Quantum technologies are transitioning from proof-of-principle experiments to practical everyday applications. As this transition unfolds, it remains unclear which quantum system will offer the most suitable platform for specific tasks. While many candidate systems offer excellent control and demonstrate rapid progress, they often require demanding operating conditions such as cryogenic temperatures down to the millikelvin range. The nitrogen-vacancy (N-V) center in diamond is a notable exception. This optically active defect allows full control of its electronic spin state at room temperature and maintains long coherence times under ambient conditions, a prerequisite for many applications. This accessibility makes the N-V center uniquely suited for translating fundamental quantum concepts into deployable technologies. The applicability of N-V centers in quantum sensing, quantum control, and quantum simulation is the central focus of this thesis. After an introductory overview of the field, the thesis introduces the experimental and theoretical concepts that are necessary to understand the presented results. It proceeds to show how the operating regime of N-V centers as sensors for AC magnetic fields and nuclear spins can be extended to low and zero magnetic bias fields. Many commonly used sensing protocols require a magnetic bias field, which can alter the investigated dynamics or may be infeasible to apply due to experimental constraints. This work describes how established sensing protocols can be adjusted for operation under these conditions, and presents novel sensing protocols that enable the detection of AC magnetic fields and nuclear spins in a regime where conventional protocols fail. These results highlight that precise control of the underlying quantum system is a must. Such control can be optimized using quantum optimal control, which allows one to improve the underlying gate operations in a closed-loop, black-box manner, provided a figure of merit can be found which reliably evaluates the gate-set's performance. As this performance can depend on the specific application, the following part of this thesis defines and benchmarks several figures of merit for closed-loop optimal control using an ensemble of N-V centers. The strengths and limitations of these figures of merit are discussed, and the dependence of the optimization performance on several control and experimental parameters is analyzed. The achieved results are applicable to a broad range of other physical platforms, and can serve as a reference for experimental groups that aim to optimize their gate operations using optimal control. Building upon these insights into quantum control, the next part of this thesis presents the fabrication, control, and application of a room-temperature quantum simulator based on thin nuclear spin layers in diamond. A detailed assessment of the growth process is provided, and the dynamics that allow nearby N-V centers to initialize and read out the nuclear spins are thoroughly investigated. Furthermore, the nuclear spin layer is employed to create and study discrete time-crystalline order. The results demonstrate a scalable route for exploring quantum many-body phenomena under ambient conditions. The thesis concludes with a summary and discussion of all results and provides an outlook on future work based on these findings. In conclusion, this thesis demonstrates and expands the versatile use of N-V centers in diamond for quantum sensing, control, and simulation, highlighting their potential as a practical platform for future quantum technologies.
Quantum technologies are transitioning from proof-of-principle experiments to practical everyday applications. As this transition unfolds, it remains unclear which quantum system will offer the most suitable platform for specific tasks. While many candidate systems offer excellent control and demonstrate rapid progress, they often require demanding operating conditions such as cryogenic temperatures down to the millikelvin range. The nitrogen-vacancy (N-V) center in diamond is a notable exception. This optically active defect allows full control of its electronic spin state at room temperature and maintains long coherence times under ambient conditions, a prerequisite for many applications. This accessibility makes the N-V center uniquely suited for translating fundamental quantum concepts into deployable technologies. The applicability of N-V centers in quantum sensing, quantum control, and quantum simulation is the central focus of this thesis. After an introductory overview of the field, the thesis introduces the experimental and theoretical concepts that are necessary to understand the presented results. It proceeds to show how the operating regime of N-V centers as sensors for AC magnetic fields and nuclear spins can be extended to low and zero magnetic bias fields. Many commonly used