Yoav Sagi's group, Gal Ness, Constantine Shkedrov and Yanay Florshaim, led an experiment-theory collaboration with Richard Schmidt's group from the Max Planck Institute of Quantum Optics, which was recently published in Physical Review X, and accompanied by a general audience synopsis.

The researchers studied the behavior of quantum impurities immersed in a Fermi sea with an ultracold Fermi gas. The quantum state of these impurities depends on their interaction with the other particles, which can be tuned in the experiment. As the impurity interacts with the majority atoms, it can locally deform the Fermi sea, generating a fermionic quasiparticle named “polaron”. At even stronger interaction, it can bound to form a bosonic quasiparticle, which is also dressed. Previous theoretical calculations, limited to a single impurity at zero temperature, predicted a first-order phase transition between these two possible ground states.

In contrast to these predictions, the researchers found a smooth crossover. Experimentally, the key to this work was the development of a novel probing technique based on a two-photon Raman transition. Theoretically, they showed that the striking difference stems from the quantum statistics of many impurities in gas at non-zero temperature. Perhaps the most intriguing observation is that there is a region of interaction-strength where there is a coexistence between the fermionic- and bosonic-like quasiparticles.

Watch Gal Ness talk about the interesting results here.