Nobel Lecture: Passion for precision*

In our highly complex and ever changing world it is reassuring to know that certain physical quantities can be measured and predicted with very high precision. Precision measurements have always appealed to me as one of the most beautiful aspects of physics. With better measuring tools, one can look where no one has looked before. More than once, seemingly minute differences between measurement and theory have led to major advances in fundamental knowledge. The birth of modern science itself is intimately linked to the art of accurate measurements. Since Galileo Galilei and Christiaan Huygens invented the pendulum clock, time and frequency have been the quantities that we can measure with the highest precision. Today, it is often a good strategy to transform other quantities such as length or voltage into a frequency in order to make accurate measurement. This is what my friend and mentor Arthur Schawlow at Stanford University had in mind when he advised his students: “Never measure anything but frequency! ” Measuring a frequency, that is, counting the number of cycles during a given time interval, is intrinsically a digital procedure that is immune to many sources of noise. Electronic counters that work up to microwave frequencies have long been available. In 1967, the Conference Generale des Poids et Mesures CGPM has defined the second, our unit of time, as the period during which a cesium-133 atom oscillates 9 192 631 770 times on a hyperfine clock transition in the atomic ground state. Today, after 50 years of continuous refinement, microwave cesium atomic clocks reach a precision of 15 decimal digits Audoin and Bernard, 2001 . Even much higher precision is expected from future optical atomic clocks which use atoms or ions oscillating at the frequency of light as the “pendulum. ” By slicing time into a hundred thousand times finer intervals, such clocks will greatly extend the frontiers of time and frequency metrology. The long missing clockwork mechanism can now be realized with a femtosecond laser frequency comb, an ultraprecise measuring tool that can link and compare optical frequencies and microwave frequencies phase coherently in a single step. Laser frequency combs provide powerful tools for new tests of fundamental physics laws. Precise comparisons of optical resonance frequencies of atomic hydrogen and other atoms with the microwave frequency of a cesium atomic clock are already establishing sensitive limits for possible slow variations of fundamental constants. Optical high harmonic generation is extending frequency comb techniques into the extreme ultraviolet, opening a new spectral territory to precision laser spectroscopy. Frequency comb techniques are also providing a key to attosecond science by offering control of the electric field of ultrafast laser pulses. Femtosecond laser frequency combs have been highlighted in the citation for the 2005 Nobel Prize in Physics. Although perfected only about seven years ago, they have already become standard tools for precision spectroscopy and optical frequency metrology in laboratories around the world. Commercial instruments have quickly moved to the market, and extensive review articles and books have been written on frequency comb techniques Udem et al., 2002; Hannaford, 2005; Ye and Cundiff, 2005 . In this lecture I will try to give my personal perspective on the evolution of these intriguing measuring tools for time and frequency. Far from attempting a comprehensive review, I have selected references that helped guide my own insights along a winding path.