With the exponential growth of global data traffic in the internet era, the demand of ultra-high-speed communications is becoming increasingly urgent. Because of its huge available bandwidth resources,terahertz communication has become one of the key technical approaches for the next generation of ultra-high-speed wireless communications. Terahertz communication systems can be divided into two types: the systems based on electronics technology and those based on photonics-assisted technology. The latter has some unique advantages, such as high communication rate, flexible frequency tunability, and seamless integration with optical fiber access networks. The basic principles of typical photonics-assisted terahertz communication systems are introduced firstly. Then, under these basic principles, the research progresses of signal generation and reception, multi-dimensional multiplexing and probabilistic constellation shaping in the systems are reviewed. Finally, the developing problems need to be solved in the photonics-assisted terahertz communication system are discussed.

Fruit charcoal is a kind of common fuel, and the gases and aerosols produced in its combustion process can affect environmental air quality and harm human health. Therefore, it is of great significance to detect and identify the air composition during the combustion process of fruit charcoal. Laser-induced breakdown spectroscopy (LIBS) is used to detect the air and aerosol during the combustion of fruit charcoal, and at the same time, fruit charcoal and its combustion ash are also detected as auxiliary analysis. The spectral lines of four samples are calibrated, and it is found that when the charcoal is burning, the carbon concentration in the air increases, and the generated aerosols contain Ca, Mg, K, Si and other elements. The elemental composition of fruit charcoal and ash is similar, both containing C, Fe, Mg, Ca, Sr, K, Na, Ba, and the intensity of C and H in fruit charcoal spectrum is higher than that in ash spectrum. Considering that the combustion of fruit charcoal produce more gas and less particulate matter, it is difficult to judge whether the fruit charcoal is burning or has burned in the room from the perspective of image recognition, so the air with and without combustion of fruit charcoal are further distinguished by principal component analysis (PCA) algorithm, and the bands where the characteristic spectral lines of C and CN are selected as the original features of cluster analysis. The results show that the two kinds of air with and without fruit charcoal combustion can be well distinguished, which proves that LIBS combined with PCA can effectively identify the combustion of fruit charcoal and can be used to detect the air pollution caused by the combustion of fruit charcoal. Furthermore, the composition of fruit charcoal and ash are also distinguished in the same way, and it is found that the distinction effect is good, which provides a reference for the recovery and utilization of fruit charcoal after combustion.

Photonic crystals (PCs) can be divided into simple PCs and compound PCs. Laser holographic interferometry is one of the important methods in the fabrication of compound PCs, in which beam polarization plays an important role. By using a four-beam configuration with certain symmetry to produce compound PCs, numerical simulations are conducted using Matlab programming to systematically study the influence of various polarization combinations (including single-beam, double-beam, three-beam, and four-beam cases) on the unitcell shape and interference contrast of the compound PCs. The research results show that the polarization degree and rotation angle of each beam have a significant effect on the compound PCs. Under different polarization conditions, a series of rich unitcell shapes, such as wavy stripes, elliptical cylinders, zigzags, are obtained. In addition, it is found that when all the interference beams are linearly polarized, the compound PCs have the optimal contrast. The results obtained in this work have certain theoretical guidance significance for the experimental fabrication of compound PCs with various unitcell shapes, as well as the development of virtual experimental research.

Aiming at the shortcomings of traditional Canny operator in filtering, which can blur the edge and need to set high and low thresholds manually, an improved adaptive threshold Canny edge detection algorithm based on 3D block matching is proposed for terahertz 3D tomography. On one hand, the algorithm improves the filtering method by replacing the Gaussian filtering algorithm with the 3D block matching (BM3D) filtering algorithm and the guided filtering algorithm to reduce the loss of image edge information. On the other hand, in view of the uncertainty of the traditional manual threshold, the maximum inter-class variance method (OTSU) is used to adaptively determine the high and low thresholds of 3D image blocks after block matching of gradient images. Finally, the edge detection of images containing noise is carried out using the algorithm. It is found that when the Gaussian noise variance is 20, the filtered peak signal-to-signal ratio (PSNR) increases from 22.202 to 27.151, which verifies the effectiveness of the algorithm in removing noise. By using BM-OTSU-Canny algorithm, the number of false edges is reduced, and at the same time, the edge points with better connectivity can be retained, and the extraction effect of edge details is improved.

