https://link.springer.com/content/pdf/10.1016%2FS1044-0305%2802%2900384-7.pdf

A two-dimensional quadrupole ion trap mass spectrometer.

Atomic clocks have been instrumental in science and technology, leading to innovations such as global positioning, advanced communications, and tests of fundamental constant variation. Timekeeping precision at 1 part in 10 18 enables new timing applications in relativistic geodesy, enhanced Earth- and space-based navigation and telescopy, and new tests of physics beyond the standard model. Here, we describe the development and operation of two optical lattice clocks, both using spin-polarized, ultracold atomic ytterbium. A measurement comparing these systems demonstrates an unprecedented atomic clock instability of 1.6 × 10 –18 after only 7 hours of averaging.

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An atomic clock with $10^{-18}$ instability

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.

Minimization of ion micromotion in a Paul trap

▪ Abstract Laser ablation of in situ metals has recently made it possible to immerse a large number of different metal atoms and ions and small clusters of metal atoms in liquid helium (He) and thus study their absorption and emission spectra in the visible region. Atoms and molecules are readily picked up by large ( ≥ 103 atoms) He droplets, and their spectra are sensitively detected through the use of either beam depletion following absorption or laser-induced fluorescence. Within the past three years, a wide variety of molecules, ranging from OCS to large organic molecules such as amino acids and a number of van der Waals complexes and even large metal clusters, have been embedded in He droplets and studied either in infrared or in the visible region. These results are discussed here in detail, and the evidence for the effect of superfluidity on the spectral features is reviewed.

Spectroscopy of atoms and molecules in liquid helium

Characteristics of mass selective axial ion ejection from a linear quadrupole ion trap in the presence of an auxiliary quadrupole field are described. Ion ejection is shown to occur through coupling of radial and axial motion in the exit fringing fields of the linear ion trap. The coupling is efficient and can result in extraction of as much as 20% of the trapped ions. This, together with the very high trapping efficiencies, can yield high sensitivity mass spectral responses. The experimental apparatus is based on the ion path of a triple quadrupole mass spectrometer allowing either the q2 collision cell or the final mass analysis quadrupole to be used as the linear trap. Space charge induced distortions of the mass resolved features while using the pressurized q2 linear ion trap occur at approximately the same ion density as reported for conventional three-dimensional ion traps. These distortions are, however, much reduced for the lower pressure linear trap possibly owing to the proposed axial ejection mechanism that leads to ion ejection only for ions of considerable radial amplitude. RF heating due to the high ejection q-value and the low collision frequency may also contribute. Two hybrid RF/DC quadrupole-linear ion trap instruments are described that provide high sensitivity product ion scanning while operated in the linear ion trap mode while also retaining all conventional triple quadrupole scan modes such as precursor ion and neutral loss scan modes. Copyright © 2002 John Wiley & Sons, Ltd.

A new linear ion trap mass spectrometer

The authors have designed and are presently testing a novel linear ion trap that permits storage of a large number of ions with reduced susceptibility to the second-order Doppler effect caused by the RF confining fields. This new trap should store about 20 times the number of ions as a conventional RF trap with no corresponding increase in second-order Doppler shift from the confining field. In addition, the sensitivity of this shift to trapping parameters, i.e., RF voltage, RF frequency, and trap size, is greatly reduced. The authors have succeeded in trapping mercury ions and xenon ions in the presence of helium buffer gas. Trap times as long as 2* 10/sup 3/ s have been measured. >

Linear ion trap for second-order Doppler shift reduction in frequency standard applications

We present design aspects of a cryogenic sapphire oscillator which is being developed for ultra-high short term stability and low phase noise in support of the Cassini Ka-band Radio Science experiment. With cooling provided by a commercial cryocooler instead of liquid helium, this standard is designed to operate continuously for periods of a year or more. Performance targets are a stability of 3/spl times/10/sup -15/ (1 second/spl les//spl tau//spl les/100 seconds) and a phase noise of -73 dB/Hz @ 1 Hz measured at 34 GHz. Test results are reported for several subsystems; including cryocooler, vibration isolation system, and ruby compensating element.

Cryo-cooled sapphire oscillator for the Cassini Ka-band experiment

: Mercury 199 Ions are confined in an RF trap and state-selected by use of optical pumping with 194 nm UV light from a (202)Hg discharge lamp. Absorption of microwave radiation at the hyperfine frequency (40.5 GHz) is signaled by atomic fluorescence of the UV light. The frequency of a 40.5 GHz oxcillator is locked to a 1.6 Hz wide atomic absorption line of the trapped ions. The measured Allan variance of this locked oscillator is currently sigma(y) (tau) = 4.4 x 10(-12) / square root of tau for 20 tau 320 seconds, indicating better short term stability than the best commercial cesium standards by almost a factor of two. This first result was achieved without magnetic shielding and without regulation of ion number.

https://ieeexplore.ieee.org/iel5/10321/32754/01537936.pdf

JPL Trapped Ion Frequency Standard Development

The /sup 199/Hg/sup +/ research frequency standards LITS-1 and LITS-2 were developed to provide continuous, reliable, high stability performance. For simplicity, a /sup 202/Hg lamp is used for state selection and a helium buffer gas for ion cooling. In a preliminary 9 day comparison between the trapped ion standards, the Allan deviation was /spl sigma//sub y/(/spl tau/)=1/spl times/10/sup -13///spl tau//sup 1/2/ and a fractional frequency stability of 6/spl times/10/sup -16/ measured for averaging times greater than 10/sup 5/ seconds. A 40 day comparison of LITS-2 against an auto-tuned H-maser referenced to UTC-NIST puts an upper limit on long term drift of LITS-2 of 1.2(1.4)/spl times/10/sup -16//day. >

Recent stability comparisons with the JPL linear trapped ion frequency standards

The authors describes an improvement in the architecture of the physics package used in the linear ion trap (LIT) based frequency standard developed at the Jet Propulsion Laboratory. This new design is based on the observation that ions can be moved along the axis of an LIT by applied DC voltages. The state selection/interrogation region can be separated from the more critical microwave resonance region where the multiplied local-oscillator signal is compared to the stable atomic transition. This separation relaxes many of the design constraints of the present units. Improvements include increased frequency stability, and a substantial reduction in size, mass, and cost of the final frequency standard. >

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G.J. Dick

Papers

Linear ion trap for second-order Doppler shift reduction in frequency standard applications

Cryo-cooled sapphire oscillator for the Cassini Ka-band experiment

JPL Trapped Ion Frequency Standard Development

Recent stability comparisons with the JPL linear trapped ion frequency standards

Improved linear ion trap physics package