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Proceedings of the National Academy of Sciences

Excitation transfer spectroscopy with two metastable 138Ba+ ions in same trap.

HANS G. DEHMELT, Xiaohui Zhao, N Yu, Warren Nagourney

Published Jul 9, 1996• 1 citation
Publisher

Abstract

In the absence of lasers approaching trapped ion clock transitions in sharpness we propose to replace the 12.49 m laser field exciting the D3/2-D5/2 transition of the single Ba+ ion A in D3/2 with the near-field of a close by identical ion B in the excited D5/2 state. We tune the frequency of the near-field by the differential Stark shift generated when the center of mass of the tuned ions is slightly moved out of the trap center by a small bias voltage. We demonstrate that the resultant resonant energy exchange can be made considerably faster than the natural lifetime of either metastable level and show how it might be detected.

DOI
10.1073/pnas.93.14.6861

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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)•Jan 1, 1998•0 citations
In the absence of lasers approaching trapped ion clock transitions in sharpness we propose to replace the 12.49 m laser field exciting the D3/2-D5/2 transition of the single Ba+ ion A in D3/2 with the near-field of a close by identical ion B in the excited D5/2 state. We tune the frequency of the near-field by the differential Stark shift generated when the center of mass of the tuned ions is slightly moved out of the trap center by a small bias voltage. We demonstrate that the resultant resonant energy exchange can be made considerably faster than the natural lifetime of either metastable level and show how it might be detected.
High-resolution optical spectroscopy of an individual trapped ion is hampered by lack of sharp lasers. This suggests the use of a second metastable excited ion as an ultrasharp light source. To this end, laser-cool two barium ions to an equilibrium distance of approximately 8 mum on the z (symmetry) axis of the trap and, in this (earth)(Ba(+))(2)-molecule, visually or photoelectrically identify them as A and B by their location. Briefly turn on a 455-nm spectral lamp until one of the ions, say the A ion, is pumped into the metastable D(5/2) level and turns invisible. Focus on the visible, spatially well-resolved B ion and turn off the blue and red illumination lasers for approximately 15 s. Then turn them back on again and check on whether the excitation by chance has been transferred to the B ion and is now in the D(5/2) level and dark while the A ion is bright. The cross section for absorption of the lambda(D(5/2) --> S(1/2)) identical with lambda(0) = 1.76 mum radiation by a stationary ion can be >lambda(0) (2)/2pi. Thus, by pushing the two ions together to approximately lambda(0)/4 by turning on a much stronger trapping field during the excitation exchange period, one might be able to detect excitation transfer in >10% of the attempts. The ions are tuned relative to each other by a 0- to 10-mV/cm variable dc field in the z direction, which displaces them axially and causes them to see different rf fields, which Stark-shifts their frequencies. In this way, a resonant transfer response as sharp as twice the natural width of the D(5/2) level, 11 mHz or a Q approximately 0.4 x 10(17), might be demonstrated.
The electric dipole-forbidden, quadrupole $6{S}_{1/2}\ensuremath{\leftrightarrow}5{D}_{3/2}$ transition in Ba+{}^{+}+ near 2051 nm, with a natural linewidth of 13 mHz, is attractive for potential observation of parity nonconservation, and also as a clock transition for a barium ion optical frequency standard. This transition also offers a direct means of populating the metastable 5D3/25{D}_{3/2}5D3/2​ state to measure the nuclear magnetic octupole moment in the odd barium isotopes. Light from a diode-pumped, solid-state Tm,Ho:YLF laser operating at 2051 nm is used to coherently drive this transition between resolved Zeeman levels in a single trapped 138{}^{138}138Ba+{}^{+}+ ion. The frequency of the laser is stabilized to a high-finesse Fabry-P\'erot cavity at 1025 nm after being frequency doubled. Rabi oscillations on this transition indicate a laser-ion coherence time of 3 ms, most likely limited by ambient magnetic field fluctuations.
