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Journal of the American Chemical Society

Design Rules, Accurate Enthalpy Prediction, and Synthesis of Stoichiometric Eu 3+ Quantum Memory Candidates

Zachary W. Riedel, Daniel P. Shoemaker

Published Jan 12, 2024• 8 citations
Publisher

Abstract

Stoichiometric Eu 3+ compounds have recently shown promise for building dense, optically addressable quantum memory as the cations’ long nuclear spin coherence times and shielded 4f electron optical transitions provide reliable memory platforms. Implementing such a system, though, requires ultranarrow, inhomogeneous linewidth compounds. Finding this rare linewidth behavior within a wide range of potential chemical spaces remains difficult, and while exploratory synthesis is often guided by density functional theory (DFT) calculations, lanthanides’ 4f electrons pose unique challenges for stability predictions. Here, we report DFT procedures that reliably reproduce known phase diagrams and correctly predict two experimentally realized quantum memory candidates. We are the first to synthesize the double perovskite halide Cs 2 NaEuF 6 . It is an air-stable compound with a calculated band gap of 5.0 eV that surrounds Eu 3+ with mononuclidic elements, which are desirable for avoiding inhomogeneous linewidth broadening. We also analyze computational database entries to identify phosphates and iodates as the next generation of chemical spaces for stoichiometric quantum memory system studies. This work identifies new candidate platforms for exploring chemical effects on quantum memory candidates’ inhomogeneous linewidth while also providing a framework for screening Eu 3+ compound stability with DFT.

