P.T.H. Fisk

Trapped-ion and trapped-atom microwave frequency standards

The stability and accuracy of atomic microwave frequency standards (atomic clocks) have been improving at a rate exceeding one order of magnitude per decade for the past 30 years. This sustained improvement has been driven mainly by the many and diverse technological applications requiring highly stable and accurate time and frequency standards. There is no reason to suspect that this rate of progress is slowing. Over the last decade, research and development in atomic frequency standards has tended increasingly towards the use of atom and ion trapping technologies to approach the supposedly ideal unperturbed atomic frequency reference consisting of a single atom or ion either motionless or in free-fall in a perfect, field-free vacuum. This work has been facilitated by the relatively recent development of techniques whereby laser light is used to cool, confine and manipulate atoms or ions. This paper reviews the current status of atomic microwave frequency standards based on trapped atoms and trapped ions, and attempts to explain the motivation for their development and the principles of their operation, with particular emphasis on the physics of factors which limit their performance.

Laser cooling of 171Yb+ ions in a linear paul trap

We report laser cooling of trapped 171Yb+ ions. The ions are confined in a linear Paul trap. Temperatures below 1 K have been achieved, and evidence of a phase transition to a crystal-like state has been observed. The metastable 2D3/2 level is drained using a transition at 609.1 nm not previously used for this purpose. Laser cooling of 171Yb+ may be of importance to the development of new microwave frequency standards.

Very high Q microwave spectroscopy on trapped /sup 171/Yb/sup +/ ions: application as a frequency standard

A microwave frequency standard based on buffer-gas cooled /sup 171/Yb/sup +/ ions confined in a linear Paul trap has been demonstrated in prototype form. The standard exhibits a fractional frequency instability characterized by an Allan deviation /spl sigma//sub y/(/spl tau/)=3.7/spl times/10/sup -13//spl tau//sup -1/2/ for /spl tau/ >

Performance of a prototype microwave frequency standard based on laser-detected, trapped171Yb+ ions

A microwave frequency standard based on buffer gas-cooled171Yb+ ions confined in a linear Paul trap has been demonstrated in prototype form. The standard exhibits a fractional frequency instability characterised by an Allan deviation of σ(y(τ) = 2.9 × 10−13τ−1/2 for τ < 2 × 104 s. Factors affecting the stability of the standard have been systematically investigated.

Temperature of laser-cooled /sup 171/Yb/sup +/ ions and application to a microwave frequency standard

Microwave frequency standards based on buffer gas-cooled /sup 171/Yb/sup +/ ions have demonstrated high stability but are limited in accuracy by the second-order Doppler shift caused by thermal motion. We have previously obtained near shot noise-limited Ramsey fringes with a laser-cooled ion cloud. Here, we present measurements confirming that the ion temperature remains <1 K throughout the microwave interrogation period for a Ramsey pulse separation of up to 10 s and longer. The potential stability of the ions as a frequency standard is better than /spl sigma//sub y/ (/spl tau/)=5/spl times/10/sup -14/ /spl tau//sup -1/2/, and estimates of the systematic offsets to the clock frequency and their uncertainties indicate that a total uncertainty of four parts in 10/sup 15/ or better is achievable.

The CSIRO trapped /sup 171/Yb/sup +/ ion clock: improved accuracy through laser-cooled operation

The microwave frequency standard based on buffer gas-cooled /sup 171/Yb/sup +/ ions at the CSIRO National Measurement Laboratory has demonstrated high stability, but has an accuracy limited by the second-order Doppler shift due to the thermal motion of the ions. We report measurements obtained with a cloud of approximately 10/sup 4/ ions laser-cooled to below 1 K, including near shot noise-limited Ramsey fringes with a pulse separation up to 10 s. Estimates indicate that a potential stability of /spl sigma//sub y/(/spl tau/)=5/spl times/10/sup -14/ /spl tau//sup -1/2/ and a total uncertainty of 5 parts in 10/sup 15/ or better are achievable for this system.

Hole burning of rare earth ions with kHz resolution

Abstract Hole burning in Y 2 O 3 : Eu 3+ is used to establish and improve laser stability. In LaF 3 : Pr 3+ 130 kHz wide holes, side holes and the spectral diffusion of the holes are studied.

Performance of a prototype microwave frequency standard based on trapped /sup 171/Yb/sup +/ ions

A microwave frequency standard based on buffer-gas cooled /sup 171/Yb/sup +/ ions confined in a linear Paul trap has been demonstrated in prototype form. The standard exhibits a fractional Allan deviation /spl sigma//sub y/(/spl tau/)=3.7/spl times/10/sup -13/ /spl tau//sup -1/2/ for /spl tau/<3000 s.<<ETX>>

Development of a /sup 171/Yb/sup +/ microwave frequency standard at the National Measurement Institute, Australia

Microwave frequency standards based on the 12.6 GHz ground state hyperfine transition in 171Yb+ have been under development at the National Measurement Institute, Australia, for many years. Using a laser-cooled ion cloud, the transition frequency was measured in 2001 to an accuracy of 8 parts in 1014, limited by the homogeneity of the magnetic field due to the stainless-steel vacuum chamber. We have designed and commissioned a new chamber in the alloy CrCu, which is non-magnetic and has good vacuum properties. The design incorporates large viewports and high-quality quartz windows for optical access. Uncertainties associated with field inhomogeneity in the new vacuum system are now below 1 part in 1015, a significant reduction which permits operation in the 10-15 accuracy range. We have also demonstrated the use of photoionization to load the trap in a preliminary experiment, including isotope-selective loading

171Yb+ Microwave Frequency Standard

Microwave frequency standards based on the 12.6 GHz ground state hyperfine transition in 171Yb+ have been under development at the National Measurement Institute, Australia, for many years. Using a laser-cooled ion cloud, the transition frequency was measured in 2001 to an accuracy of 8 parts in 1014 , limited by the homogeneity of the magnetic field. Uncertainties associated with field inhomogeneity in the new vacuum system are now below 1 part in 1015. We demonstrate that other systematic uncertainties such as AC Zeeman shift, microwave imperfections and pressure shifts permit operation in the 10-15 accuracy range. The performance of the 171Yb+ microwave standard can therefore be comparable to that of a caesium fountain.