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arXiv:1312.7617 · Journal of Physics B Atomic Molecular and Optical Physics

Direct spectroscopy of the2S1/2−2P1/2and2D3/2−2P1/2transitions and observation of micromotion modulated spectra in trapped40Ca+

Thaned Pruttivarasin, Michael T. Ramm, Hartmut Häffner

Published Jun 25, 2014• 10 citations
arXivHTMLPDF

Abstract

We present an experimental scheme to perform spectroscopy on ions and atoms with a lambda-level structure. By rapidly switching lasers between both transitions, we circumvent the complications of both dark resonances and the ac-Stark effect. We demonstrate the scheme on the 2 S 1/2 − 2 P 1/2 and 2 D 3/2 − 2 P 1/2 transitions in 40 Ca + and, within a measurement time of 10 min, extract the centre frequencies of both dipole transitions with a statistical uncertainty on the order of 200 kHz. We also apply this method to directly observe the micromotion modulated fluorescence spectra of both transitions and for the first time measure the dependence of the micromotion modulation index on the trap frequency for both transitions.

DOI
10.1088/0953-4075/47/13/135002

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Optics Letters•Dec 23, 2010•14 citations
We present an experimental scheme to perform spectroscopy on ions and atoms with a lambda-level structure. By rapidly switching lasers between both transitions, we circumvent the complications of both dark resonances and the ac-Stark effect. We demonstrate the scheme on the 2 S 1/2 − 2 P 1/2 and 2 D 3/2 − 2 P 1/2 transitions in 40 Ca + and, within a measurement time of 10 min, extract the centre frequencies of both dipole transitions with a statistical uncertainty on the order of 200 kHz. We also apply this method to directly observe the micromotion modulated fluorescence spectra of both transitions and for the first time measure the dependence of the micromotion modulation index on the trap frequency for both transitions.
This dissertation reports on precision measurements of the atomic structure of calcium ions (40Ca+), application of trapped 40Ca+ ions as a probe for a violation of fundamental symmetry and prospects of performing quantum simulations with trapped ion.We demonstrate a novel technique to perform spectroscopy on the dipole transition of 40Ca+ that circumvents usual difficulties from dark resonances and Doppler heating. The center of the atomic transition can be detected to a precision of 200 kHz or less with an integration time of 10 minutes. We apply this method to directly measure the influence of micromotion on the fluorescence spectra and confirm the dependence of the modulation index on the radial trap frequency. We measure the branching fraction of the excited 2P1/2 state of 40Ca+ to be 0.93565(7) using a simple experimental scheme readily applicable to many other ion species. Our result for 40Ca+ distinguishes well among various theoretically calculated values, which is important in guiding further developments of the theoretical work.We apply the Ramsey spectroscopy technique based on a pair of correlated ions to probe the effect of the violation of local Lorentz invariance (LLI). The energy difference between the two components of the Bell state |PsiB> = |mJ=5/2,mJ=-5/2>+|mJ=1/2,mJ=-1/2> in the D5/2 manifold of 40Ca+ is monitored for 12 hours. We found that the energy component related to the violation of LLI varies less than 17±22 mHz. Assuming a hydrogen-like model of 40Ca+, the measurement result provides us the bound of the LLI parameter C(2)0 to be 1.7±2.2x10-17.Based on numerical simulations, we show that the Aubry transition in the Frenkel-Kontorova model with trapped ion can be observed for practical experimental parameters such as the strength and wavelength of the optical lattice, which serves as an external perturbing periodic potential. Moreover, we also show that the normal mode structure of ion chain can change significantly as we vary the strength of the optical lattice.
We study the influence of micromotion on the spectrum of trapped ions with a lambda-type level scheme, leading to dark resonances due to coherent population trapping. We work with calcium ions trapped in a ring-shaped Paul trap, in which one can compensate excess micromotion for only one ion of the crystal. We observe that micromotion affects the shapes of the dark resonances and causes the appearance of “echoes” separated by intervals given by the drive frequency. We present a theoretical model that provides good fits to the measurements and can be used to estimate the amplitude of the micromotion modulation of the atomic motion. We estimate an effective temperature of the ions from the spectra and observe clear micromotion heating as well as impaired cooling for sufficiently large excess micromotion.
Micromotion of ions in Paul traps has several adverse effects, including alterations of atomic transition line shapes, significant second-order Doppler shifts in high-accuracy studies, and limited confinement time in the absence of cooling. The ac electric field that causes the micromotion may also induce significant Stark shifts in atomic transitions. We describe three methods of detecting micromotion. The first relies on the change of the average ion position as the trap potentials are changed. The second monitors the amplitude of the sidebands of a narrow atomic transition, caused by the first-order Doppler shift due to the micromotion. The last technique detects the Doppler shift induced modulation of the fluorescence rate of a broad atomic transition. We discuss the detection sensitivity of each method to Doppler and Stark shifts, and show experimental results using the last technique.
Trapped ions, as one of the pillars of progress in frequency metrology and quantum optics, require a complex experimental environment with well-defined conditions. We present that a feature called dark resonance, provided by the trapped ion itself, can be used as a versatile sensor for enhanced in-situ analysis of interacting fields. The dark resonance is formed in the lambda-type energy level scheme of a laser cooled 40Ca+ ion and corresponds to a fluorescence quenching. The method uses an analysis of the detection times of photons emitted from the upper energy level, which is excited via two optical dipole transitions. The two excitation lasers are phase locked to an optical frequency comb to reduce their linewidths and for precise control of their optical frequencies within the dark resonance. The amplitudes of interacting fields are obtained using the Fourier transform of the ion fluorescence or photon correlation measurements. This paper shows that the method can be applied for sensing of electric, magnetic and electromagnetic fields. Firstly, we present the potential for frequency analysis of the secular motion of a few-ion Coulomb crystal, which corresponds to the axial static electric field of a linear ion trap. Secondly, we demonstrate the optical frequency analysis of the employed lasers driving the two transitions. In the last case we show the analysis of an alternating magnetic field at the position of single ion.