Toward liquid cell quantum sensing: Ytterbium complexes with ultra-narrow absorption

In quantum technology (such as atomic vapor cells used in precision magnetometry), the energetic disorder induced by a fluctuating liquid environment acts in direct opposition to the precise control required for coherence-based sensing. Overcoming fluctuations requires a protected quantum subspace that only weakly interacts with the local environment. Herein, we report a ferrocene-supported ytterbium complex ((thiolfan)YbCl(THF), thiolfan = 1,1′-bis(2,4-di-tert-butyl-6-thiomethylenephenoxy)ferrocene) that exhibits an extraordinarily narrow absorption linewidth in solution at room temperature with a full-width at half-maximum of 0.625 ± 0.006 meV. A detailed spectroscopic analysis allows us to assign all near infrared (NIR) transitions to atom-centered f-f transitions, protected from the solvent environment. A combination of density functional theory and multireference methods match experimental transition energies and oscillator strengths, illustrating the role of spin-orbit coupling and asymmetric ligand field in enhancing absorption and pointing toward molecular design principles that create well-protected yet observable electronic transitions in lanthanide complexes. Narrow linewidths allow for a demonstration of extremely low-field magnetic circular dichroism at room temperature, employed to sense and image magnetic fields, down to Earth scale. We term this system an ‘atom-like molecular sensor’ (ALMS), and propose approaches to improve its performance.