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

We present initial test results for a new short-term frequency standard, the 40 K compensated sapphire oscillator (40 K CSO). Included are measurements of resonator quality factor, operational frequency, turn-over temperature, acceleration (g) sensitivity, frequency drift, and preliminary frequency stability tests. Long-term operation is facilitated use of a small cryo-cooler (1 KW wall power) with no moving cryogenic parts, and thus no preset service interval for the cold head. Initial tests with this single-stage pulse-tube cooler show a stability of 3/spl times/10/sup -14/ (1 second /spl les//spl tau//spl les/10 seconds) and a flicker floor of 2/spl times/10/sup -14/.

High stability 40 Kelvin cryo-cooled sapphire oscillator

The development of a trapped ion based fieldable frequency standard with high stability for averaging times tau >1*10/sup 4/ s is discussed. A hybrid RF/DV linear ion trap which permits storage of a large number of ions with reduced susceptibility to the second-order Doppler effect caused by the RF confining fields is described. /sup 199/HG/sup +/ ions have been confined in this trap and very high Q transitions with good signal-to-noise ratio have been measured. Stabilities of 1.6*10/sup -13// square root tau (50 or=10/sup 6/ seconds. >

Linear ion trap based atomic frequency standard

We report on tests of a frequency-stable temperature compensated sapphire oscillator (CSO) at temperatures above 77 K. Previously, high stability in sapphire oscillators had only been obtained with liquid helium cooling. Recent improvements include a more careful analysis of the ac frequency-lock "Pound" circuitry that now enables the oscillator to reliably attain a stability 6 million times better than its fractional resonator linewidth. The frequency stability of the current resonator with a quality factor of Q/spl ap/2/spl times/10/sup 6/ is approximately the same as for the very best available quartz oscillators. The apparent CSO flicker floor is 7.5/spl times/10/sup -14/ for measuring times between 3 and 10 seconds, with stability better than 2/spl times/10/sup -13/ for all measuring times between 1 and 100 seconds. We project a stability of 2/spl eta/10/sup -14/ for a resonator Q of 10/sup 7/, a value about one third of the intrinsic sapphire Q at this temperature.

Frequency stability of 1×10 -13 in a compensated sapphire oscillator operating above 77 K

We report on the design and test of a whispering gallery sapphire resonator for which the dominant (WGH/sub n11/) microwave mode family shows frequency-stable, compensated operation for temperatures above 77 kelvin. The resonator makes possible a new ultra-stable oscillator (USO) capability that promises performance improvements over the best available crystal quartz oscillators in a compact cryogenic package. A mechanical compensation mechanism, enabled by the difference between copper and sapphire expansion coefficients, tunes the resonator to cancel the temperature variation of sapphire's dielectric constant. In experimental tests, the WGH/sub 811/ mode showed a frequency turn-over temperature of 87 K in agreement with finite element calculations. Preliminary tests of oscillator operation show an Allan deviation of frequency variation of 1.4-6/spl times/10/sup -12/ for measuring times 1 second /spl les//spl tau//spl les/100 seconds with unstabilized resonator housing temperature and a mode Q of 2/spl times/10/sup 6/. We project a frequency stability 10/sup -14/ for this resonator with stabilized housing temperature and with a mode Q of 10/sup 7/. >

Temperature compensated sapphire resonator for ultra-stable oscillator capability at temperatures above 77 kelvin

A chemical polishing procedure has been developed which allows fields of 20-30 MV/m to be routinely achieved at the surfaces of superconducting resonators constructed of lead-plated copper. This has made possible the construction of lead plated resonators with accelerating fields equivalent to those achieved with niobium, since the performance of these resonators is limited by electric field breakdown.1,2 The polish itself is based on a weak acidic solution of hydrogen peroxide and EDTA, a chelating agent. Other polishes have not proven suitable for the thin (5-10?) lead layer because of their very high polishing rates. The present procedure, however, gives good polishing action even at rates as slow as 5?/min. Details of the procedure as well as comparative electric field breakdown data are presented.

G.J. Dick

Papers

High stability 40 Kelvin cryo-cooled sapphire oscillator

Linear ion trap based atomic frequency standard

Frequency stability of 1×10 -13 in a compensated sapphire oscillator operating above 77 K

Temperature compensated sapphire resonator for ultra-stable oscillator capability at temperatures above 77 kelvin

A Polishing Procedure for High Surface Electric Fields in Superconducting Lead Resonators