Students create exotic state of matter

By Bennett McIntosh

IN THE SUMMER OF 2015, Princeton students Joseph Scherrer and Adam Bowman experienced something few undergraduates can claim: they built, from scratch, a laser system capable of coaxing lithium atoms into a rare, highly excited state of matter to reveal their quantum nature.

When they joined Assistant Professor of Physics Waseem Bakr’s lab in the spring of 2014, Scherrer and Bowman had little experience in optics or quantum physics. Their task was to convince lithium atoms to enter a state of matter known as the Rydberg state. In this state, each atom has a very high-energy electron located far from the atom’s nucleus. The separation of the electron’s negative charge from the nucleus’ positive charge creates a dipole, like a magnet’s north and south poles.

To give the electrons the right amount of energy to create the Rydberg state, Scherrer and Bowman hit the atoms with two carefully tuned lasers, first blue and then red. To prove that the lithium atoms had indeed entered the Rydberg state, the two researchers needed a way to detect them. They trawled the scientific literature for a sensitive enough detection method, and eventually implemented a technique called electromagnetically induced transparency. With this technique, the Rydberg atoms interfere with the absorbance of certain wavelengths of light, so if the gas is transparent in those wavelengths, the Rydberg atoms are present.

The undergraduates designed and built the device independently, Bakr said. “I wasn’t planning on starting this, and suddenly it grew into a whole project, largely due to their efforts,” he said.

“It was a turning point in our scientific development,” said Scherrer, who graduated in 2016 with a degree in physics. “For me, it was a realization of what you can do with quantum optics.” Scherrer was awarded a Fulbright grant to join a team in Munich, Germany, where he is building electron microscopes to image the brain. He will next head to the Massachusetts Institute of Technology to pursue a Ph.D. in physics. Bowman, a physics major in the Class of 2017, continues to study the physics of electronically interesting materials, and spent his junior year and the summer of 2016 working on a new project with Ali Yazdani, Princeton’s Class of 1909 Professor of Physics. There, Bowman built a device that works like an inkjet printer for atoms to print superconductors layer-by-layer.