Spatiotemporal Mapping of Photocurrent in a Monolayer Semiconductor Using a Diamond Quantum Sensor

The detection of photocurrents is central to understanding and harnessing the interaction of light with matter. Although widely used, transport-based detection averages over spatial distributions and can suffer from low photocarrier collection efficiency. Here, we introduce a contact-free method to spatially resolve local photocurrent densities using a proximal quantum magnetometer. We interface monolayer MoS2 with a near-surface ensemble of nitrogen-vacancy centers in diamond and map the generated photothermal current distribution through its magnetic field profile. By synchronizing the photoexcitation with dynamical decoupling of the sensor spin, we extend the sensor's quantum coherence and achieve sensitivities to alternating current densities as small as 20 nA per micron. Our spatiotemporal measurements reveal that the photocurrent circulates as vortices, manifesting the Nernst effect, and rises with a timescale indicative of the system's thermal properties. Our method establishes an unprecedented probe for optoelectronic phenomena, ideally suited to the emerging class of two-dimensional materials, and stimulates applications towards large-area photodetectors and stick-on sources of magnetic fields for quantum control.Comment: 19 pages, 4 figure

Opportunities for long-range magnon-mediated entanglement of spin qubits via on- and off-resonant coupling

The ability to manipulate entanglement between multiple spatially-separated qubits is essential for quantum information processing. Although nitrogen-vacancy (NV) centers in diamond provide a promising qubit platform, developing scalable two-qubit gates remains a well-known challenge. To this end, magnon-mediated entanglement proposals have attracted attention due to their long-range spin-coherent propagation. Optimal device geometries and gate protocols of such schemes, however, have yet to be determined. Here we predict strong long-distance ($>\mu$m) NV-NV coupling via magnon modes with cooperativities exceeding unity in ferromagnetic bar and waveguide structures. Moreover, we explore and compare on-resonant transduction and off-resonant virtual-magnon exchange protocols, and discuss their suitability for generating or manipulating entangled states at low temperatures ($T\lesssim 150$ mK) under realistic experimental conditions. This work will guide future experiments that aim to entangle spin qubits in solids with magnon excitations.Comment: PRX Quantum in press, 10 pages, 5 figure

Guiding Diamond Spin Qubit Growth with Computational Methods

The nitrogen vacancy (NV) center in diamond, a well-studied, optically active spin defect, is the prototypical system in many state of the art quantum sensing and communication applications. In addition to the enticing properties intrinsic to the NV center, its diamond host's nuclear and electronic spin baths can be leveraged as resources for quantum information, rather than considered solely as sources of decoherence. However, current synthesis approaches result in stochastic defect spin positions, reducing the technology's potential for deterministic control and yield of NV-spin bath systems, as well as scalability and integration with other technologies. Here, we demonstrate the use of theoretical calculations of electronic central spin decoherence as an integral part of an NV-spin bath synthesis workflow, providing a path forward for the quantitative design of NV center-based quantum sensing systems. We use computationally generated coherence data to characterize the properties of single NV center qubits across relevant growth parameters to find general trends in coherence time distributions dependent on spin bath dimensionality and density. We then build a maximum likelihood estimator with our theoretical model, enabling the characterization of a test sample through NV T2* measurements. Finally, we explore the impact of dimensionality on the yield of strongly coupled electron spin systems. The methods presented herein are general and applicable to other qubit platforms that can be appropriately simulated.Comment: 12 pages, 6 figure

Magnon-mediated qubit coupling determined via dissipation measurements

Controlled interaction between localized and delocalized solid-state spin systems offers a compelling platform for on-chip quantum information processing with quantum spintronics. Hybrid quantum systems (HQSs) of localized nitrogen-vacancy (NV) centers in diamond and delocalized magnon modes in ferrimagnets—systems with naturally commensurate energies—have recently attracted significant attention, especially for interconnecting isolated spin qubits at length-scales far beyond those set by the dipolar coupling. However, despite extensive theoretical efforts, there is a lack of experimental characterization of the magnon-mediated interaction between NV centers, which is necessary to develop such hybrid quantum architectures. Here, we experimentally determine the magnon-mediated NV–NV coupling from the magnon-induced self-energy of NV centers. Our results are quantitatively consistent with a model in which the NV center is coupled to magnons by dipolar interactions. This work provides a versatile tool to characterize HQSs in the absence of strong coupling, informing future efforts to engineer entangled solid-state systems

Hybrid Quantum Systems of Nitrogen-vacancy Centers in Diamond Coupled to Magnetic Insulator Materials

