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We describe a micro-fabrication method to create concave features with ultra-low roughness in silica, either on optical fibers or on flat substrates. The machining uses a single CO2 laser pulse train. Parameters are chosen such that evaporation removes material while a low-viscosity melt layer produces excellent surface quality. A surface roughness σ ∼ 0.2 nm is regularly obtained. The concave depressions are near-spherical close to the center with radii of curvature between 20 and 2000 μm. The method allows fabrication of low-scatter micro-optical devices such as mirror substrates for high-finesse cavities or negative lenses on fiber tips, extending the range of micro-optical components.

Efforts to harness quantum hardware relying on quantum mechanical principles have been steadily progressing. The search for novel material platforms that could spur the progress by providing new functionalities for solving the outstanding technological problems is however still active. Any physical property presenting two distinct energy states that can be found in a long-lived superposition state can serve as a quantum bit (qubit), the basic information processing unit in quantum technologies. Molecular systems that can feature electron and/or nuclear spin states together with optical transitions are one of the material platforms that can serve as optically addressable qubits. The attractiveness of molecular systems for quantum technologies relies on the fact that molecular structures of atomically defined nature can be obtained in endless diversity of chemical compositions. Crucially, by harnessing the molecular design protocols, the optical and spin (electronic and nuclear) properties of molecules can be tailored, aiding the design of optically addressable spin qubits and quantum sensors. In this contribution, we present a concise and collective discussion of optically addressable spin-bearing molecules - namely, organic molecules, transition metal (TM) and rare-earth ion (REI) complexes - and highlight recent results such as chemical tuning of optical and electron spin quantum coherence, optical spin initialization and readout, intramolecular quantum teleportation, optical coherent storage, and photonic-enhanced optical addressing. We envision that optically addressable spin-carrying molecules could become a scalable building block of quantum hardware for applications in the fields of quantum sensing, quantum communication and quantum computing., Competing Interests: Conflict of interest: Authors state no conflicts of interest., (© 2024 the author(s), published by De Gruyter, Berlin/Boston.)

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 5 D 0 → 7 F 0 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 (NO 3 ) ligands. The Eu(III) complex-[Eu(dpphen)(NO 3 ) 3 ]-showed sensitized metal-centred emission ( 5 D 0 → 7 F J ; 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 5 D 0 → 7 F 0 transition centred at 580.9 nm. Spectral hole-burning (SHB) studies of the complex with stoichiometric Eu(III) concentration revealed a narrow homogeneous linewidth (Γ h ) of 1.55 MHz corresponding to a 0.205 μs long optical coherence lifetime (T 2opt ). Remarkably, long nuclear spin lifetimes (T 1spin ) of up to 41 s have been observed for the complex. The narrow optical linewidths and long T 1spin 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., (© 2024 The Authors. ChemPhysChem published by Wiley-VCH GmbH.)

Molecular magnetic materials based on 1,2-diamidobenzenes are well known and have been intensively studied both experimentally and computationally. They possess interesting magnetic properties as well as redox activity. In this work, we present the synthesis and investigation of potent synthons for constructing discrete metal-organic architectures featuring 1,2-diamidobenzene-coordinated metal centres. The synthons feature weakly bound dimethoxyethane (dme) ligands in addition to the 1,2-diamidobenzene. We characterize these complexes and investigate their magnetic properties by means of static and dynamic magnetometry and high-field electron paramagnetic resonance (HFEPR). Interestingly, the magnetic and magnetic resonance data strongly suggest a dimeric formulation of these complexes, viz . [M II (bmsab)(dme)] 2 (bmsab = 1,2-bis(methanesulfonamido)benzene; dme = dimethoxyethane) with M = Co, Ni, Zn. A large negative D -value of -60 cm -1 was found for the Co(II) synthon and an equally large negative D of -50 cm -1 for the Ni(II) synthon. For Co(II), the sign of the D -value is the same as that found for the known bis-diamidobenzene complexes of this ion. In contrast, the negative D -value for the Ni(II) complex is unexpected, which we explain in terms of a change in coordination number. The heteroleptic Co(II) complex presented here does not feature slow relaxation of the magnetization, in contrast to the homoleptic Co(II) 1,2-diamidobenzene complex.

