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arXiv:2606.11947

Controlled ion-ion interactions and cavity-enhanced emission of a coherent dinuclear Eu$^{3+}$ complex

Evgenij Vasilenko, Vishnu Unni Chorakkunnath, Barbora Brachnakova, Nicholas Lester Jobbitt, Senthil Kumar Kuppusamy, David Hunger, Mario Ruben

quant-phphysics.opticsSubmitted Jun 10, 2026• Announced Jun 11, 2026• 0 citations
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Abstract

Molecular rare-earth-ion complexes offer unique opportunities for quantum technologies by combining the intrinsic coherence properties of rare-earth ions with chemically tunable molecular environments. A crucial capability is the realization of multi-qubit architectures with defined qubit couplings to enable two-qubit quantum gates. Here, we investigate the optical coherence properties and excitation-induced interactions of two Eu$^{3+}$-based molecular complexes, comparing a mononuclear reference system with a dinuclear analogue in which two Eu$^{3+}$ ions are positioned at a well-defined intramolecular distance of about 7 Angstrom. Using cryogenic ensemble spectroscopy, including spectral hole burning, free-induction decay, and photon echo measurements at temperatures down to 100 mK, we demonstrate long optical coherence times $T_{2,\text{o}}$ of up to 9 $μ$s. As a key step toward scalable multi-qubit architectures, a control-target sequence was implemented to probe conditional ion-ion interactions, revealing a stronger interaction-induced dephasing in the dinuclear complex. Finally, we show the integration of the dinuclear complex into a fiber-based optical microcavity, and observe an 380-fold emission enhancement of the $\mathrm{}^5\mathrm{D}_0\rightarrow\mathrm{}^7\mathrm{F}_0$ transition. Together, these results position molecular rare-earth complexes as versatile and chemically tunable building blocks for scalable quantum technologies.

Comments
Main Manuscript 17 pages and 5 figures, Supporting Information 9 pages and 3 figures