Nitrogen-vacancy (NV) centers in diamond have been known as excellent solid-state qubit platforms in quantum information science. Hybrid quantum systems of NV centers and magnetic insulator materials are of particular interest in recent years due to their great potential in computing and sensing applications. In this dissertation, we explore several experimental and theoretical aspects of the hybrid (or composite) quantum system of NV centers and yttrium iron garnet (YIG). YIG is a ferrimagnetic insulator material extensively studied in the field of spintronics and magnonics. It shows exceptionally low magnetic damping that leads to nontrivial magnon related phenomena such as the spin Seebeck effect. The first section of this dissertation focuses on the fundamentals of quantum physics. Chapter 1 provides an introduction to quantum information science and engineering. Chapter 2 describes the basics of NV centers. Chapter 3 features the concept of boson-mediated interaction of qubits. In Chapter 4, we show that the NV center can serve as a nanoscale temperature probe of the YIG substrate and is potentially useful for the study of the spin Seebeck effect. Employing an all-optical ratiometric thermometry technique, we find that the NV-based temperature sensing can function down to liquid nitrogen temperatures without a deterioration of its temperature sensitivity, which is ideal for the sensing application of YIG. With an array of NV centers embedded into a polymer membrane, we map out a temperature gradient of YIG as a demonstration of this all-optical temperature sensing. Chapter 5 pursues a potential of the hybrid quantum architecture of NV centers and magnons in YIG, with the expectation that long-distance two-qubits gates of NV centers will be enabled. We perform a theoretical study of a practical hybrid quantum system of NV centers interacting with magnons in YIG waveguide and bar structures. With both a semi-analytic analysis and a numerical analysis, we construct a framework to compute the NV-magnon coupling strength and the NV-NV effective coupling strength mediated by magnons. This will guide future experiments and device fabrications. Lastly in Chapter 6, we use a diamond membrane that hosts ensemble of NV centers placed on top of a YIG slab and experimentally explore the coupling between the NV centers and magnons. We employ a device geometry where surface spin waves are prominent and are expected to interact with NV centers efficiently. With the longitudinal relaxometry experimental measurements, we estimate the NV-magnon coupling strength in terms of the real part of the self-susceptibility of an NV center mediated by magnons. This scheme of experimentally characterizing the hybrid quantum system’s relevant parameters will be of great use in future exploration of the NV-magnon hybrid quantum architectures

Opportunities for Long-Range Magnon-Mediated Entanglement of Spin Qubits via On- And Off-Resonant Coupling

The ability to manipulate entanglement between multiple spatially separated qubits is essential for quantum-information processing. Although nitrogen-vacancy (NV) centers in diamond provide a promising qubit platform, developing scalable two-qubit gates remains a well-known challenge. To this end, magnon-mediated entanglement proposals have attracted attention due to their long-range spin-coherent propagation. Optimal device geometries and gate protocols of such schemes, however, have yet to be determined. Here we predict strong long-distance (>μm) NV-NV coupling via magnon modes with cooperativities exceeding unity in ferromagnetic bar and waveguide structures. Moreover, we explore and compare on-resonant transduction and off-resonant virtual-magnon exchange protocols, and discuss their suitability for generating or manipulating entangled states at low temperatures (T 150mK) under realistic experimental conditions. This work will guide future experiments that aim to entangle spin qubits in solids with magnon excitations

Spatiotemporal Mapping of a Photocurrent Vortex in Monolayer MoS₂ Using Diamond Quantum Sensors

Photocurrents are central to understanding and harnessing the interaction of light with matter. Here, we introduce a contact-free method to spatially resolve photocurrent distributions using proximal quantum magnetometers. We interface monolayer MoS2 with a near-surface ensemble of nitrogen-vacancy centers in diamond and map the generated photothermal current distribution through its magnetic-field profile. By synchronizing pulsed photoexcitation with dynamical decoupling of the sensor spin, we extend the sensor’s quantum coherence and resolve time-dependent, two-dimensional current densities as small as 20 nA / μm , with a projected sensitivity of 200 nA / (μm $\sqrt{Hz}$) . Our spatially resolved measurements reveal that optical excitation can generate micron-sized photocurrent vortices in MoS2 , manifesting a photo-Nernst effect exceeding that of gate-tuned graphene at comparable magnetic fields. We further probe the rise time of the photocurrents and show that thermal diffusion determines its spatial variation. These spatiotemporal capabilities establish an optically accessed, local probe for optoelectronic phenomena, ideally suited to the emerging class of two-dimensional materials, for which making contacts is challenging and can alter the intrinsic material properties