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ChemPhysChem

Spectral Hole‐Burning Studies of a Mononuclear Eu(III) Complex Reveal Narrow Optical Linewidths of the 5D0→7F0 Transition and Seconds Long Nuclear Spin Lifetimes

Sören Schlittenhardt, Evgenij Vasilenko, Vishnu Unni Chorakkunnath, Nicholas Jobbitt, Olaf Fuhr, David Hunger, Mario Ruben, Senthil Kumar Kuppusamy

Published Jun 18, 2024• 9 citations
Publisher

Abstract

Abstract Coordination complexes of rare‐earth ions (REI) show optical transitions with narrow linewidths enabling the creation of coherent light‐matter interfaces for quantum information processing (QIP) applications. Among the REI‐based complexes, Eu(III) complexes showing the 5D0→7F0 transition are of interest for QIP applications due to the narrow linewidths associated with the transition. Herein, we report on the synthesis, structure, and optical properties of a novel Eu(III) complex and its Gd(III) analogue composed of 2,9‐bis(pyrazol‐1‐yl)‐1,10‐phenanthroline (dpphen) and three nitrate (NO3) ligands. The Eu(III) complex—[Eu(dpphen)(NO3)3]—showed sensitized metal‐centred emission (5D0→7FJ; J=0,1,2,3, 4, 5, or 6) in the visible region, upon irradiation of the ligand‐centred band at 369 nm, with the 5D0→7F0 transition centred at 580.9 nm. Spectral hole‐burning (SHB) studies of the complex with stoichiometric Eu(III) concentration revealed a narrow homogeneous linewidth (Γh) of 1.55 MHz corresponding to a 0.205 μs long optical coherence lifetime (T2opt). Remarkably, long nuclear spin lifetimes (T1spin) of up to 41 s have been observed for the complex. The narrow optical linewidths and long T1spin lifetimes obtained for the Eu(III) complex showcase the utility of Eu(III) complexes as tuneable, following molecular engineering principles, coherent light‐matter interfaces for QIP applications.

DOI
10.1002/cphc.202400280

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Abstract Coordination complexes of rare‐earth ions (REI) show optical transitions with narrow linewidths enabling the creation of coherent light‐matter interfaces for quantum information processing (QIP) applications. Among the REI‐based complexes, Eu(III) complexes showing the 5D0→7F0 transition are of interest for QIP applications due to the narrow linewidths associated with the transition. Herein, we report on the synthesis, structure, and optical properties of a novel Eu(III) complex and its Gd(III) analogue composed of 2,9‐bis(pyrazol‐1‐yl)‐1,10‐phenanthroline (dpphen) and three nitrate (NO3) ligands. The Eu(III) complex—[Eu(dpphen)(NO3)3]—showed sensitized metal‐centred emission (5D0→7FJ; J=0,1,2,3, 4, 5, or 6) in the visible region, upon irradiation of the ligand‐centred band at 369 nm, with the 5D0→7F0 transition centred at 580.9 nm. Spectral hole‐burning (SHB) studies of the complex with stoichiometric Eu(III) concentration revealed a narrow homogeneous linewidth (Γh) of 1.55 MHz corresponding to a 0.205 μs long optical coherence lifetime (T2opt). Remarkably, long nuclear spin lifetimes (T1spin) of up to 41 s have been observed for the complex. The narrow optical linewidths and long T1spin lifetimes obtained for the Eu(III) complex showcase the utility of Eu(III) complexes as tuneable, following molecular engineering principles, coherent light‐matter interfaces for QIP applications.
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Coherent light–matter interfaces are key elements for many quantum information processing architectures. Rare-earth ion (REI)-containing systems featuring narrow inter-configurational f–f transitions are suitable for creating such interfaces. Recently, narrow homogeneous linewidths (Γ h ) and long nuclear spin lifetimes ( T 1spin ) associated with two Eu 3+ complexes featuring the 5 D 0 → 7 F 0 f–f transition have been reported, elucidating that REI-molecule-based coherent light–matter interfaces can be obtained. In this study, we report a homogeneous linewidth of 2.8 MHz at 4.2 K, corresponding to an optical coherence lifetime ( T 2opt ) of 114 ns (0.11 μs), associated with the 5 D 0 → 7 F 0 transition of the prototypical charge neutral Eu 3+ complex─[Eu(trensal)]. Moreover, we have observed long nuclear spin lifetimes ( T 1spin ) up to 460 ± 80 s at 4.2 K for the complex and efficient optical pumping of hyperfine levels of the 5 D 0 ground state. The results presented in this study indicate that narrow homogeneous linewidths and long nuclear spin lifetimes could be a generic property of molecular Eu 3+ complexes featuring the 5 D 0 → 7 F 0 transition.
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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.
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.
Homogeneous linewidths below 10 kHz are reported for the first time in high-quality Eu3+ doped Y 2O3 transparent ceramics. This result is obtained on the 7F0→5D0 transition in Eu3+ doped Y 2O3 ceramics and corresponds to an improvement of nearly one order of magnitude compared to previously reported values in transparent ceramics. Furthermore, we observed spectral hole lifetimes of ∼15 min that are long enough to enable efficient optical pumping of the nuclear hyperfine levels. Additionally, different Eu3+ concentrations (up to 1.0%) were studied, resulting in an increase of up to a factor of three in the peak absorption coefficient. These results suggest that transparent ceramics can be useful in applications where narrow and deep spectral holes can be burned into highly absorbing lines, such as quantum information processing and spectral filtering.