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 superconducting split ring resonator as an accelerating structure

A cryocooled compensated sapphire oscillator (CSO), developed for the Cassini Ka-band Radio Science experiment, and operating in the 7 K-10 K temperature range was previously demonstrated to show ultra-high stability of /spl sigma//sub y/=2.5/spl times/10/sup -15/ for measuring times 200 seconds /spl les//spl tau//spl les/600 seconds using a hydrogen maser as reference. CSO-1 and CSO-3 are now both operational with new low noise receivers. We have made initial phase noise and Allan deviation measurements that show more than ten times stability improvement over the hydrogen maser for measuring times 1 second /spl les//spl tau//spl les/10 seconds, and indicate performance for the individual units of /spl sigma//sub y//spl ap/3/spl times/10/sup -15/ for measuring times from 10 to 1000 seconds. Phase noise is reduced by 20 to 28 dB over the design offset frequency range from 1 Hz to 40 Hz. Receiver design is also discussed.

Stability and phase noise tests of two cryocooled sapphire oscillators

The authors describe a spectroscopic measurement of the temperature and linear density of Hg/sup +/ ions held in a linear ion trap (LIT). The inferred temperature and number result from analysis of sidebands on the 40.5 GHz resonance line. The temperature of the ion cloud is determined by the Doppler-broadened line when microwave radiation is propagated along the axis of the LIT. When propagation is perpendicular to the trap axis, the microwave sidebands are displaced from the trap secular frequency by an amount dependent on the ion cloud size and temperature. A Monte Carlo simulation of the ion trajectories inside the cloud is used to model the position of these sidebands for a given cloud temperature, and a comparison with measured sideband position is used to determine ion number. In clock operation with 7 /spl times/ 10/sup -14/ /spl tau//sup -1/2/ stability, the method described here was used to measure a -8.8 /spl times/ 10/sup -13/ frequency offset due to second order Doppler shift at 650 K and a linear ion density of 2.5 /spl times/ 10/sup 6//75 mm. >

Doppler sideband spectra for ions in a linear trap

The standard interrogation technique in atomic beam clocks is square-wave frequency modulation(SWFM), which suffers a first-order sensitivity to vibrations as changes in the transit time of the atoms translates to perceived frequency errors. Square-wave phase modulation (SWPM) interrogation eliminates sensitivity to this noise. We present a particular scheme utilizing independent phase control of the two cavities. The technique is being considered for use with the Primary Atomic Reference Clock in Space (PARCS), a laser-cooled cesium clock scheduled to fly aboard the International Space Station in 2005. In addition to eliminating first-order sensitivity to vibrations, the minimum attack time now in this scheme is the Rabi pulse time (t), rather than the Ramsey time (T). This helps minimize dead time and the degradation of stability due to aliasing.

Phase modulation with independent cavity-phase control in laser cooled clocks in space

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.

G.J. Dick

Papers

The superconducting split ring resonator as an accelerating structure

Stability and phase noise tests of two cryocooled sapphire oscillators

Doppler sideband spectra for ions in a linear trap

Phase modulation with independent cavity-phase control in laser cooled clocks in space

Frequency stability of 1/spl times/10/sup -13/ in a compensated sapphire oscillator operating above 77 K