UC Santa Barbara

Magnetic sensing is a non-invasive imaging modality which is ubiquitous across length scales. For example, magnetic resonance imaging (MRI) is commonly performed in hospitals to image entire human organs. On the other end of the spectrum, superconducting

quantum interference devices (SQUIDs) are capable of nanometer-scaled objects. However, both of these techniques have limitations. MRI cannot be used on small length scales while SQUIDs must be operated at cryogenic temperatures and thus are incompatible with biological samples. A magnetometer with both biocompatibility and high spatial resolution could have many exciting applications. For example, such a magnetometer could image the magnetic signals associated with a neurons action potential. The most advanced methods of measuring neuronal activity, including patch clamping and voltage-sensitive dyes, rely on sensing electric fields. However, the highly invasive nature of patch clamping and the toxicity of voltage-sensitive dyes limits these methods. Alternatively, neurons produce longitudinal current and correspondingly small magnetic fields when measuring an action potential. These magnetic fields can be used to infer the neurons behavior. Sub nanoTesla/sqrt(Hz) sensitivity is necessary for sensing neuron action potentials. This thesis focuses on building a biocompatible magnetometer based on the nitrogen-vacancy (NV) center in diamond, improving the sensitivity of the magnetometer, and applying it to biological systems.

The NV center is a point defect with an atom-like energy structure that is sensitive to magnetic fields. Uniquely the NV center retains its quantum properties under ambient conditions and room temperature. Because the NV centers energy structure is optically addressable and diamond is biocompatible, NV centers are an exciting and unique candidate to measure biological magnetic fields. An outstanding challenge for using NV centers is improving the sensitivity of NVs to make them useful sensors for biological samples. The fundamental sensing limit for an NV ensemble scales inversely with the number of centers in the sensing volume (N) and, because it is a quantum sensor, scales with the inverse of its coherence time (T2*)). To achieve the requisite magnetic sensitivity, we take two main approaches: increasing the density of coherent NV centers and further increasing coherence using engineered control pulses to decouple the NV from its environment. Chapter 2 describes the basic underlying physics of an NV center which is used for the magnetometry experiments.

In order to create a dense and coherent NV center ensemble, I developed an NV center creation technique which combines plasma enhanced chemical vapor deposition (PECVD) diamond growth with electron irradiation from a transmission electron microscope

(TEM). Nitrogen is incorporated during growth, and vacancies are formed by the electron irradiation. Subsequent annealing of the diamond forms the NV center. The growth process is described in chapter 3. By using 145 keV electrons, we limit excess damage to the diamond lattice, maintaining long coherence times in our diamond while increasing the density of our NV ensemble. Additionally, this formation technique also allows us to measure the displacement energy of a carbon atom in the diamond lattice directly. Electron irradiation techniques and analysis are described in chapter 4. With members of the Jayich lab, I built a wide-field total internal reflection microscope equipped with microwaves to probe the quality of our NV center ensembles and do magnetometry experiments. The equipment used to detect ensembles of NV centers is described in chapter 5. One of the primary sources of environmental decoherence is the ensemble of nitrogen atoms not converted to NV centers known (P1 centers). The P1 centers are paramagnetic and thus can couple to NV center, decohering them. These spins are difficult to measure due to their small spin number. To understand the limiting factors of our sensitivity we developed a recipe to characterize the spin-bath in our diamond. Using double electron-electron resonance (DEER) techniques we can give quantitative values to spin bath concentrations in our diamond. These techniques are described in chapter 6. We have integrated our coherent NV ensembles in diamond with total internal reflection (TIRF) optics, microwave electronics, and patch clamping equipment to create a biocompatible magnetometer. Using the set-up, we demonstrate a proof-of-principle by plating Aplysia californica neurons on diamond and stimulating and recording action potentials with patch-clamp electronics while simultaneously recording the NV fluorescence. The current advances toward bio-imaging in the Jayich lab are described in chapter 7. This thesis concludes in chapter 8 with a path towards improving the sensitivity further to use this magnetometer to probe neurological networks and other condensed matter systems. To further improve the sensitivity of our grown diamond sample, we use spin control measurements to decouple the NV center ensemble from its environment. By applying carefully timed radio frequency pulses to the P1 center, we decouple the P1 centers from the NV centers, thus eliminating this source of decoherence. I also describe the effects of temperature changes, strain, and electric eld noise on the sensitivity of our NV ensemble and how to mitigate their decoherence. With these advances, we have achieved a 1 nT/sqrt(Hz) sensitivity in a 5x5 um2 area.