Xinye Xu

1) 1994/09-1997/06, Studying for Doctor’s degree, Key Laboratory of Quantum Optics, Shanghai Institute of Optics and Fine mechanics, Chinese Academy of Sciences。

2) 1988/09-1991/06, Studying for Master’s degree, Department of Electronics and Engineering, Hangzhou University。

3) 1980/09-1984/06, Studying for Bachelor's degree, Department of physics, Hangzhou University。

1) 2006/04-present, Professor, State Key Laboratory of Precision Spectroscopy, East China Normal University.

2) 2003/07-2006/03, Postdoctoral fellow, Department of Physics, Pennsylvania State University, USA.

3) 2000/11-2003/06, Postdoctoral fellow, JILA (Joint Institute for Laboratory Astrophysics), National Institute of Standards and Technology and University of Colorado.

4) 1998/04-2000/10, Postdoctoral fellow, Department of Physics, Seoul National University, Korea.

5) 1997/07-1998/03, Assistant researcher, Key Laboratory of Quantum Optics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences.

6) 1984/07-1994/08, Assistant、Lecturer, Department of physics, Hangzhou University.

Dr. Xinye Xu has been a professor of State Key Laboratory of Precision Spectroscopy (SKLPS) and Department of physics, East China Normal University (ECNU) since 2006. He is director of Committee of Laser Engineering and Technology, Shanghai Society of Laser. He is a member of Committee of Time and Frequency, Chinese Society of Metrology and Measurement. He is an executive member of the first council of a branch of Quantum Sensor and Precision Measuring Instrument, China Instrument and Control Society. He is a member of the expert group on fundamental physics of microgravity in the field of space science and applications of manned space engineering. He was a member of Program Committee of International Conference on Atomic Physics (ICAP) in 2010、2016 and 2020, respectively. He was vice-director of SKLPS from 2007 to 2015. He also received the fund from Shanghai Excellent Academic Leaders Program. He received his Ph.D. degree in Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences in 1997. He went to the Seoul National University, South Korea in 1998 to carry out the research on cold atoms guided by laser beam. In 2000 he moved to Joint Institute of laboratory Astrophysics (JILA) in United States and engaged in strontium optical clock research. In 2003 he went to Department of Physics in Pennsylvania State University in the United States to do cesium fountain clock research. Dr. Xinye Xu came to Department of Physics and SKLPS in ECNU in 2006, and he conducted the researches on optical clocks and atomic gyroscopes. At present, two cold ytterbium atomic optical clocks and one atomic gyroscope have been developed in ECNU. In 2020, the absolute frequency of cold ytterbium atomic clock was measured, and the measurement results were accepted by the International Metrology Commission. This is the first time for the data of a Chinese cold ytterbium atomic clock to be successfully reported to the International Metrology Organization. He presided over a number of funds supported by the national key basic research programs and the National Natural Science Foundation. His research results have been published in Nature, Phys. Rev. Lett., Phys. Rev. A and other academic journals.

1) From 2007 to 2015, deputy director of the state key laboratory of precision spectroscopy, East China Normal University.

2) From 2007 to now, a member of Committee of Time and Frequency, Chinese Society of Metrology and Measurement. 

3) From 2009 to now, he is a visiting professor and member of the academic committee of key laboratory of quantum optics, Shanghai institute of optics and fine machenics, Chinese Academy of Sciences.

4) In 2010, a member of the program committee of the International Conference on Atomic Physics (ICAP’2012).

5) From 2012 to now, director of Committee of Laser Engineering and Technology, Shanghai Society of Laser.

6) In 2012, the outstanding academic leader of Shanghai.

7) In 2016, a member of the program committee of the International Conference on Atomic Physics (ICAP’2016).

8) From 2016 to now, a member of academic committee of Key Laboratory of Time and Frequency Metrology and Standards, State Administration for Market Regulation, PRC.

9) 2018-present, a member of the academic committee of the Key Laboratory of Atomic Frequency Standard, Chinese Academy of Sciences.

10) 2018-present, a member of the editorial board of Navigation, Positioning and Timing.

