Optically Detected Magnetic Resonance on Carbene molecular Qubits

Solid-state quantum systems that utilize optical and spin degrees of freedom in a spin–photon interface have found widespread application in emerging quantum technologies. Recently, molecular qubits have gained center stage as precisely tunable entities that present a compelling alternative to well-established, yet structurally less flexible point defects in solid-state systems. In this work, we disclose ground-state triplet (GST) carbenes embedded in a molecular matrix as organic qubits comprising two unpaired electrons in close proximity that can be optically initialed and read out. In-situ photoactivation enables the precise allo- cation of the carbene position and tuning of its density in the crystalline matrix and lays the ground for optically detected magnetic resonance (ODMR) with high fluorescence contrast of more than 40%, as well as record- high spin coherence times for a molecular spin–photon interface of T2 = 157(4) μs at a temperature of around 5 K. In addition, we show how state-of-the-art quantum chemical calcu- lations including multiscale geometry predictions and complete active space self-consistent field (CASSCF) computations offer comprehensive insight into fundamental spin characteristics. With that, for the first time a series of attractive properties could be united in a single solid-state qubit material: Exclusive usage of light elements (C,H,O,N), optical spin-selective excitations and relaxation pathways, and large zero-field splitting (ZFS) parameters on the order of a few GHz resulting in protection against decoherence sources at low magnetic fields. In this study, we take first steps into a hitherto dormant playground for the design and fabrication of molecular solid-state color centers made of purely organic compounds.