Observing the creation of quasiparticles in real-time

Quasiparticles are among the most important concepts in condensed matter physics. For example, when an electron moves in solid matter, it polarises its environment due to its electrical charge. This “polarisation cloud” moves with the electron. Together, the two can be viewed as an independent quasiparticle – a so-called polaron.

“You can compare it to a skier in powder snow,” says Rudolf Grimm, professor at the University of Innsbruck (Austria). “The skier is engulfed in a cloud of snow crystals. Together they form a system with properties different from those of the skier without the snow cloud.”

However, thus far observing the formation of such quasiparticles in solid state systems in real time has been elusive. The formation processes take place on attosecond time scales – incredibly short time intervals, as an attosecond is to a second roughly what a second is to the age of the universe.

Physicists at the Technical University of Munich (TUM) and Harvard University in the USA proposed to apply the methods used in extremely accurate atomic clocks to create an ultra-cold environment in which the formation of quasiparticles takes place in slow-motion.

Under these conditions, the group led by Rudolph Grimm at the Institute of Experimental Physics of the University of Innsbruck and the Institute of Quantum Optics and Quantum Information of the Austrian Academy of Sciences succeeded in producing precisely controllable many-particle systems. This made it possible, for the first time ever, to study the creation of quasiparticles in real time.

To this end the scientists generated an ultracold quantum gas of lithium atoms containing a few potassium atoms in a vacuum chamber. For both types of atoms, they chose isotopes that, as fermions, have the same fundamental character as electrons. The interactions were controlled via magnetic fields to create fermionic polarons, which are potassium atoms dressed by a cloud of lithium.

“While the natural time scale of such quasiparticles in solid state systems is in the range of 100 attoseconds, the creation of polarons in this kind of system takes several microseconds,” says Michael Knap, Professor for Collective Quantum Dynamics at TU Munich.

“The novel methodology thus provides a new way of understanding the fundamental processes in electronics components. This insight is important, for example, for electronics in information technology and advanced imaging processes in medicine and engineering.”

The research was funded by the Austrian Science Fund (FWF) in the context of the FoQus Special Research Area and the doctoral college Atoms, Light and Molecules (ALM), the National Science Foundation (USA) and the Institute for Advanced Study at TUM, as well as the Walter Haefner Foundation, the ETH Foundation and the Simons Foundation. Further project cooperation partners included researchers of the Monash University (Australia) and the University of Amsterdam.