Quantum computing and quantum information processing are fast developing fields. As more and more qubits are integrated into the quantum computing systems, a deeper understanding and more careful modeling of the dynamics of these quantum systems in presence of driving and/or loss are necessary, not only for optimizing existing quantum devices, but also for designing and achieving novel method for more precise manipulation of these quantum systems. Our work investigating these quantum systems is inspired by quantum optics, a field focused on the quantum description of light and light-matter interactions. In this thesis, we use the theoretical toolboxes provided by quantum optics as well as condensed matter theory to investigate and model realistic quantum systems. Specifically, we focus on the Nitrogen-vacancy centers in diamond crystal, a Josephson parametric amplifier (that uses Josephson junctions as its non-linear elements), and plasmonic nanowire system. With the knowledge of the dynamics of these systems, we proposed single-photon heralded two-NV center quantum gates; designed and optimized the superconducting circuit for a Josephson parametric amplifier to improve the amplifier's saturation power; and designed new methods for robust light manipulation using topologically protected plasmonic modes. All the devices that we study are either ready to be implemented in experiments or have already been built. Further, as discussed in this thesis, our theoretical analysis of NV-centers can be extended to similar device types like solid-state defect centers, while our analysis of Josephson parametric amplifiers can be extended to other types of superconducting circuit systems.
Quantum computing and quantum information processing are fast developing fields. As more and more qubits are integrated into the quantum computing systems, a deeper understanding and more careful modeling of the dynamics of these quantum systems in presence of driving and/or loss are necessary, not only for optimizing existing quantum devices, but also for designing and achieving novel method for more precise manipulation of these quantum systems. Our work investigating these quantum systems is inspired by quantum optics, a field focused on the quantum description of light and light-matter interactions. In this thesis, we use the theoretical toolboxes provided by quantum optics as well as condensed matter theory to investigate and model realistic quantum systems. Specifically, we focus on the Nitrogen-vacancy centers in diamond crystal, a Josephson parametric amplifier (that uses Josephson junctions as its non-linear elements), and plasmonic nanowire system. With the knowledge of the dynamics of these systems, we proposed single-photon heralded two-NV center quantum gates; designed and optimized the superconducting circuit for a Josephson parametric amplifier to improve the amplifier's saturation power; and designed new methods for robust light manipulation using topologically protected plasmonic modes. All the devices that we study are either ready to be implemented in experiments or have already been built. Further, as discussed in this thesis, our theoretical analysis of NV-centers can be extended to similar device types like solid-state defect centers, while our analysis of Josephson parametric amplifiers can be extended to other types of superconducting circuit systems.
The precise control, entanglement, and measurement of quantum systems are central challenges in realizing scalable quantum information processing. This dissertation investigates how parametric modulation and engineered nonlinearity can be harnessed to both manipulate and measure superconducting qubits with high fidelity. In one part, controlled parametric drives are used to generate entanglement between qubit pairs and employ their joint states as probes of environmental dynamics. This approach enables the exploration of the non-Markovian nature of quantum environments, revealing how memory effects influence qubit coherence and energy relaxation. In a separate but conceptually related study, the same principle of parametric driving is utilized to design and analyze quantum-limited amplifiers based on three-wave mixing in weakly nonlinear Josephson circuits. An amplifier with a higher coupling quality factor than conventional JPAs is employed, allowing the two modes of a parity-time (PT) symmetric dimer to be mapped onto the in-phase (I) and quadrature (Q) components of the Josephson parametric amplifier. This mapping enables controlled exploration of non-Hermitian dynamics and mode coalescence within the amplifier’s quadrature space, offering new insights into PT-symmetric behavior and quantum-limited detection. Collectively, these investigations unify the themes of parametric control, entanglement, and amplification, advancing the broader understanding of quantum measurement and open-system dynamics.