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Controlled ion-ion interactions and cavity-enhanced emission of a coherent dinuclear Eu3+^{3+}3+ complex
Molecular rare-earth-ion complexes offer unique opportunities for quantum technologies by combining the intrinsic coherence properties of rare-earth ions with chemically tunable molecular environments. A crucial capability is the realization of multi-qubit architectures with defined qubit couplings to enable two-qubit quantum gates. Here, we investigate the optical coherence properties and excitation-induced interactions of two Eu3+^{3+}3+-based molecular complexes, comparing a mononuclear reference system with a dinuclear analogue in which two Eu3+^{3+}3+ ions are positioned at a well-defined intramolecular distance of about 7 Angstrom. Using cryogenic ensemble spectroscopy, including spectral hole burning, free-induction decay, and photon echo measurements at temperatures down to 100 mK, we demonstrate long optical coherence times T2,oT_{2,\text{o}}T2,o​ of up to 9 μμμs. As a key step toward scalable multi-qubit architectures, a control-target sequence was implemented to probe conditional ion-ion interactions, revealing a stronger interaction-induced dephasing in the dinuclear complex. Finally, we show the integration of the dinuclear complex into a fiber-based optical microcavity, and observe an 380-fold emission enhancement of the 5D0→7F0\mathrm{}^5\mathrm{D}_0\rightarrow\mathrm{}^7\mathrm{F}_05D0​→7F0​ transition. Together, these results position molecular rare-earth complexes as versatile and chemically tunable building blocks for scalable quantum technologies.
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 Eu3+^{3+}3+-based molecular complexes, with a focus on the coherent 5D0⟶ 7F0^{5}\text{D}_{0} \longrightarrow \ ^{7}\text{F}_{0}5D0​⟶ 7F0​ transition at $\sim580nm.Judd−Ofeltanalysesareperformedacrossallcomplexes,providinginsightsintoligand−fieldenvironments,localpoint−groupsymmetries,andtheresultingbranchingratiosofthecoherenttransition.Wefindthatlowerpoint−groupsymmetry(C580 nm. Judd-Ofelt analyses are performed across all complexes, providing insights into ligand-field environments, local point-group symmetries, and the resulting branching ratios of the coherent transition. We find that lower point-group symmetry (C580nm.Judd−Ofeltanalysesareperformedacrossallcomplexes,providinginsightsintoligand−fieldenvironments,localpoint−groupsymmetries,andtheresultingbranchingratiosofthecoherenttransition.Wefindthatlowerpoint−groupsymmetry(C_1,C, C,C_2$) yields higher branching ratios of the 5D0⟶ 7F0^{5}\text{D}_{0} \longrightarrow \ ^{7}\text{F}_{0}5D0​⟶ 7F0​ 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 T2opt=2.3 μsT_{\text{2}}^{\text{opt}} = 2.3~μ\mathrm{s}T2opt​=2.3 μs 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.
Comparative Optical Spectroscopy of Er3+^{3+}3+, Tm3+^{3+}3+, and Eu3+^{3+}3+ Molecular Complexes at Cryogenic Temperature
Rare-earth molecular complexes are attracting increasing interest for quantum information applications because of their structural flexibility and chemical tunability. Here, we compare the cryogenic optical properties of Er3+^{3+}3+, Tm3+^{3+}3+, and Eu3+^{3+}3+ complexes in the same ligand environment. Optical spectroscopy shows differing behavior across the three systems: spectral hole burning is observed in the Eu3+^{3+}3+ complex, whereas no measurable hole is detected in the Er3+^{3+}3+ and Tm3+^{3+}3+ complexes. We measure laser-induced local heating and rethermalization in the Er3+^{3+}3+ and Tm3+^{3+}3+ powder-like samples, observing strong local heating and slow thermal relaxation in the cold-plate cryostat. The recovery occurs over several seconds and, for some Er3+^{3+}3+ excitation conditions, extends over tens of seconds. The comparison across the three ions nevertheless suggests that heating alone is unlikely to account for the full contrast. Rather, the different behaviors of Eu3+^{3+}3+, Tm3+^{3+}3+, and Er3+^{3+}3+ point to ion-dependent relaxation and decoherence processes associated with their electronic level structure and coupling to the molecular environment. Temperature-dependent linewidth and line-shift measurements are further consistent with phonon-mediated broadening through coupling to crystal-field levels.
The success of the emerging field of solid-state optical quantum information processing (QIP) critically depends on the access to resonant optical materials. Rare-earth ions (REIs) are suitable candidates for QIP protocols due to their extraordinary photo-physical and magnetic quantum properties such as long optical and spin coherence lifetimes (T2T_2T2​). However, molecules incorporating REIs, despite having advantageous properties such as atomically exact quantum tunability, inherent scalability, and large portability, have not yet been studied for QIP applications. As a first testimony of the usefulness of REI molecules for optical QIP applications, we demonstrate in this study that narrow spectral holes can be burned in the inhomogeneously broadened 5^55D0→7_0\to^70​→7F0_00​ optical transition of a binuclear Eu(III) complex, rendering a homogeneous linewidth (ΓhΓ_hΓh​) = 22 ±\pm± 1 MHz, which translates as T2=14.5T_2 = 14.5T2​=14.5 ±\pm± 0.7 ns at 1.4 K. Moreover, long-lived spectral holes are observed, demonstrating efficient polarization of Eu(III) ground state nuclear spins, a fundamental requirement for all-optical spin initialization and addressing. These results elucidate the usefulness of REI-based molecular complexes as versatile coherent light-spin interfaces for applications in quantum communications and processing.
Abstract Europium‐doped nanocrystals constitute a promising material for a scalable future quantum computing platform. Long‐lived nuclear spin states could serve as qubits addressed via coherent optical transitions. In order to realize an efficient spin‐photon interface, we couple the emission from a single nanoparticle to a fiber‐based microcavity under cryogenic conditions. The spatial and spectral tunability of the cavity permits us to place individual nanoparticles in the cavity, to measure the inhomogeneous linewidth of the ions, and to show a multi‐modal Purcell‐enhancement of two transition in Eu 3+ . A halving of the free‐space lifetime to 1.0 ms is observed, corresponding to a 140‐fold enhancement of the respective transition. Furthermore, we observe a narrow optical linewidth of 3.3 MHz for a few‐ion ensemble in the center of the inhomogeneous line. The results represent an important step towards the efficient readout of single Eu 3+ ions, a key requirement for the realization of single‐ion‐level quantum processing nodes in the solid state.
Single quantum emitters coupled to optical cavities in the Purcell regime can be used as high-efficiency spin-photon interfaces that are essential for building a quantum network. Furthermore, the dynamical control of the spontaneous emission rate of quantum emitters can have important implications in quantum technologies, e.g. for shaping the emitted photons waveform, or for driving coherently the optical transition while preventing photon emission. Here we demonstrate the dynamical Purcell-enhanced emission of a mesoscopic ensemble of erbium ions doped into nanocrystals coupled to a fully-tunable high-finesse fiber-based optical microcavity. Erbium has excellent optical and spin coherence properties at cryogenic temperatures and, in addition, has a transition in the telecom band that can facilitate integration into existing commercial telecom fibers. We show that we can tune the cavity on and out of resonance at a rate of above 8 KHz, which is two orders of magnitude faster that the natural lifetime of the erbium ions (55 Hz), and a factor of five faster than the Purcell enhanced emission (1.8 KHz). This allows us to shape in real time the Purcell enhanced emission of the ions and to achieve full control over the emitted photon’s waveforms. With moderate improvements in our detection efficiency and cavity finesse, this capability will allow for the generation of single telecom photons with controllable wave-shape from single erbium ions and for the realization of quantum processing between rare-earth ion qubits using dipolar interactions.
The interaction of single quantum emitters with an optical cavity enables the realization of efficient spin-photon interfaces, an essential resource for quantum networks. The dynamical control of the spontaneous emission rate of quantum emitters in cavities has important implications in quantum technologies, e.g., for shaping the emitted photons' waveform or for driving coherently the optical transition while preventing photon emission. Here we demonstrate the dynamical control of the Purcell enhanced emission of a small ensemble of erbium ions doped into a nanoparticle. By embedding the nanoparticles into a fully tunable high finesse fiber based optical microcavity, we demonstrate a median Purcell factor of 15 for the ensemble of ions. We also show that we can dynamically control the Purcell enhanced emission by tuning the cavity on and out of resonance, by controlling its length with sub-nanometer precision on a time scale more than two orders of magnitude faster than the natural lifetime of the erbium ions. This capability opens prospects for the realization of efficient nanoscale quantum interfaces between solid-state spins and single telecom photons with controllable waveform, for non-destructive detection of photonic qubits, and for the realization of quantum gates between rare-earth ion qubits coupled to an optical cavity.