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Frequency Standard

Frequency Standard. Brief overview Xu Lixiang 2013/05/18. contents. History Concepts and Fundamental Previous generation of atomic clock Latest progresses. History.

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Frequency Standard

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  1. Frequency Standard Brief overview XuLixiang 2013/05/18

  2. contents • History • Concepts and Fundamental • Previous generation of atomic clock • Latest progresses

  3. History Definition: An atomic clock is a clock device that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element. Atomic clocks are the most accurate time and frequency standards known, and are used as primary standards for international time distribution services, to control the wave frequency of television broadcasts, and in global navigation satellite systems such as GPS. Quote From wikipedia 1879----The idea of using atomic transitions to measure time was first suggested by Lord Kelvin. 1930s----Magnetic resonance developed by Isidor Rabi, became the practical method for doing this. 1949----U. S. National Bureau of Standards(now NIST) built the first atomic clock---ammonia maser 1955----Louis Essen, at National Physical Laboratory, UK, built the first accurate atomic clock, a cesium standard. 1990----Laser cooling and trapping of atoms, narrow laser line widths, precision laser spectroscopy, convenient counting of optical frequencies using optical combs.

  4. Concepts and Fundamental • Fine structure, hyper-fine structure, isomer structure • Two/three-level system, pumping, population inversion, probe • Laser cooling and trapping, optical lattice • Natural linewidth, Doppler broadening, ac stark effect, integration time

  5. Example: Cesium atomic energy level

  6. Improve accuracy • To get narrow linewidth and long integrating time, we need to take measures to decrease instability. • Quality factor Q proportional to • 1)cooling: ultra-cold atoms, ion/magnetic trap(Doppler broadening) • 2)extending integrating time(long Ramsey cavity) • 3)probing more atoms/ions(optical lattice, crystal lattice) • 4)high-precision spectroscopy probing technology(Atomic fountain technology, Two-level probe) • 5)stable monochromatic laser • 6)stable and uniform electromagnetic fields

  7. Previous generation of atomic clock • Cesium atomic clock(Magnetic state selection) • Rubidium(87) atomic clock(Laser pumping) • Hydrogen maser

  8. Cesium atomic clock(Magnetic state selection) State selector A: select the special state we need by magnetic force C field: stable and uniform magnetic field State selector B: filter, let excited atoms go through Mass spectrometer: collect and count atoms Evaluation: Stable temperature of source(100) Not easy to count atoms Poor performance

  9. Hydrogen maser Maser: Microwave amplification by stimulated emission of radiation. Using special microwave to coupling with 3-level system of hydrogen.

  10. Two-level probe After the microwave interaction Step1:Standing wave, Fluorescence intensity Step2:Pushing wave, pushing away atoms at state Fg=2 Step3:Pumping wave, pumping Fg=1 to Fg=2 Step4:Standing wave, probing, Step5:Efficiency=/ pumping

  11. Rubidium(87) atomic clock(Laser pumping)

  12. New generation of atomic clocks • 1.Th(229): narrow, low-lying isomeric state  less sensitive to environmental conditions than atomic transitions. What is isomeric state: a metastable state of an atomic nucleus caused by the excitation of one or more of its nucleons (protons or neutrons). In such a metastable state, one or more of the protons or neutrons in a nucleus occupy a nuclear orbital of higher energy than an available nuclear orbital of lower energy. These states are analogous to excited states of electrons in atoms. --From wikipedia

  13. Partial Isomeric states of Th229  Ref. Single-Ion Nuclear Clock for Metrology at the 19th Decimal Place, C. J. Campbell, A. Kuzmich

  14. Two types of Thorium atomic clocks • First type : single-Ion, Ion trap, Ultra-stable interrogation light(UV) at 163nm, long integration time about 10^3~10^4 s, suppressing Zeeman shift by using mF=0mF=0 clock transition, Result: , most accurate frequency standard? • Second type: solid-state environment (host crystal: LiCAF, reasonably transparent in the VUV), high densities about 10^19 nuclei/cm^3high quality factor Result:

  15. 2.Optical Lattice Clock--Strontium (Sr 87) 1,First step: blue light 461nm, loading into 1D MOT, Zeeman-slowed thermal atomic beam, cooling to mK. 2,Second Step: red light 689nm, loading into 2D MOT, cooling to microkelvin. 3,Third step: tune the lattice to a magic wavelength for clock transition (lower ac Stark shift) 4,Fourth step: interrogate the clock transition with an ultra- stable laser, using blue laser 461nm to pumping S0 and detect the fluorescence. Result: High Density (about 10^8 nuclei/cm^3) precision Δf/f ~ Ref, Operating a Sr Optical Lattice Clock with High Precision and at high density, Matthew D. Swallows, Michael J. Martin Blue 461nm Red 689nm Intercombination? Clock Transition Hyperfine-induced mixing? 1

  16. Thank you!

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