Nobel Laureate delivers latest “Frontiers of Physics” lecture

Nobel Laureate Wolfgang Ketterle came to speak at Tech last Friday as part of the “Frontiers of Physics” lecture series hosted by the School of Physics.

Ketterle, the 2001 winner of the Nobel Prize in physics for his work with Bose-Einstein Condensates, spoke on the topic of “New Forms of Quantum Matter Near Absolute Zero Temperature.” Ketterle is currently a professor at the Massachusetts Institute of Technology, specializing in ultracold physics.

Ketterle’s work focuses on matter in the nanoKelvin temperature range. Before discussing some of the phenomena that occur when matter is lowered to such extremely low temperatures, Ketterle talked about the methods used to bring matter to those temperatures.

The first method used, Ketterle explained, is called laser cooling. When atoms absorb certain wavelengths of light, the photons that are subsequently radiated away are blue shifted—more energetic than those initially absorbed. (This work won a Nobel Prize in 1997.) In this way, specially tuned laser beams may be used to reduce the temperature of matter to as much as a thousand times colder than interstellar space.

After matter the matter is laser-cooled, a second method, called evaporative cooling, is employed to reduce the temperature of the matter to nanoKelvins, and sometimes even to picoKelvins. Scientists accomplish evaporative cooling at these levels by holding the matter in an electromagnetic potential well from which only the more energetic particles can escape. This process leaves the cooler (less energetic) particles behind, lowering the temperature of the contained matter.

The reason Ketterle’s lab group needs to achieve temperatures on the nanoKelvin scale is that decreases in temperature cause increases in the DeBroglie wavelength. When the temperature is lowered enough to make the DeBroglie wavelength greater than the interatomic spacing, the wave properties of the atoms begin to dominate. “Physics is not changing when you change one parameter—you have to change the relative importance of two parameters to change the hierarchy” said Ketterle.

Ketterle’s lab group gathers information about the velocity distributions, and thus the temperature, of particles using a $20,000, very fast and very high resolution camera to photograph the particles both before and very soon after they are released from their electromagnetic containment. Comparing the shape of the cloud of particles before release to the shape of the cloud after release gives Ketterle’s lab group information about the velocities of the particles which make up the cloud. “The hallmark of Bose-Einstein Condensates is that there’s a group of molecules at zero temperature that doesn’t expand,” said Ketterle.

When temperatures are on the order of nanoKelvins, Ketterle can manipulate matter with lasers no more powerful than a simple laser-pointer in ways which would require lasers more powerful than anyone on earth can build to accomplish at room temperature.

Much of the importance of Ketterle’s work has to do with creating and studying strong interactions between the particles in the systems Ketterle works with. “Let’s face it: Life and physics get more interesting when you have strong interactions,” said Ketterle.

Resonant interactions have infinite strength. When we put particles in resonant states, said Ketterle, we see interactions so strong that they are limited only by other physical parameters such as the Fermi energy.

At cold enough temperatures, bosons, which are particles with integer spin, condense into the same quantum state, and thus become completely correlated and incredibly strongly interacting. “Atomic clocks tick slower when you have multiple atoms in one state” said Ketterle, giving an example of the strange phenomena that occur when systems of atoms become strongly correlated and strongly interacting.

Ketterle predicts that because the atoms in the ultra-cold systems he studies are both strongly correlated and strongly interacting, his results will shed light on such topics as high temperature super-conductors and spin liquids, which are also systems in which the atoms are strongly correlated and strongly interacting.

“Ultra-cold atoms are a toolbox for designer matter,” said Ketterle. In Ketterle’s ultra-cold systems, where atoms can be very easily manipulated, there are many possibilities for creation of new configurations of matter.

“It’s been ten exciting years since the advent of Bose-Einstein condensates—I really feel that the field is brimming with excitement,” said Ketterle at the close of his lecture.

After a round of applause, Ketterle entertained some questions from the audience, then signed autographs, posed for pictures, and answered some questions from individuals.

Ketterle attributes research success to being in the right place at the right time, working with excellent people and being lucky.

“And not everybody is lucky,” he said. “We never expected fermions to behave this way—there were a lot of things I didn’t have the imagination to predict, but we just got in there and kept our eyes open.”