Indistinguishable single photons in the telecom-bandwidth of optical fibers are indispensable for long-distance quantum communication. Solid-state single photon emitters have achieved excellent performance in key benchmarks, however, the demonstration of indistinguishability at room-temperature remains a major challenge. Here, we report room-temperature photon indistinguishability at telecom wavelengths from individual nanotube defects in a fiber-based microcavity operated in the regime of incoherent good cavity-coupling. The efficiency of the coupled system outperforms spectral or temporal filtering, and the photon indistinguishability is increased by more than two orders of magnitude compared to the free-space limit. Our results highlight a promising strategy to attain optimized non-classical light sources., (© 2024. The Author(s).)

Air-stable dinuclear complexes [(bmsab)Ni II (tmsab)Ni II (bmsab)] 3- and [(bmsab)Zn II (tmsab)Zn II (bmsab)] 3- (bmsab = bis(methanesulfoneamido)benzene, tmsab = tetra(methanesulfonamido)benzene) were prepared via a synthetic route based on heteroleptic precursor complexes. The new complexes combine a distorted tetrahedral coordination environment with an open-shell bridging ligand. The Zn II species was subjected to a detailed investigation of the (spectro-)electrochemical processes. The Ni II species is a rare example of a complex that combines strong exchange coupling ( J > 440 cm -1 ) with pronounced positive zero-field splitting ( D = +72 cm -1 ). Combining SQUID magnetometry and (HF)EPR spectroscopy with ab initio calculations allowed for accurate quantification of the exchange interaction.

Photoactive chromium(III) complexes saw a conceptual breakthrough with the discovery of the prototypical molecular ruby mer -[Cr(ddpd) 2 ] 3+ (ddpd = N , N '-dimethyl- N , N '-dipyridin-2-ylpyridine-2,6-diamine), which shows intense long-lived near-infrared (NIR) phosphorescence from metal-centered spin-flip states. In contrast to the numerous studies on chromium(III) photophysics, only 10 luminescent molybdenum(III) complexes have been reported so far. Here, we present the synthesis and characterization of mer -MoX 3 (ddpd) ( 1 , X = Cl; 2 , X = Br) and cisfac -[Mo(ddpd) 2 ] 3+ ( cisfac - [3] 3+ ), an isomeric heavy homologue of the prototypical molecular ruby. For cisfac - [3] 3+ , we found strong zero-field splitting using magnetic susceptibility measurements and electron paramagnetic resonance spectroscopy. Electronic spectra covering the spin-forbidden transitions show that the spin-flip states in mer - 1 , mer - 2 , and cisfac - [3] 3+ are much lower in energy than those in comparable chromium(III) compounds. While all three complexes show weak spin-flip phosphorescence in NIR-II, the emission of cisfac - [3] 3+ peaking at 1550 nm is particularly low in energy. Femtosecond transient absorption spectroscopy reveals a short excited-state lifetime of 1.4 ns, 6 orders of magnitude shorter than that of mer -[Cr(ddpd) 2 ] 3+ . Using density functional theory and ab initio multireference calculations, we break down the reasons for this disparity and derive principles for the design of future stable photoactive molybdenum(III) complexes.

In coordination chemistry and materials science, terpyridine ligands are of great interest, due to their ability to form stable complexes with a broad range of transition metal ions. We report three terpyridine ligands containing different perfluorocarbon (PFC) tails on the backbone and the corresponding Fe II and Co II complexes. The Co II complexes display spin crossover close to ambient temperature, and the nature of this spin transition is influenced by the length of the PFC tail on the ligand backbone. The electrochemical properties of the metal complexes were investigated with cyclic voltammetry revealing one oxidation and several reduction processes. The fluorine-specific interactions were investigated by EPR measurements. Analysis of the EPR spectra of the complexes as microcrystalline powders and in solution reveals exchange-narrowed spectra without resolved hyperfine splittings arising from the 59 Co nucleus; this suggests complex aggregation in solution mediated by interactions of the PFC tails. Interestingly, addition of perfluoro-octanol in different ratios to the acetonitrile solution of the sample resulted in the disruption of the F ⋯ ${\cdots }$ F interactions of the tails. To the best of our knowledge, this is the first investigation of fluorine-specific interactions in metal complexes through EPR spectroscopy, as exemplified by exchange narrowing., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

