Electromagnetic (EM) fields can be engineered through controlled modulation of amplitude, phase, frequency, polarization, and angular momentum. Such engineered fields have enabled a wide range of promising applications in modern photonics, nanoscience, and quantum technologies. In the far-field regime, tailoring the spin and orbital angular momentum of light gives rise to structured optical fields with nontrivial topological properties, such as skyrmionic and vortex-like textures. These optical topologies have attracted considerable interest for applications in noise-resilient optical communications, super-resolution imaging, and advanced light-matter interactions. In contrast, near-field EM environments emerge from microscopic spin and charge dynamics within materials. By controlling material composition, dimensionality, and nanoscale structure, it becomes possible to engineer EM fluctuations. Probing the properties of engineered EM fields remains challenging because many of their features, including the nanoscale localization of topological features and the weak, broadband nature of near-field fluctuations, are difficult to access using conventional measurement techniques. This dissertation focuses on quantum probing of engineered EM fields using the nitrogen vacancy (NV) center in diamond, a versatile quantum sensing platform. Both far-field optical topologies and near-field EM fluctuations are investigated, demonstrating how quantum probes can be used to characterize novel EM phenomena across distinct spatial and spectral regimes.In this dissertation, the topological properties of optical beams are probed through the coherent interaction between the photonic spin texture (PST) of structured light and NV center spin qubits. The spatial distribution of photonic spin gives rise to distinct topological features of light. Through local coherent coupling with the NV spin qubit, the PST induces a measurable phase shift in the quantum superposition state of the NV center. By optically measuring the accumulated phase shift of the NV center spin qubits, the local spin texture is quantitatively reconstructed and the topological properties of the optical field are characterized. Through a coherent light-matter interaction, this work also introduces the concept of transducing topological features of light onto the phase texture of NV spin ensemble.To probe the near-field EM fluctuations, the phase coherence decay of NV center spin qubits is measured. Fluctuations arising from spin and charge dynamics within nearby materials induce random phase accumulation in the superposition state of the NV center, leading to dephasing of the spin qubit. Through frequency-selective and spectrally resolved measurements of the NV spin coherence decay rates, the local near-field EM environment is quantitatively characterized. The measured fluctuations are further connected to important materials properties and phenomena, including magnetic dynamics, superparamagnetism, and critical fluctuations in magnetic material and metastructures. Through quantum dephasing measurements, this work reveals previously unexplored material properties and near-field EM phenomena.
Electromagnetic (EM) fields can be engineered through controlled modulation of amplitude, phase, frequency, polarization, and angular momentum. Such engineered fields have enabled a wide range of promising applications in modern photonics, nanoscience, and quantum technologies. In the far-field regime, tailoring the spin and orbital angular momentum of light gives rise to structured optical fields with nontrivial topological properties, such as skyrmionic and vortex-like textures. These optical topologies have attracted considerable interest for applications in noise-resilient optical communications, super-resolution imaging, and advanced light-matter interactions. In contrast, near-field EM environments emerge from microscopic spin and charge dynamics within materials. By controlling material composition, dimensionality, and nanoscale structure, it becomes possible to engineer EM fluctuations. Probing the properties of engineered EM fields remains challenging because many of their features, including the nanoscale localization of topological features and the weak, broadband nature of near-field fluctuations, are difficult to access using conventional measurement techniques. This dissertation focuses on quantum probing of engineered EM fields using the nitrogen vacancy (NV) center in diamond, a versatile quantum sensing platform. Both far-field optical topologies and near-field EM fluctuations are investigated, demonstrating how quantum probes can be used to characterize novel EM phenomena across distinct spatial and spectral regimes.In this dissertation, the topological properties of optical beams are probed through the coherent interaction between the photonic spin texture (PST) of structured light and NV center spin qubits. The spatial distribution of photonic spin gives rise to distinct topological features of light. Through local coherent coupling with the NV spin qubit, the PST induces a measurable phase shift in the quantum superposition state of the NV center. By optically measuring the accumulated phase shift of the NV center spin qubits, the local spin texture is quantitatively reconstructed and the topological properties of the optical field are characterized. Through a coherent light-matter interaction, this work also introduces the concept of transducing topological features of light onto the phase texture of NV spin ensemble.To probe the near-field EM fluctuations, the phase coherence decay of NV center spin qubits is measured. Fluctuations arising from spin and charge dynamics within nearby materials induce random phase accumulation in the superposition state of the NV center, leading to dephasing of the spin qubit. Through frequency-selective and spectrally resolved measurements of the NV spin coherence decay rates, the local near-field EM environment is quantitatively characterized. The measured fluctuations are further connected to important materials properties and phenomena, including magnetic dynamics, superparamagnetism, and critical fluctuations in magnetic material and metastructures. Through quantum dephasing measurements, this work reveals previously unexplored material properties and near-field EM phenomena.
