Physicists build bizarre molecules called 'Rydberg polarons'

February 28, 2018, Rice University
Rice University atomic physicist Joe Whalen works on a laser cooling system for ultracold strontium gas. Credit: Jeff Fitlow/Rice University

Using lasers, U.S. and Austrian physicists have coaxed ultracold strontium atoms into complex structures unlike any previously seen in nature.

"I am amazed that we've discovered a new way that atoms assemble," said Rice University physicist Tom Killian. "It shows how rich the laws of physics and chemistry can be." Killian is the lead scientist on a new paper in Physical Review Letters (PRL) that summarized the group's experimental findings.

Killian teamed with experimental physicists from Rice's Center for Quantum Materials and theoretical physicists from Harvard University and Vienna University of Technology on the two-year project to create "Rydberg polarons" out of strontium atoms that were at least 1 million times colder than deep space.

The team's findings, which are summarized in the PRL paper and a companion theoretical study appearing this week in Physical Review A (PRA), reveal something new about the basic nature of matter, Killian said.

"The basic laws that we learn in chemistry class tell us how atoms bond together to form molecules, and a deep understanding of those principles is what allows chemists and engineers to make the materials we use in our everyday lives," he said. "But those laws are also quite rigid. Only certain combinations of atoms will form stable bonds in a molecule. Our work explored a new type of molecule that isn't described by any of the traditional rules for binding atoms together."

Killian said the new molecules are only stable at extraordinarily cold temperatures—about a millionth of a degree above absolute zero. At such low temperatures, the constituent atoms stay still long enough to become "glued together" in new, , he said.

"One amazing thing is that you can keep attaching an arbitrary number of atoms to these molecules," Killian said. "It's just like attaching Lego blocks, which you can't do with traditional types of molecules."

He said the discovery will be of interest to theoretical chemists, condensed matter physicists, atomic physicists and physicists who are studying Rydberg atoms for potential use in quantum computers.

"Nature takes advantage of a fascinating toolbox of tricks for binding atoms together to make molecules and materials," Killian said. "As we discover and understand these tricks, we satisfy our innate curiosity about the world we live in, and it can often lead to practical advances like new therapeutic drugs or light-harvesting solar cells. It is too early to tell if any practical applications will come from our work, but basic research like this is what it takes to find tomorrow's great breakthroughs."

The team's efforts centered around making, measuring and predicting the behavior of a specific state of matter called a Rydberg polaron, a combination of two distinct phenomena, Rydberg atoms and polarons.

In Rydberg atoms, one or more electrons are excited with a precise amount of energy so that they orbit far from the atom's nucleus. Rydberg atoms can be described with simple rules written down more than a century ago by Swedish physicist Johannes Rydberg. They have been studied in laboratories for decades and are believed to exist in cold reaches of deep space. The Rydberg atoms in the PRL study were up to one micron wide, about 1,000 times larger than normal strontium atoms.

Polarons are created when a single particle interacts strongly with its environment and causes nearby electrons, ions or atoms to rearrange themselves and form a sort of coating that the particle carries with it. The itself is a collective—a unified object known as a quasiparticle—that incorporates properties of the original particle and its environment.

Rydberg polarons are a new class of polarons in which the high-energy, far-orbiting electron gathers hundreds of atoms within its orbit as it moves through a dense, ultracold cloud. In the Rice experiments, researchers began by creating a supercooled cloud of several hundred thousand strontium atoms. By coordinating the timing of laser pulses with changes in the electric field, the researchers were able to create and count Rydberg polarons one by one, ultimately forming millions of them for their study.

While Rydberg polarons had previously been created with rubidium, the use of strontium allowed the physicists to more clearly resolve the energy of the coated Rydberg atoms in a way that revealed previously unseen universal characteristics.

"I give a lot of credit to the theorists," said Killian, a professor of physics and astronomy. "They developed powerful techniques to calculate the structure of hundreds of interacting particles in order to interpret our results and identify the signatures of the Rydberg polarons.

"From an experimental standpoint, it was challenging to make and measure these polarons," he said. "Each one lived for only a few microseconds before collisions with other particles tore it apart. We had to use very sensitive techniques to count these fragile and fleeting objects."

Explore further: Researchers report the creation of Rydberg polarons in a Bose gas

More information: R. Schmidt et al, Theory of excitation of Rydberg polarons in an atomic quantum gas, Physical Review A (2018). DOI: 10.1103/PhysRevA.97.022707

Related Stories

Simple model explains crystal formation of exceptional atoms

July 23, 2015

Scientists from the FOM Foundation, Eindhoven University of Technology and the University of Buenos Aires have discovered why fluctuations in the number of Rydberg atoms that forms in an ultracold gas decreases as the interaction ...

Scientists discover giant Rydberg atom molecules

June 24, 2009

A group of University of Oklahoma researchers led by Dr. James P. Shaffer, Homer L. Dodge Department of Physics and Astronomy, have discovered giant Rydberg molecules with a bond as large as a red blood cell. Determining ...

A New Type of Molecule

June 21, 2010

(PhysOrg.com) -- A Rydberg atom is one with an electron that spends most of its time far from the nucleus. Rydberg atoms, which are normally artificially produced, can have dimensions thousands of times larger than typical ...

Recommended for you

Fiber optic sensor measures tiny magnetic fields

September 19, 2018

Researchers have developed a light-based technique for measuring very weak magnetic fields, such as those produced when neurons fire in the brain. The inexpensive and compact sensors could offer an alternative to the magnetic ...

The hunt for leptoquarks is on

September 19, 2018

Matter is made of elementary particles, and the Standard Model of particle physics states that these particles occur in two families: leptons (such as electrons and neutrinos) and quarks (which make up protons and neutrons). ...

Researchers push the boundaries of optical microscopy

September 19, 2018

The field of optical microscopy research has developed rapidly in recent years. Thanks to the invention of a technique called super-resolution fluorescence microscopy, it has recently become possible to view even the smaller ...

Searching for errors in the quantum world

September 19, 2018

The theory of quantum mechanics is well supported by experiments. Now, however, a thought experiment by ETH physicists yields unexpected contradictions. These findings raise some fundamental questions—and they're polarising ...

Extremely small and fast: Laser ignites hot plasma

September 19, 2018

When light pulses from an extremely powerful laser system are fired onto material samples, the electric field of the light rips the electrons off the atomic nuclei. For fractions of a second, a plasma is created. The electrons ...

2 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

Thorium Boy
1 / 5 (2) Mar 01, 2018
Why are there no images of these atoms if they are as large as a micron? Don't the electrons describe the actual shape visible?
gkam
not rated yet Mar 01, 2018
Are you serious?

Bounce a photon off of it, and you have changed it.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.