We demonstrate laser-written concave hemispherical structures produced on the endfacets of optical fibers that serve as mirror substrates for tunable open-access microcavities. We achieve finesse values of up to 200, and a mostly constant performance across the entire stability range. This enables cavity operation also close to the stability limit, where a peak quality factor of 1.5 × 10 4 is reached. Together with a small mode waist of 2.3 µm, the cavity achieves a Purcell factor of C ∼ 2.5, which is useful for experiments that require good lateral optical access or otherwise large separation of the mirrors. Laser-written mirror profiles can be produced with a tremendous flexibility in shape and on various surfaces, opening new possibilities for microcavities.

Two homoleptic Fe(II) complexes in different spin states bearing superbasic terpyridine derivatives as ligands are investigated to determine the relationship between spin state and electrochemical/spectroscopic behavior. Antiferromagnetic coupling between a ligand-centered radical and the high-spin metal center leads to an anodic shift of the first reduction potential and results in a species that shows mixed valency with a moderately intense intervalence-charge-transfer band. The differences afforded by the different spin states extend to the electrochemical reactivity of the complexes: while the low-spin species is a precatalyst for electrocatalytic CO 2 reduction and leads to the preferential formation of CO with a Faradaic efficiency of 37%, the high-spin species only catalyzes proton reduction at a modest Faradaic efficiency of approximately 20%.

Molecular entities with doublet or triplet ground states find increasing interest as potential molecular quantum bits (qubits). Complexes with higher multiplicity might even function as qudits and serve to encode further quantum bits. Vanadium(II) ions in octahedral ligand fields with quartet ground states and small zero-field splittings qualify as qubits with optical read out thanks to potentially luminescent spin-flip states. We identified two V 2+ complexes [V(ddpd) 2 ] 2+ with the strong field ligand N,N'-dimethyl-N,N'-dipyridine-2-yl-pyridine-2,6-diamine (ddpd) in two isomeric forms (cis-fac and mer) as suitable candidates. The energy gaps between the two lowest Kramers doublets amount to 0.2 and 0.5 cm -1 allowing pulsed EPR experiments at conventional Q-band frequencies (35 GHz). Both isomers possess spin-lattice relaxation times T 1 of around 300 μs and a phase memory time T M of around 1 μs at 5 K. Furthermore, the mer isomer displays slow magnetic relaxation in an applied field of 400 mT. While the vanadium(III) complexes [V(ddpd) 2 ] 3+ are emissive in the near-IR-II region, the [V(ddpd) 2 ] 2+ complexes are non-luminescent due to metal-to-ligand charge transfer admixture to the spin-flip states., (© 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

A bimetallic triply fused copper(II) porphyrin complex (1) was prepared, comprising two monomeric porphyrin units linked through β-β, meso-meso, β'-β' triple covalent linkages and exhibiting remarkable catalytic activity for the electrochemical hydrogen evolution reaction in comparison to the analogous monomeric copper(II) porphyrin complex (2). Electrochemical investigations in the presence of a proton source (trifluoroacetic acid) confirmed that the catalytic activity of the fused metalloporphyrin occurred at a significantly lower overpotential (≈320 mV) compared to the non-fused monomer. Controlled potential electrolysis combined with kinetic analysis of catalysts 1 and 2 confirmed production of hydrogen, with 96 and 71 % faradaic efficiencies and turnover numbers of 102 and 18, respectively, with an observed rate constant of around 10 7  s -1 for the dicopper complex. The results thus firmly establish triply fused porphyrin ligands as outstanding candidates for generating highly stable and efficient molecular electrocatalysts in combination with earth-abundant 3d transition metals., (© 2022 The Authors. ChemSusChem published by Wiley-VCH GmbH.)

