Quantum Probing of Engineered Electromagnetic Fields: Far-Field Topology to Near-Field Fluctuations
Shoaib Mahmud
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.
•Jul 23, 2026•0 citations