In order to accurately measure the pulse width of 193 nm deep ultraviolet femtosecond laser, a pulse width measurement system was designed and built based on the two-photon fluorescence effect of calcium fluoride. The square dependent relationship of the two-photon fluorescence signal on incident laser intensity confirms the reliability of the two-photon fluorescence signal. The numerical calibration of the CCD detection system is carried out using insertion method, and the obtained time scale correspongding to a single pixel is 7.35 fs. In single-shot pulse measurement, the pulse width of 193 nm deep ultraviolet femtosecond laser is measured to be 476.1 fs, which is basically consistent with the result obtained through measurement and calculation using spectroscopy.

A picosecond pulse fiber front end based on a narrow-band dissipative soliton Figure-9 fiber oscillator and a single-stage single-mode fiber amplifier is reported here. By optimizing the fiber length of the fiber oscillator cavity, a self-starting single pulse mode-locked narrow-band dissipative soliton picosecond pulse with a center wavelength of about 1064 nm, a repetition rate of 10 MHz, and a pulse energy of 0.4 nJ is obtained. Then the 21.07 ps picosecond pulse generated by the fiber oscillator is amplified by a single-stage single-mode fiber amplifier, and the spectrum can still maintain a bell-shaped structure with a 3 dB spectral width of 0.31 nm and a pulse width of 19.8 ps, respectively, even the pulse energy reaches 10 nJ. The picosecond fiber front end is expected to play an important role in precision machining and other fields.

Due to its excellent characteristics of good beam quality, narrow pulse width and compact structure, all-solid-state passively Q-switched lasers have a wide application in radar detection, industrial manufacturing etc. Here, the output characteristics of passively Q-switched YAG/Nd: YAG/Cr4+: YAG laser are studied theoretically and experimentally. While being pumped by 808 nm laser with 230 μm beam diameter and 6.72 W power, a Q-switched laser output with an average power of 1.41 W and pulse width of 736 ps at the repetition rate of 8.46 kHz is obtained. Furthermore, it is shown that the symmetry of the laser beam decreases as the focus of the pump laser moves away from the Nd: YAG end face, and the laser threshold shows an upward trend when the distance between the focus of the pump laser and the Nd: YAG end face increases along the axis of the crystal.

The Mg2Si polycrystalline films are fabricated on sapphire substrates by magnetron sputtering and post annealing treatment, and then the effect of annealing temperature (375~475 oC) on the crystal structure, surface morphology, Raman spectra and optical properties of the film is investigated. The X-ray diffraction (XRD) results show that when annealing temperature is 400 oC, the (220) diffraction peak of Mg2Si film is the strongest, the crystal quality is the best, and no obvious MgO phase can be observed. The scanning electron microscopy (SEM) results show that all samples are regular hexagonal, and the annealing temperature has little effect on the morphology. The Raman spectra results show that all samples exhibit the characteristic peaks of Mg2Si films (with F2g vibration mode at near 256 cm-1, and F1u (LO) phonon mode at near 345 cm-1), indicating that all fabricated samples are Mg2Si films with good crystallization. The results of optical properties of the sample films show that, with the increase of annealing temperature, the optical band gap of the samples increases first and then decreases.

The highly precise sensitivity of atoms to magnetic fields has led to the increasing application of diamond nitrogen vacancy (NV) color centers in the field of quantum sensing. To investigate the superiority of diamond in vibration magnetometry, an integrated vibration module is used to integrate a conventional confocal system on a PCB board for vibration testing. A cylindrical permanent magnet is used as the mass block for vibration magnetometry. Due to the influence of external vibrations, the distance between the permanent magnet and the diamond NV color center will change, so the magnetic field intensity on the diamond NV color center will also change. The detection of the vibration effect is obtained by analyzing the light-detected magnetic resonance (ODMR) fluorescence signal of diamond NV color core. Integrated vibration experiments are performed to validate the vibration effect at a distance of 15 mm from the permanent magnet, and the sensitivity of the displacement noise based on the vibration sensing of diamond nitrogen vacancy color core is tested to be 11 nm /Hz1/2.

