Steven Chu

Prof. Dr. Steven Chu

United States 
Lawrence Berkeley National Laboratory 
Profs. Claude Cohen-Tannoudji and William Phillips 

Biography on the Official Web Site of the Nobel Prize


Steven Chu was born in St. Louis on February 28, 1948 and studied at the University of Rochester, where he received his bachelor’s degree in mathematics and his master’s degree in physics. He then went on to the University of Berkeley in California to work on his Ph. D., which he obtained in 1976. After two more years as a research fellow at Berkeley, he joined Bell Laboratories in Murray Hill, New Jersey, working in the technical department until 1983, when he became head of the quantum electronics research department of AT&T Bell Laboratories at Holmdel, New Jersey. In 1987, Chu was recruited by Stanford University. He served as United States Secretary of Energy from 2009 to 2013. Presently he is the William R. Kenan, Jr., Professor of Physics and Professor of Molecular & Cellular Physiology in the Medical School at Stanford University.

Chu was awarded the 1997 Nobel Prize in Physics along with his American colleague William D. Phillips and Claude Cohen-Tannoudji from France, for developing “methods to cool and trap atoms with laser light”.

At room temperature the atoms and molecules of air move vigorously and in completely disorderly fashion with velocities of around 4000 km/h. At lower temperatures they slow down so that they can be observed more closely, but they tend to condense so that the distances between their particles become too small to be studied in detail. Experiments in a vacuum are helpful, since the gases do not turn into liquids or solids, but even at -270¡C the particles continue to move too fast (ca. 400 km/h). Velocities of less than 1 km/h, which would allow measurements using sophisticated methods on free hydrogen atoms, are only achieved at one millionth of one degree above absolute zero (-273.15¡C).

The three scientists developed methods to cool and trap atoms with laser light. Atoms irradiated by a laser are slowed down by the impact of the laser photons. In 1985, Chu installed three pairs of lasers facing each other around a vacuum chamber. By using the Doppler effect, he succeeded in lowering the temperature to approx. 240 millionths of one degree above absolute zero, slowing the atoms down to approx. 30 cm/sec. Chu’s team coined the expression “optical molasses” to describe the movements in a viscous medium. Still, the atoms “dropped” out of the molasses before they could be studied. Phillips combined Chu’s method with a technique he developed for trapping sodium atoms by means of magnetic fields (“magneto-optical trap” or MOT). A further refinement of the method by Cohen-Tannoudji succeeded in lowering the temperature to 0.18 millionth of a degree, reducing the velocity to approx. 70 m/h or 2 cm/sec.

Steven Chu  is part of the exhibition project "Sketches of Science" by Volker Steger

Exhibition "Sketches of Science" by Volker Steger - Locations & Dates

Steven Chu's Sketch of Science
Steven Chu's Sketch of Science

By Volker Steger

Chu is a busy man. So, there is not much time for a Nobels-draw-pictures photo
project! He thinks carefully before starting the drawing, contemplating the
white paper for a little while. Then, with carefully chosen colours, he executes
a precise drawing of his “atom trap”. I tell him that I have photographed other
laureates whose work is related to his fi eld.
“Yeah”, he says, “now they are all using this thing!”

Chu ist ein viel beschäftigter Mann. Deshalb hat er nicht viel Zeit für ein
Nobelpreisträger-zeichnen-Bilder-und-lassen-sich-fotografi eren-Projekt!
Er denkt intensiv nach, bevor er mit der Zeichnung beginnt und betrachtet
das weiße Blatt dabei eine Zeitlang eingehend. Dann bringt er mit sorgfältig
ausgewählten Farben eine exakte Darstellung seiner „Atomfalle“ aufs Papier. Ich
erzähle ihm, dass ich andere Nobelpreisträger fotografi ert habe, deren Arbeiten
mit seinem Forschungsgebiet zusammenhängen.
„Ja“, sagt er, „jetzt arbeiten sie alle mit diesem Ding!“

Apply Light Pressure

by Adam Smith

You’re witnessing the manipulation of matter by light. Focus on that atom trapped in the centre of the drawing, where these four laser beams come together (actually there were six laser beams in the experiment, but two are directed along the z axis, out of the plane of the paper). It has encountered what Steven Chu has called ‘optical molasses’, a glue made of light that has slowed it down from travelling at, say, twice the speed of sound to meandering around at about 10 centimetres a second. This is an example of optical cooling of atoms, the ‘cooling’ referring to the reduction in the kinetic energy of the atoms caught in the trap.

So cooling atoms means reducing the random velocity of all the atoms in the sample. And although each little impulse of a photon in a laser beam scattering off an atom won’t affect the atom very much at all, if you average the effect of millions of impacts a second, then the light can have a profound effect. The trick in this technique, however, is to tune down the frequency of the lasers to take account of what’s known as the Doppler shift. Since the atoms are moving, they ‘see’ a slightly different frequency of laser light than that set by the experimenter, in just the same way as a fast-moving train coming towards you makes a higher pitched noise than that same train as it moves away from you. This Doppler shift in frequency can be used to advantage here. By suitably adjusting the laser frequency, the experimenter can ensure that the atoms will encounter a greater ‘push’ from the laser beams they are travelling towards, than the laser beams they are travelling away from. And that way, whichever way they drift, the atoms are always, on average, pushed back into the centre of the trap. The technique, therefore, is known as ‘Doppler cooling’.

The method is now being used for a wealth of applications such as making better atomic clocks, measuring very small changes in gravity and looking for oil. “It opened up a lot of things,” says Chu. “It has gone in very different directions, and it’s really exploded, so it was a thrill to be a part of the birth of this.”


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Steven Chu


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