Kharkiv Quantum Seminar

We use ultrashort laser pulses – including a unique tool known as an optical centrifuge – to coherently control and probe ultrafast, nonequilibrium many-body dynamics in superfluid helium. In this talk, I will present two complementary approaches, recently developed in my group, that exploit strong-field and ultrafast laser techniques to gain new microscopic insight into the remarkable quantum phenomenon of superfluidity.

In the first approach, we use the optical centrifuge to spin up molecules dissolved in liquid helium and investigate the decay of their rotation due to interactions between the molecular rotor and the surrounding quantum bath [1]. The ability to precisely control the rotational frequency of the molecule using tailored laser fields provides a powerful handle for studying how angular momentum and energy are exchanged between a single quantum particle and a many-body environment.

In the second approach, we exploit the high peak intensity of femtosecond laser pulses to coherently launch collective excitations known as rotons. By tracking the nonequilibrium many-body dynamics of these mysterious quasiparticles on a picosecond timescale, we probe the ultrafast response of the superfluid and explore the microscopic origin of its collective behavior from a complementary, previously inaccessible, viewpoint [2].

Taken together, the study of these two objects – laser-driven single molecular rotors and optically generated collective excitations involving many helium atoms – brings us closer to a better microscopic understanding of superfluidity.

[1] MacPhail-Bartley I., Milner A. A., Stienkemeier F., Milner V. “Control of molecular rotation in helium nanodroplets with an optical centrifuge.” Phys. Rev. Lett. 136, 033002 (2026).

[2] Milner A. A., Stamp P. C. E., Milner V., “Ultrafast non-equilibrium dynamics of rotons in superfluid helium,” PNAS 120, e2303231120 (2023).