Quantum Physics
Pump-Probe Spectroscopy of Ultracold Bose Polarons
11 September 2020
Photo: AG Schmelcher
Researchers in the University of Hamburg and the Okinawa Institute of Science and Technology have devised and employed a novel pump-probe spectroscopy technique to interrogate the formation, decay processes and coherence properties of polaronic excitations in an ultracold environment serving as a prototype quantum simulator of the respective condensed matter setup. They claim that this result paves the way for a fruitful transfer of knowledge between the realms of ultrafast dynamics, ultracold atoms and condensed matter physics.
The formation and properties of polarons drastically influence the conductivity of technologically relevant materials. Examples include the organic semiconductors found in the ecologically-friendly and emerging plastic solar cell technologies. Regarding fundamental physics, polarons are even conjectured to play a crucial role in high-temperature superconductivity. A collaboration of researchers in Hamburg and Okinawa Universities proposes a powerful spectroscopic technique that is able to capture and identify the intriguing correlation properties of these quasi-particles within the realm of ultracold quantum simulation. Their work has now been published in “Physical Review Research”.
Polarons and quantum simulation
An electron moving through a material attracts the heavy ions that form the crystal and as a consequence forces the latter to oscillate around their equilibrium positions. Landau and Pekar proposed in 1946 that if the electron and the vibrations of the crystal ions are coupled strongly enough then a quasi-particle called polaron is formed. Quasi-particles are composite structures consisting of multiple excitations (here the crystal vibrations) and/or particles (here the electrons) that collectively propagate similarly to a dressed particle. Importantly, these quasi-particles possess different physical properties from the ones corresponding to their constituents, for instance a polaron is predicted to be significantly heavier than an electron. The polarons are largely technologically relevant since, for instance, their large mass in semiconductors, such as GaAs, drastically limits the mobility of electrons and dramatically changes the conduction properties of these materials. Besides being of fundamental importance to material science, similar quasi-particles appear in distinct systems covering almost all fields of physics.
Despite the intense study of polarons several questions regarding their fundamental properties and associated correlation processes, especially in the case of strong impurity-medium coupling remain open, since they are notoriously difficult to be captured by computer simulations. However, it has been shown that the crystal vibrations can be mapped to the atom displacement of a BEC and the electrons to neutral impurity atoms. Accordingly, an analog condensed matter system is possible to be fully implemented in ultracold atom ensembles, thus realizing an example of an analogue quantum simulator as it has been envisioned by Feynman. Indeed, ultracold atoms are free from environmental defects that possibly deteriorate condensed-matter experiments and they provide an extraordinary level of control, allowing among others to selectively tune the interactions among their constituents. Nevertheless, the quantum simulation of polarons necessitates the development of novel experimental techniques for addressing the intricate properties of quasi-particles.
Pump-probe spectroscopy of ultracold impurities
The researchers from Hamburg and Okinawa have ample previous experience in understanding theoretically the properties of ultracold polarons. Among others, they have demonstrated in their past studies that the external confinement imposed in ultracold atom experiments significantly affects the behavior of polaronic excitations, leading for instance to their death for strong repulsive impurity-medium interactions, a phenomenon they has been dubbed as “temporal orthogonality catastrophe”. However, it would be challenging to directly demonstrate these emerging properties in contemporary ultracold atom experiments, since the access to some of the key observables for their characterization is not easily granted. The researchers claim that in their new work, published in Physical Review Research, the combination of the ultracold atom techniques with the ones usually employed in ultrafast spectroscopy provides the proper tools for addressing and demonstrating such correlation related phenomena, emerging in ultracold polarons.
The proposed pump-probe spectroscopy scheme relies on the preparation of a non-equilibrium state of the system by a radiofrequency pump pulse, which results in the embedding of strongly interacting impurities in a BEC. Subsequently, the system is allowed to evolve in the absence of radiofrequency fields for variable (dark) time, allowing for the formation of polarons and the development of their dynamics. Then, after this dark time, the system is interrogated by a time-delayed probe pulse, which transfers the impurity to a state that is non-interacting with its bosonic environment. For switching the interactions on and off the protocol relies on impurity atoms with spin-dependent interactions with their environment, which can be readily implemented in ultracold atom experiments.
Within their study, the formation of coherently attractive and repulsive polarons can be inferred by properly analyzing the spectra of the probe pulse for small dark times, while for longer evolution times intriguing phenomena such as the above-mentioned temporal orthogonality catastrophe or the thermalization of the ensuing impurities can be clearly observed. In addition, the strength of the proposed protocol allows for the identification of long-sought-after effects, including the induced attraction among polarons. In particular, when two impurities are close to one another each one experiences distortions of the environment caused by the other impurity, a mechanism resulting from their mutual attraction. This effect is of crucial importance to condensed-matter physics as it provides an attraction mechanism between two electrons, that has been proposed to explain a variety of phenomena, with the most striking one being the high-temperature superconductivity. However, the observation of such an attraction in ultracold atom setups has been elusive.
The authors claim that this application of pump-probe spectroscopy for the study of polarons constitutes only one out of the many possibilities of this method in the ultracold atom realm. Indeed, a similar method has been applied recently for the study of ferromagnetic properties of repulsively interacting Fermi gases which are notoriously difficult to identify. But as Prof. Schmelcher states there is a “bright future for the application of related methods in a plethora of different scenarios”. A few characteristic examples of such possible extensions involve the study of the recondensation dynamics in the excited bands of optical lattices and also in the vibrational dynamics of ultra-long range Rydberg molecules, it will enable the identification of the lifetime of such excitations.
Publication
S. I. Mistakidis, G. C. Katsimiga, G. M. Koutentakis, Th. Busch, and P. Schmelcher,
"Pump Probe Spectroscopy of Bose Polarons: Dynamical Formation and Coherence"