Ultranarrow Optical Inhomogeneous Linewidth in a Stoichiometric Rare-Earth Crystal

Design Rules, Accurate Enthalpy Prediction, and Synthesis of Stoichiometric Eu3+ Quantum Memory Candidates

Stoichiometric Eu3+ 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 Cs2NaEuF6. It is an air-stable compound with a calculated band gap of 5.0 eV that surrounds Eu3+ 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 Eu3+ compound stability with DFT.

Ultra-narrow inhomogeneous spectral distribution of telecom-wavelength vanadium centres in isotopically-enriched silicon carbide

Spin-active quantum emitters have emerged as a leading platform for quantum technologies. However, one of their major limitations is the large spread in optical emission frequencies, which typically extends over tens of GHz. Here, we investigate single V4+ vanadium centres in 4H-SiC, which feature telecom-wavelength emission and a coherent S = 1/2 spin state. We perform spectroscopy on single emitters and report the observation of spin-dependent optical transitions, a key requirement for spin-photon interfaces. By engineering the isotopic composition of the SiC matrix, we reduce the inhomogeneous spectral distribution of different emitters down to 100 MHz, significantly smaller than any other single quantum emitter. Additionally, we tailor the dopant concentration to stabilise the telecom-wavelength V4+ charge state, thereby extending its lifetime by at least two orders of magnitude. These results bolster the prospects for single V emitters in SiC as material nodes in scalable telecom quantum networks.

Slow light and photon echoes in the 7F0→5D0 transition of Sm2+ in single-crystal BaFCl

Slow light effects induced by transient spectral hole-burning in the 7F0→5D0 transition of Sm2+ in BaFCl at 688 nm are reported and a probe pulse delay of 1.25 μs was observed through a 5 mm thick crystal. This delay corresponds to a reduction of the group velocity vG of the transmitted light to ∼4000 m/s. An analysis of the dependence of the slow light effect on the probe pulse timing indicates some broadening of the spectral hole caused by relatively fast excitation energy transfer. We also demonstrate two-pulse (2PE) and (three-pulse) stimulated photon echoes (SPE) for the first time for Sm2+ in the solid state and a homogenous linewidth of 16 kHz (∼2.5·10-8 nm) was obtained at 1.8 K. The echoes in the optically dense medium were very efficient and revealed spectral diffusion on the 100-μs time scale possibly due to flipping of the fluorine and chlorine nuclear spins in the environment of the Sm2+ ions. Furthermore, the SPE also indicates relatively fast energy transfer, commensurate with the hole decay.

Quantum Mimicry With Inorganic Chemistry

Quantum objects, such as atoms, spins, and subatomic particles, have important properties due to their unique physical properties that could be useful for many different applications, ranging from quantum information processing to magnetic resonance imaging. Molecular species also exhibit quantum properties, and these properties are fundamentally tunable by synthetic design, unlike ions isolated in a quadrupolar trap, for example. In this comment, we collect multiple, distinct, scientific efforts into an emergent field that is devoted to designing molecules that mimic the quantum properties of objects like trapped atoms or defects in solids. Mimicry is endemic in inorganic chemistry and featured heavily in the research interests of groups across the world. We describe a new field of using inorganic chemistry to design molecules that mimic the quantum properties (e.g. the lifetime of spin superpositions, or the resonant frequencies thereof) of other quantum objects, "quantum mimicry." In this comment, we describe the philosophical design strategies and recent exciting results from application of these strategies.

Observation of the hyperfine structure and anticrossings of hyperfine levels in the luminescence spectra of LiYF4:Ho3

Resolved hyperfine structure and narrow inhomogeneously broadened lines in the optical spectra of a rare-earth-doped crystal are favorable for the implementation of various sensors. Here, a well-resolved hyperfine structure in the photoluminescence spectra of LiYF₄:Ho single crystals and the anticrossings of hyperfine levels in a magnetic field are demonstrated using a self-made setup based on a Bruker 125HR high-resolution Fourier spectrometer. This is the first observation of the resolved hyperfine structure and anticrossing hyperfine levels in the luminescence spectra of a crystal. The narrowest spectral linewidth is only 0.0022 cm^-1. This fact together with a large value of the magnetic g factor of several crystal-field states creates prerequisites for developing magnetic field sensors, which can be in demand in modern quantum information technology devices operating at low temperatures. Very small random lattice strains characterizing the quality of a crystal can be detected using anticrossing points.

Elimination of noise in optically rephased photon echoes

Photon echo is a fundamental tool for the manipulation of electromagnetic fields. Unavoidable spontaneous emission noise is generated in this process due to the strong rephasing pulse, which limits the achievable signal-to-noise ratio and represents a fundamental obstacle towards their applications in the quantum regime. Here we propose a noiseless photon-echo protocol based on a four-level atomic system. We implement this protocol in a Eu³⁺:Y₂SiO₅ crystal to serve as an optical quantum memory. A storage fidelity of 0.952 ± 0.018 is obtained for time-bin qubits encoded with single-photon-level coherent pulses, which is far beyond the maximal fidelity achievable using the classical measure-and-prepare strategy. In this work, the demonstrated noiseless photon-echo quantum memory features spin-wave storage, easy operation and high storage fidelity, which should be easily extended to other physical systems.

Gigahertz-bandwidth optical memory in Pr3+:Y2SiO5

We experimentally study a broadband implementation of the atomic frequency comb (AFC) rephasing protocol with a cryogenically cooled Pr³⁺:Y₂SiO₅ crystal. To allow for storage of broadband pulses, we explore a novel, to the best of our knowledge, regime where the input photonic bandwidth closely matches the inhomogeneous broadening of the material (∼5GHz), thereby significantly exceeding the hyperfine ground and excited state splitting (∼10MHz). Through an investigation of different AFC preparation parameters, we measure a maximum efficiency of 10% after a rephasing time of 12.5 ns. With a suboptimal AFC, we witness up to 12 rephased temporal modes.

