Low-noise, narrow-linewidth lasers are of great importance for applications demanding exceptional spectral purity, for example, laser cooling. While the cold atom systems usually use large, expensive, and sophisticated lasers to achieve certain wavelengths, photonic integrated circuits can enable portable laser frequency locking systems. This work presents a more compact 780 nm laser stabilization scheme using an on-chip silicon nitride Mach-Zehnder interferometer as a frequency reference. Through the Pound-Drever-Hall locking technique, the system reduces frequency noise by 48.5 dB and narrows the integral linewidth to 15.4 kHz. This demonstrates a compact, high-performance solution for laser stabilization, showing the potential for rubidium atom cooling and quantum manipulation.

The traditional global navigation satellite system (GNSS) satellite inter-frequency clock bias (IFCB) estimation methods rely on sequential calculations per epoch and then model fitting, which introduces redundant steps, increases data storage and transmission costs, and reduces the timeliness of real-time services (RTS). To address these limitations, this paper proposes a direct parameterization estimation method for GNSS satellite IFCB, which estimates model coefficients directly from the combined carrier phase and pseudorange observations without first generating the intermediate per-epoch IFCB sequence. This method simplifies the processing flow, reduces computational complexity, and generates a compact parameterized IFCB product with the minimum storage requirements. Validation experiments were conducted using 30 d multi-system GNSS data from 85 international GNSS service (IGS) stations and 3 d data from 20 independent IGS stations for IFCB model building and precise point positioning (PPP) evaluation. The results show that the proposed method has comparable IFCB estimation accuracy to the traditional cycle-based and fitting methods, with errors at the millimeter level. The IFCB model correction significantly improves the convergence and positioning accuracy of PPP: compared to the uncorrected solutions, the three-dimensional root mean square error of GPS static PPP is decreased by approximately 33%, and that of BDS-2 is decreased by approximately 29%. For RTS, direct coefficient estimation reduces processing delay and enables efficient IFCB extrapolation and broadcasting. Real-time PPP tests confirmed that this method is comparable in accuracy to traditional methods while improving timeliness. This method is applicable to GPS, BDS-2, BDS-3, and Galileo systems, providing an efficient and universal solution for multi-frequency GNSS high-precision positioning in post-processing and real-time scenarios.

Efficient and precise measurement of electromagnetic signals is crucial for both fundamental science and practical applications. Demodulation is a widely used technique for receiving and recovering modulated signals. Here we show a quantum demodulator using two heteronuclear ions to simultaneously detect the amplitudes and frequencies of unknown periodic signals, overcoming the constraint of double-parameter optimization. With reinforcement learning, the quantum Fisher information scales as t4 for frequency estimation and as t2 for amplitude estimation, achieving inverse-quartic and inverse-quadratic temporal scaling, respectively. This work demonstrates the potential of heteronuclear ions in efficient quantum sensing and highlights the role of reinforcement learning in advancing quantum control.

Millimeter-level transient displacements caused by extreme natural disasters are crucial for studying their dynamic processes. However, at present, such signals are rarely captured in Chinese mainland by GNSS. In October 2014, the passage of Typhoon Vongfong generated a significant storm surge, causing sea levels to rise rapidly within hours. Using 3-h displacements from GNSS stations of the Crustal Movement Observation Network of China (CMONOC) along the eastern coast of China, we evaluated non-tidal ocean loading (NTOL) effects during October 2014 by comparing the GNSS observations with model predictions. According to the NTOL predictions, vertical deformation in coastal areas in eastern China reached 2 cm, but subsidence could only be marginally detected by GNSS. Only at the Zhejiang Zhoushan station (ZJZS), the root mean square (RMS) in the vertical was reduced by 1.64 mm after removing the NTOL predictions. In the future, to capture transient signals, we will focus on integrating multi-GNSS to improve the accuracy of high-rate GNSS positioning.

