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We have developed a simple time-bin phase encoding quantum key distribution system, using the optical injection locking technique. This setup incorporates both the merits of simplicity and stability in encoding, and the immunity to channel disturbance. We have demonstrated, to our knowledge, the longest field implementation of quantum key distribution over deployed aerial fiber with a distance of 87 km. During the 70-day field test, we achieved approximately a 1.0 kbps secure key rate with stable performance. Our work takes an important step toward widespread implementation of QKD systems in diverse and complex real-life scenarios.

The mechanical, electronic structure and optical properties of aluminium based double halide perovskite were calculated by density functional theory. The formation energy and elastic constant confirm the stability of the cubic perovskite materials. The materials are all ductile and suitable for flexible photovoltaic and optoelectronic devices. The band gap values vary from 0.773 eV to 3.430 eV, exactly corresponding to the range of ideal band gap values for good photoresponse. The band structure analysis shows that all the materials possess small effective mass, which indicates a good transport of carriers. And these materials have a broad energy range of optical absorption for utilization and a detector of photons. Moreover, less expensive K

Recently, halide double perovskite materials have attracted more and more attention due to their structural stability and excellent photoelectric properties. In this work, the electronic, mechanical and optical properties of halide double perovskite Cs2AgGaX6 (X = Cl, Br) have been studied by first-principle calculations. The results show that the Goldschmidt’s tolerance factor t of these compounds is close to 1, which is the ideal value of structurally stable cubic perovskite. Moreover, their thermodynamic stability is also ensured by the calculated negative formation energy. The Cs2AgGaX6 (X = Cl, Br) is a direct band-gap semiconductor with values of 2.038 eV and 0.967 eV, respectively. Compared with many halide double perovskites, both Cs2AgGaX6 (X = Cl, Br) compounds exhibit the advantage of low effective carrier mass. Finally, their reflectance, absorption coefficient and dielectric function in the range of 0~30 eV were studied. The results show that these compounds have certain application value in optoelectronic devices.

The current theoretical analysis model for high-dimensional quantum key distribution (QKD) was developed without carefully considering the dead time and after-pulse of the single-photon detectors. Here, we propose a new model that accounts for these two effects. With this model, we analyze the effects of various properties of the single-photon detector on the performance of the high-dimensional quantum key distribution and show numerically that the high-dimensional quantum key distribution can be achieved up to 180 km with InGaAs/InP-type single-photon detectors. This work can improve our understanding of the factors that determine the performance of high-dimensional quantum key distribution and will help to promote the application of single-photon detectors based on avalanche photodiodes in high-dimensional quantum key distribution.

Epigallocatechin gallate (EGCG or (2S,3R)-5,7-dihydroxy-2-(3,4,5-trihydroxyphenyl)-3,4-dihydro-2H-chromen-3-yl 3,4,5-trihydroxybenzoate) and its isomers gallocatechin gallate (GCG or (2R,3R)-2-(3,4,5-trihydroxyphenyl)-3,4-dihydro-1(2H)-benzopyran-3,5,7-triol 3-(3,4,5-trihydroxybenzoate) are the main antioxidant components in tea polyphenols and tea. In this paper, the B3LYP density functional theory (DFT) was used to optimize the EGCG and GCG molecular configuration at the 6‑311G(d,p) level of theory implemented in Gaussian software. The result is convergent and has no imaginary frequency, indicating a stable structure that reaches minimum energy value. Multiwfn wave function analysis software was used to study the frontier orbitals of EGCG and GCG. The electrophilic and nucleophilic sites of the two molecules were predicted by the contribution of each atom to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The results show that the electrophilic reaction is mainly on No.1 benzene ring, and the main contribution is the C atom, especially the para‑C atom of the C atom connected by the two phenolic hydroxyl groups on the benzene ring. At the same time, the activation of the phenolic hydroxyl group on the para- and ortho-C atoms on the benzene ring was verified. When there is a phenolic hydroxyl group on the benzene ring, its para- and ortho-C atoms become more active, and they are easier to lose electrons. So substitution or other reactions occurs. This site is also the main site against oxygen free radicals which is an accelerated aging substance. The nucleophilic sites are mainly around C=O and no. 3 benzene rings, so it has certain ability to obtain electrons. Finally, the active sites of EGCG were analyzed by using the two methods of Fukui function and mean partial ionization energy, and the results were basically consistent with the above.

