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Dense Atomic Vapors

Steve Cundiff studies the behavior of dense atomic vapors at temperatures ranging from 300–800 °C. In his group’s initial experiments, researchers directed two or three excitation laser pulses into dense vapors of potassium atoms (39K). The group used a reflection cell to study the signal beam generated by coherent interaction between the excitation pulses in the vapor. This method is similar to using a stroboscope, which uses pulses of ordinary light to make tennis balls appear stationary as they fly through the air. One major goal of this and subsequent experiments is to test an 1873 prediction of a fundamental interaction of light (known as the Lorentz-Lorenz shift) with a dense ensemble of oscillators. The Cundiff group’s results suggest that the interactions are more complicated than Hendrik Lorentz predicted more than 130 years ago.

The first set of experiments showed that the first laser pulse synchronized resonance frequencies of the emitted light from the 39K atoms (i.e., created coherence); additional pulses gathered information about the dissipation of the coherence caused by atomic collisions. By varying the amount of time between pulses, the group monitored what occurred as atoms approached each other, collided, and flew apart. The group also studied the change of the decay rate of the signal with different laser powers.

In the next set of experiments, the Cundiff group used transmission cell in a JILA MONSTR (Multidimensional Optical Nonlinear SpecTRometer) to perform two-dimensional Fourier-transform spectroscopy of the dense atomic vapor. This technique allowed the researchers to observe how a laser pulse interacts with the 39K atoms. It also made it possible to discover new phenomena and investigate their properties. This research continues to shed light on the collision behavior of 39K atoms in a dense vapor.

JILA MONSTR Helps Solve the Schrödinger Equation for Hot K Atoms

The Cundiff group recently came up with an experimental technique to measure key parameters needed to solve the Schrödinger equation for detailed spectra of a gas of hot (180 °C) potassium (K) atoms. The researchers obtained the spectra by using the JILA MONSTR to perform optical three-dimensional (3D) Fourier-transform spectroscopy on the gas. The spectra allowed them to see what was happening inside the quantum world of the atoms in their experiment.

The researchers were able to disentangle all possible pathways between specific initial conditions such as excited states or superposition states. Once all pathways had been identified, the researchers were able to make the measurement necessary for characterizing the pathways. With this information, they were able to figure out some pieces of the Hamiltonian they needed. The Hamiltonian is a key part of the Schrödinger equation that describes the time-dependent evolution of quantum states in a physical system such as a gas of hot K atoms. 

This technique opens up many possibilities, including realizing the dream of coherently controlling chemical reactions. Coherent control requires an understanding of all possible quantum pathways in a particular reaction. The fact that optical 3D Fourier-transform spectroscopy made it possible to identify all the pathways in this experiment is a big step forward in realizing this dream. The new technique also opens the door to experimentally determining a Hamiltonian for an even more complex system.