In atom interferometer gravity experiments, Raman laser preparation commonly uses an optical phase-locked loop method, in which the master and slave laser beat signal is mixed with a 6.8 GHz microwave signal source firstly, then frequency discrimination is performed with direct digital frequency synthesis signal generator, and lastly the feedback signal obtained is used to control the low-noise Raman optical output. So the phase noise of Raman output will directly affect the sensitivity of the atom interference gravimeter. This design uses the STM32F103C8T6 microcontroller to program and control the LMX2594 digital phase-locked loop chip, and obtains a 6.8 GHz microwave signal source through the phase-locked loop frequency synthesis technology. The final experimental results show that the phase noise of the microwave signal source is 65.2 dB@1 Hz, and -95.3 dB@1 kHz, Allan deviation (ADEV) is 2.72 × 10-11@1 s, and the output power is greater than 10 dBm. When the pulse interval time (T ) is 100 ms, influence of signal source on sensitivity of the atom interference gravimeter is 8 × 10-8 m/s2/Hz1/2, and on the resolution of the atom interference gravimeter is 2×10-8 g@600 s. It is shown that the design has the advantages of high frequency stability and low phase noise, which can meet the requirements of microwave reference sources for atom interferometer.

Spin squeezing and quantum entanglement are very important and widely used in quantum information processing. Therefore, it is meaningful to produce spin squeezing and quantum entanglement using the multi-body interaction in Bose-Einstein condensation. The effects of three-body and four-body interactions on spin squeezing and quantum entanglement at high density Bose-Einstein condensation are investigated, and the spin squeezing parameters and two entanglement parameters are analytically calculated using short time approximation. The results show that in the dynamic process, three- and four-body interactions can produce spin squeezing and quantum entanglement. Moreover, four-body interaction can produce stronger spin squeezing and better entanglement than the three-body interaction.

The spatial two-body correlations of Bose-Fermi mixture with equal masses of bosons and fermions in Tonks-Girardeau regime under periodic boundary conditions are theoretically investigated. Combining the exact solution of the quantum many-body system, the analytical formula of the spatial correlation functions for Bose-Fermi mixture at both absolute zero temperature and finite low temperatures are derived using some calculating techniques. The results show that, for a fixed number of bosons (fermions) at absolute zero temperature, the number of fermions (bosons) has little effect on the spatial correlation function of bosons (fermions). By contrast, at finite temperature, the number of fermions (bosons) has a greater effect on the spatial correlation function of bosons (fermions), and with the increase of temperature, the influence of the other particles number on the correlation results will be more significant. In addition, when the total particle number is fixed, the correlation results of the same number of bosons and fermions are consistent whether at absolute zero temperature or at a finite temperature.

Quantum key distribution (QKD) has unconditional security in the information theory. The phase-matching quantum key distribution (PM-QKD) is one of the variants of twin-field quantum key distribution (TF-QKD), which has been proposed recently to overcome the rate-distance limits of point-to-point protocol without quantum repeaters. In view of the fact that the infinite decoy states are not available in practice, four-intensity decoy-state PM-QKD protocol is more practical. Therefore, a novel PM-QKD protocol with four-intensity decoy states, namely four-intensity decoy-state PM-QKD protocol, is propsed in this work. The secure key rate formula of the proposed protocol is presented and its performances are analyzed through numerical simulations to prove its validity.

The application of fiber-based quantum key distribution (QKD) depends on the compatibility with existing optical networks. The use of wavelength division multiplexing (WDM) technology for co-fiber transmission of classical data and quantum signals has the advantages of security, economy, and practicality. Aiming at the prominent problems that affect the application of the classical-quantum signal co-fiber transmission system, such as the difficulty in optimizing and calculating the average photon number of signal states, the number of decoy states and other parameters, and the slow running speed, the channel noise components are modelel and analyzed, and the finite-length effect and the decoy state method are evaluated considering the statistical fluctuations. Furthermore, based on the experimental data set, the back propagation (BP) neural network is trained to predict the system parameters such as the average photon number of signal states under different channel noise conditions. The results show that the predicted average photon number by the BP network is basically consistent with the original curve, and the training error is less than 10-3. It is indicated that the network can be used as an effective model for practical prediction of the parameters of the decoy-state classical-quantum co-fiber transmission system, which is of great practical significance for the development of quantum-secure communication towards high speed, large capacity and intelligence.