Micro-Newton cold-gas thrusters are promising actuators for precision space missions, but their performance is strongly influenced by the integrated head architecture. This study presents the design, fabrication, and experimental validation of a piezoelectrically controlled cold-gas microthruster head for space-based gravitational-wave detection missions. The proposed head integrates a cone-needle throttle, a micro-orifice interface, and a downstream micro-nozzle, thereby converting actuator displacement into a regulated mass flow and ultimately into thrust. One-dimensional theory was first used for preliminary sizing, and Direct Simulation Monte Carlo (DSMC) analysis of the complete throttle-nozzle geometry was then applied to determine the final design parameters under rarefied-flow conditions. The selected design uses a throat radius of 29 μm and a needle half-angle of 10 degrees. Following fabrication and structural characterization, the integrated device was validated through mass-flow calibration and vacuum thrust testing. The experimental results show that the pressure-decay-based calibration provides a consistent mapping between actuation command, calibrated flow rate, and thrust output. The measured flow-thrust relation preserves the high linearity predicted by simulation, while the experimentally evaluated specific impulse meets the specified design target over the tested range. In addition, thrust-resolution testing at a baseline thrust of approximately 98.4 micro-Newton demonstrates a minimum resolvable step of 50 nano-Newton, and the measured thrust-noise amplitude spectral density remains below 0.07 micro-Newton/sqrt(Hz) over the 10 mHz-1 Hz band for the tested thrust levels. These results support the feasibility of the proposed integrated cold-gas microthruster head and its device-level validation approach for future space-based gravitational-wave detection applications.

This study investigates the performance of a new compact (55 cm x 56 cm x 48 cm) in situ spin-exchange optical pumping 3He neutron spin filter (NSF) system developed at the China Spallation Neutron Source. The enclosed NSF cell, filled with 3He at 2.53 bar, achieved an initial 3He polarization of approximately 60%. After subsequent improvements in the magnetic field and heating system, this in situ system achieved a 3He polarization of 75.66%+/- 0.09%, resulting in 96.30% neutron polarization at 2 & Aring;. This highly compact in situ system is equipped with self-supportive features, pre-pumping capabilities, polarization maintenance, and a low-noise nuclear magnetic resonance system. These advantages significantly reduce the preparation time and simplify polarized neutron experiments, making it suitable for various neutron beamlines in China, particularly those with a limited sample space. These characteristics establish it as a quasi-standardized system that plays a vital role in polarized neutron experiments, including those involving polarized neutron imaging, neutron reflection, the performance calibration of polarized neutron instruments, and the neutron optics parity and time reversal experiment.

Highly efficient and high-precision measurement of electromagnetic signals is an important issue in both fundamental science and practical application. The demodulation is a widely used technique for receiving and recovering the modulated electromagnetic signals. Here we report the first experimental execution of a quantum demodulator using two heteronuclear ions - $^{40}\text{Ca}^+$ and $^{43}\text{Ca}^+$ - to simultaneously detect the amplitudes and frequencies of unknown periodic signals, breaking the constraint of double-parameters optimization. The $^{40}\text{Ca}^+$ ion serves as the sensor for detecting unknown frequencies, while the $^{43}\text{Ca}^+$ ion focuses on measuring unknown amplitudes. Under the optimization of reinforcement learning, the quantum Fisher information for unknown-frequency estimation scales as \(t^4\), achieving the ``super-Heisenberg'' scaling, while for unknown-amplitude estimation scales as \(t^2\), approaching the Heisenberg scaling. These observations present the practical application of two heteronuclear ions in efficient quantum sensing an highlight the advantage of reinforcement learning in improving quantum control techniques.

