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

Abstract

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.

Observation of Narrow Optical Homogeneous Linewidth and Long Nuclear Spin Lifetimes in a Prototypical [Eu(trensal)] Complex

“…It would be also interesting to study Yb 3+ and Er 3+ complexes that emit in the near-infra-red region because such complexes are useful for realizing molecule-based quantum communication architectures. , Two recent reports elucidate narrow optical linewidths associated with Yb 3+ complexes, , implying the utility of Ln 3+ complexes for creating coherent light–matter interfaces with quantum technological implications.…”

Elucidating Ultranarrow 2F7/2 to 2F5/2 Absorption in Ytterbium(III) Complexes

“…They have different chemical structures and spectral features, including YbCp3 with a high (𝐷 3 ) symmetry, K3[Yb(BINOL)3] and Yb(trensal) with a relatively high (𝐶 3 ) symmetry, and the asymmetric (thiolfan)YbCl(THF) reported recently. 9 We extracted their experimental absorption spectra for the SOCF fitting operations. For (thiolfan)YbCl(THF), the local charges are directly assigned based on a LoProp charge calculation from SOC-CASSCF/RASSI theory, removing the charge assignment step.…”

“…The linewidth for the transition from E(1,2) to E(9,10) for (thiolfan)YbCl(THF) is found to be 0.615 meV, consistent with reported linewidth measured from a narrowband laser. 9 Because of resolution from typical UV-Vis spectrometers, this linewidth is artificially broadened to 3. which shows excellent agreement. We also observe a strong correlation coefficient of 0.9991 between the energies of ground (states 1 and 2) and excited states (states 9 and 10) corresponding to the narrowest linewidth transition in the experiment (Figure 5c).…”

“…In general, whether a material is used as a quantum sensor or a qubit, a very narrow optical linewidth aids in state preparation and readout. 9 In this manuscript, we explore how to modulate the narrow f-orbital-centered absorption properties of lanthanide complexes. The frontier orbitals of lanthanide(III) complexes consist of the valence electrons in the 4f manifold and are highly protected from their local environment.…”

“…We implement a spin-orbit crystal field (SOCF) atomic-level calculation as a fitting mechanism to capture the spectral positions and relative and absolute oscillator strengths of several YbCp3 (Cp = cyclopentadienyl). 9,[14][15][16] We use SOCF to describe the f-f transition energies and combine Judd-Ofelt theory, which describes how 4f and 5d orbitals mix upon ligand perturbation, with the magnetic dipole computations to compute total oscillator strengths. Using the orbitals defined in the SOCF fitting procedure, we also develop an "atomic similarity" metric that describes how well each state is described by specific spin-orbit "atomic" orbitals.…”