We present the design and implementation of a fringe detection pipeline for the Lunar Orbit VLBI Experiment (LOVEX) on board China’s Queqiao-2 cislunar relay satellite. To address the challenge posed by limited space–ground time synchronization accuracy, we developed a dedicated initial clock offset search scheme based on a modified version of the Chinese VLBI Network software correlator. Key features of the pipeline include raw data validation, wide-range clock offset search, iterative fringe fitting, and GPU-accelerated processing. The pipeline was successfully applied to the initial clock offset determination in LOVEX observations. In particular, we report the detection of fringes for blazar AO0235+164 in the s5123m session, where a projected baseline length of 5.5 Earth diameters was achieved. We further discuss technical approaches for enhancing sensitivity to weak signals, with an emphasis on the need for advanced phase calibration techniques. The potential application of tensor core acceleration is also discussed.

In this work, we propose and experimentally demonstrate hybrid transmission of ultra-stable optical frequency reference and quantum key distribution (QKD) over homogeneous MCF. The scheme achieves optical frequency stabilization at the remote site by beating the loop-back frequency signal with a local reference to extract link phase noise, which is then compensated via voltage-controlled oscillator (VCO) feedback. Prior to the experiment, we first analyze the critical challenges in MCFs including inter-core crosstalk and inter-core spontaneous Raman scattering. To minimize the crosstalk between the signals, the quantum signal and the optical frequency propagate through separate fiber cores. Successful hybrid transmission of optical frequency references and QKD is achieved across 20km seven-core fiber. The frequency instability of 1.47x10(-17 )at 1s and 5.06x 10(-19) at 10000s, and secure key rate of 317.28 kbps can be realized in this integrated transmission system. The scheme benefits from two merits, ultra-stable optical frequency offers high-precision clocks to QKD, and QKD provides one-time-pad keys to encrypt the time data. These results demonstrate the feasibility and tolerance of our QKD system with high-precision frequency reference and the viability of multi-core fibers for next-generation quantum networks.

The advent of artificial intelligence, cloud services, and big data applications has propelled the evolution of next-generation high-capacity datacenters. It is highly anticipated that coherent detection will penetrate further into datacenters in the next decade. However, the large number of tunable lasers in data centers makes the optical module structure complex and adds additional thermal power consumption. Meanwhile, optical frequency combs serve as high-precision frequency resolution and wide spectral coverage lasers, holding tremendous potential for the field of wavelength division multiplexing optical communication. Here, we present an innovative global frequency-synchronous optical network (global-FSON) architecture for coherent short-reach optical interconnects by synchronizing optical frequency combs to a global positioning system disciplined oscillator (GPSDO) and distributing them. The global-FSON architecture can share GPS-referenced optical frequency combs as master lasers among different servers, which eliminates the need for a massive number of tunable lasers in datacenter optical interconnects. On this basis, we achieve a reset-free carrier phase recovery analog coherent receiver in the optical domain and demonstrate dual-polarization coherent signals demultiplexing without coherent silicon application-specific integrated circuits. We introduce the global-FSON architecture that provides a laser source with absolute stability for all transponders in datacenter coherent optical interconnects. Its implementation would bolster the potential applicability of coherent optical communication in next-generation datacenter coherent optical interconnects.

We present a quantum computing framework for VLBI data correlation. We point out that a classical baseband time series data of length N can be embedded into a quantum superposition state using amplitude encoding with only log_2 N qubits. The basic VLBI correlation and fringe fitting operations, including fringe rotation, Fourier transform, delay compensation, and cross correlation, can be implemented via quantum algorithms with significantly reduced computational complexity. We construct a full quantum processing pipeline and validate its feasibility and accuracy through direct comparison with a classical VLBI pipeline. We recognize that amplitude encoding of large data volumes remains the primary bottleneck in quantum computing; however, the quantized nature of VLBI raw data helps reduce the state-preparation complexity. Our investigation demonstrates that quantum computation offers a promising paradigm for VLBI data correlation and is likely to play a role in future VLBI systems.

We demonstrate a free-space co-transmission of optical frequency and communication signals at the same wavelength. The system achieves 100-Gb/s DP-QPSK signals over free-space link with the optical frequency instability of 1.47×10-17@1s and 7.61×10-19@1000s.

Our weekly cadence radio monitoring campaign captured a bright flare in 2025 from the microquasar GRS 1915+105, observed simultaneously in the S- and X-bands (2.25 and 8.42 GHz) with a short single baseline of two radio telescopes in Shanghai. Through high time resolution analysis, we detected a significant and short-lived quasiperiodic oscillation (QPO) at similar to 0.03 Hz and its harmonic (similar to 0.06 Hz) in both radio bands of two consecutive observations on MJD 60765 (>5.9 sigma) and MJD 60772 (2.8 sigma). Crucially, the QPO frequency is identical in both radio bands and matches oscillations detected in previous years. The recurrence and wavelength independence of the QPO frequency suggest an intrinsic characteristic timescale of the accretion-jet system.