11) From 2019 to now, an executive member of the first council of a branch of Quantum Sensor and Precision Measuring Instrument, China Instrument and Control Society.

12) From 2019 to now, a member of the expert group on fundamental physics of microgravity in the field of space science and applications of manned space engineering.

    My research areas are atomic‚ molecular and optical physics. My major research interests include laser cooling and trapping of neutral atoms‚ guiding of cold atoms‚ fountain atomic clocks‚ optical atomic clocks‚ atom interferometers‚ atomic gyroscopes, nonlinear optics‚ ultra-stable and ultra-narrow laser systems‚ quantum coherent control with optical frequency combs‚ interaction of cold atoms with ultra-fast pulses‚ high-resolution photoassociative spectroscopy of cold atoms‚ precision spectroscopy and precision measurements. My group is currently developing the ytterbium optical atomic clock for its use in metrology‚ communication and precision measurement, and studying the atomic gyroscopes applied for the inertial navigation.The following is the introduction of several research topics being carried out by my research group:

Project 1: Study of Ytterbium Optical Clocks

1. Introduction of Yb

At present, the neutral atomic optical clocks in the world mostly use alkaline earth metals, such as Sr, Yb and Hg atoms. And Yb has been one of the best atoms for optical lattice clocks in many institutes worldwide due to the several merits, for instance, numerous isotopes, the simple and clear energy level structure, available lasers for cooling and clock interrogation.

2. Energy levels

The ground state electronic structure of Yb atoms is [Xe]4f¹⁴6s², and there are two electrons in the outermost electron layer. The relevant energy levels and transitions of Yb are shown in Fig. 1. The relevant transitions of the optical clock experiment are described as follows:

Fig.1 Relevant energy level of ¹⁷¹Yb

6s²¹S₀-6s6p ¹P₁: natural linewidth about 29 MHz, ¹P₁ state lifetime 5.5 ns, used for the first-stage Doppler cooling, Zeeman slower and fluorescence detection.

6s²¹S₀-6s6p ³P₀: this is a double forbidden transition, the nuclear magnetic moment of the ³P₀ state has hyperfine interactions with the ¹P₁ and ³P₁ states, and gains a finite lifetime and non-zero decay rate to ground. The natural linewidth is nearly 10 mHz, which is favorable for high-performance clocks.

6s²¹S₀-6s6p ³P₁: natural linewidth about 182 kHz, upper-level lifetime 873ns.Using the narrow transition for the second-stage Doppler cooling of ytterbium atoms, a lower temperature of the cold atoms can be obtained. Meanwhile, it is also used to spin-polarization of the atoms.

6s6p ³P_{0, 2}-6s7s ³S₁: used to repump the atoms on 6s6p ³P₀ and 6s6p ³P₂ back to the 6s²¹S₀ ground state to achieve normalized detection.

3. Brief introduction of Yb optical clock

The Yb optical lattice clock consists of ultracold atoms system, ultra-narrow clock laser system, and optical frequency comb system (Fig. 2). Ultracold atoms system is the pivotal part of optical clock. With the complicated system and advanced technology, optical clocks have been one of the most challenging scientific projects.

Fig.2 The experimental setup of a ¹⁷¹Yb optical clock system

Fig.3 The schematic diagram of a ¹⁷¹Yb optical clock

1) Cooling of ¹⁷¹Yb

(1) The Yb oven is heated to 400 ℃. And the atoms are collimated by two direction optical molasses(2D-OM);

(2) Then the atoms with a speed about 300 m/s are slowed down to approximate 10 m/s by Zeeman slower, and confined in 399 nm magneto-optical-trap (Blue MOT).

(3) The atoms are cooled to ~1 mK in 399 nm MOT, then loaded into 556 nm MOT (Green MOT) and cooled to below 10 uK. At last, the atoms are loaded into optical lattice (OL, 759 nm), and clock transition interrogation are operated in OL. The fluorescence signal is collected by photomultiplier (PMT) and intensified charge-coupled device (ICCD).

