One Atom to Rule Them All: A New Class of Quantum Matter Observed

Scientists have observed a new class of quantum matter at the very smallest scales in one of the coldest environments ever made. This discovery could pave the way for new technologies including innovations in superconductivity and other cutting-edge fields.

The researchers examined the behavior of matter on the atomic and subatomic scales – known as “quantum matter” – where a large number of particles interact with each other.

This latest discovery reveals a new state of quantum matter called a “Rydberg polaron,” a relatively giant particle containing many atoms that behaves in some ways like a single massive particle.

The experiment, initiated by the theoretical work at the Institute for Theoretical Atomic Molecular and Optical Physics (ITAMP) at the Harvard-Smithsonian Center for Astrophysics (CfA) and Harvard Physics, was performed in the laboratory of Thomas Killian at Rice University, where the electrons were given so much energy, on the verge of being pulled away from the nucleus. These highly excited atoms were immersed in a gas that had been cooled to just a millionth of a degree above absolute zero. The result is the creation of a “Rydberg atom,” which is about one hundred billionth of a meter across. This makes it about ten thousand times larger than a typical atom.

“By putting Rydberg atoms in these conditions, we saw that the atoms and molecules can configure themselves in ways that we’ve never seen before,” said Richard Schmidt of ITAMP, who led the theoretical work, along with Hossein Sadeghpour at ITAMP and Eugene Demler at Harvard Physics.

In this quantum environment, atoms and molecules can be stacked to form heavier molecules, much in the way Lego pieces are arranged. In the new work, the team applied this process to Rydberg atoms, where increasingly heavy Rydberg molecules were formed by adding large numbers of surrounding atoms until as many as 160 atoms were added to a Rydberg atom.

“The emergence of complexity in nature is often due to the appearance of new properties or behaviors. However, like an increasingly large and precarious Lego tower, such an arrangement will crumble unless a new property emerges to stabilize the structure,” said co-author Hossein Sadeghpour.

Quantum interactions – that is similar behavior by different quantum particles – between the Rydberg atom’s electron and surrounding atoms enable this quantum system with many different particles to hold together.  In the process, this object changes its character to become what scientists call a "Rydberg polaron".

The polaron becomes shrouded by surrounding atoms that move along with it because of these quantum interactions, developing an effective mass that is larger than the mass of the atoms occupying it. At this point, the Ryberg polaron stops behaving like a molecule and starts acting more like a single massive particle. An analogy is that of a horse galloping along and gradually becoming covered in a cloud of dirt particles, which obscures and changes the appearance of the animal.

“It’s possible to make larger and larger polarons until the objects stops having quantum behavior and starts acting classically,” said Sadeghpour.

Applications of this work include the potential to gain a better understanding of room-temperature superconductivity and many-body interactions. The work may also aid in designing new materials, and help act as a spectroscopic probe of weak correlations in quantum many-body matter.

The Physical Review Letter describes the experimental work conducted at Rice University by Tom Killian, and the theoretical work conducted at Harvard University by Eugene Demler, at the Harvard-Smithsonian Center for Astrophysics by Richard Schmidt and Hossein Sadeghpour, Eugene Demler at Harvard Physics, and at Vienna University of Technology (https://arxiv.org/abs/1706.03717). The Physical Review A article explains the details of the theory by generalizing the polaron concept to a much more strongly interacting system, paving the ground to explore the properties of this novel quantum state of matter, such as its effective mass and the nature of interactions between polarons (https://arxiv.org/abs/1709.01838).

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

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