Information and communication technology has continuously driven the demand for higher data transmission rates. At the same time, frequency synchronization technology also needs to continually adapt to the high-precision frequency references required between equipment in high-speed optical communication systems. However, existing time and frequency transmission technologies, which rely on the hardware-timestamp functions specified in IEEE1588, cannot meet the accuracy requirements for Precision Time Protocol (PTP) devices in 5G + or future 6G communications. Multi-core fiber, with its characteristics such as multi-channel transmission, superior symmetry, high integration, and versatility, is poised to become the preferred choice for next-generation communication fibers. There is a need to investigate the co-transmission of RF references and data signals based on multi-core fiber to further expand the capacity of communication data transmission and provide precise RF references for 5G + and future 6G communications. This paper proposes and experimentally demonstrates a novel approach for RF clock references and data signals co-transmission over a seven-core fiber on the same wavelength. By inserting an RF standard tone into the data signal spectrum through spectral modulation, we achieve co-transmission of a 10-MHz RF standard and 224-Gb/s dual-polarization 16-QAM signals over 1 km and 10 km seven-core fiber links based on frequency-synchronous optical network (FSON) architecture. The RF and data signals are received and demultiplexed entirely in the optical domain using a radio frequency and data signal demultiplexing (RFDSD) module. The measured 10-MHz frequency stability over 1 km and 10 km seven-core fiber links is better than commercial rubidium atomic clocks and it demonstrates the potential for picosecond-level clock dissemination within short-reach optical interconnects scenario. This work shows good performance in coherent demultiplexing of 224-Gb/s DP-16QAM signals with all tributaries demultiplexed below the 7% FEC threshold at receiver optical power levels of -19 dBm and − 18.5 dBm for 1 km and 10 km seven-core fiber links, respectively. Our approach provides a promising solution and theoretical foundation for next-generation high-speed, high-capacity, picosecond-level physical delay short-reach coherent optical interconnect applications.

The 1.3 mm ground-based very long baseline interferometry (VLBI) array Event Horizon Telescope (EHT), is limited by Earth's diameter, restricting its black hole shadow imaging to only M87* and Sgr A*. Extending baselines to the Moon would achieve 0.7 uas angular resolution at 230 GHz, enabling shadow detection for a much larger sample of supermassive black holes (SMBHs). The concept is motivated by space VLBI missions and lunar exploration, including the ongoing Lunar Orbit VLBI EXperiment (LOVEX) aboard QueQiao-2 (Chang'E-7) and the planned International Lunar Research Station (ILRS). We assess shadow detectability for 31 SMBHs with predicted large angular sizes, exploring different telescope location and antenna size. Assuming a telescope at the lunar antipode, we simulate the Moon-Earth (u,v) coverage and show that sources with direction near the Moon's orbital plane yield projected baselines spanning from short to long, enabling sampling of the first visibility null - a key shadow signature. Using a geometric ring model, we identify six shadow-detectable candidates for Moon-Earth VLBI. Among these, M104, NGC 5077, and NGC 1052 are detectable with a 5 m lunar-based telescope; PGC 049940 requires 10 m; NGC 524 requires 20 m; and NGC 5252 requires 40 m. Furthermore, if space telescopes fill the baseline coverage gaps between Moon and Earth, the n=2 photon ring region is detectable for Sgr A*, M87* with a 10 m lunar-based telescope, and 12 candidates are detectable for the n=1 photon ring region using a lunar-based telescope of up to 40 m. These results provide a clear scientific and technical motivation for lunar-based telescopes in future black hole shadow studies.

Space Very Long Baseline Interferometry (SVLBI) can significantly improve angular resolution by extending interferometric baselines beyond Earth's diameter. China is actively promoting the development and scientific applications of SVLBI, including the proposal to deploy a radio telescope on the lunar surface through the International Lunar Research Station (ILRS), as well as the long-term goal of establishing a cislunar SVLBI array. In this research, the \enquote{lunar far side configuration} refers to placing a telescope directly at the lunar south pole, while the \enquote{multiple space configuration} broadly encompasses telescopes in lunar orbit, in Earth orbit, and at equilibrium points such as the Earth-Moon L2 or Sun-Earth L2. Based on detailed simulations with multiple configurations derived from the Event Horizon Telescope (EHT) ground network, we systematically evaluate the uv coverage and angular resolution of the proposed SVLBI array. The results show that: (1) in the lunar far side configuration, deploying a telescope extends the baseline and improves uv coverage, but only yields limited gains in angular resolution; (2) in the multiple space configuration, deploying telescopes achieves substantial improvements in both $uv$ coverage and angular resolution, owing to the significantly longer baselines. These findings provide important theoretical and technical support for the future development of SVLBI in China.