This thesis describes a series of experiments on the control of the optical properties of the nitrogen-vacancy (NV) center in diamond, and on control of the electron and nuclear spin states associated with the NV center. The NV center is a fluorescing atomic defect center in diamond, consisting of a substitutional nitrogen atom adjacent to a vacancy in the diamond carbon lattice. The electron spin of the NV center can be initialized and read out optically, and coherently manipulated with high fidelity even at room temperature. This unique combination of properties has attracted much attention to the NV center in recent years for application in quantum information technologies. The experiments in this work focus on exploiting the intrinsic hybrid nature of the NV center. The NV center forms a hybrid spin register, as its electron spin is always coupled to the nuclear spin of its own nitrogen nucleus, and can furthermore be coupled to additional nuclear carbon-13 spins in the diamond. In addition, the electronic spin state can be mapped onto the state of a photon, in principle allowing long range transmission of quantum information and measurement-based entanglement of distant NV center registers. The long-term goal is to achieve a large scale quantum network where the different constituents each perform the task they are most well suited for: the stable nuclear spins store the quantum information, photons transfer quantum information over long distances, and the fast electron spin interfaces between spin and photon states. For measurement-based entanglement of distant NV centers it is of primary importance to maximize the emission and detection of coherent photons emitted by the NV center. Spontaneous emission can be controlled and enhanced by coupling an emitter to a photonic cavity. Chapters 3, 4, and 5 of this thesis describe experiments aimed at maximizing the coherent photon emission rate of an NV center by coupling the NV center to a photonic crystal cavity. One of the main challenges is to position a single NV center into the nanometer-sized cavity mode. Chapter 3 describes the development of a nanopositioning technique, which allows diamond nanocrystals ~50 nm in size containing single NV centers to be placed at arbitrary locations on a chip, with nanometer precision. By using a sharp etched tungsten tip, individual nanocrystals can be picked up, moved and placed under real-time imaging with an electron microscope. We explicitly demonstrate that the unique optical and spin properties of the NV center are conserved by the nanopositioning process. In chapter 4, we apply the nanopositioning technique to couple single NV centers contained in a diamond nanocrystals to gallium phosphide photonic crystal cavities. These cavities resonate in the visible with high quality factors, and are designed to have the lowest energy mode coincide with the zero-phonon line of the NV center. Efficient coupling is evidenced by a strong enhancement of NV center emission at the cavity wavelength. The high quality factor of a photonic crystal cavity is a result of careful design and fabrication. The introduction of a nanocrystal may therefore be expected to have a detrimental effect on the optical properties of a cavity. In chapter 5 we investigate the effect of a nanocrystal on the optical properties of photonic crystal cavities. Our simulations and measurements show that the effect of a nanocrystal is in fact only minor, a promising result for future work on cavity-QED systems in the solid state and with diamond defect centers in particular. Understanding and mitigating decoherence is a key challenge for quantum science and technology. The main source of decoherence for solid-state spin systems is the uncontrolled spin bath environment. Chapter 6 describes experiments that exploit quantum control of the NV center and recently developed dynamical decoupling techniques to study the properties of the surrounding bath of spins belonging to single substitutional nitrogen atoms. The coherence properties of a single NV center are affected by changes in the state of the bath spins, allowing a single NV center to be used as a probe to detect bath spin resonances and study the quantum dynamics of the spin bath. We use quantum control of the spin bath to extend the dephasing time of the NV center, important for the application of the NV center in DC magnetometry. In chapter 7 we describe the development and implementation of decoherence protected quantum gates for the hybrid spin register formed by the NV center electron spin that is coupled to the nuclear spin of its own nitrogen atom. Electron and nuclear spins evolve and decohere at vastly different rates, making it challenging to use such a hybrid system as a fully functional quantum register. We create a universal set of two-qubit quantum gates by integrating dynamical decoupling techniques into the gate operation. We demonstrate the power of our gate design by implementing for the first time Grover's quantum search algorithm on a solid-state spin register.
