As one of the core carriers of the modern high-precision frequency standard, the research on time-keeping cesium clocks holds strategic significance for the construction of national time-frequency systems. It not only provides nanosecond-level synchronization capabilities for the frontier fields such as BeiDou navigation, high-speed communication and deep space exploration and but also serves as a critical cornerstone for safeguarding national information security and advancing the independent controlment of science and technology.
The CAP group of Peking University’s Institute of Quantum Electronics is one of the first groups to develop a time-keeping optically pumped cesium beam microwave clock in China. By leveraging interaction between microwave and Ramsey interference technology, the clock frequency is locked on the hyperfine energy level transition spectral line of the ground state of cesium atoms, which is also the foundation of the modern definition of the second. With the support of a number of major national projects and after nearly 20 years of intensive exploration, the frequency stability has been certified by the China Institute of metrology as short-term @1s, long-term @100,000s. It has achieved five times more than the cesium clock (5071A high-performance cesium tube) used in the U.S. GPS, which has reached the leading level in the field of international time-keeping cesium clock. The optically pumped microwave cesium clock developed by the group successfully breaks through the stranglehold technology of ultra-high-precision atomic clock. It establishes a time-frequency reference system with completely independent intellectual property rights for major projects such as the new generation of satellite navigation, deep space exploration and quantum sensing.
Recent works:
Recently, our group is making some research on using two lasers with different frequencies, that is, two-laser optical pumping, to significantly improve the atomic utilization efficiency and further explore the performance potential of optically pumped cesium clock.
At present, the two-laser pumping mode adopted by our group is one laser of transition for pumping and the other laser of transition can exploit the forbidden transition between energy levels to drive the atoms to converge into the target Zeeman sublevel. By optimizing the frequency, polarization, light intensity and other parameters of the two lasers, the population of atoms in the clock transition state can reach 100% theoretically, which increases the maximum atomic utilization efficiency by 7 times. Consequently, the frequency stability limit of entire clock can be improved by 2-3 times. The breakthrough of this technology is expected to achieve a high-precision cesium clock with short-term stability comparable to hydrogen clock while maintaining superior long-term stability.
A novel optical lattice microwave clock scheme based on neutral cold atomic clusters combines optical lattice manipulation of atomic external momentum states with microwave Ramsey interference. This approach simultaneously achieves a high signal-to-noise ratio from a large population of atoms and ultra-narrow spectral linewidth from extended free evolution time.
In the ground environment, cold atomic clusters are rapidly prepared (10 milliseconds) via electromagnetically induced transparent cooling and loaded into an optical lattice. A Bloch laser transfers atomic momentum adiabatically and manipulates the external momentum state of cold atoms. This method can offset the free fall caused by gravity, which allows atoms to maintain a similar "suspension" effect for a long time within centimeter-scale in the center of the cavity. It can also maintain atomic coherence of atomic internal states for Ramsey interference, enabling an ultra-long free evolution time of the order of seconds. The linewidth of Ramsey fringes can be narrowed to the order of 0.1Hz. Combined with the high signal-to-noise ratio brought by a large number of atoms participating in the transition, it is projected to achieve a frequency stability of , and a long-term stability of the order of . This frequency stability index is equivalent to the current best large fountain clock (meter-scale height), while the system volume is reduced by an order of magnitude.