The Lunar Orbital VLBI Experiment (LOVEX) is a scientific component of the Chinese Lunar Exploration Project (CLEP) Chang’E-7. The spaceborne component of LOVEX is implemented onboard the relay satellite QueQiao-2, which was launched on 20 March 2024, and later placed into an elliptical selenocentric orbit. The LOVEX-specific payload consists of an X-band cryogenic receiver, a hydrogen maser frequency standard, and VLBI data formatting and acquisition electronics. Several components of the QueQiao-2 nominal onboard instrumentation, such as the 4.2-m antenna, the data storage device, and the downlink communication system, contribute to the overall spaceborne VLBI instrumentation. This allows us to form a space radio telescope capable of co-observing with Earth-based radio telescopes in VLBI mode. In this space VLBI system, the length of the baseline extends up to approximately 380000 km. This paper presents the LOVEX scientific objectives, architecture, instrumentation, prelaunch tests, in-flight verification and calibration, and the first in-flight detections of interferometric response (“fringes”) achieved through observations of the quasar AO 0235+164 and the Chang’E-6 orbital module, positioned at the Sun-Earth Lagrange point L2. These initial results demonstrate the successful performance of LOVEX, verifying its capability for both astronomical and spacecraft tracking observations at ultra-long VLBI baselines.

For some space radio telescopes,the orbit determination accuracy is not high enough,the time synchronization accuracy provided by the satellite platforms is low,and GNSS devices are not available.As a result,a traditional method that relies on GNSS devices to obtain an initial clock offset followed by performing correlation with the calibration source may fail to obtain fringes.Moreover,a brutal force search across the 2D clock offset and fringe rate search plane is computationally expensive.In light of these challenges,we propose a novel time synchronization method that utilizes the spacecraft's telemetry tone signal.This method employs frequency polynomials derived from Doppler tracking for fringe rotation during the correlation process.By aligning the frequency of the target station precisely with that of the reference station,it is only necessary to split the clock offset search range into multiple time windows,perform correlation for each window,and identify the window with the highest signal-to-noise ratio(SNR).The precise clock offset is determined by combining the residual delay with the initial offset.To validate the method,we observe the Tianwen-1 telemetry signal with the 4.5 m small telescope in the Tianma campus of Shanghai Astronomical Observatory and 40 m telescope in Kunming.The results demonstrate that our method can accurately determine clock offset for a time range as wide as±10 ms,with an SNR slightly higher than that achieved with the delay model.This method is suitable for wide-range time synchronization for space Very Long Baseline Interferometry observations,especially in scenarios involving small antennas with low sensitivity and poor orbit determination accuracy.

We report the first experimental demonstration hybrid transmission of ultra-stable optical frequency references and quantum key distribution (QKD) signals over a homogeneous multi-core fiber (MCF). The frequency transfer system achieves frequency stabilization at the remote site via active phase noise compensation. A QKD system implementing the BB84 phase-encoding protocol with decoy states is integrated within the platform. Successful hybrid transmission of optical frequency references and quantum signals is achieved over 10 km 7-core fiber. The integrated transmission system achieves a frequency stability of 8×10−18 at 1s and 3.45×10−19 at 10000 s, with a quantum bit error rate (QBER) of 2.54%.

The Lunar Orbital VLBI Experiment (LOVEX) aims to utilize the 4.2-m diameter antenna on the Queqiao-2 relay satellite of the Chang’E-7 mission, which is equipped with an X-band cryogenic receiver, an H-maser, and a VLBI data acquisition backend, thereby forming a space radio telescope in lunar orbit. The lunar orbital telescope will collaborate with Earth-based telescopes to conduct VLBI observations, thus forming a lunar-Earth space VLBI network. The length of the baseline will extend up to approximately 380000 km, which will be the longest VLBI baseline to date. This paper introduces the LOVEX VLBI data correlation system. The system is capable of generating VLBI delay models for both the lunar orbital and Earth-based telescopes; it also performs the initial clock search within a large clock offset window ranging from −10 ms to +10 ms for the lunar-Earth baseline leveraging both wideband blazar signals and spacecraft (SC) differential one-way ranging (DOR) signals—a distinctive feature of the system. To fulfill the scientific objectives of astrophysics, astrometry, and orbit determination of SC, the data correlation system outputs visibilities in various formats. Additionally, the system can directly output the VLBI residual delay and delay rate of SC after bandwidth synthesis. Anticipating the inclusion of more Earth-based stations in the future, the system is designed with the capability to correlate digital signals observed by 10 stations, with a bandwidth of 512 MHz and dual-polarization, running on an off-the-shelf central processing unit (CPU) + graphics processing unit (GPU) cluster. This system was applied to both the ground verification system and the first LOVEX observation experiment, and successfully detected VLBI fringes for signals from blazars and SC after fringe fitting.