Negative refraction in terbium at ultraviolet frequencies

One of the key challenges in the development of negative index metamaterials is creating a sufficiently strong magnetic response in the material. Rare-earth ions can contain a strong optical magnetic response even in the ultraviolet region of the spectrum, which can be enhanced using magneto-electric cross-coupling. Using energies, transition strengths, and linewidths from atomic structure software, along with realistic inhomogeneous broadenings and densities in a solid, we simulate a negative index scheme using a terbium crystal at a wavelength of 282 nm.

Atomically dispersed Eu(III) sites in natural deep eutectic solvents based fluorescent probe efficient identification of Fe3+ and Cu2+ in wastewater

Heavy metal ions in wastewater have brought serious environmental pollution. To improve the detection efficiency, it is important to find useful fluorescent probes. The emerging green natural deep eutectic solvents (NADESs) offer attractive option for "green" detection for its good biocompatibility, easy preparation, and high sensitivity. In this study, a multi-functionalized fluorescent probe with atomically dispersed EuCl₃·6H₂O in amino acid-based NADESs (l-Glutamic acid/Glycerol, l-Glu/Gly) was synthesized by metal-ligand coordination interactions with a mass ratio of 15:1. Combined with the NADESs and rare earth metal, the l-Glu/Gly/EuCl₃·6H₂O could form the amino site and Eu²⁺ site fluorescent centers. Under the excitation wavelength of 370 nm, it had dual emission peaks at 425 nm and 470 nm with efficient resonance energy transfer. The stable optoelectronic properties of l-Glu/Gly/EuCl₃·6H₂O under external factors, such as mass ratio (13,1 to 18:1), temperature (30-50 °C), pH (1 to 14) and storage time ( >42 days), approved l-Glu/Gly/EuCl₃·6H₂O an excellent fluorescence probe. In the application of water-quality monitoring, Fe³⁺ and Cu²⁺ could react with l-Glu/Gly/EuCl₃·6H₂O in different reactive patterns. The blue fluorescence was quenched by Fe³⁺ and enhanced by Cu²⁺, thus metal ions could be distinguished with high sensitivity. The detective process was determined and the fluorescent mechanism was also proposed. l-Glu/Gly/EuCl₃·6H₂O fluorescent probe was demonstrated to be an efficient fluorescent probe for metal detection avoiding the hydrothermal process, and the cumbersome of ilter, dialysis, freeze drying.

Electromagnetically induced transparency in inhomogeneously broadened solid media

We study, theoretically and experimentally, electromagnetically induced transparency (EIT) in two different solid-state systems. Unlike many implementations in homogeneously broadened media, these systems exhibit inhomogeneous broadening of their optical and spin transitions typical of solid-state materials. We observe EIT lineshapes typical of atomic gases, including a crossover into the regime of Autler-Townes splitting, but with the substitution of the inhomogeneous widths for the homogeneous values. We obtain quantitative agreement between experiment and theory for the width of the transparency feature over a range of optical powers and inhomogeneous linewidths. We discuss regimes over which analytical and numerical treatments capture the behavior. As solid-state systems become increasingly important for scalable and integratable quantum optical and photonic devices, it is vital to understand the effects of the inhomogeneous broadening that is ubiquitous in these systems. The treatment presented here can be applied to a variety of systems, as exemplified by the common scaling of experimental results from two different systems.

Soft Quantum Control for Highly Selective Interactions among Joint Quantum Systems

We propose a quantum control scheme aimed at interacting systems that gives rise to highly selective coupling among their near-to-resonance constituents. Our protocol implements temporal control of the interaction strength, switching it on and off again adiabatically. This soft temporal modulation significantly suppresses off-resonant contributions in the interactions. Among the applications of our method we show that it allows us to perform an efficient rotating-wave approximation in a wide parameter regime, the elimination of side peaks in quantum sensing experiments, and selective high-fidelity entanglement gates on nuclear spins with close frequencies. We apply our theory to nitrogen-vacancy centers in diamond and demonstrate the possibility for the detection of weak electron-nuclear coupling under the presence of strong perturbations.

Analysis of the potential for non-invasive imaging of oxygenation at heart depth, using ultrasound optical tomography (UOT) or photo-acoustic tomography (PAT)

Despite the important medical implications, it is currently an open task to find optical non-invasive techniques that can image deep organs in humans. Addressing this, photo-acoustic tomography (PAT) has received a great deal of attention in the past decade, owing to favorable properties like high contrast and high spatial resolution. However, even with optimal components PAT cannot penetrate beyond a few centimeters, which still presents an important limitation of the technique. Here, we calculate the absorption contrast levels for PAT and for ultrasound optical tomography (UOT) and compare them to their relevant noise sources as a function of imaging depth. The results indicate that a new development in optical filters, based on rare-earth-ion crystals, can push the UOT technique significantly ahead of PAT. Such filters allow the contrast-to-noise ratio for UOT to be up to three orders of magnitude better than for PAT at depths of a few cm into the tissue. It also translates into a significant increase of the image depth of UOT compared to PAT, enabling deep organs to be imaged in humans in real time. Furthermore, such spectral holeburning filters are not sensitive to speckle decorrelation from the tissue and can operate at nearly any angle of incident light, allowing good light collection. We theoretically demonstrate the improved performance in the medically important case of non-invasive optical imaging of the oxygenation level of the frontal part of the human myocardial tissue. Our results indicate that further studies on UOT are of interest and that the technique may have large impact on future directions of biomedical optics.