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Related Concept Videos

¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

2.3K
The coupling interactions of nuclei across four or more bonds are usually weak, with J values less than 1 Hz. While these are usually not observed in spectra, the presence of multiple bonds along the coupling pathway can result in observable long-range coupling.
In alkenes, spin information is communicated via σ–π overlap, as seen in allylic (four-bond) and homoallylic (five-bond) couplings. These coupling interactions are stronger when the σ bond is parallel to the alkene...
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Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)01:20

Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)

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Two NMR-active nuclei bonded to a central atom can be involved in geminal or two-bond coupling. Geminal coupling is commonly seen between diastereotopic protons in chiral molecules and unsymmetrical alkenes, among others.
The central atom need not be NMR-active because its electrons are affected by the electron polarization of the spin-active atoms. However, spin information is transmitted less effectively than in one-bond coupling, and 2J values are usually weaker than 1J values. The energy of...
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Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)01:22

Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)

1.3K
Vicinal or three-bond coupling is commonly observed between protons attached to adjacent carbons. Here, nuclear spin information is primarily transferred via electron spin interactions between adjacent C‑H bond orbitals. This generally favors the antiparallel arrangement of spins, so 3J values are usually positive.
The extent of coupling depends on the C‑C bond length, the two H‑C‑C angles, any electron-withdrawing substituents, and the dihedral angle between the involved orbitals. The...
1.3K
Propagation Speed of Electromagnetic Waves01:30

Propagation Speed of Electromagnetic Waves

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Electromagnetic waves are consistent with Ampere's law. Assuming there is no conduction current Ampere's law is given as:
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Spin–Spin Coupling: One-Bond Coupling01:17

Spin–Spin Coupling: One-Bond Coupling

1.2K
Coupling interactions are strongest between NMR-active nuclei bonded to each other, where spin information can be transmitted directly through the pair of bonding electrons. While nuclei polarize their electrons to the opposite spins, the bonding electron pair has opposite spins. Configurations with antiparallel nuclear spins are expected to be lower in energy. When coupling makes antiparallel states more favorable, J is considered to have a positive value. The one-bond coupling constant, 1J,...
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Single-mode fiber coupling with a M-SPGD algorithm for long-range quantum communications.

Kui-Xing Yang, Maimaiti Abulizi, Yu-Huai Li

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    Researchers developed an adaptive optics system to improve single-mode fiber coupling for satellite quantum communication. This technology enhances signal stability and efficiency in free-space quantum key distribution (QKD) under atmospheric turbulence.

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    Area of Science:

    • Quantum communication
    • Free-space optics
    • Adaptive optics

    Background:

    • Global-scale quantum networks rely on satellite-based quantum communication.
    • Efficient single-mode fiber coupling is crucial for daylight quantum key distribution (QKD) and fiber compatibility.
    • Strong atmospheric turbulence poses a significant challenge to stable free-space quantum channel coupling.

    Purpose of the Study:

    • To develop and test an adaptive optics (AO) system for enhancing single-mode fiber coupling efficiency.
    • To address the challenge of atmospheric turbulence in free-space quantum communication.
    • To improve the performance of daylight quantum key distribution (QKD).

    Main Methods:

    • Development of a single-mode receiver incorporating an adaptive optics (AO) system.
    • Implementation of a modal version of the stochastic parallel gradient descent (M-SPGD) algorithm within the AO system.
    • Performance testing over an 8 km urban terrestrial free-space channel under strong atmospheric turbulence.

    Main Results:

    • The M-SPGD AO system achieved approximately 3.7 dB improvement in single-mode fiber coupling efficiency.
    • Significant suppression of signal fluctuation was observed under strong atmospheric turbulence.
    • Demonstrated enhanced performance for daylight quantum key distribution (QKD).

    Conclusions:

    • The developed M-SPGD AO system effectively improves single-mode fiber coupling efficiency in challenging atmospheric conditions.
    • This technology is applicable to free-space long-range quantum communication systems.
    • Enhanced coupling efficiency and stability pave the way for more robust quantum networks.