Sulforaphane is the plant active substance with the best anticancer effect and antioxidant found in vegetables. In this paper, based on density functional theory(DFT), the B3LYP/6-311G+(d,p) theoretical level was used to optimize the molecular structure of sulforaphane in gas phase. On the basis of optimization, the first 30 excited states of molecule in methanol were calculated by time-density functional theory(TD-DFT) in SMD solvent model. To analy the excited states of sulforaphane molecule by calculate UV spectrum and hole-electron. Finally, we predicted the active sites of sulforaphane molecule by calculating frontier orbital and fukui function. The results show that: In the UV spectrum, the absorption peaks at 204.4602 and 336.5590nm are the absorption bands of hetero atomic groups of conjugate molecules, which are generated by n-π* transition and belong to R absorption band. According to hole-electron analysis, S0→S18 is the local excitation, and S0→S7, S0→S25 and S0→S29 are the charge transfer excitation. In addition, the O and S atoms on sulfuryl group of sulforaphane are the site of electrophilic reaction; the C and S atoms on the cyanide group are sites of nucleophilic reaction. This study provides a theoretical basis for better understanding the antioxidant and anticancer mechanisms of sulforaphane.

This paper provides an intriguing electronic and magnetic properties of the Perovskite solar cell materials (MAPbI3, CsGeI3) which are analyzed by using density functional theory, where MAPbI3 is the abbreviation of CH3NH3PbI3. Herein, we mainly discuss the influence of ferromagnetism and antiferromagnetism on the crystal structure, band gap and electronic density of states of perovskite (MAPbI3) (CsGeI3). The magmom values of the applied magnetic field are from −6 μB to 6 μB, respectively (The negative sign here represents the opposite direction of the original magnetic field). The lattice parameters and volume of the crystal under different magnetic fields are obtained. It can be seen from the free energy that the stability of the crystal is compared with non-magnetic field, when the applied magnetic fields are at 4 μB and 5 μB, the free energy of the crystal is the lowest and its relative stability is better. At the same time, the magnetic field of CsGeI3 increases from −5 μB to 5 μB. The results show that the density of states of CsGeI3 moves towards the direction of higher energy, and the change of MAPbI3 crystal is more obvious. Similarly, we should pay attention to that when applying a magnetic field to the crystals, it is found that the band gap of both MAPbI3 and CsGeI3 decreases obviously after increasing the magnetic field, which avoids the situation of spin up and spin down overlap of MAPbI3, which is helpful to adjust the band gap size. From the perspective of crystal spectral absorption characteristics, when considering the ferromagnetic effect on CsGeI3 with the magnetic moment of a single atom is set to (1 μB, 2 μB, 3 μB, 4 μB, 5 μB, 6 μB), the width of absorption spectrum tends to decrease, but the impact on MAPbI3 is small.

Based on DFT, B3LYP method was used to optimize the structure of 3-tert-butyl-4-hydroxyanisole (BHA) molecule at the level of 6-311g(d,p) set level. On this basis, the first 50 excited states were calculated by using B3LYP/def 2-TZVP method based on TD-DFT with ethanol as solvent. Multiwfn 3.6 software was used to plot the UV spectrum, and three methods including atomic charges, Fukui function (FF) and electrostatic potential (ESP) were used to predict its electrophilic reactive sites. Finally, bond energy of O10–H11 and C12–O13 were investigated by the ωB97XD/TZVP method. Comparing the experimental and theoretical UV spectra, it was found that the overall agreement was good. The oxygen atom on the phenolic hydroxyl group and the oxygen atom on the ether bond are the electrophilic reactive sites. And the C12–O13 bond on the ether bond is more prone to dissociation. They are easily reacted with peroxide radicals to have oxidation resistance.

The optical properties indicate that the potential of cm-CsPbBr3 (cm-CsPbI3) as light-absorbing materials is close to that of pn-CsPbBr3 (pn-CsPbI3).

Quantum key distribution (QKD)1,2 offers a long-term solution to secure key exchange. Due to photon loss in transmission, it was believed that the repeaterless key rate is bounded by a linear function of the transmittance, O(η) (refs. 3,4), limiting the maximal secure transmission distance5,6. Recently, a novel type of QKD scheme has been shown to beat the linear bound and achieve a key rate performance of $$O(\sqrt{\eta })$$ (refs. 7–9). Here, by employing the laser injection technique and the phase post-compensation method, we match the modes of two independent lasers and overcome the phase fluctuation. As a result, the key rate surpasses the linear bound via 302 km and 402 km commercial-fibre channels, over four orders of magnitude higher than existing results5. Furthermore, our system yields a secret key rate of 0.118 bps with a 502 km ultralow-loss fibre. This new type of QKD pushes forward long-distance quantum communication for the future quantum internet. Phase-matching quantum key distribution is implemented with a 502 km ultralow-loss optical fibre. The fluctuations of the laser initial phases and frequencies are suppressed by the laser injection technique and the phase post-compensation method.

