Synopsis: Photons gained, photons lost
Studying spectra resulting from excitations is a standard tool to investigate many-body systems. In Bragg spectroscopy of ultracold atomic gases—a scattering process where a driving field subjects a gas to a perturbation—such spectra can be directly compared with theory. The system’s response to the perturbation is usually measured with a time-of-flight imaging technique of the atomic cloud. This method, however, is not particularly useful for low-momentum excitations and strong interactions, where the response of the system to the driving field can be hard to quantify.
In a paper published in Physical Review A, J. M. Pino and colleagues at JILA and at the University of Colorado get around this difficulty with a new technique, which measures the probing field’s complementary reaction to the gas. They count the photons gained or lost in one of the laser fields used to drive the excitations, using techniques that improve sensitivity by minimizing the effects of noise.
An interesting and powerful new feature of the photon-counting measurement technique is that, in contrast to the time-of-flight technique, it can be used to probe the dynamics of Bragg excitations during a single laser pulse. – Franco Dalfovo and Jihane Mimih