Spin Manipulation of the Nitrogen Vacancy Center and its Applications

Das Stickstoff-Fehlstellen-Zentrum (NV-Zentrum) in Diamant ist eines der vielver- sprechendsten Spinsysteme für Anwendungen im Bereich Quanten-Computing, -Information und -Sensorik. Die Abhängigkeit der Fluoreszenzintensität vom Spinzu- stand ermöglicht dabei das rein optische Auslesen des Spinzustandes. Für alle Anwendungen, die auf aktive Spinmanipulation angewiesen sind, ist Mikrowellen- strahlung unverzichtbar. Die Fähigkeit, den Spinzustand von NV-Zentren vollständig zu kontrollieren, wird durch die Richtung, Intensität und Polarisation der Mikrow- ellenstrahlung definiert. Es gibt verschiedene Ansätze, um geeignete Mikrowellen- strahlung zu erzeugen, aber oft ist die Feldintensität zu gering oder es gibt andere Einschränkungen, z.B. eine geringe Frequenzbandbreite. Im ersten Teil meiner Arbeit untersuche ich transparente Leiter auf Basis von Indium- Zinn-Oxid (ITO), um die Mikrowellenansteuerung von NV-Zentren zu optimieren. Dabei wird eine detaillierte Analyse von ITO auf Diamant bezüglich einzelner NV-Zentren vorgestellt. Ein mathematisches Modell wurde entwickelt, um die Feldverteilung vorherzusagen. Zusätzlich wird eine Methode zur Kontrolle der Mikrowellenpolarisation mit einer transparenten ITO-Struktur vorgestellt, die zu einer vollständigen Kontrolle des Spinzustands des NV-Zentrums führt. Weiterhin werden Simulationen in Kombination mit einem analytischen Modell verwendet, um optimale Mikrowellenparameter für die Spinkontrolle vorherzusagen. Für eine kommerzielle Anwendung von NV-Zentren als Magnetfeldsensor sind Pro- duktionskosten und Bauteilkomplexität wichtige Faktoren, die in der Forschung oft vernachlässigt werden. Der zweite Teil meiner Arbeit konzentriert sich da- her auf einen mikrowellenfreien Ansatz zur Magnetometrie mit NV-Zentren. Der Einfluss der Laseranregung auf den magnetischen Kontrast wird an einzelnen NV- Zentren, Ensembles von NV-Zentren und Nano-Diamantpulver mit einer hohen NV- Zentrenkonzentration dargestellt und nachfolgend zur Demonstration von isotropen Magnetfeldmessung verwendet. Abschließend wird die Anwendbarkeit durch die Konstruktion eines Magnetfeldsensors aus Komponenten der Automobilbranche gezeigt.The nitrogen vacancy center (NV center) in diamond is one of the most promising spin systems for applications in quantum computing, information and sensing. The dependency of the fluorescence intensity on the spin state allows a purely optical readout of the spin state. A green laser can be used to pump the NV center in the spin ground state while microwave radiation can manipulate the spin state of the NV center. For all applications depending on active spin manipulation, microwave radiation is indispensable. The ability to fully control the spin state of NV centers is defined by direction, strength and polarization of the microwave radiation. Different approaches exist to deliver the microwave radiation, but they often lack in strength or have other restrictions, e.g. a small frequency band width. In the first part of my thesis, I investigate transparent conductors based on indium tin oxide (ITO) to optimize microwave delivery. In this process a detailed analysis of ITO on diamond concerning confocal microscopy through this transparent film is presented. A mathematical model was developed and tested to predict the field distribution in possible applications. Additionally a method to control microwave polarization with a transparent ITO structure is shown which results in full spin state control of the NV center. Furthermore simulations combined with a analytical model are used to predict optimal microwave parameters for spin control. For a commercial application of NV centers as a magnetic field sensor, important factors are production cost and device complexity which are often neglected in research. The second part of my thesis therefore focuses on a microwave free approach of NV center magnetometry for industry applications. The influence of laser excitation on magnetic contrast was studied on single NV centers, ensembles of NV centers and nano diamond powder with a high NV center concentration. The findings were used to demonstrate isotropic magnetic field sensing. Finally, the applicability was shown by constructing a magnetic field sensor from automotive grade components