Newport Inc. was licensed recently by the National Renewable Energy Laboratory of the United States to update the highest efficiency of the perovskite solar cell (PSC) certification of PSCs by 23.7%. Exploring new hole transfer layer is the key to the future development of PSC. In this paper, we constructed seven organic hole transport material molecules such as copper-phthalocyanine (CuPc), 2',7'-bis(bis(4-methoxyphenyl)amino)spiro[cyclopenta-[2,1-b:3,4-b']dithiophene-4,9'-fluorene] (FDT), Poly-triarylamine (PTAA), poly(3,4-ethylenedioxy thiophene)/poly(styrenesulfonate) (PEDOT/PSS) poly(3-hexylthiophene) (P3HT) and six in-organic hole transport material molecules such as CuCSN, CuI, InCuS2, CuO, Cu2O, NiO with Material Studio software. By the structure optimization, their energy band, density of state (DOS), HOMO/lowest unoccupied orbit (LUMO) energy level and absorption spectrum were calculated. Furthermore, the HOMO/LUMO electron cloud distribution map of FDT molecule was analyzed in detail. The results show that the electron cloud is closer to the nucleus with the increase of the isopotential surface value. From the absorption spectra, the absorption wavelengths of most inorganic hole transport materials are mainly concentrated at about 200 nm, which is relatively short. But the absorption wavelengths of organic hole transport materials are distributed in long wavelength region, most of them are above 2000 nm. Only the absorption spectra of PTAA, Spiro OMetad and CuPc are in the range of solar spectrum. The HOMO energy levels of seven organic hole transport materials are slightly higher than the values of valence band of CH3NH3PbI3 and NH2CH = NH2PbI3, which are favorable for carrier injection and transport. The band gap of inorganic hole transport materials CuCSN and CuI is wider. From the energy band structure curve, the effective mass of NiO, CuO, Cu2O carriers is smaller, which the carrier transport rate is relatively high. The hole transport material must have high hole mobility and hole conductivity so as to ensure the effective transport of the hole at the interface between the hole transport layer and the perovskite layer.

The electron-transport layer in planar perovskite solar cells plays an important role in improving photoelectric conversion efficiency. At present, the main electronic transmission materials in perovskite solar cells include TiO2, ZnO, WO3, ZrO2, SnO2, ZnO2, etc. This work mainly studies the electron-transport characteristics of six different electron-transport layers in perovskite solar cells. Based on the density functional theory, the electron-transport model of a solar cell doped with formamidinium iodide lead compound perovskite under six different electron-transport materials was constructed, and their effective electron mass and the mobility of carriers were obtained by optimizing the structure and theoretical calculation. The results show that the mobility of electrons in TiO2 crystal is slightly higher than that of FA0.75Cs0.25Sn0.5Pb0.5I3 carriers. Because of their high matching degree, it can be reasonably explained that titanium dioxide has been widely used in perovskite solar cells and achieved higher photoelectric conversion efficiency. In addition, the mobility of carriers in WO3 and SnO2 crystals is also high, so they also have great advantages in carrier transport. Due to its abundant, nontoxic, and low-pollution content, TiO2 has become the most widely used electronic transmission layer material for solar cells. Furthermore, we have explored eight new semiconductor materials that have not yet been used in perovskite solar cells as the electron-transport layer. The calculation results show that Ta2O5 and Bi2O3 are promising materials for the electron-transport layer. This study provides a theoretical basis for seeking better electronic transmission materials for solar cells in the future.

Quantum key distribution (QKD) can provide information theoretically secure key exchange even in the era of quantum computer. However, QKD requires the classical channel to be authenticated, and the current method is pre-sharing symmetric keys. For a QKD network of $n$ users, this method requires $C_n^2 = n(n-1)/2$ pairs of symmetric keys to realize pairwise interconnection. In contrast, with the help of mature public key infrastructure (PKI) and post-quantum cryptography (PQC) with quantum resistant security, each user only needs to apply for a digital certificate from certificate authority (CA) to achieve efficient and secure authentication for QKD. We only need to assume the short-term security of the PQC algorithm to achieve the long-term security of the distributed keys. Here, we experimentally verified the feasibility, efficiency and stability of the PQC algorithm in QKD authentication, and demonstrated the advantages when new users join the QKD network. Using PQC authentication we only need to believe the CA is safe, rather than all trusted relays. QKD combined with PQC authentication will greatly promote and extend the application prospects of quantum safe communication., Comment: 8 pages, 6 figures

First principles calculation based on density functional theory (DFT) with the generalized gradient approximation (GGA) are carried out to investigate the electronic band structures of Si x Ge ( 1 − x ) alloys nanofilms. The calculation results show that the band gaps of (100), (110) and (111) surfaces Si x Ge ( 1 − x ) alloy films with different thickness first increase with the increase of Si content, then flatten out, and finally decrease. At the same time, the transformation of direct band gap and indirect band gap occurs when the thickness of films and Si content of the three surface Si x Ge ( 1 − x ) alloy films changes to a certain critical condition. It will be a good way to obtain direct-gap band emission in Si x Ge ( 1 − x ) alloys materials.

