New frequency standards

Abstract. We present a review of the helium spectroscopy, related to transitions between 23S and 23P states around 1083 nm. A detailed description of our measurements, that have produced the most accurate value of the 23Po — 23Pi fine structure interval, is given. It could produce an accurate determination (34 ppb) of the fine structure constant a. Improvements in the experimental set up are presented. In particular, a new frequency reference of the laser system has been developed by frequency lock of a 1083 nm diode laser to iodine hyperfine transitions around its double of frequency. The laser frequency stability, at 1 s timescale, has been improved of, at least, two orders of magnitude, and even better for longer time scales. Simultaneous 3He —4 He spectroscopy, as well as absolute frequency measurements of 1083 nm helium transitions can be allowed by using the Li-locked laser as frequency standard. We discuss the implication of these measurements for a new determination of the isotope and 23 5 Lamb shifts. 

The two-photon 243 nm lS S-i/2 transition and other transitions in the hydrogen atom have recently been included in a new list of approved radiations for the practical realisation of the metre [2]. The particular interest of these transitions lies in the fact that, uniquely among present optical frequency standards, they may be calculated in terms of the Rydberg constant with an accuracy approaching that with which they have been measured. The latest measurement of... 

In this contribution we speculate on some new techniques of optical frequency comparison by use of modulated lasers, and on a possible new optical frequency standard based on the methane stabilised HeNe laser. He start by reviewing our... 

The rapid progress in recent years in the spectroscopy of the hydrogen atom has renewed pressure for a much better optical frequency standard. This in itself would not be enough to solve the measurement problem. New techniques of comparing optical frequencies are needed. He have developed methods of modulating lasers which can be used for frequency differences in excess of 2THz. 

New optical frequency standards based on harmonics of methane stabilised lasers will mean that we will never be more than 44 THz away from a reference frequency. New techniques of making frequency interval measurements of this magnitude will then be needed. 

The deposition of noble metals onto oscillating quartz crystals of the thickness shear type, for fine adjustment of their frequency, has already been carried out for many years by frequency standard manufacturers. The idea of using the frequency decrease by mass deposition to determine the weight of the coating is comparatively new. Sauerbrey [35] and Lostis [36] were the first to propose the quartz-crystal microbalance. The AT-cut crystal oscillating in a thickness shear mode was found to be best suited for this purpose. The thickness xq of an infinite quartz plate is directly related to the wavelength A. of the continuous elastic transverse wave, the phase velocity vq of that wave and the frequency vq (i.e. the period xq) of the oscillating crystal, as shown in Fig. 4 ... 

In this Section, we will describe briefly the most recent projects of atomic clocks involving/based on ion traps as described above. The first part concerns micro-wave clocks, while the one following will be dedicated to optical frequency clocks. Performances of atomic standards can be evaluated only by comparison (frequency beatings) with another devices. When a new atomic standard can be presumed to out-perform the norm, it can be evaluated only from the comparison with a second system, which must be build in a similar way. It is worth noting that performances of each scheme depend on the local oscillator a quartz (eventually, cryogenic) oscillator for the microwave range, and a laser for the optical one. 

Audoin, Claude, and Bernard Guinot. The Measurement of Time Time, Frequency, and the Atomic Clock. Translated by Stephen Lyle. New York Cambridge University Press, 2001. Offers information on the physics behind time measurement, with sections on atomic frequency standards, types of oscillators and atomic clocks, and scientific and industrial applications of time measurement. Includes appendixes listing global time laboratories and definitions of SI units. 

J.P. Gordon, H.J. Zeiger, Ch.H. Townes The maser - new type of microwave amplifier, frequency standard and spectrometer. Phys. Rev. 99, 1264 (1955)... 

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