Fig. 4 Images of atomic cloud in MOTs  and OL

   2) State preparation and clock interrogation

(1)¹⁷¹Yb with nuclear spin I=1/2, yields two sub-states () in each clock level, leading the atoms distributed between the two energy levels. A polarization laser at 556 nm is applied for higher excitation fraction in one single state.

(2)The linewidth and stability of the clock laser are important factors that limit the short stability of the atomic optical clock. The 1156 nm laser is lock onto an ultra-stable FP cavity with 290000 fineness to obtain sub-Hz linewidth. Frequency doubled clock laser performed clock interrogation,then 649 nm and 770 nm laser are applied to repump atoms back to ¹S₀ state.

   3) Evaluation of clock performance

The sideband spectrum and carrier spectrum of 578 nm clock transition is shown in Fig. 5, with a Rabi probe time of 600 ms, we obtain 1.9 Hz linewidth atomic spectrum, which is slightly wider than the Fourier-limited linewidth. And then, the 578 nm laser was locked on the clock transition spectrum of the cold ytterbium atom. The in-loop instability reaches 9×10^-18 after an averaging over a time of 20000 s. By synchronous comparison between two clocks, we have achieved the fractional instability of a single system assessed to 4.6×10^-16/τ^1/2, as shown in Fig. 6.

Fig. 5 (L)The sideband and carrier spectrum of the clock transition;  (R) the carrier spectrum of the clock transition

Fig. 6 (L)The in-loop instability of the ytterbium optical clock; (R)The instability of a single clock.

4. Absolute frequency measurement

The absolute frequency measurement is one of the important contents of optical clock experiment, and also marks the final establishment of the optical clock. Generally, optical frequency should refer to atomic transition and optical comb is used to link optical frequency and microwave frequency standards. In our experiment, the 578nm laser frequency was measured by optical frequency comb with respect to the H-maser in the closed-loop locking process, and calibrated through the GPS carrier phase frequency transmission link between East China Normal University (ECNU) and China Metrology Research Institute (NIM), so as to realize the traceability of the optical frequency to the SI second. After 15 days of continuous measurement, the uncertainty of the entire experimental system was evaluated, the absolute frequency was determined to be 518 295 836 590 863.30(38) Hz with a fractional uncertainty of 7.3 × 10^-16. The work was successfully published in Metrology (Metrologia 57, 065017 (2020)) in October 2020, and was highly praised by the reviewers, “The result is in good agreement with the recommended value in neutral Yb as a secondary representation of the SI second endorsed by the International Committee for Weights and Measures (CIPM).” In addition, we also submitted the absolute frequency measurement value to the Consultative Committee for Time and Frequency (CCTF), and received the acceptance notice by the Frequency Standards Working Group (WGFS) in November 2020.

(a)                                                                       (b)

Fig.7 (a)Schematic of frequency calibration of the ECNU Yb1 optical lattice clock via NIM; (b) the absolute frequency values of ¹⁷¹Yb clock transition measured by different laboratories.

Yb1                                                                   Yb2

Fig. 8 Two Yb atomic optical clock systems at ECNU

5. Prospect

Thanks to the rapid progress of cold-atom precision spectroscopy technology, optical clocks have made a great progress in recent years, and the instability and uncertainty of the best optical clocks in the world have been below level. The optical clock is expected to replace the cesium fountain clock to redefine SI second.The development of high-precision optical clocks based on cold-atoms can lay the foundation for our country to establish a new generation of time and frequency measurement system based on optical clocks, which is important to have voice and gain the initiative in the international redefinition of the second in the future. In addition, using the high-precision optical clock as a tool can realize the accurate measurement of many other physical quantities and physical parameters, which improve the measurement accuracy by several orders.In terms of the application of optical clock, the space optical clocks based on space stations can be used to verify the relativity at higher precision, ,and in addition, combined with the gravity redshift effect, the gravitational potential field on the Earth's surface can be measured precisely; Taking advantage of the excellent working environment in space and the high precision of optical clocks, many new physics experiments can be conducted, such as the search for dark matter and the detection of gravitational waves. Meanwhile, the development of space optical clock paves the way for the next generation GPS and also lay a solid foundation for our country's development of precise navigation of spacecraft and deep space exploration.The development of miniaturized optical atomic clock has great significance for widely application of optical clock. The miniaturizations of atom system, laser system and frequency comb system are important. The development of ultra-high precision optical clock will accelerate the development of related disciplines and scientific research fields.