Superparamagnetism in nanoscale magnetic layers is a critical property for a wide range of spintronic-based sensor and computing applications. While conventional magnetization measurements can detect superparamagnetic signatures, they often require the application of high perturbative fields and are difficult to implement for magnetic layers integrated within functional devices. In this study, we non-invasively investigate the superparamagnetic spin dynamics of a nanoscale CoFeB layer of thickness 1.1 nm, deposited on a diamond substrate, by probing its low-frequency near-field electromagnetic (EM) fluctuations using nitrogen-vacancy (NV) centers-based quantum dephasometry. Our measurements reveal an unconventional, non-monotonic temperature dependence of the dephasing time of NV centers, which we attribute to EM fluctuations produced by thermally driven superparamagnetic domain flipping in CoFeB. Our findings are further supported by the theoretical interpretation of the dephasing dynamics of NV centers and the complementary SQUID-based magnetization characterizations of the CoFeB layer. Additionally, exploiting the technique of NV dephasometry, we extract the spectral density of the EM fluctuations in CoFeB, which is used to isolate different components of the EM fluctuations acting on NV centers. We also measure the CoFeB-to-NV distance-dependent coherence times of NV centers to investigate the effect of the dimensionality of the CoFeB layer on the generated near-field EM fluctuations. These results provide critical insight into the magnetization dynamics and near-field EM environment of nanoscale magnetic layers. It also has significant implications for the development of hybrid quantum spintronic devices and applications involving nanoscale opto-magnetic materials.
Abstract Photonic spin texture (PST), the spatial distribution of the spin angular momentum of light, is connected to unique properties of light, such as optical skyrmions and topological optical N-invariants. There has been recent progress on the generation and manipulation of PST using various methodologies. However, a challenge remains for the sub-wavelength characterization of PST. Here, we demonstrate nitrogen–vacancy (NV) centers in diamond as nanoscale quantum sensors for imaging the PST of a beam with orbital angular momentum. Leveraging the coherent interaction between photon spin and NV center electron spin at cryogenic temperature (77 K), and using the Hahn-echo magnetometry technique, we experimentally demonstrate the imprinting of the PST on the quantum phase shift of NV centers. Our work can lead to the development of a quantum imaging platform capable of characterization of the spin texture of light at sub-wavelength scales.
The nitrogen-vacancy (NV) center in diamond has garnered great interest over the past decade as its electronic spin shows promise as a quantum bit (qubit) and nanoscale sensor. Consisting of a substitutional nitrogen adjacent to a vacant site within the carbon lattice of diamond, this defect exhibits millisecond-long spin coherence times extending beyond room temperature, spin-dependent optical addressability, coupling to intrinsic and nearby nuclear spins, and it can be controlled and manipulated through electrical, magnetic, and optical means. In particular, at cryogenic temperatures (T < 25 K), the NV center's excited state becomes sharp and optically resolvable, providing a solid-state quantum optical testbed. In this thesis, I describe several experiments that explore this quantum optical interface to facilitate the development of a photonic network of single spins linked and controlled by light. We begin by exploring how electric fields tune the orbital levels within the NV center through the DC Stark effect, finding a surprising photo-induced field that aids in the ability to tune multiple NV centers' optical transitions to degeneracy. We then develop techniques to fully control the spin state of the NV center by coupling through a lambda system, an energy configuration consisting of two lower levels coupled to one of higher energy. When a lambda system is optically driven, the spin becomes trapped in a dark state, or the eigenstate of the system that is not coupled to the light fields through destructive interference, forming the basis for the various types of control demonstrated. We demonstrate arbitrary-basis initialization and readout of the spin state through coherent population trapping, as well as the ability to rotate about any arbitrary basis through stimulated Raman transitions. Combining these techniques, we measure the NV center's spin coherence through a completely optical measurement. We then extend these lambda system techniques to adiabatically move the dark state in trajectories around the Bloch sphere. Such trajectories accumulate a quantum mechanical phase that depends only on the geometry of the path enclosed, not on the energetics or time of the interaction. We characterize the interaction, measure this phase, known as Berry phase, and explore the limits of its control and resilience to noise. Finally, we demonstrate another all-optical control technique that uses strong ultrafast pulses of light to transfer the spin between the ground and excited states, deriving spin manipulation from the excited state dynamics. This technique also provides time-resolved spectroscopy of the excited state and its various decay and decoherence mechanisms. These experiments advance the progress toward the development of photonic networks coupling and controlling defects through light-matter interactions.