SummaryWe propose and experimentally demonstrate a hybrid transmission scheme that combines radio frequency standards with coherent optical communication on a same wavelength over 1-km 7-core fiber. We use a pilot tone assisted Costas Loop (PACL) module for DSP-free polarization and coherent demultiplexing of 224-Gb/s DP-16QAM signals in the optical domain based on frequency-synchronous optical network (FSON). This PACL module can also be employed for frequency standards comparison between the transmitter and the receiver. The system achieves an Allan deviation (ADEV) of 4.7×10-12 of 10-MHz radio frequency at 1s and 6.8×10-15 at 10000s, which can support frequency standards comparison at the level of commercial rubidium atom clock.

The filter bank multi-carrier with offset quadrature amplitude modulation (FBMC-OQAM) waveform demonstrates strong anti-interference capabilities due to its sub-carrier-level filtering. As a result, it is increasingly being adopted for underwater acoustic communication (UWAC) in challenging transmission environments. This paper presents a closed-form analytical expression for the bit error rate (BER) performance of FBMC-OQAM in shallow water, long-distance horizontal communication scenarios. The expression considers the residual Doppler frequency offset, multipath propagation, and ambient noise characteristic of underwater acoustic channels, accounting for the limitations of traditional non-uniform Doppler coarse compensation methods. Theoretical numerical results and Monte Carlo simulation results show that under any number of symbols, the derived closed-form expression consistently exhibits high computational accuracy, when facing the impact of Doppler spread or compression at any scale.

We demonstrate a simultaneous transmission of time-frequency and data over a 160-km urban business network in Shanghai. The signals are transmitted through a cascaded optical link consisting of 48 km and 32 km, which are connected by an optical relay. The metrological signals are inserted into the communication network using dense wavelength division multiplexing. The influence of the interference between different signals has been discussed. The experimental results demonstrate that the radio frequency (RF) instability can reach 2.1×10-14 at 1 s and 2.3×10-17 at 10,000 s, and the time interval transfer of one pulse per second (1 PPS) signal with less than 10 ps at 1 s is obtained. This work paves the way for the widespread dissemination of ultra-stable time and frequency signals over the communication networks.

Future inter-satellite clock comparison on high orbit will require optical time and frequency transmission technology between moving objects.Here,we demonstrate robust optical frequency transmission under the condition of variable link distance.This variable link is accomplished by the relative motion of a single telescope fixed on the experimental platform to a corner-cube reflector(CCR)installed on a sliding guide.Two acousto-optic modulators with different frequencies are used to separate forward signal from backward signal.With active phase noise suppression,when the CCR moves back and forth at a constant velocity of 20 cm/s and an acceleration of 20 cm/s2,we achieve the best frequency stability of 1.9 × 10-16 at 1 s and 7.9 × 10-19 at 1000 s indoors.This work paves the way for future studying optical frequency transfer between ultra-high-orbit satellites.

In the Chang'E-7 mission of the fourth stage of China's Lunar Exploration Project, the relay satellite will carry a 4.1-m aperture X-band parabolic telescope. This and the ground-based telescopes will form a lunar orbit very long baseline interferometry (VLBI) experimental system with the space-ground baseline up to 400000 km. To improve the sensitivity of this baseline, we propose a weighted full-spectrum signal combining method. First, we derive the optimal weighting coefficient ratio for combining the signals of the two telescopes. Then, we propose the VLBI weighted full-spectrum combining method whose optimal weight coefficient is determined by investigating the data, and the obtained result is consistent with the theoretical derivation. Finally, we use Chang'E-4 original VLBI observation data for method testing. The results show that the signal combining method proposed in this work can greatly improve the signal-to-noise ratio for VLBI observations.

SummaryWe demonstrate an ultra-precision optical frequency transfer scheme over 70 m free-space. The space link is extended by an optical angular reflector which is placed on a mobility platform. The mobility platform is driven by a computer numerically controlled electro-motor and moves the reflector back and forth. Two different acoustic optical modulators (AOM) with different frequencies are used to separate forward and backward signals. When the transfer system is stabilized, the transferred modified Allan deviation (MDEV) of optical frequency can be 2.7E-16@1s and 1.3E-18@4000s.