sensing protocols require a magnetic bias field, which can alter the investigated dynamics or may be infeasible to apply due to experimental constraints. This work describes how established sensing protocols can be adjusted for operation under these conditions, and presents novel sensing protocols that enable the detection of AC magnetic fields and nuclear spins in a regime where conventional protocols fail. These results highlight that precise control of the underlying quantum system is a must. Such control can be optimized using quantum optimal control, which allows one to improve the underlying gate operations in a closed-loop, black-box manner, provided a figure of merit can be found which reliably evaluates the gate-set's performance. As this performance can depend on the specific application, the following part of this thesis defines and benchmarks several figures of merit for closed-loop optimal control using an ensemble of N-V centers. The strengths and limitations of these figures of merit are discussed, and the dependence of the optimization performance on several control and experimental parameters is analyzed. The achieved results are applicable to a broad range of other physical platforms, and can serve as a reference for experimental groups that aim to optimize their gate operations using optimal control. Building upon these insights into quantum control, the next part of this thesis presents the fabrication, control, and application of a room-temperature quantum simulator based on thin nuclear spin layers in diamond. A detailed assessment of the growth process is provided, and the dynamics that allow nearby N-V centers to initialize and read out the nuclear spins are thoroughly investigated. Furthermore, the nuclear spin layer is employed to create and study discrete time-crystalline order. The results demonstrate a scalable route for exploring quantum many-body phenomena under ambient conditions. The thesis concludes with a summary and discussion of all results and provides an outlook on future work based on these findings. In conclusion, this thesis demonstrates and expands the versatile use of N-V centers in diamond for quantum sensing, control, and simulation, highlighting their potential as a practical platform for future quantum technologies.
The recent decade has witnessed exciting developments in quantum science and technology. Quantum sensing is a rapidly growing branch, which exploits the fragility of quantum systems to detect small external signals. The extreme control at the single-atom level and the remarkably long coherence time of engineered quantum systems enable unprecedented sensitivity and precision, beating the performance of classical techniques. The nitrogen-vacancy (NV) center in diamond is one of the most promising quantum sensors. Its unique advantages have attracted considerable attention, including a long spin coherence time throughout a broad temperature range, nanoscale resolution limited by the sensor-sample distance, convenient optical control of its spin states and remarkably versatile sensing capabilities. Its properties are well studied under a bias magnetic field oriented along its quantization axis. However, its sensing capabilities in the presence of an off-axis magnetic field are much less explored. My research aims to unlock the full potential of NV quantum sensor by developing new sensing modalities under a magnetic field acting perpendicular to the NV axis, and furthermore, integrating them into a scanning probe microscope. In this dissertation, I will report our progress toward building a scanning NV multimodal quantum imaging platform, which will open up exciting opportunities in measuring nanoscale phenomena. The first chapter provides an introduction to the background of quantum sensing with NV centers, followed by a discussion on its basic properties and established sensing techniques. The second and third chapters are dedicated to two experiments performed under a perpendicular magnetic field where traditional NV magnetometry based on the Zeeman effect fails. In the second chapter, I will introduce a new sensing strategy that uses the entanglement between the electron and nuclear spins to restore the magnetic field sensitivity under this unfavored condition. This allows us to sense small changes in the magnetic field angle relative to the NV axis, and to detect anisotropic magnetic noise in the local environment. In the third chapter, I will present our latest development of scanning NV electrometry, where a single NV at the apex of a diamond scanning probe images external alternating (AC) and direct (DC) electric fields with sub-100 nm resolution. In the last chapter, I will discuss our efforts on four experiments that were left unfinished. I will present the results we have achieved, analyze the challenges that we faced and propose potential solutions to overcome them.
