Molecular Engineering Optically Addressable Spin Qubits

The field of quantum information processing aims to leverage the unique physics of quantum mechanics to develop new strategies for applications such as computation, communications, and sensing. Two subfields have emerged as compelling platforms: (i) optically-addressable defect spins in the solid-state (which offer an optical interface, but limited chemical control) and (ii) molecular spin qubits (which offer chemical control, but lacked an optical interface). In this thesis, we aim to extend the optical spin initialization and readout schemes developed for spin defects in solid-state crystals to the class of chemically-synthesized molecular spin qubits. We begin by providing a brief historical context on motivations for the field of quantum information, and highlight a few parallel developments within the fields of optically-addressable defect spins and molecular spin qubits. In Chapter 2, we provide theoretical background on the electronic strucutre of metal ions in solids and organometallic coordination complexes, and discuss considerations specific to the cases of both the transition metal and lanthanide series. By doing so, we aim to build an intuitive picture to model the electronic structure of the ground and first few excited states of these metal ion systems in distinct chemical environments. In Chapter 3, we discuss the temporal dynamics of these systems, briefly covering both molecular photophysics and spin dynamics, with the aim of outlining the necessary conditions to develop a spin-photon interface for molecules. Next, we present experimental results that deliver on our objective of creating optically-addressable molecular spin qubits. Chapter 4 presents optical initialization and readout of an ensemble of Cr^{4+} molecular spins through a spin-selective optical pumping scheme, which represents the first (to our knowledge) optical readout of a molecular ground state spin qubit. Additionally, we show that this strategy applies to multiple compounds, and that chemical design allows control of the intrinsic spin properties. Chapter 5 covers our efforts to develop an off-resonant readout scheme for these tetravalent chromium complexes, which leverages polarization selection rules to yield spin-selective pumping. Finally, Chapter 6 discusses a strategy to push down to the single spin level, outlining an approach by which single molecular Ce^{3+} spins could be probed optically via polarization selection rules. Finally, we conclude by discussing general outlook on unique opportunities that optically-addressable molecular spins might present, and suggest future research directions that may prove fruitful