To obtain precise and accurate timing signals across a wide area, the technique of fiber-optic time transfer is widely applied. As one of the main concerns in time transfer systems, transmission stability can be improved by applying filtering algorithms, and the Kalman filter (KF) usually plays a key role. The KF can be applied to enhance the precision of the transmission delay measurements within the system, and further improving the accuracy of delay compensation. Although the KF is optimal for linear Gaussian systems, it is not always effective for fiber-optic time transfer due to the nonlinear, nonstationary nature of fiber link time delay drift. The difficulty lies in the fact that the fixed state transition matrix in KF cannot track the unexpected fiber delay drift to achieve unbiased predictions. This limitation poses a potential risk of long-term stability degradation when applied to delay compensation. To address this issue, a time-varying state transition matrix is necessary. This article proposes a solution by introducing a data-driven neural network, specifically the long short-term memory (LSTM) model. The proposed method is experimentally validated over optical fiber transmission links spanning distances from 160 to 1280 km, demonstrating its ability to enhance short-term stability while preserving long-term stability, distinguishing it from the traditional KF. The short-term stability optimization is close to 1/root 2 across three different fiber lengths, which is equivalent to a 3-dB signal-to-noise ratio (SNR) improvement of the received one pulse-per-second (1PPS) signal. This result offers a promising solution for enhancing the precision and reliability of fiber-optic time transfer systems in a variety of applications.

As one of the most precise timekeeping instruments ever developed, the optical clock will be used as the measuring equipment for the next generation of second definition. The demand for the miniaturization of optical clocks is progressively urgent. In this paper, a multi-channel radio frequency (RF) module with a 20% volume of the commercial module is designed and implemented for the transportable 40Ca+ ion optical clock. Based on the double-crystal oscillator interlocking technique, a 1 GHz low-phase noise reference source is developed for direct digital synthesis. Through the simulation and optimization of the signal link design, the frequency range of the low phase-noise RF signal can reach 0–400 MHz with a 4 μHz resolution. Through two-stage power amplifying with different kinds of filters, it can achieve an output power of up to +33 dBm (2 W) at 100 MHz with a 25 dB phase noise lower than the commercial module at 1 Hz, and its third harmonic suppression ratio has been reduced by more than 20 dB at the frequency point of 300 MHz. This multi-channel RF module is used for the power stability and timing control test of a 729 nm clock laser to meet the requirements of the transportable 40Ca+ optical clock. Additionally, this module can also be applied to other quantum systems such as the quantum absolute gravimeter, quantum gyroscopes, and quantum computers.

Accurate monitoring of the horizontal tilt is essential to suppress low-frequency noise in the measurement system of micro-thrust—a requirement for spacebased gravitational wave measurements. Unfortunately, the sensitivity of modern inclinometers is fundamentally limited by the noise floors of their physical and electronic components.To overcome these limitations is a new inclinometer design which consists of a gravitational compound pendulum and a displacement sensor that reach ultrahigh sensitivity. Our inclinometer implements a mechanical amplification mode of a quasi-zero stiffness system.By critically coupling the pendulum’s gravitational restoring moment with the stiffness of a flexure pivot, the system efficiently transforms minute tilt angles into measurable displacements. Theoretical modelling clarifies the mechanics of this angular amplification and guides our strategy for regulating the gravitational restoring moment.Experimental characterization demonstrates a resolution of 0.87 nrad over a 0–109 µrad range, the tunable amplification factor can be increased to exceeding 160 and the nonlinearity error as low as 1.18%. This compact and versatile design provides a strong motivation to deploy ultra-precision tilt measurement on advanced metrology and adaptive devices.

As one of the most accurate instruments in history, the optical clock will be used as the measuring equipment for the next generation of seconds. The demand for miniaturization of optical clock is progressively urgent. In this paper, a multi-channel radio frequency module with a 20% volume of the commercial module is designed and implemented for the transportable 40Ca+ ion optical clock. Based on the double crystal oscillator interlocking technique，1 GHz low-phase noise reference source is developed for direct digital synthesis. By simulation and optimization of the signal link design, a frequency range of the low-phase noise RF signal can reach 0-400 MHz with a 4 μHz resolution. Through two-stage power amplified with different kinds of filters, it can get an output power up to +33 dBm (2 W) at 100 MHz with a 25 dBc/Hz phase noise lower than the commercial module at 1 Hz, and its third harmonic suppression ratio has been reduced by more than 20 dBm at the frequency point of 300 MHz. This multi-channel RF module is used for the power stability and timing control test of 729 nm clock laser, to meet the requirements of the transportable 40Ca+ optical clock. Without this, this module can also be applied to other quantum systems such as quantum absolute gravimeter, quantum gyro and quantum computer.

