Targeted energy supply with a laser can create a kind of “giant atom”. An electron of such an atom is forced onto an orbit that is so far outside the atomic shell that it inflates the atom to gigantic dimensions. The result is a so-called Rydberg atom. The average distance of such an electron to its nucleus can be hundreds of nanometers.

Thomas C. Killian, Rice University, Houston, TX, USA, and colleagues have used a Rydberg atom to experimentally create a particularly exotic state of matter, a so-called Rydberg polaron. They cooled a cloud of strontium atoms to such an extent that they formed a Bose-Einstein condensate. In this state, the atoms lose their independence and swing in a way in a common mode. The scientists then used laser irradiation to create a Rydberg atom. The electron of the Rydberg atom no longer orbits only its own nucleus, but the shell of the giant atom swallows several neighboring atoms. Depending on the radius of the Rydberg atom and the density of the Bose-Einstein condensate, the shell may contain up to 160 additional strontium atoms. They fit together with their shell in the orbit of the outer electron. Because these strontium atoms are electrically neutral, they are not repelled by the charge of the electron.

A kind of weak binding state is formed between the “swallowed” atoms and the surrounding Rydberg atom. However, this bond is much weaker than the bond between the atoms in a crystal and it is detectable only at ultracold temperatures. According to the researchers, this new, weakly bound state of matter is an exciting way to better understand the physics of ultracold atoms.

• Creation of Rydberg Polarons in a Bose Gas,
  F. Camargo, R. Schmidt, J. D. Whalen, R. Ding, G. Woehl, Jr., S. Yoshida, J. Burgdörfer, F. B. Dunning, H. R. Sadeghpour, E. Demler, T. C. Killian,
  Phys. Rev. Lett. 2018.
  https://doi.org/10.1103/PhysRevLett.120.083401