Cesium Fountain Atomic Clock

An important milestone in the development of NRC's first Cesium Fountain Primary Frequency Standard was reached in April 2009 when the atomic clock standard, known as NRC-FCs1, began producing a time signal. This new fountain clock, developed in the Frequency and Time (F&T) laboratories at the Institute for National Measurement Standards, will be used to participate in the synthesis of the International Atomic Time (TAI), and to improve frequency measurements of atomic properties useful to fundamental physics, metrology and astrophysics.

The term "fountain clock" is used to describe this type of clock because cesium atoms are launched vertically and fall back down following a trajectory that resembles the pattern of a water fountain. Other types of clocks in the laboratories at NRC’s Institute for National Measurement Standards, such as hydrogen masers and cesium beam clocks, have good performance for short time intervals but do not offer the long time accuracy of the Fountain clock. NRC's best time standard currently in use, an ensemble of cesium beam clocks, will not gain or lose more than one millionth of a second in three years, an impressive stability. However, the NRC Fountain Clock will be ten times more stable: it will not gain or lose more than one millionth of a second in 30 years.

To reach such a high accuracy, the circuitry of NRC-FCs1 must be able to obtain a very precise time measurement from cesium atoms. By launching these atoms vertically to a height of about one meter, more time is available to measure precisely the magnetic oscillations of the atoms. The magnetic oscillations occur at a rapid rate of 9192631770 times per second, and therefore require a high performance microwave generator to obtain the most precise measurements. The cesium atoms must also be cooled to a temperature below 3mK (three millionth of a degree above absolute zero). Lasers are used to achieve this low temperature, as well as to launch and detect the atoms once they have been excited by the microwave synthesizer. These atom oscillations, observed in the latest measurements, produced the Ramsey fringes shown on the graph.

This milestone is the latest in a series of achievements that includes the development of a Ramsey cavity to probe the cesium atoms, the assembly of a magnetically shielded vacuum chamber, the trapping and detection of cooled atoms, and a controlled vertical launch of the atomic clouds to a height of over a meter.

Now that the clock is ticking, it has to go through an evaluation process before it can become a Primary Standard. The evaluation includes characterizing the internal operating parameters of the clock such as the number of atoms, the power of the microwave radiation, spurious modes of the Ramsey cavity, and the stability of the magnetic fields. Also, the evaluation process requires measuring the effect of external perturbations such as sensitivity on temperature variations, laser intensity fluctuations, and vertical alignment of the launch. The effects of all these parameters must be understood, controlled precisely, and compensated for in order to achieve the accuracy required for contribution to the international time scale.

Parallel to the development of the Fountain Clock, the Frequency and Time Group is developing a new ion trap that will allow more precise measurements of the strontium ion. The higher precision of NRC-FCs1 will be required to perform these new measurements which will link optical to microwave frequencies via the frequency comb. The strontium ion is a candidate for the future optical atomic clocks, which will improve the stability of time generation by another factor ten.

The additional accuracy of the Fountain Clock will also allow a higher stability and better utilization of the GPS time transfer system and give NRC a better time metrology link to the international community. Our participation will support international efforts to maintain the best realization of the SI second and to continue research in sensitive measurements in fundamental science, such as the study of a possible relation between the changing size of the universe and the changing oscillation frequency of atoms.

• More information about the Cesium Fountain Clock

Related Information

Institutes:

• NRC Institute for National Measurement Standards