This study introduces proposes a new closed-loop dynamic micro-thrust measurement method based on torsional pendulum, addressing the poor real-time performance and insufficient precision inherent in traditional open-loop techniques that rely on extreme displacement values to estimate thrust. This method achieves proportional-integral-derivative (PID) closed-loop control through electromagnetic force compensation, tracking the thrust to be measured in real-time. The measurement results are directly inverted from the compensation force. Experimental results show that the dynamic thrust of the electromagnetic calibration system within 0.8 Hz is highly consistent with the calibration results of a high-precision balance with R-2=0.99895. In closed-loop mode, the phase lag for 20- mu N dynamic thrust measurement is only 0.5425 degrees, with a root-mean-square error (RMSE) as low as 1.82%. The system's expanded uncertainty is 1.79% below the intrinsic frequency of 11.57 mHz, providing a reliable ground-based verification solution for spacecraft drag-free control and high-frequency dynamic thrust measurement.

Objective In microgravity, atoms can be cooled to very low temperatures, manipulated by a trap with a novel topology structure, and observed over long timescales. This phenomenon has garnered considerable attention, leading to exploration of ultracold atomic physics and its applications in microgravity. Over the past two decades, various state-of-the-art ground-based microgravity facilities and highly reliable ultracold atomic physics experimental systems have been developed to explore the lower temperature limit and applications of cold atoms in microgravity. However, space-based platforms, such as sounding rockets and space stations, have evolved into ideal environments because of their long free-fall time and stable microgravity environment. With the development of the Chinese Space Station (CSS), a Cold Atom Physics Rack (CAPR) that uses an all-optical approach has been deployed to investigate low-temperature and novel physical phenomena in microgravity based on the ultracold quantum degenerate gas of Rb-87 Bose-Einstein condensate (BEC). In addition, the CAPR serves as an open experimental platform for studying ultracold atomic physics and performing precision measurements in microgravity, with the major aim of cooling atoms at the pico-Kelvin scale through two-stage crossed beam cooling (TSCBC). Methods The CAPR needs to satisfy the restrictions on its size, weight, and power consumption. In addition, it needs to withstand the vibrations and impact during its launch as well as operate well after the launch. A highly reliable and integrated CAPR that integrated all the hardware for preparing, manipulating, and probing the Rb-87 BEC was designed. The designed CAPR included a physical system, a cooling laser system, an optical trap and lattice laser system, an electronic control unit, and a rack supporting system with dimensions of 1820 mmx1050 mmx815 mm. The dimensions and mass of the assembled physical system were approximately 590 mmx930 mmx510 mm and 170 kg, respectively. This system could provide a high-vacuum, optical, and magnetic environment for ultracold atoms. The cooling laser system consisted of a repumping laser, cooling laser, and probing laser, which provided three high-power outputs for cyclic cooling of Rb-87 atoms to temperatures of tens of microkelvins as well as for detecting the atoms. The optical trap and lattice laser system provided eight high-power outputs for evaporative cooling to attain the BEC, deep cooling via TSCBC, and manipulation of the ultracold atoms in the optical lattice. The electronic control unit controlled the experimental sequences as well as stored the experimental results and engineering parameters. The sizes and weights of the laser cooling system, optical trap and lattice laser system, and electronic control unit were similar (550 mmx470 mmx270 mm and less than 50 kg, respectively). To achieve the mission target, BEC and TSCBC tests were conducted on the ground before the launch.The realization of the Rb-87 BEC and the TSCBC were crucial and confirmed that the output of all the subsystems fulfilled the experimental requirements for the preparation, regulation, and detection of ultracold atoms. Results and Discussions The vacuum apparatus is the main part of the physical system and includes a two-dimensional magneto-optical trap (2D-MOT) chamber and science chamber for atomic cooling, manipulation, and probing. In addition, all the magnetic coils and optical modules, which provide the required magnetic and optical fields for the ultracold atoms, are fixed on the vacuum chambers. In the laser cooling system, the powers of the repumping, cooling, and probing lasers are 200,600,800 mW,respectively. The repumping laser is locked to the Rb-87 D2 |5(2)S(1/2), F=1 >->|5(2)S(3/2), F'=0,1 > crossover transition via modulation transfer spectroscopy (MTS), which is 193 MHz red-detuned from the repumping transition. The frequencies of the cooling and probing lasers are red-detuned by a few natural linewidths (Gamma=2 pi x6.065(9) MHz, which is the natural linewidth of the Rb-87 D2 line)from the Rb-87 D2 |5(2)S(1/2), F=2 >->|5(2)S(3/2), F'=3 > transition. The MOT loading process takes 10 s and more than 1.5x10(9 )atoms can be trapped with a temperature below 500 mu K. Furthermore, the atoms can be cooled to a temperature below 30 mu K using optical molasses, demonstrating the performance of the 780 nm cooling laser system. As to the optical trap and lattice laser system, the capability of the tight-confining laser is confirmed by loading more than 1.2x10(6) atoms and successfully cooling more than 1x10(5 )atoms via evaporative cooling to the BEC at a temperature below 30 nK. The performance of the loose-confining laser is verified by deeply cooling the ultracold atoms to 2.4 nK via TSCBC. Additionally, the CAPR performs well in space environmental qualification certification tests. Conclusions The CAPR flight model (FM) was installed in the Mengtian laboratory module, which was launched into the CSS on October 31,2022. The CAPR investigates low-temperature and novel physical phenomena in microgravity based on the quantum degenerate gas of Rb-87 BEC. Here, we report the design of the integrated CAPR, which includes a physical system, a cooling laser system, an optical trap and lattice laser system, an electronic control unit, and a rack supporting system. Ground based experiments have been conducted to confirm the ability of the CAPR to realize the Rb-87 BEC and lower its temperature from 30 nK to 2.4 nK with the TSCBC.