We report observation of laser excitation of a narrow, magnetic-dipole-allowed, infrared transition in a single trapped ion. Quantum jumps of a Ba+{\mathrm{Ba}}^{+}Ba+ atom were observed using a diode laser tuned in resonance with the excited-state 5d 2^{2}2D3/2{\mathit{D}}_{3/2}D3/2​--5d 2^{2}2D5/2{\mathit{D}}_{5/2}D5/2​ transition at 12.48 \ensuremath{\mu}m. Single-ion resonance line shapes were obtained and a transition frequency of \ensuremath{\nu}=24 012 033\ifmmode\pm\else\textpm\fi{}11 MHz was determined. Calculated quantum jump rates of the D-D transition for the applied laser spectral brightness yield agreement with experimentally observed values.
Optical manipulation of Ba+ ions is performed mainly by a 493 nm laser for the S1/2−P1/2 transition and a 650 nm laser for the P1/2−D3/2 transition. Because the branching ratio between the 493 and 650 nm transitions of a single Ba+ ion is comparable, stabilization systems of both lasers are equally important for Doppler cooling, sub-Doppler cooling, optical pumping, and state detection. The stabilization system of a 493 nm laser to an absolute Te2 reference has been well established. However, the stabilization of a 650 nm laser to the absolute reference of an atomic or molecular spectrum has not been presented before, to our knowledge. Here, we experimentally obtain the I2 spectrum near 650 nm and provide the experimental connection to the spectrum of Ba+. We present the 20 spectral lines of I2 in the range of 0.9 GHz above the resonance of the P1/2−D3/2 transition. We stabilize the 650 nm laser through the optical cavity to the lowest one among these lines, which is about 350 MHz apart, as the absolute frequency reference. Finally, we measure the frequency differences between these iodine lines and the Ba+ resonance through the fluorescence excitation spectrum with well-resolved dark states, which is in agreement with the theoretical expectation. The presented stabilization scheme enables us to perform precise experiments with Ba+ ions.
We have employed the method of shelving to measure Zeeman resonances of a single trapped Ba+ ion. We use optical pumping to place the ion in a selected magnetic sublevel of either the 6S(1/2) ground state or the 5D(3/2) metastable state. The ion is exposed to an rf field, and a probing/shelving mechanism detects whether spin-flip transitions have taken place. We have observed rf transitions with linewidths of 15 Hz, limited by magnetic-field noise. We have also observed the shift in the Zeeman frequency when the ion is illuminated by off-resonant light. A simultaneous measurement of such light shifts in two atomic states of Ba+ will permit a precise determination of the ratio of transition matrix elements.
Single ions trapped and laser cooled provide ideal objects for high resolution spectroscopy and frequency standards. For the latter purpose we are investigating the 5s2 1S0 → 5s5p 3P0 transition of In+. With a natural linewidth of only 1.1 Hz this resonance offers very high resolution and is highly immune to frequency shifts due to external electromagnetic fields, because it connects two levels with vanishing electronic magnetic momenta [1]. The wavelength of this clock transition is 236.5 nm and is technically very convenient, since it coincides with the fourth harmonic of the 946 nm Nd:YAG laser line. So this intrinsically frequency stable solid-state laser can be used to excite the transition [2]. For laser cooling and fluorescence detection of the indium ion the 5s2 1So -> 5s5p 3P1 transition at 230.6 nm can be employed. The use of the relatively narrow intercombination line for laser cooling allows us to study optical sideband cooling in the strong binding regime, where the oscillation frequencies of the ion in the trap (around 1 MHz) are larger than the optical linewidth of 360 kHz. In this parameter range laser cooling is possible to the quantum ground state of the vibrational motion in the trap corresponding to a temperature of roughly 100 μK. The same low temperature was obtained with sideband cooling of a small Coulomb crystal consisting of two ions, creating an interesting new quantum few-particle system.