Quantum light-matter interactions, described by the theory of quantum electrodynamics (QED), have motivated numerous scientific discoveries and technological developments over the past few decades. Considerable effort has been made to develop different QED platforms, e.g., free space, waveguides, cavities, and photonic crystals, for engineering quantum light-matter interactions. This thesis explores a new frontier, deep microscopic QED, which leverages the deep microscopic response of materials to reveal new topological, quantum, and thermal photonic phenomena with applications to quantum sensing and information processing. In the first part of this thesis, we investigate unique topological photonic effects arising in the deep microscopic QED regime. Previous efforts focused on topological photonic phases and edge states arising from artificial spatial structures of materials (e.g., photonic crystals or metamaterials). One fundamental question is whether topological photonic states can also arise from the intrinsic response of many-body systems and do not rely on materials' macroscopic structures. Here, we first discuss a new topological electromagnetic phase - ‘optical N-phase’ arising in two-dimensional (2D) viscous hydrodynamic electrons in graphene, which originates from the microscopic response of 2D materials and does not rely on materials' macroscopic structures. We then introduce the associated edge states, optical N-plasmons, which are topologically protected ultra-subwavelength edge magneto-plasmons. We further show how these topological plasmons can be harnessed to design chiral radio-frequency nonreciprocal circuit components, topological hydrodynamic circulators, that interface quantum and classical hardware.In the second part of this thesis, we focus on collective atom-photon interactions in the deep microscopic QED regime. In atom interactions with photonic environments, there exist two non-unitary processes -- pure dephasing and spontaneous emission. Despite considerable interest in collective effects in spontaneous emission since the 1950s, one long-standing problem is whether collective effects in pure dephasing can arise in atom-photon interactions. This question is particularly important in the quantum information era since pure dephasing is the main obstacle in current quantum information technologies. Here, we first introduce a new collective quantum phenomenon, ‘nanophotonic superdephasing,’ occurring in atom/spin ensembles in nanoscale photonic environments. This nanophotonic superdephasing phenomenon originates from low-frequency photonic environment correlations, and leads to collectively accelerated disentanglement without photon emission, in stark contrast to conventional collective spontaneous emission/superradiance effects. We further reveal the effects of nanophotonic environment mediated correlated dynamics on quantum computing and sensing. We demonstrate that correlations of nanoscale EM environments in spin qubit quantum computing devices can induce correlated errors and limit quantum gate fidelity. Meanwhile, based on nanophotonic collective dephasing, we also propose a multiqubit quantum sensing protocol that exploits spatially correlated noise near materials to enable order parameter symmetry characterization of materials beyond single-qubit methods.In the final part, we explore thermal photonic phenomena in the deep microscopic regime. Conventional techniques in engineering near-field radiative heat transfer typically rely on phonons, plasmons, and electromagnetic modes in structured materials. Here, we reveal a new approach to enhance near-field thermal radiation: leveraging intrinsic quantum coherence in superconducting materials. We reveal the quantum fingerprint of superconductivity in thermal photonics, ‘superconducting coherence peak’ in near-field thermal radiation, which originates from quantum coherence of Bogoliubov quasiparticles, with potential applications in thermal engineering in superconducting circuits.Overall, the thesis establishes a deep microscopic QED platform for new quantum, topological, and thermal photonic phenomena in many-body systems, with applications in quantum sensors and devices.
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.
One central theme in quantum photonics is tailoring the interactions between atoms/spins and their electromagnetic (EM) environments. Considerable effort has focused on engineering spontaneous emission by shaping EM environments, known as the Purcell effect. However, photonic environment control of pure dephasing, which is a complementary paradigm of non-unitary atom/spin couplings with EM environments, remains largely unexplored. Here, we introduce a nanophotonic approach to modify qubit pure dephasing dynamics. Unlike Purcell engineering that tailors photonic environments at qubit resonance frequencies (typically optical/near-infrared), we develop ultra-subwavelength spin noise metasurfaces for efficient broadband control of low-frequency (e.g., ∼MHz) photonic environments far off-resonant with atoms/spins for dephasing engineering. We experimentally demonstrate our approach using lithographically defined CoFeB metasurfaces and shallow nitrogen-vacancy (NV) centers in diamond. Instead of modified spontaneous emission, we observe modified NV pure dephasing dynamics near different spin noise metasurfaces. We further isolate metasurface-controlled dephasing from other dephasing mechanisms (e.g., spin bath) by measuring the NV ensemble dephasing noise spectrum with dynamical decoupling spectral decomposition techniques. Our results establish a new frontier in engineering quantum light-matter interactions with nanophotonic structures.