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His thesis and postdoctoral work at Berkeley, the observation of parity non-conservation in atomic transitions in 1978, was one of the earliest atomic physics confirmations of the Weinberg-Salam-Glashow theory that unifies the weak and electromagnetic forces.
While at Bell Laboratories he and Allen Mills did the first laser spectroscopy of positronium, the bound state of an electron and positron in 1982. They went on to measure the 1s-2s energy level splitting of that atom to an accuracy of a few parts per billion. They also made the first measurement of the corresponding transition in muonium, an atom consisting of ?+ and an electron. He also worked on exciton energy transfer in solids and picosecond pulse propagation in solids.
In 1985, he led the group that showed how to first cool and then trap atoms with light. The optical trap was also used to trap microscopic particles in water: these so-called "optical tweezers" are widely used in biology. The first optical trapping was followed by the demonstration of the magneto-optic trap, the most commonly used atom trap. After joining the Stanford Physics Department in 1987, Chu (and independently, Dalibard and Cohen-Tannoudji) explained how multi-level atoms can be cooled far below the minimum temperature predicted by the theory of two-level atoms. His group also demonstrated the first atomic fountain and then made the first atomic fountain frequency standard to exceed the short term stability of atomic clocks maintained by standards laboratories. They developed a novel atom interferometer that has already exceeded the accuracy of the most accurate commercial inertial sensors.
Using the optical tweezers, Chu developed methods to simultaneously visualize and manipulate single bio-molecules. Using this new technique, his group have used single DNA molecules to address a number of long standing problems in polymer science. His group is also applying methods such as fluorescence energy transfer, optical tweezers and atomic force microscope methods to study the interactions and motion of individual bio-molecules.
Chu has been awarded the Herbert Broida Prize for Spectroscopy (American Physical Society, 1987), Richtmyer Memorial Prize Lecturer (APS/AAPT, 1990), co-winner of the King Faisal International Prize for Science (1993), the Arthur Schawlow Prize for Laser Science (APS, 1994), the William Meggers Award for Laser Spectroscopy (Optical Society of America, 1994), the Science for Art Prize (Louis Vitton - M”et Hennesey, 1995), and co-winner of the Nobel Prize in Physics (1997). Chu received a Humboldt Senior Scientist award (1995) and the Guggenheim Fellowship (1996).
He is a member of the National Academy of Sciences, the American Philosophical Society, the American Academy of Arts and Sciences, the Academica Sinica, and is a foreign member of the Chinese Academy of Sciences and the Korean Academy of Science and Engineering.
Steven Chu is the Theodore and Frances Geballe Professor of Physics and Applied Physics at Stanford University. Professor Chu's research is primarily in atomic physics, quantum electronics, polymer and bio-physics.