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Proceedings of the National Academy of Sciences

Metastability transfer spectroscopy with two like ions in the same trap

HANS G. DEHMELT, Warren Nagourney

Published Oct 1, 1988• 7 citations

Abstract

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.

DOI: 10.1073/pnas.85.20.7426

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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.

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.

The velocities of atoms can be manipulated by near-resonant laser light. After many absorption-spontaneous emission cycles momentum is transferred to the atoms in the direction of the laser beam. In this way atoms can be slowed down and cooled to milliKelvin temperatures and below. In three mutually orthogonal pairs of counter-propagating laser beams in combination with a magnetic field atoms can be cooled and trapped. In this so-called magneto-optical trap (MOT) two atoms can be excited to a molecular state by absorption of a photon. This technique is called photoassociation spectroscopy. In our case we study metastable helium atoms, which have a large internal energy. When two metastable helium atoms approach each other, one of the atoms ionizes and the ions are detected by a microchannelplates detector. When a probe laser beam is directed onto the MOT cloud and scanned in frequency, atom pairs can be excited to several ro-vibrational states. In the excited state the metastable helium atoms are accelerated towards each other and ionize at short internuclear distance. The ro-vibrational states appear as peaks in the ionization signal and from the position of these states information on the excited state interaction potential can be extracted.

With two ions in a spheroidal Paul trap, the harmonic trapping potential turns, by Coulomb repulsion, into a bistable well. Two Ba ions have been confined, laser-cooled, and observed, via their laser-excited resonance scattering, by a spatially resolving photomultiplier, or intensifying CCD camera. A repump laser releases the ions from a metastable state. Well-cooled ions are found localized. When the laser is detuned, the Raman cooling rate and the ions’ temperature vary. A transition from the crystallized state to a toroidal gas takes place. The ions are discriminated when one (i) gets excited into a non-fluorescing metastable state, or (ii) is of another isotopic species. Hopping rates of 1 s −1 are found at high thermal excitation. Elsewhere, the rates drop by two orders of magnitude, and vary resonantly with temperature near potential energy/kinetic energy = 15, where the bright ion’s hopping upstream of the laser beam is five times more likely than downstream. Two discernible ions in a trap represent a microscopic model system for the study of reaction kinetics.

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.

Abstract The kinetics of two-step photoionization of barium atoms through excited P 1 1 , 3 levels was studied by numerical simulation method in the optically dense approximation. The first laser at wavelength λ 1 = 553.5 nm ( λ 1 = 791 nm) pumps transition S 0 1 ↔ P 1 1 ( S 0 1 ↔ P 1 3 ). The second laser with λ 2 = 266 nm ( λ 2 = 310 nm) ionizes atoms from the level P 1 1 ( P 1 3 ). The third probe laser with wavelength λ i = 493.41 nm causes resonant fluorescence of barium ions. It is shown that the metastable D 2 3 from the D 1 , 2 , 3 3 triplet in barium is strongly populated through P 1 <

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.