The current work has investigated the mechanical, electronic and optical properties of Pb-free vacancy-ordered double perovskites K2PdCl6 and K2PdBr6 by using first-principles calculations based on the framework of density functional theory (DFT). The calculated lattice constants of K2PdCl6 and K2PdBr6 are close to the experimental data. It is determined by calculating the Goldschmidt’s tolerance factors and elastic constants of K2PdCl6 and K2PdBr6 that they can be stabilized into 3D cubic crystal structures. The calculated Poisson and Pugh’s ratios indicate that K2PdCl6 is a brittle material, while K2PdBr6 exhibits ductile behavior. Both K2PdCl6 and K2PdBr6 are indirect band gap semiconductors, which show suitable band gaps of 2.151 eV and 1.368 eV for optoelectronic devices, respectively. In addition, the optical properties of K2PdCl6 and K2PdBr6 in the photon energy range of 0–6 eV further reveal the application potential of these compounds in single-junction and tandem solar cells as well as other optoelectronic devices.

Recently, more and more vacancy-ordered double perovskites have been proposed as functional materials in optoelectronic devices. In this paper, a simulation work on the structural, electronic and optical properties of vacancy-ordered double perovskites Tl2TiX6 (X ​= ​Cl, Br, I) has been carried out by using first-principles calculations to reveal their potential application in optoelectronic devices. The Goldschmidt's tolerance factors of these compounds are very close to the ideal value of 1 for cubic perovskites, which confirm their structural stability. Moreover, the formation energies of these compounds were also calculated to evaluate their thermodynamic stability. The theoretically predicted lattice constants of Tl2TiCl6, Tl2TiBr6 and Tl2TiI6 are 10.060 ​A, 10.553 ​A and 11.320 ​A, respectively, which can provide references for future experimental research. The calculated electronic properties indicate that Tl2TiCl6, Tl2TiBr6 and Tl2TiI6 have direct bandgaps of 3.39 ​eV, 2.57 ​eV and 1.58 ​eV, respectively, showing ideal bandgap nature for optoelectronic applications. Compared with many vacancy-ordered double perovskites, the three Tl2TiX6 compounds all show the advantage of small effective mass of holes. In addition, the investigation of optical properties, such as reflectivity, absorption coefficient and energy loss function in the photon energy range of 0–25 ​eV further confirms the potential utilization of these Tl2TiX6 compounds in optoelectronic devices.

CsGeI3 has been widely studied as an important photoelectric material. Based on the density functional theory (DFT), we use first-principles to study the photoelectric properties of CsGeI3 by applying successive hydrostatic pressure. It has been found that CsGeI3 has an optimal optical band gap value of 1.37 eV when the applied pressure is &minus, 0.5 GPa, so this paper focuses on the comparative study of the photoelectric properties when the pressure is &minus, 0.5 GPa and 0 GPa. The results showed that CsGeI3 has a higher dielectric value, conductivity, and absorption coefficient and blue shift in absorption spectrum when the pressure is &minus, 0.5 GPa. By calculating and comparing the effective masses of electrons and holes and the exciton binding energy, it was found that their values are relatively small, which indicates that CsGeI3 is an efficient light absorbing material. CsGeI3 was found to be stable under both pressure conditions through multiple calculations of the Born Huang stability criterion, tolerance factor T, and phonon spectrum with or without virtual frequency. We also calculated the elastic modulus of both pressure conditions and found that they are both soft, ductile, and anisotropic. Finally, the thermal properties of CsGeI3 under two kinds of pressure were studied. It was found that the Debye temperature and heat capacity of CsGeI3 increased with the increase of thermodynamic temperature, and the Debye temperature increased rapidly after pressure, while the heat capacity slowly increased and finally stabilized. Through the calculation of enthalpy, entropy, and Gibbs free energy of CsGeI3, it was found that the Gibbs free energy decreases faster with the increase of temperature without applied pressure, which indicates that CsGeI3 has a higher stability without pressure. Through the comparative analysis of the photoelectric properties of CsGeI3 under pressure, it was found that CsGeI3 after applied pressure is a good photoelectric material and suitable for perovskite solar cells (PSCs) material.