Quantum Design (Germany)

ABSTRACT Quantum key distribution (QKD) enables secure communication by harnessing the fundamental principles of quantum physics, which inherently guarantee information‐theoretic security and intrinsic resistance to quantum computing attacks. However, the secure key rate of QKD typically decreases exponentially with increasing channel distance. In this work, through the development of a novel polarization‐state preparation method, an ultra‐low time‐jitter laser source, and superconducting nanowire single‐photon detectors, a 5‐GHz integrated QKD system featuring ultra‐low quantum bit error rates (QBERs) is demonstrated. The system achieves secure key rates of 1.076 Mbps at 150 km and 105 kbps at 200 km over standard single‐mode fiber channels, respectively. The system substantially enhances the secure key rate, enabling the implementation of high‐resolution video calls with one‐time‐pad encryption over intercity backbone QKD links. This work represents a significant step forward in the development of high‐performance practical QKD systems.

ABSTRACT Nanoscale quantum sensing with solid‐state spins enables high‐spatial‐resolution detection of microscopic physical phenomena, benefiting from their surface proximity, atomic‐scale dimensions and optical addressability. The miniaturization of such sensing platforms is critical for broadening the applications. High‐resolution optical imaging of solid‐state spins typically requires bulky multi‐lens objectives. Here, we report a single dielectric ultraflat metalens capable of both exciting and collecting fluorescence from spin defects. The high spatial resolution, comparable with traditional objective, is demonstrated through the optical imaging of spin defects in diamond. The nanoscale electromagnetic field sensing with spin defects in constrained space is subsequently realized, due to the high numerical aperture and small size of the metalens. Our work provides a platform for developing integrated nanoscale quantum sensing devices. The results will promote the compatibility of solid‐state spin quantum sensor in various practical environment.

ABSTRACT Topological defects play a central role in spontaneous symmetry breaking, profoundly influencing the order parameter and emergent states. These effects are particularly complex in low‐dimensional systems with strong electronic correlations and dimensional crossovers, necessitating detailed and sophisticated analyses. Using CuTe as a prototype, with charge density wave (CDW) states near room temperature (Tc = 335 K), this study reveals a novel CDW induced by topological defects, specifically CDW dislocations. Temperature‐dependent transmission electron microscopy was employed to visualize the emergence of this novel CDW at topological singularities, where the amplitude is significantly suppressed. The formation and melting of this CDW exhibit an unconventional, temperature‐dependent hysteresis near Tc, which is further corroborated by clearly observed anomalous hysteresis in electrical transport. This work offers a comprehensive picture of emergent CDW states in CuTe and paves the way for the potential application of CDW systems in information storage technologies.

We systematically investigate the electronic structure factors underpinning the distinct photochemical behaviors in a series of structurally related Fe(III)–azido complexes ( 1 ), ( 2 ), and ( 3 ), using density functional theory (DFT), time‐dependent DFT, and ab initio ligand field theory together with the angular overlap model. It had been shown in experimental studies that the ground spin state of these complexes influences the photochemical reaction pathways, specifically photoreductive and redox‐neutral ligand dissociation, and photooxidation to yield high‐valent nitrenoid species. In addition, 3 shows a unique acetato‐ligand decarboxylation channel not observed in the low‐spin analogs. In this first comparative in silico study of the three complexes, we show how the steric demands of the methylated ligand in 3 lead to a weakened equatorial ligand field resulting in a high spin ground state which in turn influences the excited state manifold. With a simple approach, wherein we explore the evolution of the electronically excited states along specific vibrational modes, we find the onset of dissociative photooxidation paths in 1 and 2 which are absent in the high‐spin species 3 . Similarly, we can rationalize why photoinduced decarboxylation is only observed in 3 .

