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The nitrogen-vacancy (NV) center in diamond has been extensively studied in recent years for its remarkable quantum coherence properties that make it an ideal candidate for room temperature quantum computing and quantum sensing schemes. However, these schemes rely on spin-spin dipolar interactions, which require the NV centers to be within a few nanometers from each other while still separately addressable, or to be in close proximity of the diamond surface, where their coherence properties significantly degrade. Here we demonstrate a method for overcoming these limitations using a hybrid yttrium iron garnet (YIG)-nanodiamond quantum system constructed with the help of directed assembly and transfer printing techniques. We show that YIG spin-waves can amplify the oscillating field of a microwave source by more than two orders of magnitude and efficiently mediate its coherent interactions with an NV center ensemble. These results demonstrate that spin-waves in ferromagnets can be used as quantum buses for enhanced, long-range qubit interactions, paving the way to ultra-efficient manipulation and coupling of solid state defects in hybrid quantum networks and sensing devices., 7 pages, 4 figures

The negatively charged nitrogen-vacancy center in diamond has become a prominent room temperature spin qubit platform for quantum information and sensing applications owing to its inherent optical addressability and long spin coherence times. In particular, a major research effort has focused on exploiting the metrology capabilities of the nitrogen-vacancy center to investigate the details of biological and magnetic systems at the nanoscale using diamond micro- and nanoparticles (we will mostly refer to these as nanoparticles in what follows). These particles behave like highly localized, low-thermal-mass spin systems and can therefore be used for high spatial resolution, high sensitivity mapping of their environment. Nonetheless, some outstanding challenges limit the sensing capabilities of these nanosensors. On the one hand, the nitrogen-vacancy centers contained in nanoparticles present strongly degraded coherence properties compared to what is common in bulk diamond due to a higher density of structural defects and impurities. On the other hand, in order to investigate systems that produce a signal that decay fast with distance (e.g. single or small ensembles of spins), the qubit needs to be placed within a few nanometer of the diamond surface. Because of the presence of magnetic and electric noise resulting from electronic spin and charges at the interface, the coherence of the qubits is further diminished. As the nitrogen-vacancy center’s coherence properties ultimately limit the achievable sensitivity, improving thse properties has been a central research focus in the scientific community. In this thesis two main approaches to extend the capabilities of diamond nanoparticles as nanoscale sensors are investigated. Firstly, we discuss a fabrication process developed to produce nanodiamonds with controlled crystal properties, shape, size, and nitrogen-vacancy center placement and density. We show that these accomplishments are conducive to enhanced photoluminescence properties and bulk-like spin coherence times. We additionally demonstrate that, thanks to their precisely controlled geometry, the nanoparticles can be optically trapped within a microfluidic system and behave as highly stable, contactless nanoprobes in solution. In a separate effort, we study hybrid quantum architectures that promise to improve the sensitivity of the nitrogen-vacancy centers by enabling the amplification of the signal to be measured. In this way, the requirements on the position of the qubits with respect to the diamond’s interface can be relaxed. In particular, we first develop a technique to create portable arrays of nanoparticles embedded within a flexible and transparent matrix that can be placed in contact with a sample and provide two-dimensional mapping capabilities. We then use this sensing platform to create a ferromagnet-nanodiamond hybrid system to study the interactions between collective spin excitations (spin waves) in the ferromagnet and ensembles of nitrogen-vacancy centers in the nanodiamonds. We show that surface confined spin waves interact strongly with the qubits’ spins and can mediate and enhance their coherent magnetic interactions with a microwave source. These results pave the way to the use of spin wave mediated coupling as a way to improve the sensitivity of the nitrogen-vacancy centers both in nanoparticles and bulk material.