This thesis explores novel strategies for advancing superconducting quantum computing through a series of experimental investigations and device innovations. Grounded in the development of circuit quantum electrodynamics, the work addresses the challenges posed by the limitations of coherent control, coherence time and scaling of superconducting qubits. Focusing on transmon qubits, the thesis first reviews the foundational principles of circuit quantum electrodynamics and the critical role of the transmon in achieving robust qubit performance. Three experimental studies form the core of this work. The first study demonstrates a universal quantum gate set for strongly coupled transmons. By exploiting the residual ZZ interaction, the work introduces a novel two-axis gate protocol for single-qubit operations and implements both CZ and CNOT gates with high fidelities, highlighting the potential for fast, low-error quantum operations in the strong coupling regime. The second study investigates a dual-rail qubit architecture using parametrically coupled transmons, showcasing a hardware-efficient approach to quantum error correction. This experiment converts single-photon loss errors into detectable erasures and leverages mid-circuit detection to enhance qubit coherence, thereby advancing fault-tolerant quantum computing schemes in the NISQ era. The final study focuses on the development of merged-element transmons (MET) and their FinMET variants. By integrating the Josephson junction and shunt capacitor into a single trilayer device, these innovations achieve a dramatic reduction in device footprint while addressing loss channels through advanced materials engineering, ultimately paving the way for scalable, high-coherence quantum circuits. Collectively, these contributions demonstrate significant progress in quantum gate design, error mitigation, and qubit architecture, offering promising routes toward the realization of largescale, fault-tolerant quantum computers
Individual nitrogen vacancy (NV) centers in diamond have recently emerged as a leading candidate for a building block for quantum information processing systems. Individual NV centers have many properties that are desirable for qubits. For example their spin states can be manipulated, initialized and measured even at room temperature. In order to build a quantum information processing system that consists of more than a single NV center, these individual NV centers have to be connected via quantum channels that distribute entanglement and allow quantum state transfer between NV centers. This thesis explores how the optical properties of the NV center can be used and manipulated to form these quantum channels. Likewise we show how the same properties can be used to better control the nuclear spin environment surrounding the NV center. We first introduce, review and experimentally study the relevant optical properties of the NV center. These quantum optical techniques are then used to experimentally demonstrate spin-photon entanglement between the electronic spin of an individual NV center and a single photon. We next demonstrate quantum interference of two photons produced by NV centers in distinct diamond samples separated by two meters. These two demonstrations pave the way for entanglement generation, leading to quantum channels, between remote NV centers. Finally we demonstrate optical cooling, real-time measurement and conditional preparation of the nuclear spin environment around an NV center using all-optical manipulation of its electronic spin. This technique can lead to better control of individual NV centers and thus more robust quantum information processing systems.
Quantum technology stands at the forefront of a technological revolution and has become a central focus of modern physics in recent decades. By exploiting the principles of quantum mechanics, such as superposition and entanglement, it enables capabilities beyond the reach of classical devices. These include enhanced computational power (quantum computing), secure information transfer (quantum communication), and high-precision measurements (quantum sensing and metrology). These capabilities often rely on the precise generation, control, and conversion of microwave photons. However, many aspects of their manipulation remain unexplored and require further study to uncover new possibilities. This thesis focuses on one class of devices known as the Josephson parametric converter (JPC). In its conventional implementation, the JPC leverages three-wave mixing to realize quantum-limited amplification and noiseless frequency conversion. Since these applications have been extensively studied, this work considers a modified variant with one junction removed, thereby enabling four-wave mixing. The aim is to analyze this variant to determine whether photon doubling can be achieved with such a relatively simple circuit. Using the normal-mode formalism, the analysis shows that the photon-doubling coefficients are maximized when the junction is biased at half a flux quantum. Numerical simulations relied on a Floquet approach and Suzuki-Trotter decomposition to model the time evolution of the circuit and verify the occurrence of photon doubling. The simulations indicate that the minimal three-junction JPC can theoretically achieve unity conversion for photon doubling, as verified for initial photon populations n=1 and n=2. However, achieving full conversion requires accounting for Stark shifts, conversion speed, and drive frequency precision. Since this device now employs four-wave mixing, its potential should be further explored. With these new capabilities, it is worth investigating whether it can support operations beyond photon doubling, such as the generation of multiphoton states and squeezed states.
In this Thesis we design radiation patterns capable of creating effective light-matter interactions suited to applications in quantum computing, quantum simulation and quantum sensing. On the one hand, we have used dynamical decoupling techniques to design quantum operations that are robust against errors in environmental and control fields, achieving high-fidelity quantum logic in trapped ions and energy-efficient nuclear magnetic resonance at the nanoscale with nitrogen-vacancy centers in diamond. On the other hand, we have studied generalised models of light-matter interaction, leading to the discovery of selective multi-photon interactions in the Rabi-Stark model and a proposal for preparing non-classical quantum states using the nonlinear quantum Rabi model. Finally, we have shown how the appropriate tailoring of interactions among ultracold atoms in optical lattices could lead to solve the boson sampling problem faster than the best supercomputers, thus demonstrating quantum supremacy. In this manner, we believe the results presented here significantly expand our knowledge on the control of light-matter interactions, and provide optimal scenarios for current quantum devices to generate the next-generation of quantum applications.