Transition Metal Ions as Optically Addressable Spin Qubits

This thesis investigates transition metal ions (TMIs) as an emerging platform to create quantum bits (qubits). The electronic orbitals of TMIs can be tuned and designed using basic guiding principles to form various structures that contain long lived qubits in the ground state and spin selective optical transitions that form a photonic interface to initialize and readout the quantum states. This thesis starts with describing the physics behind electron spin qubits and TMI orbitals that host the electrons. We then narrow the focus to a few symmetry configurations based on basic design principles from understanding TMI physics. Through optical spectroscopy we confirm the expected electronic structure that allows for a spin-photon interface of chromium ions within two different semiconducting hosts: silicon carbide (SiC) and gallium nitride (GaN). We observe a striking similarity of the optical and spin structure between the qubits in these different semiconducting hosts. This is a result of the similar local environment of the defects, demonstrating the portability of TMI qubits across various hosts. We then further narrow our focus to TMI qubits within SiC, a technologically mature semiconducting material widely used in industry. We create qubits in this material on demand through ion implantation and annealing, a first step towards scalable quantum devices based on transition metal qubits within solid state. In this system, we show long spin lifetimes for ensembles of chromium ions with high optical initialization and readout fidelities, which demonstrates their viability as spin qubits with optical addressability. We also characterize vanadium ion defect spins that possess an optical interface within the telecom O-band compatible with single-ion detection, showing promise for using them in long distance quantum communication devices that can utilize the existing low-loss telecommunications fiberoptic infrastructure. Finally, we leverage a new synthetic chemistry approach to atomistically design the local ligand environment of TMIs to generate optically addressable molecular spin qubits and show their coherent control. By changing the ligand environment and/or the core transition metal type, we can keep the physics of the qubit the same while changing both the optical transition over 200 nm and the microwave transitions over 100 GHz, showing the promise of transition metal based molecules as tailor-made qubits for different quantum applications that can range from self-assembled individually addressable spin arrays for quantum computation to highly multiplexed quantum sensors that are a nanometer in size. The demonstrations of long coherence times with high initialization and readout fidelities, detection of single emitters within the telecom band, and synthetically tunable molecular qubits highlight the promise of TMIs as a flexible, emerging quantum technology platform with potential applications in quantum communication, computation, and sensing