It's About Time

Recognizing the Contributions of Agilent's own Time Lord, Dr. Len Cutler

Len Cutler is recognized worldwide as an expert in atomic frequency standards and quantum-mechanical effects. Since joining HP in May 1957, he has contributed to many products and received numerous patents and awards. But he is probably best known for his work on the HP cesium beam clocks that are the atomic time-keepers for the world.

In 1964, Len and Al Bagley unveiled the Hewlett-Packard 5060A Cesium Beam Clock at the International Conference on Chronometry in Lausanne, Switzerland. The "portable" and extremely accurate clock was used to synchronize U.S. Naval Observatory and Swiss time to within a microsecond (previous techniques had been limited to millisecond resolution). In 1967, the "flying clock" went to 53 locations in 18 countries and correlated time around the world to an accuracy of about 0.1 microseconds. That same year, the cesium hyperfine transition was adopted as the basis for the SI second, the internationally accepted standard for a time interval. A few years later, in 1972 and 1976, HP's cesium clocks were flown in tests that helped verify Einstein's theories of special and general relativity.

Len's ongoing work led to the inventions of HP's laser interferometer jointly with Al Bagley and Joe Rando, the development of rubidium and mercury ion frequency standards, and precision quartz resonators and oscillators. In 1991, HP released the 5071A cesium beam, developed with Len as technical leader. This clock is presently the world's best commercial clock and has stability of about 1 second in 1.6 million years. One hundred of the 215 clocks that contribute to the stability of the International Atomic Time Standard are 5071A's. While representing about half of the total number of clocks, these 100 clocks account for about 82 percent of the weight given in combining the readings of the various clocks. (The other 115 clocks are hydrogen masers, primary standards, other vendors' cesium standards, and older HP cesium clocks.)

Len has many honors. He is an IEEE Fellow, was elected to the National Academy of Engineering in 1987, received the 1984 IEEE Morris E. Leeds award, the 1984 IEEE Centennial award, the 1989 IEEE Rabi award for his consistent contributions to frequency standards, and, along with Robin Giffard and Curt Flory, the 1993 American Institute of Physics Prize for Industrial Application of Physics. In November 1990 Len was named HP's first Distinguished Contributor. He was also the subject of a front-page profile in The Wall Street Journal March 19, 1997 and in August 2000 was named Inventor of the Week by the Massachusetts Institute of Technology, an honor bestowed as part of the Lemelson-MIT National Program in Invention, Innovation and Creativity.

Interview

The following interview with Len appeared in the July 1996 issue of Inforum, an internal HP Labs newsletter.

On New Year's Eve 1991, the 5071A was unveiled on TV to mark the start of the new year. I heard we were off by a couple of seconds. What happened?

That was a bit embarrassing. We were exactly on the second, just unfortunately the wrong second. There are basically two parts to having a clock. One is getting a very stable frequency source to count the passing of each second, and the second is saying what time it is. Cesium beam clocks have automated mechanisms for synchronizing to the second and other mechanisms are used to set which second it is. We apparently set the wrong starting point and didn't realize it until we got up to the top floor of the Transamerica building. Fortunately, we were able to set it to the right second before the broadcast.

What technology changes have made your work possible?

I'm not sure I can answer the question that way. Some important things have been economic issues: GPS receivers have come down dramatically in price -- about $200 for an eight-channel receiver. Others have been advances in basic research, such as cesium fountains, optical pumping, and better understanding of the quantum-mechanical effects. The advancement in the accuracy of analog-digital converters (ADCs) had a very positive effect. We now have 24-bit ADCs that are self-calibrating and capable of greatly reducing thermal effects. We are now able to build low-cost ovens for our crystal oscillators that can maintain the temperature of the crystal to within a millidegree and thus improve the stability of the resonant frequency. High-performance ovens have traditionally been a very expensive part of the system. The current new oven is about 3 inches in diameter.

What are the current inhibitors?

While cesium clocks have excellent long-term stability, the greatest weakness is the short-term stability caused by shot noise in the cesium beam. The approach we are taking to this is to use optical pumping rather than the traditional magnetic selection to present a higher number of cesium atoms in the "right state" into the microwave cavity and detector. At a high level, the way a traditional cesium clock works is to create a beam of cesium atoms in a particular energy state, pass the atoms into a microwave cavity that causes state transitions in cesium, and then select the atoms that have transitioned to a new energy state. The frequency of the microwave cavity is adjusted via a feedback loop to maximize the number of transitions, resulting in a very stable frequency. If magnetic selection is used, only 1 atom in 16 is in the right state. With optical pumping, we excite the atoms until they fall into "dark states" -- basically once in a dark state, atoms cannot leave the state by additional pumping. Nature was very kind here --- it is easy to pump all the cesium atoms into the dark state that is needed to make the clock. Thus, using optical pumping, we'll get a much more intense beam and improve the signal-to-shot-noise ratio and consequently the short-term stability.

Acknowledgement
The basis of this feature originally appeared on the HP Labs website and is used here with permission.

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