UKRI-NSF Coherent Optical Control of Triplet Spin States in Organic Molecule Quantum Emitters

We propose to discover and characterize the elusive triplet state in dibenzoterrylene (DBT) molecules using novel sensitizer-enhanced spectroscopy. By combining chemical and quantum optical approaches, we will measure the triplet state's energy structure, develop coherent preparation protocols, and optimize spin coherence times. This work will establish DBT as a promising quantum memory platform while advancing our understanding of molecule-phonon coupling in quantum systems. Single DBT molecules embedded in poly-aromatic hydrocarbon matrices have emerged as exceptional quantum emitters, demonstrating remarkable optical coherence properties at cryogenic temperatures. These molecular quantum systems offer several advantages over epitaxial quantum dots, including their identical nature, ease of fabrication, and opportunities for functionalization. While DBT molecules demonstrate exceptional quantum optical properties, their ground and excited states are both spin singlets, limiting their use in quantum memory applications. A triplet state would provide the desired spin degree of freedom, offering a route to quantum memory in these excellent emitters. The triplet state has remained unobserved due to two key challenges: the difficulty in calculating accurate theoretical predictions of its energy levels, and the extremely weak intersystem crossing rates (approximately 10-7) that make direct experimental detection nearly impossible through conventional methods. We propose a novel approach combining chemical and quantum optical techniques across three main objectives: I. Triplet State Discovery and Characterization: We will employ molecular sensitizers with strong intersystem crossing to enhance population transfer to DBT's triplet state. Using broadband spectroscopy in-solution phase, we will locate the triplet energy level, predicted to lie at infrared wavelengths. This chemical approach circumvents the limitations of direct optical excitation. II. Energy Level Structure, Dynamics and Coherent State Preparation: Once identified, we will employ Autler-Townes spectroscopy to precisely determine the triplet state's energy, spin structure, and linewidths. We will measure the radiative and non-radiative rates of the pathways to and from the triplet state. This spectroscopy will also serve as an excellent test for density functional theory predictions of triplet states in complex molecules. The weak intersystem crossing prevents efficient spontaneous population of the triplet state. We will develop a two-photon coherent Raman transfer scheme to directly prepare the triplet state, enabling controlled access to this quantum resource. III. Spin Coherence Engineering: Initial spin coherence measurements will be performed in a cryostat under high magnetic fields. We will systematically study decoherence mechanisms, particularly focusing on interactions with lattice phonons and surface charges. Drawing parallels with epitaxial quantum dots, we will employ charge engineering techniques and dynamical decoupling protocols to extend coherence times, and photonic engineering to enhance light-matter interaction and branching ratios for chosen decay paths. Our long-term vision is to develop DBT into a quantum memory platform capable of storing and manipulating entangled photonic states through spin operations. Our approach unites expertise in chemical synthesis, ultrafast spectroscopy, and quantum optics to address fundamental questions about molecule-vibration-electron coupling while advancing practical quantum technologies.