Abstract
Spontaneous emission has been studied by physicists for decades and continues to reveal exciting physics. We describe an experiment to study the spontaneous emission of a monoatomic vapor at room temperature as a fundamental experiment for upper-level physics undergraduates. The experiment begins by exciting a population of Rb atoms in a room temperature vapor cell with a laser pulse shorter than the average lifetime of the excited states. The resulting fluorescence signal is recorded as a function of time, and the excited state lifetime can be determined by measuring the decay rate. By analyzing the Fourier transform of the time-dependent polarized fluorescence signal, quantum interference (i.e., quantum beating) is observed among the hyperfine energy levels. This experiment can be completed by upper-level undergraduates in physics to demonstrate and connect hands-on experiments with concepts in atomic and quantum physics classes.
Editor's Note: This paper presents a modern take on the quantum beat experiment. The authors use a commercially available distributed feedback laser, along with electro-optical and acousto-optical modulators, to generate short laser pulses. These laser pulses are used to excite rubidium atoms into a superposition of multiple atomic states, and the resulting polarized fluorescence decay data are collected. The necessary quantum theory for analyzing these data is presented and then used to determine the lifetime of the Rb excited state. In addition, the oscillatory behavior present in the decay data is analyzed via a Fourier transform, yielding values for the frequency differences of the excited Rb hyperfine levels. This project offers a clear demonstration of important quantum concepts and is suitable as an instructional laboratory for advanced undergraduates.