The study of quantum mechanics, together with the ability to coherently control and manipulate quantum systems in the lab has led to a myriad of discoveries and real world applications. In this thesis we present experiments demonstrating precise control of an individual long-lived spin qubit as well as sensing applications for biology and investigation of quantum many-body dynamics.\n\nStable quantum bits, capable both of storing quantum information for macroscopic time scales and of integration inside small portable devices, are an essential building block for an array of potential applications. In the second chapter of this thesis we demonstrate high-fidelity control of a solid-state qubit, which preserves its polarization for several minutes and features coherence lifetimes exceeding 1 second at room temperature.\n\nSensitive probing of temperature variations on nanometer scales is an outstanding challenge in many areas of modern science and technology. In chapter three we show how nitrogen vacancy centers in diamond can be used as a robust, high sensitivity temperature probe. We furthermore demonstrate biological compatibility by introducing nano-sized diamonds into living cells and measuring externally induced sub-cellular temperature gradients.\n\nUnderstanding the dynamics of interacting many-body quantum systems with on-site potential disorder has proven one of the biggest challenges in quantum physics to investigate both in theory and experiment. In chapter four we demonstrate how coherent control techniques can be utilized to probe the many-body dynamics of a strongly interacting NV spin ensemble. Specifically, we show how a long-range interacting dipolar spin system exhibits characteristically slow thermalization in the presence of tunable disorder.\n\nThe presented works offer up many new areas to investigate, including complex quantum many-body effects of large, disordered spin systems, as well as applications of NV centers as bio-compatible nano-scale temperature probes.
The study of quantum mechanics, together with the ability to coherently control and manipulate quantum systems in the lab has led to a myriad of discoveries and real world applications. In this thesis we present experiments demonstrating precise control of an individual long-lived spin qubit as well as sensing applications for biology and investigation of quantum many-body dynamics.\n\nStable quantum bits, capable both of storing quantum information for macroscopic time scales and of integration inside small portable devices, are an essential building block for an array of potential applications. In the second chapter of this thesis we demonstrate high-fidelity control of a solid-state qubit, which preserves its polarization for several minutes and features coherence lifetimes exceeding 1 second at room temperature.\n\nSensitive probing of temperature variations on nanometer scales is an outstanding challenge in many areas of modern science and technology. In chapter three we show how nitrogen vacancy centers in diamond can be used as a robust, high sensitivity temperature probe. We furthermore demonstrate biological compatibility by introducing nano-sized diamonds into living cells and measuring externally induced sub-cellular temperature gradients.\n\nUnderstanding the dynamics of interacting many-body quantum systems with on-site potential disorder has proven one of the biggest challenges in quantum physics to investigate both in theory and experiment. In chapter four we demonstrate how coherent control techniques can be utilized to probe the many-body dynamics of a strongly interacting NV spin ensemble. Specifically, we show how a long-range interacting dipolar spin system exhibits characteristically slow thermalization in the presence of tunable disorder.\n\nThe presented works offer up many new areas to investigate, including complex quantum many-body effects of large, disordered spin systems, as well as applications of NV centers as bio-compatible nano-scale temperature probes.
Improved control and manipulation of quantum systems lies at the heart of modern quantum science and technology, with broad applications ranging from quantum metrology to quantum information science and many-body physics. In this thesis, I will introduce a novel framework for robust Hamiltonian engineering in interacting spin systems, which enables the efficient design of Floquet engineering pulse sequences for qubit and qudit systems that out-perform state-of-the-art control methods by orders of magnitude. As applications of this framework, we utilize a high density ensemble of nitrogen vacancy (NV) centers in diamond, and demonstrate significantly improved control over the disordered, interacting many-body spin system. This enabled the first solid state spin ensemble quantum sensor operating beyond the sensitivity limit imposed by interactions between sensing particles, as well as the observation and understanding of novel dynamical many-body phenomena, including discrete time crystalline order and thermalization dynamics in long-range interacting XXZ spin models.
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.
