Evaluation of the frequency instability limited by Dick effect in the microwave 199Hg＋ trapped-ion clock

In the microwave 199Hg＋ trapped-ion clock, the frequency instability degradation caused by the Dick effect is un- avoidable because of the periodical interrogating field. In this paper, the general expression of the sensitivity function g（t） to the frequency fluctuation of the interrogating field with Nπ-pulse （N is odd） is derived. According to the measured phase noise of the 40.5-GHz microwave synthesizer, the Dick-effect limited Allan deviation of our 199Hg＋ trapped-ion clock is worked out. The results indicate that the limited Allan deviations are about 1.75 ×10-13/√τ and 3.03 ×10-13/√τ respectively in the linear ion trap and in the two-segment extended linear ion trap under our present experimental parameters.

Zeeman decoherence effect of trapped ^(199)Hg^(+) ion Ramsey spectra

In the ^(199)Hg^(+) ion microwave clock,the Zeeman decoherence effect caused by the overlapping of Zeeman sidebands and the radial secular motion sidebands will decrease the contrast of the Ramsey fringe,thus reducing the signal-to-noise ratio of the spectra.In this paper,the Zeeman decoherence effect is analyzed theoretically and investigated experimentally.A simplified model is built to describe the Ramsey spectral probability,in which the transverse relaxation time T2 is introduced to characterize the influence of the Zeeman decoherence effect phenomenologically.The experiments were carried out on a linear quadrupole trap ^(199)Hg^(+) ion clock.The results show that the probability model matches well with the experimental data,and the magnetic field value should be more than 150 mGs(1 Gs=10-4 T)to avoid the Zeeman decoherence effect.

Experimental Determination(∼mHz)of the Ground-State Hyperfine Separation of Trapped ^(199)Hg^(+) in a Hyperbolic Paul Trap

The Ramsey fringes on the ground-state hyperfine transition of ^(199)Hg^(+)(ΔF=1,ΔmF=0)ions trapped in a hyperbolic Paul trap are firstly observed with the method of time-separated oscillatory fields.The full width at half maximum of central Ramsey fringes is about 25 mHz and the corresponding quality factor Q of the line is greater than 10^(12) for the trapped ^(199)Hg^(+)microwave frequency standard.The hyperfine transition frequency shifted by magnetic field is also measured by the high-resolution Ramsey fringes.The final result isΔνhfs=40507347997.3(0.5)Hz,which is corrected to zero magnetic field.

Comparison experiments of neon and helium buffer gases cooling in trapped ^(199)Hg^+ ions linear trap

The influences of different buffer gas, neon and helium, on 199^Hg^＋ clock transition are compared in trapped 199^Hg^＋ linear trap. By the technique of time domain＇s Ramsey separated oscillatory fields, the buffer gas pressure frequency shifts of 199^Hg^＋ clock transition are measured to be （df/dPNe）（1/f） = 1.8 × 10^-8 Torr^-1 for neon and （df/dPHe） （1/f） = 9.1 × 10^-8 Torr^-1 for helium. Meanwhile, the line-width of 199^Hg^＋ clock transition spectrum with the buffer gas neon is narrower than that with helium at the same pressure. These experimental results show that neon is a more suitable buffer gas than helium in 199^Hg^＋ ions microwave frequency standards because of the 199^Hg^＋ clock transition is less sensitive to neon variations and the better cooling effect of neon. The optimum operating pressure for neon is found to be about 1.0 × 10^-5 Torr in our linear ion trap system.

Observation of Photoassociation Spectra of Ultracold ^(174)Yb Atoms at ^(1)S_(0)-^(3)P_(1) Inter-Combination Line

Photoassociation(PA)of cold atoms is a key tool widely used for accurate determination of long-range molecular potentials,measurement and tuning of interatomic interactions,as well as creation of desired cold molecules.We report the observation of PA spectra of ultracold ytterbium(Yb)atoms at the ^(1)S_(0)–^(3)P_(1) inter-combination line by detecting the resonant loss of atoms trapped in a crossed far-off-resonance trap(crossed-FORT).Eight vibrational levels in the ^(1)0^(+)_(u) molecular state are observed for PA laser detunings up to 1 GHz with respect to the atomic resonance.The photoassociation resonances have a Lorentzian shape and sub-MHz linewidths due to the ultra low temperature of the atoms,and the resonance positions agree well with that predicted by the semiclassical formula.

A Method of Measuring the Axial Secular Motion Temperature of Trapping Large Size Ion Clouds

A large cloud of ^(40)Ca^(+) is successfully trapped and manipulated in a linear ion trap.The axial length of the ion cloud is measured under a series of end-cap voltages.We propose a method of measuring the axial secular motion temperature of the ion cloud by analyzing its image on an electron-multiplying CCD.The method is based on the Boltzmann equation that the axial density distribution of ions at secular motion temperature T satisfies.The axial secular motion temperature of the ion cloud is also obtained by measuring the Doppler broadened line width.For the same trapping parameters,the axial secular motion temperature by analyzing the image of ion cloud is 840 K and by fitting the experimental resonance line profile is 700 K.

Frequency locking of a 399-nm laser referenced to fluorescence spectrum of an ytterbium atomic beam

The laser cooling of ytterbium（Yb） atoms needs a 399-nm laser which operates on the strong1S0-1P1 transition and can be locked at the desired frequencies for different Yb isotopes.We demonstrate a frequency locking method using the fluorescence spectrum of an Yb atomic beam as a frequency reference.For unresolved fluorescence peaks,we make the spectrum of the even isotopes vanish by using the strong angular-dependence of the fluorescence radiations;the remained closely-spaced peaks are thus clearly resolved and able to serve as accurate frequency references.A computer-controlled servo system is used to lock the laser frequency to a single fluorescence peak of interest,and a frequency stability of 304 kHz is achieved.This frequency-locked laser enables us to realize stable blue magneto-optic-traps（MOT） for all abundant Yb isotopes.