Optically Addressable Ground State Molecular Spins

Quantum systems are revolutionizing the fields of sensing and communication. 1,2 In the area of quantum sensing, the exquisite sensitivity of the quantum states of electron spins to electromagnetic fields enables high fidelity detection. 3 The largest body of work in quantum sensing focuses on defects in semiconductors or so-called "color centers."These systems feature spin-optical interfaces that enable initialization and readout of spin state.Applications of color centers range from elucidating biological processes in cells 4-8 to imaging magnetic phenomena in condensed matter systems. 9,10itialization and readout of color centers can be broadly classified as resonant or off-resonant.An off-resonant scheme for an S = 1 system involves the promotion of the spins populating both the M s = 0 and 1 sublevels to the first triplet excited state.While the spins in the M s = 0 excited state immediately relax to the M s = 0 ground state, the M s = 1 spins undergo spin-selective intersystem crossing (ISC) and eventually collect in the M s = 0 sublevel, as depicted in Fig. 1. 11 Conversely, in a resonant excitation scheme, narrow-linewidth lasers excite the spins in two of the three spin sublevels of the S = 1 ground state to the first S = 0 excited state, which then non-selectively decays and shelves population within the remaining unprobed spin sublevel.The iconic color center, the anionic nitrogen vacancy pair defect in diamond (NV -), can be optically initialized and readout through either an off-resonant or resonant excitation scheme (Fig. 1).However, many systems are restricted to resonant polarization, for example, Cr 4+ defects in silicon carbide (SiC). 12Optical polarization of the electron spins in these solid-state platforms then allows for microwave manipulation of the ground state.The ability to optically control and readout coherences of bright, single spins is key for developing protocols for quantum technologies, such as generating correlated spin pairs for entanglement-enhanced quantum sensing. [13][14][15][16] Molecules featuring a spin optical interface comprise an attractive platform for quantum information science (QIS) applications, in particular quantum sensing.These systems can be realized by harnessing synthetic chemistry to create analogous electronic structures to solid-state spin defects featuring a spin-optical interface.Molecules featuring ground state spin control enable rational tuning of electron spin properties and the construction of precise multi-qubit arrays, opening a new area of molecular color centers primed for quantum sensing and communication..The advent of this area opened a new area of molecular color centers primed for quantum sensing and communication applications.Indeed, spin-optical interfaces have been realized across a variety of coordination complexes and organic radicals.For each system, we describe the electronic structure considerations and specific mechanisms that enable optical addressability and highlight the diverse optical initialization and readout schemes in the literature, providing inspiration for future compounds that could improve and expand upon the current generation of molecular color centers.