Image charge detection statistics relevant for deterministic ion implantation

Image charge detection is a non-perturbative pre-detection approach for deterministic ion implantation. Using low energy ion bunches as a model system for highly charged single ions, we experimentally studied the error and detection rates of an image charge detector setup. The probability density functions of the signal amplitudes in the Fourier spectrum can be modelled with a generalised gamma distribution to predict error and detection rates. It is shown that the false positive error rate can be minimised at the cost of detection rate, but this does not impair the fidelity of a deterministic implantation process. Independent of the ion species, at a signal to-noise ratio of 2, a false positive error rate of 0.1% is achieved, while the detection rate is about 22

Weak electron irradiation suppresses the anomalous magnetization of N-doped diamond crystals

Several diamond bulk crystals with a concentration of electrically neutral single substitutional nitrogen atoms of ≲80 ppm, the so-called C or P1 centers, are irradiated with electrons at 10 MeV energy and low fluence. The results show a complete suppression of the irreversible behavior in field and temperature of the magnetization below 30 K, after a decrease in ≲40 ppm in the concentration of C centers produced by the electron irradiation. This result indicates that magnetic C centers are at the origin of the large hysteretic behavior found recently in nitrogen-doped diamond crystals. This is remarkable because of the relatively low density of C centers, stressing the extraordinary role of the C centers in triggering those phenomena in diamond at relatively high temperatures. After annealing the samples at high temperatures in vacuum, the hysteretic behavior is partially recovered

Optimization of a Diamond Nitrogen Vacancy Centre Magnetometer for Sensing of Biological Signals

Sensing of signals from biological processes, such as action potential propagation in nerves, are essential for clinical diagnosis and basic understanding of physiology. Sensing can be performed electrically by placing sensor probes near or inside a living specimen or dissected tissue using well-established electrophysiology techniques. However, these electrical probe techniques have poor spatial resolution and cannot easily access tissue deep within a living subject, in particular within the brain. An alternative approach is to detect the magnetic field induced by the passage of the electrical signal, giving the equivalent readout without direct electrical contact. Such measurements are performed today using bulky and expensive superconducting sensors with poor spatial resolution. An alternative is to use nitrogen vacancy (NV) centers in diamond that promise biocompatibilty and high sensitivity without cryogenic cooling. In this work we present advances in biomagnetometry using NV centers, demonstrating magnetic field sensitivity of ∼100 pT/√Hz in the DC/low frequency range using a setup designed for biological measurements. Biocompatibility of the setup with a living sample (mouse brain slice) is studied and optimized, and we show work toward sensitivity improvements using a pulsed magnetometry scheme. In addition to the bulk magnetometry study, systematic artifacts in NV-ensemble widefield fluorescence imaging are investigated

Optimisation of a diamond nitrogen vacancy centre magnetometer for sensing of biological signals

Sensing of signals from biological processes, such as action potential propagation in nerves, are essential for clinical diagnosis and basic understanding of physiology. Sensing can be performed electrically by placing sensor probes near or inside a living specimen or dissected tissue using well established electrophysiology techniques. However, these electrical probe techniques have poor spatial resolution and cannot easily access tissue deep within a living subject, in particular within the brain. An alternative approach is to detect the magnetic field induced by the passage of the electrical signal, giving the equivalent readout without direct electrical contact. Such measurements are performed today using bulky and expensive superconducting sensors with poor spatial resolution. An alternative is to use nitrogen vacancy (NV) centres in diamond that promise biocompatibilty and high sensitivity without cryogenic cooling. In this work we present advances in biomagnetometry using NV centres, demonstrating magnetic field sensitivity of approximately 100 pT/$\sqrt{Hz}$ in the DC/low frequency range using a setup designed for biological measurements. Biocompatibility of the setup with a living sample (mouse brain slice) is studied and optimized, and we show work toward sensitivity improvements using a pulsed magnetometry scheme. In addition to the bulk magnetometry study, systematic artifacts in NV-ensemble widefield fluorescence imaging are investigated

Optimisation of a diamond nitrogen vacancy centre magnetometer for sensing of biological signals