In noisy intermediate-scale quantum (NISQ) devices, the reliability of quantum circuits is affected by quantum noise. In order to realize the efficient and reliable execution of controlled-NOT(CNOT) quantum circuit on a quantum chip, a cost measurement method for calculating the minimum Steiner noise path length is presented, taking the interaction error rate of adjacent qubits as the weight. Then based on this method, a noise-aware nearest neighbor synthesis algorithm for CNOT quantum circuits is proposed. The experimental results show that, compared with the existing methods, the proposed algorithm can effectively reduce the number of CNOT gates used in the synthesis process on the premise of ensuring the reliability of the circuit. The average optimization rate of CNOT gate cost reaches 27.7%, and the optimization rate of 200-gate CNOT quantum circuits reaches 93.79%.

In order to solve the problem of mapping quantum circuits to two-dimensional architecture and realizing qubit nearest neighbor, a quantum circuit layout and optimization method in two-dimensional architecture is proposed. Firstly, according to the execution order and interaction of quantum gates in quantum circuit, a depth-first search qubit mapping order based on the weight of qubits is proposed, then the initial qubit mapping is realized by taking into account the relationship between the put qubits in the mapping order, the qubits to be put in and the unput qubits. Secondly, the selection of the same look-ahead quantum cost in the nearest neighbor process is optimized, then according to the optimized cost results, SWAP gates are inserted to realize the nearest neighbor of all double quantum gates. Finally, the proposed method is verified by experiments and compared with the existing methods, and it is shown that the average optimization rate of the propsed method reaches 18% on the small and medium-sized Benchmark and 17% on the medium and large-scale Benchmark.

Quantum battery is a kind of quantum devices of storage energy, which has wide potential applications in many quantum small systems, such as quantum sensors, molecular motors and so on. Based on the framework of collision model, the effects of coherence of non-equilibrium environment on the storage energy of quantum battery have been studied when the battery is being charged in a non-equilibrium coherent environment. By deriving the quantum master equation describing the dynamics of system, the analytical expression for storage energy of quantum battery at steady state is given, and then the effects of squeezing coherence and heat-exchange coherence on the storage energy of quantum battery are discussed. It demonstrates that the squeezing coherence of auxiliary unit has no contribution to the storage energy of battery, but the heat-exchange coherence (including coherence magnitude and relative phase) can improve the storage energy of battery efficiently, and can be regarded as a kind of useful "fuel" resource for battery.

In view of the problems existed in the traditional analysis devices of dissolved gas in transformer oil, such as large size,complicated oil circuit and susceptibility to electromagnetic interference, an analysis system based on optical fiber photoacoustic sensing is designed to detect trace acetylene gas dissolved in transformer oil. In the system, the laser wavelength is optimized through spectral line analysis, and a thermostatically controlled laser driving module is designed. The pump laser and probe light are transmitted into the photoacoustic sensor probe by two single-mode fibers. The photoacoustic signal generated by gas molecules absorbing the pump laser energy is detected by a Fabry-Perot interference cantilever, and then demodulated by a self-designed spectral measurement and signal processing module. Experimental results show that the designed optical fiber photoacoustic sensor system can effectively detect photoacoustic signal of acetylene gas dissolved in transformer oil, with a detection limit of 0.5 μL/L.

The localized surface plasmon resonance of bowtie gold dimer are numerically simulated by finite element method in this work. It is found that when the geometry remains unchanged, the resonant peak shifts of gold dimer decay exponentially with increasing particle spacing, and the decay length is size-independent as the shift and gap are scaled respectively by the peak wavelength and particle size. The electric field distribution of the nanostructures under the resonance condition is further analyzed. It is found that the electric field distribution along the x and y axes of the single obeys the law of negative exponential decay, and the intensity of the electric field is related to the polarization direction of the incident electromagnetic wave. While due to the coupling effect of surface plasmon between dimers, the gap between dimers has a great influence on the attenuation rate of the electric field. Finally, the resonance absorption inensity of dimers is also studied, and it is found that the relationship between the absorption inensity and the gap of dimer is still a negative exponential function. The results of this paper have important guiding significance for micro sensing, particle capture, and so on.