Presentation Speech by Professor Bengt Nagel of the Royal Swedish Academy of Sciences, December 10, 1997.

Translation of the Swedish text.

Your Majesties, Your Royal Highnesses, Ladies and Gentlemen,

The Royal Swedish Academy of Sciences has decided to award this year’s Nobel Prize for Physics jointly to three physicists for “development of methods to cool and trap atoms with laser light”.

The air around us consists of molecules moving back and forth with breathtaking velocities around a mean of about 500 m/s, distributed around this velocity roughly according to a bell curve, so dear to statisticians. A molecule collides with some other molecule about one billion times per second, and there are around twenty-five trillion (= million million million) molecules in each cubic centimeter. As result of this disordered rapid motion we get properties such as air pressure, the ability to transmit sound, a (fairly bad) capacity for conducting heat, etc.

If we want to study the individual or small numbers of molecules or atoms we must slow down their motion, both in order to get time to observe them, and also because the properties of our communication with them, which occurs with the help of light, are influenced by their velocity. The frequency of the light that a moving atom sends out or can absorb changes because of the Doppler effect, with which we are familiar from acoustics. We can make an analogy with an absurd opera performance – maybe something for this year’s prize winner in literature – where the opera star sings her aria while moving at great velocity around the stage, colliding with and rebounding from her fellow actors. Visually this might be a spectacular performance, but acoustically it could be a disaster: the audience would think that the star didn’t sing in tune. (If somebody would like to try this out, I can mention that if the singer moves with the speed of a good sprinter, the maximum variation in pitch would be half a tone interval.)

Last year’s Nobel Prize in physics dealt with low temperatures, and with conventional cooling methods one might cool the atoms down to some millionths of a degree above absolute zero, which formally would correspond to velocities of some centimeters per second. The problem is that long before this temperature is reached, the atoms or molecules would have condensed into a liquid and finally a solid body. Atoms in a chorus sing different tunes from solo, and it is the solo performance we want. We must reduce the motion of the atoms while keeping them apart.

The idea of using laser light and the Doppler effect to achieve this was discussed as early as the 1970s and is based on the fact that when an atom absorbs a light particle, a photon, it receives an impulse from the absorbed photon, and if it has a velocity opposite to that of the photon, its motion is slowed down. We adjust the frequencey of the laser beam so that the photon resonates with, and hence can be absorbed only by, atoms moving in the opposite direction from that of the laser beam. – This basic idea has demanded many years of extensions and experimental developments to result in an effective cooling and trapping of atoms. An important device is the magneto-optical trap, where atoms are slowed down in an “optical molasses” consisting of three pairs of mutually opposite laser beams, and are kept trapped by a magnetic field. In the latest development, subrecoil cooling, equivalent temperatures of fractions of a millionth of a degree have been obtained.

Besides the direct importance for increasing our knowledge of the interaction between atoms and radiation which is a result of increased control of the atomic motion, one could of course ask about possible practical uses of these new advances.

The classical answer: “One day, sir, you may tax it” was given in the 1850s by Michael Faraday in reply to a question by William Gladstone, then British minister of finance – Chancellor of the Exchequer – if electricity had any practical value. Faraday’s studies of electricity and magnetism laid the foundations of electrotechnology.

I cannot yet see any directly taxable object resulting from the contributions awarded this year’s physics prize, but I believe it will come.

Because we can make atoms practically stand still, we can design much more precise atomic clocks; this enables us, for example, to make a more accurate position determination on earth via satellites. The awarded cooling techniques supplemented by other cooling techniques, have led to achievement of what could be called the “dream mile” of atomic physics, Bose-Einstein condensation of gases, a phenomenon predicted by Einstein more than 70 years ago. This in turn can lead to the construction of “atomic lasers”, coherent intense atom beams, which among other things could be used for the construction of very small electronic components.

Professors Steven Chu and Claude Cohen-Tannoudji, Dr. William Phillips.

You have been awarded this year’s Nobel Prize in Physics as leaders and representatives of the most successful groups and collaborations in the extensive work of developing methods for cooling and trapping of atoms, which has opened up a new area of control and study of atoms and atomic gases.

It is my pleasure and my privilege to convey to you the felicitations of the Royal Swedish Academy of Sciences and to ask you to step forward and receive the Prize from the hands of His Majesty the King.