Project 2: Research on NMR Gyroscope

Inertial navigation system (INS), as a robust self-contained navigation system without the need for external references, provides high concealment and exceeds the limit of time, environment and region. In specific environment (such as caves and depths), where GPS and BDS can’t function well, it can still achieve the precise navigation of the carrier. As one of the key components of inertial navigation and inertial measurement, the gyroscope is widely used in aviation, aerospace, navigation and other military and civil fields.

Fig.1 Applications of gyroscopes

The gyroscope in using limits the further development of the INS owing to it can’t meet high precision and compact sizeat the same time. With the advantages of high precision, small size, low cost and insensitive to acceleration, the nuclear magnetic resonance gyroscope(NMRG) is developing towards chip scale and strategic precision, which has become a new research hotspot in inertial navigation field.

1. Introduction of atoms

Nuclear magnetic resonance gyroscopes mostly adopt the scheme of alkali-metal and noble-gas. The most common alkali-metal atoms are Rb, Cs, and K. Owing to the Cs atoms have higher vapor pressure at the same temperature compared to other alksali-metal atoms andthere is only one stable isotope of Cs, ¹³³Cs is selected in the experiment. The Xe atoms have two stable isotopes-¹²⁹Xe and ¹³¹Xe, which can eliminate the interference of static magnetic field. Therefore, the ensemble of ¹³³Cs-¹²⁹Xe/¹³¹Xe is selected in the experiment.

2. Energy levels

In the experiment, the lasers mainly interact with ¹³³Cs atoms. The relevant energy levels and transitions of ¹³³Cs are shown in Fig. 2.

6²S_1/2-6²P_1/2: The transition is the D1 line of ¹³³Cs atoms. The circularly polarized light realizes the optically pumped polarization of ¹³³Cs atoms.

6²S_1/2-6²P_3/2: The transition is the D2 line of ¹³³Cs atoms. By forming a magnetometer with the linearly polarized light, we detect the precession of noble gas atoms.

Fig.2 Relevant energy levels of ¹³³Cs

3. Composition and working process of NMRG

Fig.3 Schematic diagram of the experimental setup

Fig.4 Photograph of the experimental setup

The key part of NMRG is the vapor cell containing a droplet of alkali metal (Cs), noble gas(¹²⁹Xe/¹³¹Xe), and buffer gas(N₂). The working principle of NMRG can be divided into three parts: Optical Pumping Polarization, Nuclear Magnetic Resonance Process and Precession Frequency Detection.

1) Optical Pumping Polarization: The pumping beam polarizes the electronic spins of alkali metal atoms, and the noble gas atoms are polarized by alkali metal atoms through spin-exchange collisions.

2) Nuclear Magnetic Resonance Process: An oscillating magnetic field is utilized to drive the nuclear spins deviate from the static magnetic field and precess with the Larmor frequency.

3) Precession Frequency Detection: We detect the nuclear precession frequency by applying an atomic magnetometer. When the gyroscope rotates, the precession frequency detected by the atomic magnetometer would change. By measuring the corresponding frequencies, the rotation angular velocity of the gyroscope can be obtained.

Fig.5 Schematic diagram of nuclear magnetic resonance gyroscope

4. Experimental results

At present, the NMRG experimental system including the three-axis atomic magnetometer has been built. Magnetic field sensitivities of 100 fT/Hz^1/2 in x and y axes and 20 fT/Hz^1/2 in the z axis are achieved. Compared with other similar three-axis atomic magnetometers, the realized magnetic field sensitivity and frequency bandwidth are better, and it is more suitable for precise measurement of the angular velocity. The research results have been published in the Applied Physics Letters (Appl. Phys. Lett., 2020, 116(3): 034001). As a key technology for optimizing the performance of NMRG, the high-sensitivity atomic magnetometer would produce significant application value in the field of inertial navigation.

