Rydberg

A team of JQI researchers and their colleagues have won in the quantum category of the UMD Invention of the Year Award. They are honored for developing a new method for counting particles of light—photons—without destroying them.

Photons—the quantum particles of light—normally don’t have any sense of personal space. A laser crams tons of photons into a tight beam, and they couldn’t care less that they are packed on top of each other. Two beams can even pass through each other without noticing. This is all well and good when making an extravagant laser light show or using a laser level to hang a picture frame straight, but for researchers looking to develop quantum technologies that require precise control over just one or two photons, this lack of interaction often makes life difficult. Now, a group of UMD researchers has come together to create tailored interactions between photons in an experiment where, at least for photons, two’s company but three’s a crowd. The technique builds on many previous experiments that use atoms as intermediaries to form connections between photons that are akin to the bonds between protons, electrons and other kinds of matter. These interactions, along with the ability to control them, promises new opportunities for researchers to study the physics of exotic interactions and develop light-based quantum technologies.

Flashlight beams don’t clash together like lightsabers because individual units of light—photons—generally don’t interact with each other. Two beams don’t even flicker when they cross paths. But by using matter as an intermediary, scientists have unlocked a rich world of photon interactions. In these early days of exploring the resulting possibilities, researchers are tackling topics like producing indistinguishable single photons and investigating how even just three photons form into basic molecules of light. The ability to harness these exotic behaviors of light is expected to lead to advances in areas such as quantum computing and precision measurement. In a paper recently published in Physical Review Research, JQI Fellow Alexey Gorshkov, JQI postdoctoral researcher Przemyslaw Bienias, and their colleagues describe an experiment that investigates how to extract a train of single photons from a laser packed with many photons.  In the experiment, the researchers examined how photons in a laser beam can interact through atomic intermediaries so that most photons are scattered out of the beam and only a single photon is transmitted at a time. They also developed an improved model that makes better predictions for more intense levels of light than previous research focused on. The new results reveal details about the work to be done to conquer the complexities of interacting photons. 

The behavior of a few rubidium atoms in a cloud of 40,000 hardly seems important. But a handful of the tiny particles with the wrong energy may cause a cascade of effects that could impact future quantum computers. Some proposals for quantum devices use Rydberg atoms—atoms with highly excited electrons that roam far from the nucleus—because they interact strongly with each other and offer easy handles for controlling their individual and collective behavior. Rubidium is one of the most popular elements for experimenting with Rydberg physics. Now, a team of researchers led by JQI Fellows Trey Porto, Steven Rolston and Alexey Gorshkov have discovered an unwanted side effect of trying to manipulate strongly interacting rubidium atoms: When they used lasers to drive some of the atoms into Rydberg states, they excited a much larger fraction than expected. The creation of too many of these high-energy atoms may result from overlooked “contaminant” states and could be problematic for proposals that rely on the controlled manipulation of Rydberg atoms to create quantum computers.

From NIST TechBeat--It’s not lightsaber time, not yet. But a team including theoretical physicists from JQI and NIST has taken another step toward building objects out of photons, and the findings, recently published in Physical Review Letters, hint that weightless particles of light can be joined into a sort of “molecule” with its own peculiar force. Researchers show that two photons, depicted in this artist’s conception as waves (left and right), can be locked together at a short distance. Under certain conditions, the photons can form a state resembling a two-atom molecule, represented as the blue dumbbell shape at center. The arrangement is akin to the way that two hydrogen atoms sit next to each other in a hydrogen molecule.

Rydberg atoms, atoms whose outermost electrons are highly excited but not ionized, are potentially important for storing an processing quantum information since they can preserve a superpostion of states for a long time. One problem with Rydberg atoms is that in they are often difficult to handle. One approach is to search for special wavelengths---“magic wavelengths”—at which atoms can be trapped and excited into Rydberg states without disturbing them. JQI researchers with collaborators from U. Delaware and U. Nevada compare precision calculations of these special wavelengths with experimental measurement. This work is featured as an Editor's Suggestion in the journal Physical Review A.