This thesis investigates the theoretical foundations, material properties, and experimental realization of quantum sensors based on the Nitrogen-Vacancy (NV) centers in diamond, a solid-state system that combines quantum coherence with room-temperature operability. The first chapter introduces the principles of quantum sensing, outlining the fundamental definitions, operating protocols, and physical processes underlying the quantum measurement paradigm. Emphasis is placed on the quantum sensing Hamiltonian and the distinction between various classes of quantum sensors. The second chapter focuses on diamond as a host material and on the NV center as a quantum defect. The electronic structure, optical transitions, and spin properties of the NV center are analyzed in detail. Several techniques of NV-based magnetometry are presented, including continuouswave and pulsed optically detected magnetic resonance (ODMR). The discussion also addresses coherence times (T1, T2, and T∗2 ) and methods of diamond synthesis together with NV-center generation through irradiation and annealing. Chapter three develops the physical theory describing the NV center. The Hamiltonian formalism includes spin-spin, hyperfine, and quadrupolar interactions, as well as couplings to external magnetic fields. The chapter further introduces the Bloch sphere representation, magnetic resonance phenomena, and models of lightmatter interaction, ranging from the semiclassical to the fully quantum Rabi framework. A seven-level rate-equation model is proposed to capture the spin dynamics and optical cycling processes of the NV center. The fourth chapter reports the experimental results. After describing materials and methods, several ODMR-based vector magnetometry experiments are presented, highlighting different spectral regimes and the dependence of resonance features on the external magnetic field geometry. Additional studies demonstrate temperature sensing and microwave-free magnetometry techniques for mapping magnetic field distributions. Sensitivity analysis and calibration procedures are also discussed.
In modern-day quantum metrology, quantum sensors are widely employed to detect weak magnetic fields or nanoscale signals. Quantum devices, exploiting quantum coherence, are inevitably connected to physical constants and can achieve accuracy, repeatability, and precision approaching fundamental limits. As a result, these sensors have shown utility in a wide range of research domains spanning both science and technology. A rapidly emerging quantum sensing platform employs atomic-scale defects in crystals. In particular, magnetometry using nitrogen-vacancy (NV) color centers in diamond has garnered increasing interest. NV systems possess a combination of remarkable properties, optical addressability, long coherence times, and biocompatibility. Sensors based on NV centers excel in spatial resolution and magnetic sensitivity. These diamond-based sensors promise comparable combination of high spatial resolution and magnetic sensitivity without cryogenic operation. The above properties of NV magnetometers promise increasingly integrated quantum measurement technology, as a result, they have been extensively developed with various protocols and find use in numerous applications spanning materials characterization, nuclear magnetic resonance (NMR), condensed matter physics, paleomagnetism, neuroscience and living systems biology, and industrial vector magnetometry. In this chapter, NV centers are explored for magnetic sensing in a number of contexts. In general, we introduce novel regimes for magnetic-field probes with NV ensembles. Specifically, NV centers are developed for sensitive magnetometers for applications where microwaves (MWs) are prohibitively invasive and operations need to be carried out under zero ambient magnetic field. The primary goal of our discussion is to improve the utility of these NV center-based magnetometers.
