Coherent Control of Rotons in Superfluid Helium

Abstract:

Coherent control is the use of light to control the evolution of an initial state of a quantum system to some defined final state. A femtosecond laser pulse is used to create elementary excitations within superfluid helium-4. Most notable are those corresponding to the maximum, minimum, also known as maxons and rotons, and plateau region of the dispersion curve, which relates these excitations to their respective energies and momenta. Light scattering experiments have only seen roton-pairs and a very broad feature in the spectrum assigned to excitations near the plateau. 

In our experiment, we detect small but defined peaks in a time resolved approach assigned to the maxons and the plateau. Controllability is demonstrated by recording the response as pulse energy increases. Coherent control is used to suppress the dominant roton-pair resonance and reveal the small outlying peaks. These features are then fitted in the time domain to learn their relative phase at t=0.