The synchronisation of time and frequency between remote locations is crucial for many important applications. Conventional time and frequency dissemination often makes use of satellite links. Recently, the communication fibre network has become an attractive option for long-distance time and frequency dissemination. Here, we demonstrate accurate frequency transfer and time synchronisation via an 80 km fibre link between Tsinghua University (THU) and the National Institute of Metrology of China (NIM). Using a 9.1 GHz microwave modulation and a timing signal carried by two continuous-wave lasers and transferred across the same 80 km urban fibre link, frequency transfer stability at the level of 5×10−19/day was achieved. Time synchronisation at the 50 ps precision level was also demonstrated. The system is reliable and has operated continuously for several months. We further discuss the feasibility of using such frequency and time transfer over 1000 km and its applications to long-baseline radio astronomy.

Tetrapod gold nanocrystals, to be the core of Surface-enhanced Raman spectroscopy (SERS) nanoprobes, with tunable localized surface plasmon resonance (LSPR) from 650 nm to 785 nm in Vis-NIR region have been successfully prepared by a facile seeded growth approach. The local electromagnetic field distribution and their huge extinction cross section of tetrapod gold nanocrystals were simulated by finite-difference time-domain method. Both the calculated and experimental results reveal that the LSPR property of tetrapod gold nanocrystals is closely dependent on the morphology features of their tips where strong field enhancement appears. These tetrapod nanocrystals have exhibited good capability not only for Raman signal enhancement but also successfully utilizing as NIR SERS bioimaging nanoprobes. In vitro SERS imaging of stained breast cancer cells has been also demonstrated. The tetrapod gold nanocrystals developed here with precisely tunable LSPR offer advantages of enhanced signal quality, good stability and better biocompatibility in SERS imaging, which has great potential for various biomedical applications.

We demonstrate a fiber-based multiple-access ultrastable frequency dissemination scheme over an 83 km fiber link. As a performance test, we reproduce the disseminated 9.1 GHz radio-frequency modulation signal at an arbitrary point in the dissemination channel. Relative frequency stability of 7×10(-14)/s and 5×10(-18)/day is obtained. Highly synchronized frequency signals can be regenerated along the entire fiber pathway and its applications are discussed.

The Square Kilometre Array (SKA) project is an international effort to build the world’s largest radio telescope, with a one-square-kilometre collecting area. In addition to its ambitious scientific objectives, such as probing cosmic dawn and the cradle of life, the SKA demands several revolutionary technological breakthroughs, such as ultra-high precision synchronisation of the frequency references for thousands of antennas. In this report, with the purpose of application to the SKA, we demonstrate a frequency reference dissemination and synchronisation scheme in which the phase-noise compensation function is applied at the client site. Hence, one central hub can be linked to a large number of client sites, thus forming a star-shaped topology. As a performance test, a 100-MHz reference frequency signal from a hydrogen maser (H-maser) clock is disseminated and recovered at two remote sites. The phase-noise characteristics of the recovered reference frequency signal coincide with those of the H-maser source and satisfy the SKA requirements.

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