lifetime than electronic spins.This advantage has a price: it is more difficult to access their reading.To do this, I have made a molecular transistor to connect a single molecule possessing two magnetic centers, the Tb2Pc3, to the source and drain electrodes. The monomolecular magnet used has two magnetic centers (the Tb3 + ions), whose electronic spins J = 6, are coupled to each other via a dipolar interaction. In addition, each of them is coupled to its nuclear spin I = 3/2 via the hyperfine interaction. We thus have a pair of two qudits (d = 4), which brings the size of the Hilbert space to 16, and this inside a single molecule.At first, I have developed the Zeeman diagram of the molecule, which is its energy response to an external magnetic field. Then, I detail the manufacture of the samples, and in particular the use of the electromigration technique. Next, I present the electrical transport measurements, at very low temperatures (milliKelvins) and under a magnetic field, which make it possible to detect the reversal of the electronic spins, which position is dependent on the state of the pair of nuclear spins: it is how the reading of the states of qudits couple is performed.A study of the dynamics of the system is then carried out by correlation measurements among the position of the reversals of the electronic spins between two consecutive scans. This gives a better visualization of the states of the system, but also its relaxation.Finally, I was able to extract its effective temperature, using a Maxwell-Boltzmann distribution. Of the order of 300 mT, it is consistent with the literature, as well as with those extracted on two other molecular transistors obtained at other times of my thesis.In summary, this thesis shows for the first time the use of a single-molecule transistor to access reading of a qudits couple. The large number of existing molecules, and the large number of qubits or qudits that could be coupled inside one of them, makes molecular spintronics a very promising way for possible future molecular quantum computers.The next step will be to operate the coherent manipulation of such a system, in particular via the use of the Stark effect, as it has already been done using a molecule having only a magnetic center.

Indiana University-Purdue University Indianapolis (IUPUI) / In the first part of this thesis, we investigate the propagation of quantum fluctuations in a nonlinear interferometer comprising under conditions of harmonic generation by computer simulations. This investigation assumes idealized conditions such as lossless and uniform nonlinear media, an ideal cavity and ideal photodetectors. After linearizing wave equations for harmonic generation with a coherent state input, we obtain equations for one dimensional spatial propagation of the mean field and quantum fluctuations for initial conditions set by arbitrary interferometer phase. We discover that fluctuations are de-squeezed in the X and Y quadratures as the interferometer phase is tuned. However, we discover that there is are quadratures P-Q obtained by rotating the X-Y quadratures for which squeezing is improved by factors of 10^9. We present a practical idea to implement rotation of X quadrature fluctuations to the Q quadrature by using an ideal empty optical cavity. Signal-to-Noise ratio of the nonlinear interferometer was calculated and compared with that of a linear interferometer with coherent state input. We calculated a maximum performance improvement of a factor of 60 for a normalized propagation length ζ0 = 3 under ideal conditions. In the second part of this thesis, we investigate experimentalarrangements to transfer atomic coherence from light to cesium atoms. We discuss the experimental arrangement to generate coherence under conditions of electromagnetically induced transparency (EIT). We measure a continuous wave EIT width of 7.18 MHz and present results for pulsed arrangements.