Optical-Relayed Entanglement Distribution Using Drones as Mobile Nodes.

Entanglement distribution has been accomplished using a flying drone, and this mobile platform can be generalized for multiple mobile nodes with optical relay among them. Here we develop the first optical relay to reshape the wave front of photons for their low diffraction loss in free-space transmission. Using two drones, where one distributes the entangled photons and the other serves as relay node, we achieve entanglement distribution with Clauser-Horne-Shimony-Holt S parameter of 2.59±0.11 at 1 km distance. Key components for entangled source, tracking, and relay are developed with high performance and are lightweight, constructing a scalable airborne system for multinode connectio and toward mobile quantum networks.

Drone-based entanglement distribution towards mobile quantum networks.

Satellites have shown free-space quantum-communication ability; however, they are orbit-limited from full-time all-location coverage. Meanwhile, practical quantum networks require satellite constellations, which are complicated and expensive, whereas the airborne mobile quantum communication may be a practical alternative to offering full-time all-location multi-weather coverage in a cost-effective way. Here, we demonstrate the first mobile entanglement distribution based on drones, realizing multi-weather operation including daytime and rainy nights, with a Clauser-Horne-Shimony-Holt S-parameter measured to be 2.41 ± 0.14 and 2.49 ± 0.06, respectively. Such a system shows unparalleled mobility, flexibility and reconfigurability compared to the existing satellite and fiber-based quantum communication, and reveals its potential to establish a multinode quantum network, with a scalable design using symmetrical lens diameter and single-mode-fiber coupling. All key technologies have been developed to pack quantum nodes into lightweight mobile platforms for local-area coverage, and arouse further technical improvements to establish wide-area quantum networks with high-altitude mobile communication.

A three-dimensional cadmium(II) coordination polymer: poly[[[µ-4,4'-(propane-1,3-diyl)dipyridine-κ(2)N:N](µ-3,3'-thiodipropionato-κ(3)O,O':O)cadmium(II)] monohydrate].

In the title coordination polymer, {[Cd(C6H8O4S)(C13H14N2)]·H2O}n, the Cd(II) atom displays a distorted octahedral coordination, formed by three carboxylate O atoms and one S atom from three different 3,3'-thiodipropionate ligands, and two N atoms from two different 4,4'-(propane-1,3-diyl)dipyridine ligands. The Cd(II) centres are bridged through carboxylate O atoms of 3,3'-thiodipropionate ligands and through N atoms of 4,4'-(propane-1,3-diyl)dipyridine ligands to form two different one-dimensional chains, which intersect to form a two-dimensional layer. These two-dimensional layers are linked by S atoms of 3,3'-thiodipropionate ligands from adjacent layers to form a three-dimensional network.

Poly[µ7-sulfanediyldiacetato-disilver(I)].

In the title coordination polymer, [Ag2(C4H4O4S)], each AgI cation is four-coordinated by three of the four carboxylate O atoms and the S atom from symmetry-related sulfanediyldiacetate ligands, thus defining a distorted tetrahedral geometry at the metal centre. The AgI cations are bridged by sulfanediyldiacetate groups, leading to a two-dimensional layer structure. These layers are interconnected via Ag-S bonds to form a three-dimensional coordination polymer network overall.

Bis-(1H-benzimidazole-κN)bis(4-methyl-benzoato-κO,O')cobalt(II).

In the title mononuclear complex, [Co(C(8)H(7)O(2))(2)(C(7)H(6)N(2))(2)], the Co(II) atom is coordinated by four carboxylate O atoms from two 4-methyl-benzoate ligands and two N atoms from two benzimidazole ligands in an octa-hedral coordination geometry. The molecules are assembled via inter-molecular N-H⋯O hydrogen-bonding inter-actions into a three-dimensional network.

Poly[[µ(2)-aqua-aqua-(µ(3)-3,5-dinitro-salicylato)barium(II)] monohydrate].