We utilize cavity-enhanced extinction spectroscopy to directly quantify the optical absorption of defects in MoS 2 generated by helium ion bombardment. We achieve hyperspectral imaging of specific defect patterns with a detection limit below 0.01% extinction, corresponding to a detectable defect density below 1 × 10 11  cm -2 . The corresponding spectra reveal a broad subgap absorption, being consistent with theoretical predictions related to sulfur vacancy-bound excitons in MoS 2 . Our results highlight cavity-enhanced extinction spectroscopy as efficient means for the detection of optical transitions in nanoscale thin films with weak absorption, applicable to a broad range of materials.

Control of the spin state of metal complexes is important because it leads to a precise control over the physical properties and the chemical reactivity of the metal complexes. Currently, controlling the spin state in metal complexes is challenging because a precise control of the properties of the secondary coordination sphere is often difficult. It has been shown that non-covalent interactions in the secondary coordination sphere of transition metal complexes can enable spin state control. Here we exploit this strategy for fluorinated triazole ligands and present mononuclear Co II and Fe II complexes with "click"-derived tripodal ligands that contain mono-fluorinated benzyl substituents on the backbone. Structural characterization of 1 and 2 at 100 K revealed Co-N bond lengths that are typical of high spin (HS) Co II complexes. In contrast, the Fe-N bond lengths for 3 are characteristic of a low spin (LS) Fe II state. All complexes show an intramolecular face-to-face non-covalent interaction between two arms of the ligand. The influence of the substituents and of their geometric structure on the spin state of the metal center was investigated through SQUID magnetometry, which revealed spin crossover occurring in compounds 1 and 3. EPR spectroscopy sheds further light on the electronic structures of 1 and 2 in their low- and high-spin states. Quantum-chemical calculations of the fluorobenzene molecule were performed to obtain insight into the influence of fluorine-specific interactions. Interestingly, this work shows that the same fluorinated tripodal ligands induce SCO behavior in both Fe II and Co II complexes.

Reaction between a carbazole-based mesoionic carbene ligand and manganese(II) iodide results in the formation of a rare air-stable manganese(IV) complex after aerobic workup. Cyclic voltammetry reveals the complex to be stable in five oxidation states. The electronic structure of all five oxidation states is elucidated chemically, spectroscopically (NMR, high-frequency EPR, UV-Vis, MCD), magnetically, and computationally (DFT, CASSCF).

Rare-earth ions (REIs) are promising solid-state systems for building light-matter interfaces at the quantum level 1,2 . This relies on their potential to show narrow optical and spin homogeneous linewidths, or, equivalently, long-lived quantum states. This enables the use of REIs for photonic quantum technologies such as memories for light, optical-microwave transduction and computing 3-5 . However, so far, few crystalline materials have shown an environment quiet enough to fully exploit REI properties. This hinders further progress, in particular towards REI-containing integrated nanophotonics devices 6,7 . Molecular systems can provide such capability but generally lack spin states. If, however, molecular systems do have spin states, they show broad optical lines that severely limit optical-to-spin coherent interfacing 8-10 . Here we report on europium molecular crystals that exhibit linewidths in the tens of kilohertz range, orders of magnitude narrower than those of other molecular systems. We harness this property to demonstrate efficient optical spin initialization, coherent storage of light using an atomic frequency comb, and optical control of ion-ion interactions towards implementation of quantum gates. These results illustrate the utility of rare-earth molecular crystals as a new platform for photonic quantum technologies that combines highly coherent emitters with the unmatched versatility in composition, structure and integration capability of molecular materials., (© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)