Quantum heat engines and refrigerators are open quantum systems, whose dynamics can be well understood using a non-Hermitian formalism. A prominent feature of non-Hermiticity is the existence of exceptional points (EPs), which has no counterpart in closed quantum systems. It has been shown in classical systems that dynamical encirclement in the vicinity of an EP, whether the loop includes the EP or not, could lead to chiral mode conversion. Here, we show that this is valid also for quantum systems when dynamical encircling is performed in the vicinity of their Liouvillian EPs (LEPs), which include the effects of quantum jumps and associated noise-an important quantum feature not present in previous works. We demonstrate, using a Paul-trapped ultracold ion, the first chiral quantum heating and refrigeration by dynamically encircling a closed loop in the vicinity of an LEP. We witness the cycling direction to be associated with the chirality and heat release (absorption) of the quantum heat engine (quantum refrigerator). Our experiments have revealed that not only the adiabaticity breakdown but also the Landau-Zener-Stückelberg process play an essential role during dynamic encircling, resulting in chiral thermodynamic cycles. Our observations contribute to further understanding of chiral and topological features in non-Hermitian systems and pave a way to exploring the relation between chirality and quantum thermodynamics.

The Efimov effect and its universal property are of paramount importance in quantum few-body physics. Despite this, the predicted ground state Efimov resonance has not yet been observed in 39,40,41K-87Rb mixtures within the currently available observation window. Cooling atoms in the microgravity environment of outer space might overcome this limitation, whereas the residual curvature of the strong magnetic fields may result in significant atom leakage. In this work, we propose an optical method based on far-detuned time-averaged dipole potential to counteract the three-dimensional inhomogeneous field. The target intensity distribution can be conveniently obtained by modulating the central position of the quasi-1D print beam using acoustic optical modulators. Within a volume of 300 × 300 × 400 µm3, the residual potential fluctuations can be reduced by two orders of magnitude to less than 100 pK. The proposed approach offers a realistic prospect of investigating the Efimov-type resonance in the 40K-87Rb Bose-Fermi mixture.