Sensing of signals from biological processes, such as action potential propagation in nerves, are essential for clinical diagnosis and basic understanding of physiology. Sensing can be performed electrically by placing sensor probes near or inside a living specimen or dissected tissue using well established electrophysiology techniques. However, these electrical probe techniques have poor spatial resolution and cannot easily access tissue deep within a living subject, in particular within the brain. An alternative approach is to detect the magnetic field induced by the passage of the electrical signal, giving the equivalent readout without direct electrical contact. Such measurements are performed today using bulky and expensive superconducting sensors with poor spatial resolution. An alternative is to use nitrogen vacancy (NV) centres in diamond that promise biocompatibilty and high sensitivity without cryogenic cooling. In this work we present advances in biomagnetometry using NV centres, demonstrating magnetic field sensitivity of approximately 100 pT/$\sqrt{Hz}$ in the DC/low frequency range using a setup designed for biological measurements. Biocompatibility of the setup with a living sample (mouse brain slice) is studied and optimized, and we show work toward sensitivity improvements using a pulsed magnetometry scheme. In addition to the bulk magnetometry study, systematic artifacts in NV-ensemble widefield fluorescence imaging are investigated

Magnetic field and angle-dependent photoluminescence of a fiber-coupled nitrogen vacancy rich diamond

Here, we investigate the magnetic field dependent photoluminescence (PL) of a fiber-coupled diamond single crystal with a high density of nitrogen vacancy (NV) centers. Angle-dependent magnetic field sweep measurements between 0 and 111 mT were performed using an oscillating illumination combined with lock-in techniques. Besides the expected superposed PL of differently oriented NV centers, a zoo of features in the PL are found. These features can be associated with level anti-crossings and cross relaxations. In particular, PL measurements allowed us to detect auto-cross relaxation between coupled NV centers. Moreover, the PL measurements at low magnetic fields show dips suggesting an interaction of NV centers with additional spin defects. The results presented here are not only a study for NV-based fiber-coupled sensors made of diamond, but also show a way to investigate with manageable effort and purely an optical multispin interaction with at least one NV center as a constituent

Image charge detection statistics relevant for deterministic ion implantation

Image charge detection is a non-perturbative pre-detection approach for deterministic ion implantation. Using low energy ion bunches as a model system for highly charged single ions, we experimentally studied the error and detection rates of an image charge detector setup. The probability density functions of the signal amplitudes in the Fourier spectrum can be modelled with a generalised gamma distribution to predict error and detection rates. It is shown that the false positive error rate can be minimised at the cost of detection rate, but this does not impair the fidelity of a deterministic implantation process. Independent of the ion species, at a signal to-noise ratio of 2, a false positive error rate of 0.1% is achieved, while the detection rate is about 22

Image charge detection statistics relevant for deterministic ion implantation

Image charge detection is a non-perturbative pre-detection approach for deterministic ion implantation. Using low energy ion bunches as a model system for highly charged single ions, we experimentally studied the error and detection rates of an image charge detector setup. The probability density functions of the signal amplitudes in the Fourier spectrum can be modelled with a generalised gamma distribution to predict error and detection rates. It is shown that the false positive error rate can be minimised at the cost of detection rate, but this does not impair the fidelity of a deterministic implantation process. Independent of the ion species, at a signal to-noise ratio of 2, a false positive error rate of 0.1% is achieved, while the detection rate is about 22

Robust nuclear hyperpolarization driven by strongly coupled nitrogen vacancy centers

Nuclear magnetic resonance techniques are widely used in the natural sciences but they lack sensitivity. Therefore, large sample volumes or long measurement times are necessary. In this work, we investigate the polarization of bulk 13C nuclei in a diamond above the thermal equilibrium at room temperature. Previously studied mechanisms utilize direct coupling to nitrogen vacancy centers or the additional assistance of substitutional nitrogen impurities for this purpose. We exploit strongly coupled nitrogen vacancy centers as polarization sources. We study two approaches to transfer the optically induced polarization of the electron spins of the nitrogen vacancy centers to nearby nuclear spins. First, the electron-nuclear polarization transfer is achieved by energy matching conditions or, second, by magnetic field sweeps inducing Landau–Zener-like transitions. Simulations according to a quantum mechanical system consisting of two coupled nitrogen vacancy centers and a weakly coupled 13C spin show an excellent agreement with the experimental data. Both approaches allow a reduction of the measurement time by roughly three orders of magnitude