Fig.6 Responses in three axes with respect to the fields

Fig.7 Magnetic field sensitivity of the three-axis atomic magnetometer

Project 3: Quantum Computation and Quantum Simulation

1. Brief introduction of topological quantum computation and Majorana Fermions

Quantum computation is the use of quantum phenomena such as superposition and entanglement to perform computation. Because quantum computing has the characteristics of parallel computing, its potentially computing efficiency far exceed the traditional calculation. However, standard quantum computation has a serious problem that the quantum state can easily be disturbed by the environment and then decoherence. The topological quantum computing employing non-Abelian anyons would be fault tolerance guaranteed by physics at the hardware level and can be effectively protected against the disturbing of the local environment. Majorana fermion is an electrically neutral fermion, which is its own antiparticle. In topological superconductor and topological superfluid, Majorana fermions can exist as emergent quasiparticles that is the simplest non-Abelian anyons following non-Abelian statics. An exchange（braid）of Majorana fermions can contribute not just phase change , but can send the system into a different state. The operation of the quantum gate of topological quantum computing can be realized by braiding Majorana fermions and is independent of the details of the trajectories.

Fig.1 Schematic diagram of braiding Majorana Fermion

2. Quantum simulation with cold atoms

Quantum simulation means artificial constructing a quantum system, which is easier to manipulate and study to simulate the behavior of another quantum system. With the development of experimental techniques in cold atomic systems, ultracold atomic systems can be used to simulate some important models in condensed matter and to study novel states of matter. Cold atomic systems are highly flexible and controllable, and the systems are much cleaner compared to solid systems. In cold atomic systems, an optical lattice with periodic potential can be experimentally constructed by the interference of laser field, and atoms are trapped in the optical lattices. An optical lattice is a versatile tool to perform quantum simulations. which is analogous to the lattice structure of solid-state systems. In addition, the Feshbach resonance technique provides a powerful tool for effectively tuning the interactions between atoms. Cold atoms with synthetic spin-orbit coupling provides a convenient quantum simulation platform for the study of topological states of matter. The cold atomic system is an ideal platform for generating and detecting Majorana fermions。

 Fig.2 Schematic diagram of synthetic spin-orbit coupling induced by clock laser

3. Research contents

This project focuses on the theoretical and experimental study of topological states and Majorana fermions in cold ytterbium atomic systems. The main research contents of the project include:

(1) The control of interactions between cold ytterbium atoms. We plan to use orbital Feshbach resonance and other techniques to realize the tuning of the scattering length between ytterbium atoms, and to investigate problems such as BEC-BCS crossover in cold ytterbium atomic systems

(2) Synthetic spin-orbit coupling induced by the clock laser. Compared with the Raman-induced spin-orbit coupling scheme, Synthetic spin-orbit coupling exploiting the clock transition can effectively suppress the heating of atoms. We aim to generate synthetic spin-orbit coupling in ytterbium optical lattice clock by optical clock transition. On this basis, we plan to simulate the topological state of matter and explore the novel topological phase.

(3) Study of Majorana fermions in cold ytterbium atomic systems. We are theoretically analyzing our scheme for the generation of Majorana fermions in quasi-one-dimensional or quasi-two-dimensional cold ytterbium system with synthetic spin-orbit coupling and estimating the experimental parameters. Then, we desire to experimentally realize the preparation of cold ytterbium atoms in the topological superfluid phase. Majorana fermions would appear in the topological defects such as the end of 1D atomic chain and 2D vortex which could be detected by radio frequency spectroscopy. On this basis, we would braid Majorana fermions and study their non-abelian quantum statistical properties.