DOI
10.1021/jacs.3c11615

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Stoichiometric Eu 3+ compounds have recently shown promise for building dense, optically addressable quantum memory as the cations’ long nuclear spin coherence times and shielded 4f electron optical transitions provide reliable memory platforms. Implementing such a system, though, requires ultranarrow, inhomogeneous linewidth compounds. Finding this rare linewidth behavior within a wide range of potential chemical spaces remains difficult, and while exploratory synthesis is often guided by density functional theory (DFT) calculations, lanthanides’ 4f electrons pose unique challenges for stability predictions. Here, we report DFT procedures that reliably reproduce known phase diagrams and correctly predict two experimentally realized quantum memory candidates. We are the first to synthesize the double perovskite halide Cs 2 NaEuF 6 . It is an air-stable compound with a calculated band gap of 5.0 eV that surrounds Eu 3+ with mononuclidic elements, which are desirable for avoiding inhomogeneous linewidth broadening. We also analyze computational database entries to identify phosphates and iodates as the next generation of chemical spaces for stoichiometric quantum memory system studies. This work identifies new candidate platforms for exploring chemical effects on quantum memory candidates’ inhomogeneous linewidth while also providing a framework for screening Eu 3+ compound stability with DFT.
Three phases of stoichiometric europium(III) iodate compounds are probed as candidates for optical quantum memory to determine the impact of reaction conditions on phase formation and to relate crystal structure to their ability to host persistent excited states relevant to quantum memory. Hydrothermal synthesis procedures for growing macroscopic single crystals of rectangular-block α-Eu(IO 3 ) 3 ( P 2 1 / c ), yellow hexagonal plate β-Eu(IO 3 ) 3 ( P 2 1 / n ), and transparent plate NaEu(IO 3 ) 4 ( Cc ) are presented. The ability to burn narrow and long-lived spectral holes on the 7 F 0 → 5 D 0 optical transitions is a necessary condition for implementing optical quantum memory. α-Eu(IO 3 ) 3 was determined to be incapable of hole-burning due to the presence of insufficient asymmetry of the europium site, which forbids the required optical transition. β-Eu(IO 3 ) 3 was determined to be incapable of storing information due to a lack of observed spectral hole burning, likely due to decoherence caused by the closely packed edge-sharing Eu(III) polyhedra. Analysis of each phase’s crystal structure suggests that the Eu–Eu nearest neighbor distance and distortion of europium site symmetry are critical material design parameters for quantum memory applications.
We obtain a low optical inhomogeneous linewidth of 25 MHz in the stoichiometric rare-earth crystal EuCl_{3}·6H_{2}O by isotopically purifying the crystal in ^{35}Cl. With this linewidth, an important limit for stoichiometric rare-earth crystals is surpassed: the hyperfine structure of ^{153}Eu is spectrally resolved, allowing the whole population of ^{153}Eu^{3+} ions to be prepared in the same hyperfine state using hole-burning techniques. This material also has a very high optical density, and can have long coherence times when deuterated. This combination of properties offers new prospects for quantum information applications. We consider two of these: quantum memories and quantum many-body studies. We detail the improvements in the performance of current memory protocols possible in these high optical depth crystals, and describe how certain memory protocols, such as off-resonant Raman memories, can be implemented for the first time in a solid-state system. We explain how the strong excitation-induced interactions observed in this material resemble those seen in Rydberg systems, and describe how these interactions can lead to quantum many-body states that could be observed using standard optical spectroscopy techniques.
Stoichiometric materials of Eu(III) offer a promising platform for quantum memories attributable to their unique capability to display a distinctive, nondegenerate 5 D 0 → 7 F 0 transition, which enables precise mapping of optical quantum states into their energy-level manifold─a prerequisite for reliable storage and retrieval on demand. However, placing Eu(III) into chiral polar structures, which are necessary for achieving narrow spectral line widths and long optical lifetimes, is a daunting task. Here, we discover Eu 2 (SeO 3 ) 2 (SO 4 )(H 2 O) 2, a rare Eu(III) material that exhibits chiral polar symmetries encompassing both local and global structures. This unique structure is shaped by an appropriate combination of asymmetric ligands. Chirality fosters dipole–dipole interactions and J -mixing, as evidenced by second-harmonic generation, photoluminescence, and magnetic susceptibility. The broken inversion symmetry is supported by the phase-matching behavior in second-harmonic generation. The 5 D 0 → 7 F 0 transition is observed at 578 nm (2.145 eV) with 7.42 meV line width and 3.39 μs lifetime at T = 78 K. The analysis of magnetic susceptibility data using Van Vleck’s theory results in an effective magnetic moment of ρ eff = 3.33 μ B /Eu 3+ and J -mixing. Heat capacity data reveal underlying phonon dynamics in the material. This study demonstrates a pathway to realizing new chiral stoichiometric Eu 3+ compounds with potential for optically addressable quantum memory applications.
Stoichiometric materials of Eu(III) offer a promising platform for quantum memories attributable to their unique capability to display a distinctive, nondegenerate J = 0 transition, which enables precise mapping of optical quantum states into their hyperfine structure for reliable storage and retrieval on demand. However, placing Eu(III) into chiral polar structures, which are necessary for achieving narrow spectral linewidths and long optical lifetimes, is a daunting task. Here, we discover Eu2(SeO3)2(SO4)(H2O)2, a rare Eu(III) material that exhibits chiral polar symmetries encompassing both local and global structures. This unique structure is shaped by an appropriate combination of asymmetric ligands. The chirality fosters dipole-dipole interactions and J-mixing, as characterized by second-harmonic generation, photoluminescence, and magnetic susceptibility. The broken inversion symmetry is supported by the phase-matching behavior of second-harmonic generation. The J = 0 transition is observed at 578 nm with a narrow linewidth at 78 K and a microsecond-scale optical lifetime. The analysis of magnetic susceptibility data using Van Vleck theory results in an effective magnetic moment of 3.33 μB/Eu3+ and J-mixing. Heat capacity data reveal underlying phonon dynamics in the material. This study demonstrates a pathway toward realizing new stoichiometric Eu3+ compounds with potential for optically addressable quantum memory applications.
Materials science has an important role to play in the successful establishment of quantum technologies as they are expected to develop suitable material platforms for these highly demanding applications. Among different promising solid-state materials, rare-earth ions embedded in some host crystals show outstandingly narrow optical line widths and long-lived spin states at cryogenic temperatures. In this manuscript, we explore a promising rare-earth (RE)-based material for use in quantum applications: monazite-phase LaPO 4:Eu 3+ crystals, providing the first comprehensive, high-resolution optical coherence and spin lifetime characterization across the full concentration range, starting from 1% Eu 3+ and up to the fully stoichiometric compound EuPO 4 . The results show optical coherence times of the order 100 μs and nuclear spin lifetimes up to 2 h at 4 K, setting this material among the best reported so far, while experimental conditions for its synthesis with appropriate quality consist of a rather simple solid-state reaction. This opens promising perspectives for its use in quantum applications.