Judd-Ofelt analysis and coherent spectroscopy of europium-based molecular complexes for quantum information applications

AI summary

Overview

Research area: Quantum information science, specifically rare-earth-ion (REI) molecular crystals as a materials platform, combining coordination chemistry with cryogenic optical spectroscopy.

Technical level: Advanced. The paper assumes familiarity with Judd-Ofelt theory, crystal-field/point-group symmetry, branching ratios, inhomogeneous versus homogeneous linewidths, and coherent optical techniques (spectral hole burning, free-induction decay, two-pulse photon echo).

Scope (one sentence): A comprehensive optical characterisation of twelve Eu³⁺-based molecular complexes — Judd-Ofelt parameters, branching ratios, lifetimes, quantum efficiencies, inhomogeneous linewidths, and coherence measurements on one complex (5-Eu) — aimed at identifying structure-property relations for quantum information applications.

What This Paper Is About

Traditional rare-earth-ion crystals doped into inorganic hosts have excellent coherence but suffer from low oscillator strengths and branching ratios of their coherent transitions, plus a random ion distribution that complicates scaling into registers. Molecular crystals built from Eu³⁺ complexes are an alternative because they self-assemble into ordered single crystals of identical copies and their ligand shell can be chemically tuned. The paper asks which structural features of a Eu³⁺ complex make the nominally forbidden ⁵D₀ → ⁷F₀ transition at ~580 nm strong enough to use, while preserving the narrow linewidths and long coherence needed for quantum protocols.

Key Contributions

1. A unified optical survey of twelve Eu³⁺ complexes (1-Eu through 12-Eu), spanning mononuclear and binuclear species with point-group symmetries from C₁ and C₂ up to C₃, C₃ᵥ, C₂ᵥ, and distorted D₄d/D₅h environments, measured under one consistent set of methods.
2. Judd-Ofelt analyses for the whole series, yielding Ω₂, Ω₄, Ω₆ parameters, radiative lifetimes, room-temperature optical lifetimes, intrinsic quantum efficiencies, and branching ratios for the ⁵D₀ → ⁷F_J (J = 0, …, 6) transitions, including comparison of Judd-Ofelt-derived and directly measured branching ratios.
3. New cryogenic inhomogeneous linewidth data for previously unreported complexes (1-Eu to 6-Eu), including a 6.4(0.2) GHz line for 5-Eu at 4.2 K, plus the synthesis and crystallographic characterisation of the new complex 6-Eu.
4. First coherent optical spectroscopy of 5-Eu, giving T₂^opt = 2.3(0.3) μs at 4.2 K from two-pulse photon echo, alongside spectral hole burning and free-induction decay measurements

Authors’ abstract

Rare-earth-ion based molecular crystals are emerging as a promising platform for quantum information applications, combining long optical coherence times with the chemical tunability of coordination chemistry. Here we present a comprehensive optical characterisation of twelve Eu$^{3+}$-based molecular complexes, with a focus on the coherent $^{5}\text{D}_{0} \longrightarrow \ ^{7}\text{F}_{0}$ transition at $\sim$580 nm. Judd-Ofelt analyses are performed across all complexes, providing insights into ligand-field environments, local point-group symmetries, and the resulting branching ratios of the coherent transition. We find that lower point-group symmetry (C$_1$, C$_2$) yields higher branching ratios of the $^{5}\text{D}_{0} \longrightarrow \ ^{7}\text{F}_{0}$ transition, exceeding 1\% in several complexes. Optical inhomogeneous linewidths measured at 4.2 K reveal narrow linewidths for some complexes of below ten GHz, indicative of a highly ordered crystalline environment. Optical coherence measurements of one of the complexes, 5-Eu, yields an optical coherence time of $T_{\text{2}}^{\text{opt}} = 2.3~μ\mathrm{s}$ at 4.2 K, comparable to the best-performing molecular systems reported to date. Systematic characterisation and quantitative comparison will be key to find structure-property relations guiding rational design of optimised complexes for quantum information applications.