This dissertation investigates applications of the Nitrogen vacancy in diamond (NV). The NV is a unique quantum system allowing for optical polarization and read out even at ambient conditions. Its electron spin (S=1) has coherence times in the order of milliseconds and is therefore an ideal candidate to investigate the foundations of quantum mechanics as well as exploiting quantum effects in metrology and information processing applications. To utilize the NV for quantum technologies, first the interaction of the NV with its surroundings has to be investigated and controlled. Here dynamical decoupling sequences (CPMG) could be utilized to extend the NV coherence time to a few milliseconds by decoupling the NV from its surrounding spin bath (given by the 13C isotope, natural abundance 1.1%). But not only decoupling sequences can be used to suppress the spin bath interaction, also eigen state tailoring can suppress the bath interaction. As long as strain is the dominant interaction, the NV eigen energies are not susceptible to small magnetic field changes. Therefore the magnetic dipole interaction with the spin bath is suppressed. But the spin bath is not only a nuisance limiting NV coherence times, but also a potential resource for quantum technologies. In order to harness the individual 13C nuclear spins, their interaction with the NV and among themselves has to be investigated. To not be limited by the NV coherence times, a new spectroscopy method, only limited by the NV lifetime, was developed. This can in principle be extended to the second range by using a nuclear spin memory. First advances in quantum technology were demonstrated in the field of metrology, where the Zeeman effect of the NV center allowed for precise nanoscale measurement of magnetic fields. Here this concept was extended to us the Stark effect to detect electric fields. A sensitivity of η=142.6±3.6 V/(cmHz1/2) was demonstrated. This is equivalent to the detection of a single elemental charge at a distance of 150 nm in one second. By using two NVs with a distance of less then 20 nm, a demonstration of nanoscale single fundamental charge detection was feasible. Here one NV was used as a controllable fundamental charge while the other NV was used as sensor NV, detecting the electric field of the fundamental charge. This allowed for the detection of a single electronic charge at ambient conditions. Another keystone of quantum technology is the reliable on demand or heralded creation of entanglement. In this dissertation, the successful entanglement of two electron spins in solid state at ambient conditions is demonstrated. The fundamental interaction used to create an entangled state was dipolar coupling between the two electron spins. In order to be able to harness weaker coupling strengths, an entanglement protocol based on dynamical decoupling was developed. A fidelity of F=0.67±0.04 was demonstrated. By using the intrinsic nuclear spins of the nitrogen it was possible to store the entangled state on the millisecond time scale. However, the theoretical limit for the fidelity given by the polarization and the coherence times is F=0.849. The discrepancy can be explained by pulse errors. These can be avoided by using optimal control yielding a fidelity of F=0.824±0.015. Also optimal control allowed for nuclear spin entanglement by storing the electron spin entanglement on the nitrogen nuclear spins. These experiments are first steps towards a room temperature quantum register, where the electron spins are used as bus to transport information and the nuclear spins act as a memory.
In recent years, the field of quantum sensing has garnered increasing attention due to its potential to revolutionize various scientific and technological domains. Among the different quantum sensors, the nitrogen-vacancy (NV) color center in diamond stands out for its ease of use, ability to be read out and initialized with a laser, and long coherence times even at room temperature. Over the past years, numerous quantum control sequences have been developed to successfully deploy NV sensors in diverse situations, such as measuring nearby spin clusters, classical AC signals, and static magnetic fields. However, the NV center faces limitations when coupling to high frequency signals. More specifically, as the frequency of the target signal increases, stronger driving fields over NVs are needed, ultimately reaching the limits of current experimental capabilities. In this thesis, we propose several protocols to address this high-frequency problem in different scenarios.
Quantum sensing using nitrogen-vacancy (NV) centers in diamond has emerged as a powerful platform for detecting ultra-low magnetic fields under ambient conditions. Owing to their long spin coherence times, optical addressability, and compatibility with aqueous environments, it has found widespread applications in biosensing and bio-imaging. This review presents the fundamental principles and recent advances in NV-based quantum magnetometry for biosensing applications, with a particular focus on measurements in aqueous medium and at cellular and molecular length scales. We discuss the underlying spin physics of NV centers and highlight two primary detection modalities: optically detected magnetic resonance (ODMR) and T1 relaxometry-based sensing and how these approaches aids in the detection of both static magnetic fields and dynamic magnetic noise arising from biological processes. The review explores key application areas, including nanoscale nuclear magnetic resonance (NMR), monitoring of neural activity, detection of abnormal or rogue cells using NV-based platforms etc. In addition, strategies for enhancing sensitivity, such as surface functionalization of nanodiamonds, femtosecond (fs) laser-written photonic structures, and integration with microfluidic and lab-on-chip systems have also been discussed in depth. We also address the critical challenges, including surface-induced decoherence, charge-state instability, and signal-to-noise limitations in biofluids associated with NV-based biosensing applications. Finally, we outline future prospects, highlighting how NV-based magnetic biosensing provides a promising pathway for translating quantum sensing technologies into practical biomedical applications.