Scooping the time from a fountain

The race against the time and for the time is well underway. Many metrology institutes worldwide participate, and among them is the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig. The objective: to construct the most precise clock of the world; a clock which outshines the atomic clocks available to date. The method: to construct a fountain for cold atoms. Interim report from PTB: the fountain clock begins to tick.

Looking at the heart of an atomic fountain clock: six lasers radiate their light on a gas of cesium atoms (in a vacuum chamber shown on the left, below the copper coil visible here), thus producing a source of cold atoms. The lasers can also give the atoms a well-aimed push: like water drops in a fountain the atoms fly upward little less than one meter until gravity pulls them down again. It is expected that a clock of such design will be wrong by less than one second in 10 million years. (Photograph: PTB)

Clocks are to go as steadily as time itself, the second ticks are to be always of the same well-defined length. Today's atomic clocks already achieve this in good approximation: the best of them differ by less than one millionth of a second per year. Why then measure the time even more precisely?

Precise clocks are not only found in the laboratories of the metrology institutes; they are also in the stars: pulsars which emit electromagnetic radiation at short, regular intervals are extremely steady "beacons" in the sky. Clocks on earth must be as precise as is at all possible in order that the variations of the pulsars' clock pulse can be determined. Astrophysicists, too, welcome highly precise clocks, as they want to measure with great precision the effects of Einstein's relativity theory predicted by him. For only if the best clocks are used as references will it ever be possible to quantitatively compare Einstein's predictions and other gravitation theories. But atomic clocks play an important part also in everyday life on earth: The Global Positioning System (GPS) which is based on measurements of the travelling time of radiosignals between satellites and receivers on earth, i.e. the comparison of clocks is at its basis, can function the more precisely the more stable the clocks in the satellites and the control stations are.

All this - and a great deal of scientific and technical ambition - are the mainsprings of the race for the world's most precise clock. A race which has been given further impetus by the 1997 Nobel Prize in physics awarded to Steven Chu, Claude Cohen-Tannoudji and William Phillips for the development of a method by which they had succeeded in extremely cooling down atoms with the aid of laser light and trapping them. The atoms - which ambient temperature had before accelerated to high-speed zig-zaggery - move only leisurely in such a "trap". And slow atoms allow also the precision of atomic clocks to be further improved.

The atoms flit through the PTB's cesium atomic clocks horizontally (parallel to the earth's surface), at a rate of about 100 meters per second, so that they travel the decisive distance inside the apparatus (about one meter long) in one hundredth of a second. To obtain a sharper resonance signal - the signal from which information about the duration of the second is derived - this travelling time must be prolonged, and this can be achieved either by extending the distance or by reducing the atoms' speed. As the size of atomic clocks cannot be arbitrarily large, the atoms' speed must be reduced instead. However, in particular in the case of slow atoms gravity makes itself felt: the atoms simply drop downward - a drawback of these atomic clocks. The fountain design, on the other hand, allows intelligent use to be made of gravity.

Using an ingenious arrangement of several laser beams, the PTB's clock experts irradiate a gas of cesium atoms. This irradiation does not, however, "heat up" the atoms, it rather cools them down so that a gas which was initially at room temperature turns into an atomic ensemble with a temperature of a few millikelvin. The atoms "freeze" in a way and then move only at a rate of more or less one centimeter per second - a source of cold atoms has been formed. When the lasers are "detuned" in relation to one another, for a short time and in a defined way, the cooled and trapped atoms can be given a well-aimed "push" upward: they fly upward, rise until gravity has consumed their motive energy and drop down again along the same way. A scenario that reminds one of a fountain.

Just as in a conventional atomic clock the energy state of the atoms is manipulated and checked. During their upward and downward movement the atoms fly through the microwave field of a resonator, and this changes the condition of the atom's electron shell. However, the time of interaction with the microwave field has become significantly longer: a stone thrown up one meter needs little less than one second to return into the hand. This is the time for which also the atoms in a fountain clock are in contact with the microwave field so that the resonance signal measured is correspondingly sharper. The aim of fixing the second "more sharply" is within easy reach.

Accuracies of "10 to the power of minus 15" seconds are concerned in the fountain clock, that is to say, time errors which are so small that such a clock (in the mental experiment) would be wrong by one second in more than 10 million years. A great number of small effects may prevent success. A fruitful competition is going on in the national metrology institutes in Germany, France, Great Britain, Japan, the USA and elsewhere, with different ideas for, and approaches to, the construction of such a clock.

The colleagues at the Paris Laboratoire Primaire du Temps et des Fréquences are a bit ahead of all, they recognized earlier that the new development has a promising future. However, it is important for all applications that the clocks' accuracy is proved by comparison among one another. For this reason alone is it worthwhile participating in the competition and developing several clocks.

The fountain clock at PTB on which scientists and technicians at PTB have been working for two years will begin to tick one of these days. The results of first comparison measurements with the other PTB clocks fully come up to the expectations. They also show, however, that many investigations are still required before time can in fact be scooped from the fountain. But one thing is certain: the time of the fountain clocks will come.

Additional information can be obtained from:
Dr. Andreas Bauch
Unit of Time section, PTB
telephone: ++49 531 592-4320
e-mail: time(at)ptb.de