 Fig.3 Sketch of the scheme for creating Majorana fermion

Project 4: High Precision Time & Frequency Transfer

1.     Introduction of high precision time-frequency transfer

The time and frequency standard with high stability and high accuracy is the goal that people have always been pursuing. Since the invention of the first atomic clock, the performance of atomic frequency standards has been continuously improved. To date, the research focus has shifted from microwave atomic clocks to the optical counterparts (i.e., optical clocks). However, the traditional free-space-based microwave signal transfer can no longer meet the remote synchronization and comparison for atomic clocks with higher precision. For instance, the comparison system represented by the satellite two-way time-frequency transfer (TWSTFT) cannot transfer highly stable optical clock signals. It has been shown that the optical fiber network is the most promising medium for precision optical frequency transfer.

In addition to transfer optical carriers, currently available signals also include optical frequency comb and radio/microwave signals. Although the accuracy of radio/microwave transfer is no better than that of the optical carrier transfer, radio/microwave-based applications have higher flexibility, which makes the transfer system easier to deploy. Based on the optical fiber link, our team carries out high-precision radio/microwave transfer, high-precision optical frequency transfer and high-precision time transfer. In this way, the optical-clock signal of our team can be effectively distributed and shared, and the transfer system is geared to preparing remote optical-clock comparison and related applications.

2.     Research contents

2.1   Radio/microwave frequency transfer

For the optical fiber transmission of high precision RF frequency, the main problem is to eliminate the negative effects of transmission delay and drift accumulation caused by the disturbance of optical fiber link such as vibration, temperature, fiber aging, and so on. Two-way-time-frequency transfer (TWTFT), optical-mechanical temperature compensation, and electronic phase compensation methods are common schemes. Based on the electronic phase compensation method, the principle prototype of 100MHz RF transmission over a 50 km optical fiber link is established, and the stability of the transmission loop is better than 5×10^-14 τ^-1. The scheme is used for long-term hydrogen clock comparison.

Figure 1 Diagram of high precision time-frequency transfer based on optical fiber link

2.2   Optical frequency transfer

Benefit from our optical atomic clock, optical frequency comb and optical fiber phase noise suppression technology. The high-precision optical frequency standard transfer link can transmit the frequency stability of the optical clock to the remote end directly through the optical fiber.

Figure 2 Diagram of high precision optical frequency standard transfer link

2.3   Time pulse transfer

On the basis of high precision radio/microwave frequency transfer, the key of high precision time pulse transfer is to accurately measure the optical fiber delay and to accurately offset or compensate the optical fiber delay at picosecond level. The time delay compensation method can be realized by optical fiber delay line or radio frequency delay line.

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    Aspiring students are welcome to apply for our research group's master and doctoral candidates! We also sincerely welcome those who are willing to engage in scientific research to work in our research group, including conducting post-doctoral research.

Please contact Prof. Xu at xyxu@phy.ecnu.edu.cn

1. Postgraduate courses: 《Modern Atomic Physics》.

2. Undergraduate course: 1)《Theoretical Mechanics》; 2)《Experiments of Precision Optics I》.

1) Shanghai science and technology major project, subtask: development and application of cold ytterbium optical clocks, 2019/07-2024/06.

2) National key research and development project of China, 2016YFA0302103, subject: “quantum optical frequency standards”, 2016/07-2021/06.

3) National high technology research and development program (863), 2014AA123401, subject: “nuclear-spin magnetic resonance technology”, 2014/09-2017/12.

4) Key project of national natural science foundation of China, 11134003, “study on deep cooling and precise control of ytterbium atoms applied in optical clocks”, 2012/01-2016/12.

5) National key basic research and development project of China (973), 2012CB821302, subject: “optical lattice frequency standards”, 2012/01-2016/08.

6) Shanghai excellent academic leaders program, 2012, “study on precision spectroscopy and measurement based on cold atoms”.

7) General project of national natural science foundation of China, 10774044, “theoretical and experimental study on three-dimensional optical lattice of optical atomic clocks”, 2008/1-2010/12.

Selected Publications:

1) In 1993, awarded the third prize of Zhejiang Science and Technology Progress Award, fifth accomplisher.

2) In 2012, selected as an outstanding academic leader of Shanghai.

3) In 2020, awarded the Individual Contribution Award for the 50th Anniversary of Shanghai Laser Society.