The fine-tuning of intermolecular or intramolecular non-covalent interactions (NCIs) and thus the precise synthesis of metal complexes in which the spin states can be controlled by NCIs remains challenging, even though several such complexes have been intensively studied. In this regard, we present mononuclear cobalt(II) and iron(II) complexes with "click"-derived tripodal ligands that contain fluorinated benzyl substituents in the secondary coordination sphere. The complexes were co-crystallized with different solvent molecules to decipher the effect of the crystallized solvents on NCIs, and on the spin state of the metal ion. Additionally, the fluorine-specific interactions in the secondary coordination sphere were examined. We present a first structure-property correlation between the nature of interaction of the (per)fluorinated aromatic substituents on the ligand periphery, and the spin state of the metal complexes. In particular, the TF 5 TA containing ligand show interesting stacking motifs depending on the used solvent, and these interactions have an influence on the spin state of the cobalt(II) complexes. Furthermore, the iron(II) complex thereof, Fe(TF 5 TA) 2 (BF 4 ) 2 ·2EtOH displays spin crossover (SCO).

The dynamics of nanosystems in solution contain a wealth of information with relevance for diverse fields ranging from materials science to biology and biomedical applications. When nanosystems are marked with fluorophores or strong scatterers, it is possible to track their position and reveal internal motion with high spatial and temporal resolution. However, markers can be toxic, expensive, or change the object's intrinsic properties. Here, we simultaneously measure dispersive frequency shifts of three transverse modes of a high-finesse microcavity to obtain the three-dimensional path of unlabeled SiO 2 nanospheres with 300 μs temporal and down to 8 nm spatial resolution. This allows us to quantitatively determine properties such as the polarizability, hydrodynamic radius, and effective refractive index. The fiber-based cavity is integrated in a direct-laser-written microfluidic device that enables the precise control of the fluid with ultra-small sample volumes. Our approach enables quantitative nanomaterial characterization and the analysis of biomolecular motion at high bandwidth., (© 2021. The Author(s).)

Metal-metal bonding is a widely studied area of chemistry 1-3 , and has become a mature field spanning numerous d transition metal and main group complexes 4-7 . By contrast, actinide-actinide bonding, which is predicted to be weak 8 , is currently restricted to spectroscopically detected gas-phase U 2 and Th 2 (refs. 9,10 ), U 2 H 2 and U 2 H 4 in frozen matrices at 6-7 K (refs. 11,12 ), or fullerene-encapsulated U 2 (ref. 13 ). Furthermore, attempts to prepare thorium-thorium bonds in frozen matrices have produced only ThH n (n = 1-4) 14 . Thus, there are no isolable actinide-actinide bonds under normal conditions. Computational investigations have explored the probable nature of actinide-actinide bonding 15 , concentrating on localized σ-, π-, and δ-bonding models paralleling d transition metal analogues, but predictions in relativistic regimes are challenging and have remained experimentally unverified. Here, we report thorium-thorium bonding in a crystalline cluster, prepared and isolated under normal experimental conditions. The cluster exhibits a diamagnetic, closed-shell singlet ground state with a valence-delocalized three-centre-two-electron σ-aromatic bond 16,17 that is counter to the focus of previous theoretical predictions. The experimental discovery of actinide σ-aromatic bonding adds to main group and d transition metal analogues, extending delocalized σ-aromatic bonding to the heaviest elements in the periodic table and to principal quantum number six, and constitutes a new approach to elaborate actinide-actinide bonding., (© 2021. The Author(s), under exclusive licence to Springer Nature Limited.)

Whilst quantum computing has recently taken great leaps ahead, the development of quantum memories has decidedly lagged behind. Quantum memories are essential devices in the quantum technology palette and are needed for intermediate storage of quantum bit states and as quantum repeaters in long-distance quantum communication. Current quantum memories operate at cryogenic, mostly sub-Kelvin temperatures and require extensive and costly peripheral hardware. It is demonstrated that ensembles of weakly coupled molecular spins show long coherence times and can be used to store microwave pulses of arbitrary phase. These studies exploit strong coupling of the spin ensemble to special 3D microwave resonators. Most importantly, these systems operate at room temperature., (© 2021 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

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.