Solid-state spin defects, particularly the nitrogen-vacancy (NV) center in diamond, have emerged as powerful platforms for exploring quantum many-body physics and quantum metrology. Their exceptional coherence properties and strong dipolar interactions offer unique opportunities to simulate complex quantum dynamics and engineer entangled states for enhanced sensing beyond the standard quantum limit. However, achieving scalable entanglement in these systems is notoriously difficult due to intrinsic disorder and the complex nature of long-range dipole-dipole interactions. This thesis presents both theoretical and experimental advancements in harnessing quantum dynamics in interacting spin ensembles for quantum-enhanced metrology. A central result is the experimental realization of spin squeezing in a dense ensemble of NV centers, demonstrating a clear path toward overcoming classical limits in solid-state platforms. To further formalize and scale these phenomena, this work develops theoretical frameworks for generating robust spin squeezing under realistic experimental conditions. We show that finite-temperature easy-plane magnetism can be actively leveraged to create scalable squeezed states for the dipolar spin systems on regular lattices. Building upon this, we further extend the result to spatially disordered quantum dipoles. We demonstrate that strongly coupled spin pairs suppress squeezing through dynamical heating, and we propose a selective shelving protocol to mitigate this problem. Furthermore, we also propose a universal protocol for quantum-enhanced sensing that exploits information scrambling, utilizing the natural, scrambling dynamics of strongly interacting many-body systems to effectively amplify weak external signals. Together, the findings detailed in this thesis establish a foundational paradigm for controlling and utilizing solid-state spin ensembles, paving the way for entanglement-enhanced quantum sensors.
The field of quantum science and technology has generated many ideas for new revolutionary devices that exploit the quantum mechanical properties of small-scale systems. Isolated solid state spins play a large role in quantum technologies. They can be used as basic building blocks for a quantum computer or as ultra-sensitive magnetic-field probes which can detect the extremely weak magnetic field generated by a single proton. A major hurdle for realizing these applications is the loss of quantum coherence resulting from uncontrolled interactions with spins in the environment. In the experiments described in my thesis we studied spins associated with defect centers in diamond and used new strategies for mitigating decoherence involving advanced quantum control techniques and for fundamental studies of decoherence. We show that we can prolong the coherence time of a single spin associated with a Nitrogen-Vacancy (NV) defect center in diamond with dynamical decoupling techniques. Our experiments are accurately reproduced theoretically and from this theory we conclude that, with dynamically decoupling, the spin environment can in principle be made irrelevant for the decoherence of a single spin. This removes a major obstacle for using solid-state spins in quantum science and technology. Furthermore, the dynamics in the spin environment and its influence on the NV spin is thoroughly experimentally studied. By better understanding the mechanisms behind decoherence we may one day find the answer to unresolved fundamental issues in quantum physics such as the quantum measurement problem.
Quantum sensors encompass a diverse class of devices that exploit quantum coherence to detect weak or nanoscale signals. As their behavior is tied to physical constants, quantum sensors can achieve accuracy, repeatability, and precision approaching fundamental limits [1, 2]. As a result, these sensors have shown utility in a wide range of applications in both science and engineering leading to innovations. Among many solid-state spin systems, nitrogen-vacancy (NV) centers in diamonds have attracted considerable attention as a leading platform in many fields of quantum technologies, from quantum sensing to information processing and communication applications. They have superior physical properties, and their quantum coherence is preserved even at room temperature. The energy levels of NV centers are sensitive to magnetic fields, electric fields, strain, and temperature, and the spatial resolution is scalable from nanometers to millimeters. Therefore, diamond quantum sensors are distinctive sensors with multimodality and span scalable application fields. To realize an ideal quantum sensor for applications, the technical challenges in the sensor materials and the quantum control techniques, such as coherent microwave spin manipulation, effective optical spin-state initialization, and readout, have been developed. This chapter briefly explains the sensing principle using NV centers and then introduces sensor systems for biomedical applications, and monitoring of batteries for electric vehicles and internal power devices.
The nitrogen vacancy (NV) center in diamond is a unique quantum system that combines solid state spin qubits with coherent optical transitions. The spin states of the NV center can be initialized, read out, and controlled with RF fields at room temperature. It can be coupled to other spin systems in the environment while at the same time maintaining an extraordinary degree of quantum coherence. Experiments utilizing the NV center's spin states have led to a wide range of demonstrations from quantum error correction to high-sensitivity magnetometry. This thesis, however, focuses on creating an interface between NV centers and light in the visible domain by making use of its optical transitions. Such an interface connects the quantum system consisting of NV centers and nuclear spins to photons, which can then be used to both manipulate the spin qubits themselves or transport quantum information over large distances.