Optical spectrometers are indispensable tools across various fields, from chemical and biological sensing to astronomical observations and quantum technologies. However, the integration of spectrometers onto photonic chips has been hindered by the low spectral resolution or large device footprint with complex multiple channel operations. Here, we introduce a chip-integrated spectrometer by leveraging the acoustically-stimulated Brillouin scattering in a hybrid photonic-phononic chip. The Brillouin interaction provides a dynamic reflection grating with a high reflectivity up to 50% and a fast switching time on the microsecond scale, achieving a spectral resolution of 0.56 nm over a 110 nm bandwidth using just a single 1 mm-long straight waveguide. This performance approaches the fundamental limit of resolution for a given device size, validating the potential of the hybrid photonic-phononic device for efficient and dynamically-reconfigurable spectral analysis, and thus opens up the avenues for advanced optical signal processing and sensing applications.

Erbium-doped crystals are promising candidates for fiber-compatible quantum memories due to their optical transition in the telecommunication C-band. Here, we investigate the optical decoherence dynamics of isotopically enriched 167Er3+:Y2SiO5 (YSO) at sub-Kelvin temperatures and low magnetic fields (<1 T). We observed an optical homogeneous linwidth of 100 Hz at 100 mK and 0.2 T – significantly narrower than previous values obtained at 7 T in a sample with the same 167Er3+ concentration. We analyze the dependence of coherence properties on magnetic field orientation and temperature, identifying spectral diffusion resulted from the host spin bath as key decoherence mechanisms. We further identified that superhyperfine interactions between the 167Er3+ and the host bath spins pose challenges for long-term storage protocols requiring precise spectral tailoring. To address this issue, we propose a strategy that combined the Stark modulation memory scheme with noiseless photon echo storage protocol, which is expected to achieve spin-wave quantum storage with lower noise and higher efficiency.

Abstract Ultra-high performance concrete (UHPC) surpasses conventional concrete in performance. However, ensuring consistent mechanical properties during production, even with identical recipes, remains challenging. Using experimental data, this study investigates how material quality, environmental conditions, measurement errors in material dosing, and mixing and curing conditions influence the mechanical properties of UHPC. This broad scope of influencing factors and production conditions increases data dimensionality and, coupled with the high cost of UHPC experiments, results in a sparse dataset. Traditional evolutionary algorithms, though effective in feature selection, struggle with high-dimensional small-sized datasets. To address this, a search-space-constraining method for the non-dominated sorting genetic algorithm II (NSGA-II) is introduced, incorporating domain-specific knowledge into population initialization to reduce dimensionality and thus enhance prediction accuracy and solution stability. Comparative evaluations using various machine learning algorithms on the UHPC dataset demonstrate that population initialization to constrain the search space of NSGA-II outperforms the standard NSGA-II. Finally, the significance of each examined factor in the UHPC manufacturing process for the properties of the final product is discussed.

Identifying two-dimensional (2D) materials with the desired electrochemical performance for lithium-ion batteries is of great interest in developing next-generation energy devices. Motivated by the successful synthesis of multiferroic 2D materials, Cu(CrX2)2 (X = S, Se, and Te), which exhibit simultaneous ferroelectricity and ferromagnetism, we performed first-principles calculations to investigate their potential as anodes for lithium-ion batteries. It is first demonstrated that the multiferroic Cu(CrX2)2 2D nanosheets exhibit metallic characteristics with a large density of states (DOS) at the Fermi level, making them promising candidates for achieving good electrical conductivity. Next, we comprehensively investigate the electrochemical properties of the predicted systems and demonstrate that lithium exhibits sufficient mobility on their surface, with appreciable stability. For instance, the binding energy (Eb) of the lithium adatom on Cu(CrS2)2 is −4.034 eV, with a diffusion barrier as low as 0.212 eV. As a consequence, the maximum theoretical specific capacity for lithium adatoms reaches as high as 1089 and 666 mAh g–1 respectively, for Cu(CrS2)2 and Cu(CrSe2)2, which can be attributed to a much higher storage capacity of lithium adatoms compared to previously identified 2D anode materials. All of these remarkable properties, including high binding energy (Eb), low diffusion barrier, high specific capacity, and good electrical conductivity, suggest that the proposed multiferroic Cu(CrX2)2 2D materials could be promising electrode materials for lithium-ion batteries.