In the title coordination polymer, {[Ba(C(7)H(2)N(2)O(7))(H(2)O)(2)]·H(2)O}(n), the Ba(II) atom is ten-coordinated by seven O atoms from four 3,5-dinitro-salicylatate ligands, two µ(2)-bridging aqua ligands and one water mol-ecule. The coordination mode is best described as a bicapped square-anti-prismatic geometry. The 3,5-dinitrosalicylatate ligands bridge three Ba atoms. Centrosymmetrically related dinuclear barium units, with a Ba⋯Ba separation of 4.767 (5) Å, form infinite chains, which are further self-assembled into a supra-molecular network through inter-molecular O-H⋯O hydrogen-bonding inter-actions between O atoms of 3,5-dinitro-salicylatate ligands and water mol-ecules.

Poly[aqua-(µ(2)-oxalato)(4-oxidopyri-din-ium)erbium(II)].

The title complex, [Er(C(5)H(5)NO)(C(2)O(4))(H(2)O)](n), is a new erbium polymer based on oxalate and 4-oxidopyridinium ligands. The Er(II) center is coordinated by six O atoms from three oxalate ligands, one O atom from a 4-oxidopyridinium ligand and one water mol-ecule, and displays a distorted square-anti-prismatic coordination geometry. The oxalate ligands are both chelating and bridging, and link Er(II) ions, forming Er-oxalate layers in which the attached water and 4-oxidopyridinium units point alternately up and down. A mirror plane passes through the Er atom, one C, the oxide O and two oxalate O atoms. The layers are assembled into a three-dimensional supra-molecular network via inter-molecular hydrogen bonding and π-π stacking inter-actions [centroid-centroid distances of 3.587 (2) Šbetween parallel pyridinium rings]. Both the water mol-ecule and the 4-oxidopyridinium ligand are disordered over two sites in a 1:1 ratio.

Diaqua-bis(benzyl-oxyacetato)copper(II).

In the title mononuclear complex, [Cu(C(9)H(9)O(3))(2)(H(2)O)(2)], the Cu(II) ion, located on an inversion center, is hexa-coordinated by four O atoms from two benzyl-oxyacetate ligands [Cu-O = 1.9420 (14) and 2.2922 (14) Å] and two water mol-ecules [Cu-O = 2.0157 (15) Å] in a distorted octa-hedral geometry. In the crystal structure, inter-molecular O-H⋯O hydrogen bonds link the mol-ecules into layers parallel to the bc plane.

Diaqua-bis[2-(benzyl-oxy)acetato]cobalt(II).

In the mononuclear title complex, [Co(C(9)H(9)O(3))(2)(H(2)O)(2)], each Co(II) atom is located on an inversion center and is hexa-coordinated by four O atoms from two benzyl-oxyacetate ligands [Co-O bond lengths = 2.0487 (9) and 2.1090 (9) Å] and two water mol-ecules [Co-O bond length = 2.0873 (9) Å] in a distorted octa-hedral geometry. In the crystal structure, inter-molecular hydrogen bonds and π-π stacking inter-actions [centroid-centroid distance between phenyl rings = 3.692 (2) Å] link the mol-ecules into a supra-molecular structure.

Tetra-aqua-(1,10-phenanthroline-κN,N')magnesium(II) bis-[(2,4-dichloro-phen-yl)acetate].

In the mononuclear title complex, [Mg(C(12)H(8)N(2))(H(2)O)(4)](C(8)H(5)Cl(2)O(2))(2), each Mg(II) ion is hexa-coordinated by two N atoms from a 1,10-phenanthroline ligand [Mg-N = 2.233 (2) Å] and four water mol-ecules [Mg-OW = 2.033 (2) and 2.043 (1) Å] in a distorted octa-hedral geometry. A twofold rotation axis passes through the Mg atom. In the crystal structure, the cations and anions are linked by inter-molecular O-H⋯O hydrogen bonds and π-π stacking inter-actions [centroid-centroid distance = 3.804 (2) Å] into layers parallel to the ac plane.

Erratum: Poly[aqua-(µ(2)-oxalato)(4-oxidopyri-din-ium)erbium(II)]. Corrigendum.

The chemical name in the title, the formula and the scheme of the paper by Gu, Hao, Song, Lin & Ma [Acta Cryst. (2008), E64, m649-m650] are corrected.[This corrects the article DOI: 10.1107/S1600536808009380.].