Time Transfer in a 1839-km Telecommunication Fiber Link Demonstrating a Picosecond-Scale Stability

An implementation of high-precision time transfer over a 1839-km field fiber loop back link between two provincial capitals of China,Xi’an and Taiyuan,is reported.Time transfer stabilities of 6.5 ps at averaging time of 1 s and 4.6 ps at 40000 s were achieved.The uncertainty for the time transfer system was evaluated,showing a budget of 56.2 ps.These results stand for a significant milestone in achieving high-precision time transfer over a field fiber link spanning thousands of kilometers,signifying a record-breaking achievement for the real-field time transfer in both stability and distance,which paves the way for constructing the nationwide high-precision time service via fiber network.

Atomic optical spatial mode extractor for vector beams based on polarization-dependent absorption

Vector beams with spiral phase and spatially varying polarization profiles have many applications from optical micromanipulation to materials processing. Here, we propose and demonstrate an atomic spatial mode extracting scheme for the vector beam based on polarization-dependent absorption in the atom vapor. By employing the linear polarization pump beam which induces polarization sensitive absorption in the atomic ensemble, a counter-propagated weak probe vector beam is extracted by spatial absorption, and extracted part still maintains the original polarization and the vortex phase.The topological charges of the extracted mode are verified by interfering with the Gaussian beam, and it can be found that the orbital angular momentum is conserved in the extracting process. Our work will have potential applications in non-destructive spatial mode identification, and is also useful for studying higher-dimensional quantum information based on atomic ensembles.

Coherent Optical Frequency Transfer via a 490 km Noisy Fiber Link

We demonstrate the coherent transfer of an ultrastable optical frequency reference over a 490 km noisy field fiber link.The fiber-induced phase noise power spectrum density per-unit-length at 1 Hz offset frequency can reach up to 510 rad^(2)·Hz^(-1)·km^(-1),which is much higher than the fiber noise observed in previous reports.This extreme level of phase noise is mainly due to the fiber link laying underground along the highway.Appropriate phaselocked loop parameters are chosen to complete the active compensation of fiber noise by measuring the intensity fluctuation of additional phase noise and designing a homemade digital frequency division phase discriminator with a large phase detection range of 2^(12)πrad.Finally,a noise suppression intensity of approximately 40 d B at1 Hz is obtained,with fractional frequency instability of 1.1×10^(-14 )at 1 s averaging time,and 3.7×10^(-19) at10000 s.The transfer system will be used for remote atomic clock comparisons and optical frequency distribution over a long-distance communication network established in China.

Surpassing the classical limit of the microwave photonic frequency fading effect by quantum microwave photonics

With energy-time entangled biphoton sources as the optical carrier and time-correlated single-photon detection for high-speed radio frequency(RF)signal recovery,the method of quantum microwave photonics(QMWP)has presented the unprecedented potential of nonlocal RF signal encoding and efficient RF signal distilling from the dispersion interference associated with ultrashort pulse carriers.In this paper,its capability in microwave signal processing and prospective superiority are further demonstrated.Both QMWP RF phase shifting and transversal filtering functionality,which are the fundamental building blocks of microwave signal processing,are realized.Besides good immunity to the dispersion-induced frequency fading effect associated with the broadband carrier in classical MWP,a native two-dimensional parallel microwave signal processor is provided.These results well demonstrate the superiority of QMWP over classical MWP and open the door to new application fields of MWP involving encrypted processing.

Robust Transfer of Optical Frequency over 500km Fiber Link with Instability of 10^(−21)

Our primary objective is to mitigate the adverse effects of temperature fluctuations on the optical frequency transmission system by reducing the length of the interferometer.Following optimization,the phase-temperature coefficient of the optical system is reduced to approximately 1.35 fs/K.By applying a sophisticated temperature control to the remained“out-of-loop”optics fiber,the noise floor of the system has been effectively lowered to 10−21 level.Based on this performance-enhanced transfer system,we demonstrate coherent transmission of optical frequency through 500-km spooled fiber link.After being actively compensated,the transfer instability of 4.5×10^(−16) at the averaging time of 1 s and 5.6×10^(−21) at 10000 s is demonstrated.The frequency uncertainty of received light at remote site relative to that of the origin light at local site is achieved to be 1.15×10^(−19).This enhanced system configuration is particularly well suited for future long-distance frequency transmission and comparison of the most advanced optical clock signals.

单光子微波光子学

伴随着微波光子学技术的迅速发展,超低功率下的高速微波光子信号检测、处理是一个限制微波光子应用领域的主要难点.考虑到单光子探测器具有超微弱光信号检测能力的同时还具有极低的时间抖动特性,将单光子检测与经典微波光子处理技术相结合是一种突破上述难点的潜在解决方案.基于此,本文提出了一种单光子微波光子系统.该系统使用单光子信号作为载波,通过微波强度改变光子探测几率,再通过单光子探测器得到光子探测几率复原微波信号.该系统成功实现了对单光子强度的微波光子信号进行滤波、移相的操作,而且系统的处理速度仅由单光子探测器时间抖动决定.此外,得益于系统所使用的信号恢复技术,该系统具有的优秀的抗干扰能力能够有效滤除噪声.该系统展示了一个微波光子学和量子光学学科交叉的范例,为之后量子微波光子学的发展奠定基础.

Quantum microwave photonics in radio-over-fiber systems

As the main branch of microwave photonics,radio-over-fiber technology provides high bandwidth,low-loss,and long-distance propagation capability,facilitating wide applications ranging from telecommunication to wireless networks.With ultrashort pulses as the optical carrier,a large capacity is further endowed.However,the wide bandwidth of ultrashort pulses results in the severe vulnerability of high-frequency radio frequency(RF)signals to fiber dispersion.With a time-energy entangled biphoton source as the optical carrier combined with the singlephoton detection technique,a quantum microwave photonics method in radio-over-fiber systems is proposed and demonstrated experimentally.The results show that it not only realizes unprecedented nonlocal RF signal modulation with strong resistance to the dispersion but also provides an alternative mechanism to distill the RF signal out from the dispersion effectively.Furthermore,the spurious-free dynamic ranges of the nonlocally modulated and distilled RF signals have been significantly improved.With the ultra-weak detection and the high-speed processing advantages endowed by the low-timing-jitter single-photon detection,the quantum microwave photonics method opens new possibilities in modern communication and networks.