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<jats:p>The precision measurement of Doppler frequency shifts is of great significance for improving the precision of speed measurement. This paper proposes a precision measurement scheme of tiny Doppler shifts by a parametric amplification process and squeezed vacuum state. This scheme takes a parametric amplification process and squeezed vacuum state into a detection system, so that the measurement precision of tiny Doppler shifts can exceed the Cramér–Rao bound of coherent light. Simultaneously, a simulation study is carried out on the theoretical basis, and the following results are obtained: for the signal light of Gaussian mode, when the amplification factor <jats:italic>g</jats:italic> = 1 and the squeezed factor <jats:italic>r</jats:italic> = 0.5, the measurement error of Doppler frequency shifts is 14.4% of the Cramér–Rao bound of the coherent light in our system. At the same time, when the local light mode and squeezed vacuum state mode are optimized, the measurement precision of this scheme can be further improved by <jats:inline-formula> <jats:tex-math><?CDATA $\sqrt{(2n+1)/(n+1)}$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msqrt> <mml:mrow> <mml:mrow> <mml:mo>(</mml:mo> <mml:mrow> <mml:mn>2</mml:mn> <mml:mi>n</mml:mi> <mml:mo>+</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> <mml:mo>)</mml:mo> </mml:mrow> <mml:mo>/</mml:mo> <mml:mrow> <mml:mo>(</mml:mo> <mml:mrow> <mml:mi>n</mml:mi> <mml:mo>+</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:msqrt> </mml:mrow> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cpb_30_7_074202_ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> times, where <jats:italic>n</jats:italic> is the mode-order of the signal light.</jats:p>

<jats:p>We propose and demonstrate an alternative method for spectral filtering and frequency stabilization of both 780-nm and 960-nm lasers using a high-finesse length-tunable cavity (HFLTC). Firstly, the length of HFLTC is stabilized to a commercial frequency reference. Then, the two lasers are locked to this HFLTC using the Pound–Drever–Hall (PDH) method which can narrow the linewidths and stabilize the frequencies of both lasers simultaneously. Finally, the transmitted lasers of HFLTC with each power up to about 100 μW, which act as seed lasers, are amplified using the injection locking method for single-atom Rydberg excitation. The linewidths of obtained lasers are narrowed to be less than 1 kHz, meanwhile the obtained lasers’ phase noise around 750 kHz are suppressed about 30 dB. With the spectrally filtered lasers, we demonstrate a Rabi oscillation between the ground state and Rydberg state of single-atoms in an optical trap tweezer with a decay time of (67 ± 37) μs, which is almost not affected by laser phase noise. We found that the maximum short-term laser frequency fluctuation of a single excitation lasers is at ∼3.3 kHz and the maximum long-term laser frequency drift of a single laser is ∼46 kHz during one month. Our work develops a stable and repeatable method to provide multiple laser sources of ultra-low phase noise, narrow linewidth, and excellent frequency stability, which is essential for high precision atomic experiments, such as neutral atom quantum computing, quantum simulation, quantum metrology, and so on.</jats:p>

<jats:p>We investigate the influence of the birefringence on the high-order harmonics in an a-cut ZnO crystal with mid-infrared laser pulses. The high harmonics exhibit strong dependence on the alignment of the crystal with respect to the laser polarization. We introduce the Jones calculus to counteract the birefringent effect and obtain the harmonics with polarization corrections in ZnO. We show that the birefringent effect plays an important role in the orientation dependence of HHG.</jats:p>

<jats:p>The translucent GGAG:Ce/glass composites are prepared successfully by ball-milling, tableting, and pressureless sintering. The thickness of composites is about 400 μm. The x-ray diffraction (XRD), differential scanning calorimetry (DSC), density of composite materials are measured and discussed systematically. The scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) elemental mapping are employed to analyze the particle size, the shape of powders, and the distribution of GGAG:Ce particles in the glass matrix, respectively. The decay time, ultraviolet, (UV), x-ray excitation luminescence spectra, and temperature spectra are studied. The results show that the composite materials have high light output, good thermostability, and short decay time. The method adopted in this work is an effective method to reduce the preparation time and cost of the sample. The ultralow afterglow indicates that the composite materials have an opportunity to be used for x-ray detection and imaging.</jats:p>

<jats:p>We study the steady optical response of a square lattice in which all trapped atoms are driven by a probe and a coupling fields into the ladder configuration of electromagnetically induced transparency (EIT). It turns out to be a many-body problem in the presence of van der Waals (vdW) interaction among atoms in the upmost Rydberg state, so Monte Carlo (MC) calculation based on density matrix equations have been done after introducing a sufficiently large cut-off radius. It is found that the absorption and dispersion of EIT spectra depends critically on a few key parameters like lattice dimension, unitary vdW shift, probe Rabi frequency, and coupling detuning. Through modulating these parameters, it is viable to change symmetries of the absorption and dispersion spectra and control on demand depth and position of the transparency window. Our MC calculation is expected to be instructive in understanding many-body quantum coherence effects and in manipulating non-equilibrium quantum phenomena by utilizing vdW interactions of Rydberg atoms.</jats:p>