The performance of real-time precise point positioning (PPP) relies primarily on the availability and quality of orbit and clock corrections. In this research, we collected data streams from 12 real-time mount points of IGS Real-Time Service (RTS) or analysis centers for a one-month period and conducted a performance assessment, including product latency and data availability, accuracy of orbit, clock and positioning performance. The epoch availability of GPS, GLONASS, Galileo and BDS was more than 98.5%, 95.79%, 94.20% and 85.9%, respectively. In addition, the orbit and clock errors of different real-time corrections was investigated. Then, PPP in static and kinematic for 16 IGS stations was conducted. The results show the real-time PPP for different products has a longer convergence time and a slightly worse accuracy than those of the post-processing PPP. For static PPP over 24 h, the real-time products of WHU had the best performance, with a mean RMSE of 1.0 cm in the horizontal and vertical directions and a median convergence time of 12.0 min. The products of CAS had the faster convergence speed due to the shortest product latency. Regarding real-time kinematic PPP for GPS only in an hourly batch, the real-time products of WHU and ESA performed best with a mean RMSE of 10.8 cm and 9.5 cm in the horizontal and vertical directions, respectively. Additionally, the PPP for different real-time products with the multi-GNSS combination obtained higher accuracy than those with GPS only in post-processing or real-time mode, and the PPP with the GPS/GLONASS/Galileo/BDS combination had the fastest convergence speed and best positioning performance. The hourly based kinematic PPP results of CAS, DLR, GFZ and WHU with the GREC combination had positioning errors smaller than 5.2 cm.

We experimentally measured the ultra-narrow momentum width of an optical trapped Bose-Einstein condensate (BEC) in situ based on matter-wave interference, which validates our previous theoretical work [arXiv: 2205.02416]. By sweeping the interval of double stand-wave pulses, the BEC wave packet was splitted into different diffraction orders and then we counted the oscillation curve of the population of zero-momentum state to calibrate the momentum width. Compared with our simplified theory, we observed an accelerated evolution of interference fringes in time-domain. We evaluated this interference process minutely by numerically calculating the Gross-Pitaevskii equation and using Wigner function to intuitively demonstrate the influence of the external potential and nonlinear term. We confirmed that the reduction of interference fringe evolution period actually originates from the synergistic cooperation of the mean-field interaction of the BEC and spatial density modulation caused by the interference between different momentum states. Our approach could be generalized to other ultra-cold atomic gases with different momentum distributions, and in principle a single shot can obtain the result. This quantum thermometry is particularly suitable for momentum width calibration in practical deep cooling experiments, while for atomic samples at pK level the mean-field interaction can be safely ignored.

In the development of the Cold Atom Physics Research Rack (CAPR) on board the Chinese Space Station, the laser system plays a critical role in preparing the all-optical 87 R b Bose-Einstein condensates (BECs). An all-fiber laser system has been developed for CAPR to provide the required optical fields for atom interaction and to maintain the beam pointing in long-term operation. The laser system integrates a 780 nm fiber laser system and an all-fiber optical control module for sub-Doppler cooling, as well as an all-fiber 1064 nm laser system for evaporative cooling. The high-power, single-frequency 780 nm lasers are achieved through rare-Earth doped fiber amplification, fiber frequency-doubling, and frequency stabilization technology. The all-fiber optical control module divides the output of the 780 nm laser system into 15 channels and regulates them for cooling, trapping, and probing atoms. Moreover, the power consistency of each pair of cooling beams is ensured by three power tracking modules, which is a prerequisite for maintaining stable MOT and molasses. A high-power, compact, controlled-flexible, and highly stable l064 nm all-fiber laser system employing two-stage ytterbium-doped fiber amplifier (YDFA) technology has been designed for evaporative cooling in the optical dipole trap (ODT). Finally, an all-optical 87 R b BEC is realized with this all-fiber laser system, which provides an alternative solution for trapping and manipulating ultra-cold atoms in challenging environmental conditions.