Abstract:

Abstract:High-precision time transfer plays an important role in the areas of fundamental research and applications. Accompanying with the remarkable improvements in the ability of generating and measuring high-accuracy time-frequency signal, seeking for new time-transfer techniques between distant clocks with much further improved accuracy attracts attentions world-widely. The time-transfer technique based on optical pulses has the highest precision presently, and the further improvement in the accuracy is heavily dependent on the time-domain properties of the pulse as well as the sensitivity of the applied measurement on the exchanged pulse. The application of optical frequency comb in time transfer for a precision up to femtosecond level are currently the focus of much interest, and has recently achieved many breakthroughs. Further investigations show that, utilizing quantum techniques, i.e. quantum measurement technique and quantum optical pulse source, can lead to a new limit on the measured timing information. Furthermore, it can be immune from atmospheric parameters, such as pressure, temperature, humidity and so on. Such quantum improvements on time-transfer have a bright prospect in the future applications requiring extremely high-accuracy timing and ranging. The potential achievements will form a technical basis for the future realization of sub-femtosecond time transfer system.

Abstract:The SI “second” is realized by caesium primary frequency standards (PFSs) using laser cooled atoms in a fountain configuration. Four sub systems and operation procedure of the NTSC-F1 primary frequency standard are introduced in the paper. The frequency stability of NTSC-F1 is 3.0×10-13/τ-1/2 compared to hydrogen maser. Four terms of frequency shift and uncertainty including second order Zeeman frequency shift, cold collision shift, gravity shift and blackbody shift are evaluated. The improvement of NTSC-F1 is introduced.

Abstract:We report on studies of an optical lattice for a strontium clock performed at the National Time Service Center. Following two-stage laser cooling and trapping, 88Sr cold atoms with population of 105 and a longitudinal temperature of 8.4 μK are loaded into a one-dimensional optical lattice. Spectroscopic analysis of the 1S0−3P0 transition gives a linewidth of 180 Hz measured using magnetic field-induction, which mixes the 3P1 state with the 3P0 state. Rabi oscillations are observed. Because of the inhomogeneous excitation among the atoms, the Rabi π-pulse excitation at 5 ms shows a near 65% excitation of atoms. The transverse velocity distribution of the atomic beam and the absolute frequencies of the four inter-combination transitions of the isotopes was measured precisely using velocity-selective fluorescence spectroscopy. By optical injection of two cascade external-cavity diode lasers, a single comb line at 689 nm from an optical femtosecond laser comb is filtered and amplified with a 37-dB side-mode suppression and a linewidth of less than 240 Hz. We describe recent work on a space optical clock concerning the physical vacuum system, thermal analysis, and a permanent-magnet Zeeman slower for a space strontium optical clock. The first six mode frequencies are obtained and the corresponding oscillation modes are described in detail. We also simulate and analyze thermal profiles for both the physical and optical units installed in the cooling system. When the injected cooling water has a temperature of 21°C or 28°C, the units meet operational requirements for temperatures in a space environment. Using a series of permanent magnets, a Zeeman slower is built that can withstand space launching and operating conditions for the space optical clock.

Abstract:We propose an approach of long-term stabilization of optical fiber phase by controlling a piezo-based phase modulator and a Peltier component attached to the fiber via a phase-locked loop(PLL) circuit with dual proportional-integral- derivative(PID) adjustment. With this approach, we can suppress the fast disturbance and slow drifting of optical fiber to satisfy the requirements of optical phase long-term locking. In theory, a mathematical model of an optical fiber phase control system is established. The disturbance term induced by environment influence is considered into the PLL model. The monotonous and continuous changing environment disturbance will cause a steady-state error in this theory model. The experimental results accords well with the theory. The steady-state performance, adjusting time, and overshoot can be improved by using the dual PID control. As a result, the long-term, highly stable and low noise fiber phase locking is realized experimentally.

Abstract:This paper presents a new type of cold atom interferometry gravimeter based on Bragg diffraction, which is able to increase the gravity measurement sensitivity and stability of common Raman atom gravimeters significantly. By comparing with Raman transition, the principles and advantages of Bragg diffraction-based atom gravimeters have been introduced. The theoretical model for a time-domain Bragg atom gravimeter with atomic incident direction parallels to the wave vector of Bragg lasers has been constructed. Some key technical requirements for an nth-order Bragg diffraction-based atom gravimeter have been deduced, including the temperature of atom cloud, the diameter, curvature radius, frequency, intensity, and timing sequence of Bragg lasers, etc. The analysis results were verified by the existing experimental data in discussion. The present study provides a good reference for the understanding and construction of a Bragg atom gravimeter.

Abstract:The recent advances of atom interferometer and its application in precision inertial measurement are reviewed. The principle, characteristics and implementation of atom interferometer are introduced and it can be used to measure gravitational acceleration, gravity gradient and rotation for its high sensitivity. We also present the principle, structure and new progress of gravimeter, gravity gradiometer and gyroscope based on atom interferometer.

Abstract:Large momentum transfer (LMT) beamsplitting in atom interferometry is reviewed, focusing on the use of Bloch Oscillations to achieve high momentum separation without loss of visibility. Phase sensitivity with a fringe visibility of 7% is observed in a horizontally guided, acceleration-sensitive atom interferometer with a momentum separation of 80ℏk between its arms. In addition, a 510ℏk beamsplitter is demonstrated.

Abstract:We give a simple introduction to the theoretical treatment of atoms interacting strongly with electromagnetic fields in the radiofrequency, microwave and laser domains. In particular, we discuss the concept of dressed atoms, which considers the combination of the atom and photons as a composite physical system. This powerful concept has a wide range of applications in atomic physics and we give a few examples of its use in the manipulation of ultracold atoms in adiabatic potentials. These examples are selected from experimental work conducted by our research team in Oxford but there are numerous other applications and we outline some future possibilities.