<jats:p>High-quality Fe-doped ZnS films have been fabricated by electron beam evaporation. After the doping, the fabricated films still maintain the preferential crystalline orientation and phase purity of the host ZnS. According to the observation of surface morphology, the root mean-square roughness of the samples increases slightly with the increase of doping content. All of the prepared samples are in cubic zinc blende structure of ZnS. Transmission spectrum confirms a more obvious dip near 3 μm with higher dopant concentration and it can be attributed to the typical <jats:sup>5</jats:sup>E→<jats:sup>5</jats:sup>T<jats:sub>2</jats:sub> transition of Fe<jats:sup>2+</jats:sup>. Fe-doped ZnS film is also successfully used for <jats:italic>Q</jats:italic>-switched Er:ZBLAN fiber laser.</jats:p>

<jats:p>We propose a physical model of estimating noise and asymmetry brought by high isolation Bi-directional erbium-doped fiber amplifiers (Bi-EDFAs), no spontaneous lasing even with high gain, in longdistance fiber-optic time and frequency (T/F) synchronization system. It is found that the Rayleigh scattering noise can be suppressed due to the high isolation design, but the amplified spontaneous emission (ASE) noise generated by the high isolation Bi-EDFA and the bidirectional asymmetry of the transmission link caused by the high isolation Bi-EDFA will deteriorate the stability of the system. The calculated results show that under the influence of ASE noise, the frequency instability of a 1200 km system composed of 15 high isolation Bi-EDFAs is 1.773 × 10<jats:sup>−13</jats:sup>/1 s. And the instability caused by asymmetry is 2.6064 × 10<jats:sup>−16</jats:sup>/30000–35000 s if the total asymmetric length of the bidirectional link length is 30 m. The intensity noises originating from the laser and detector, the transfer delay fluctuations caused by the variation in ambient temperature and the jitter in laser output wavelength are also studied. The experiment composed of three high isolation Bi-EDFAs is done to confirm the theoretical analysis. In summary, the paper shows that the short-term instability of the T/F synchronization system composed of high isolation Bi-EDFAs is limited by the accumulation of ASE noise of amplifiers and the laser frequency drift, while the long-term instability is limited by the periodic variation in ambient temperature and the asymmetry of the amplifiers. The research results are useful for pointing out the direction to improve the stability of the fiber-optic T/F synchronization system.</jats:p>

<jats:p>To overcome the inherent limits of traditional single wave imaging for nondestructive testing, the multi-wave focusing and imaging method is thoroughly studied. This method makes the compressional waves and shear waves focused in both emission and reception processes, which strengthens the focusing energy and improves the signal-to-noise ratio of received signals. A numerical model is developed to study the characteristics of a multi-wave focusing field. It is shown that the element width approaching 0.8 wavelengths of shear waves can keep a balance between the radiation energy of two waves, which can achieve a desirable multi-wave focusing performance. And an experiment using different imaging methods for a linear phased array is performed. It can be concluded that due to the combination of the propagation and reflection characteristics of two waves, the multi-wave focusing and imaging method can significantly improve the imaging distinguishability of defects and expand the available sweeping range to a sector of –65° to 65°.</jats:p>

<jats:p>Near space has been paid more and more attentionin recent years due to its military application value. However, flow characteristics of some fundamental configurations (e.g., the cavity) in near space have rarely been investigated due to rarefied gas effects, which make the numerical simulation methods based on continuous flow hypothesis lose validity. In this work, the direct simulation Monte Carlo (DSMC), one of the most successful particle simulation methods in treating rarefied gas dynamics, is employed to explore flow characteristics of a hypersonic cavity with sweepback angle in near space by considering a variety of cases, such as the cavity at a wide range of altitudes 20–60 km, the cavity at freestream Mach numbers of 6–20, and the cavity with a sweepback angle of 30°–90°. By analyzing the simulation results, flow characteristics are obtained and meanwhile some interesting phenomena are also found. The primary recirculation region, which occupies the most area of the cavity, causes pressure and temperature stratification due to rotational motion of fluid inside it, whereas the pressure and temperature in the secondary recirculation region, which is a small vortex and locates at the lower left corner of the cavity, change slightly due to low-speed movement of fluid inside it. With the increase of altitude, both the primary and secondary recirculation regions contract greatly and it causes them to separate. A notable finding is that rotation direction of the secondary recirculation region would be reversed at a higher altitude. The overall effect of increasing the Mach number is that the velocity, pressure, and temperature within the cavity increase uniformly. The maximum pressure nearby the trailing edge of the cavity decreases rapidly as the sweepback angle increases, whereas the influence of sweepback angle on velocity distribution and maximum temperature within the cavity is slight.</jats:p>

<jats:p>The nanoparticles suspended in a shear flow are subjected to a shear lift force, which is of great importance for the nanoparticle transport. In previous theoretical analysis on the shear lift, it is usually assumed that the particle temperature is equal to the temperature of the surrounding gas media. However, in some particular applications, the particle temperature can significantly differ from the gas temperature. In the present study, the effect of particle temperature on the shear lift of nanoparticles is investigated and the corresponding formulas of shear lift force are derived based on the gas kinetic theory. For extremely small nanoparticles (with radius <jats:italic>R</jats:italic> &lt; 2 nm) or large nanoparticles (<jats:italic>R</jats:italic> &gt; 20 nm), the influence of the particle temperature can be neglected. For the intermediate particle size, the relative error induced by the equal gas-particle temperature can be significant. Our findings can bring an insight into accurate evaluation of the nanoparticle transport properties.</jats:p>