Tiny atomic collisions unleash unexpected energy bursts

Rubidium atoms can behave unpredictably during atomic collisions, when hit by the right color of laser light. They might even eject themselves from carefully controlled traps, surprising researchers who aim to study them in near-zero-temperature conditions.

A recent study has made it possible to control collisions between these atoms more precisely than ever. The research was led by Professor Cindy Regal from the University of Colorado Boulder and Jose D’Incao from the University of Massachusetts.

The experts joined forces to examine what happens when laser light nudges these tiny particles to collide with each other.

Understanding cold atomic collisions

Experiments involving ultracold atomic gases have revealed intriguing surprises about how atoms bounce off one another. Researchers often operate at temperatures near to absolute zero, where standard thermal motion is dramatically slowed.

This allows quantum effects to dominate, including special collision behaviors that are sensitive to small energy shifts in atoms.

Regal’s group explored light-assisted collisions, which occur when a laser photon briefly creates a quantum superposition state in two atoms. In this arrangement, either atom could have absorbed the photon, which leads to unexpected energy release that often kicks both atoms out of their trap.

A spotlight on hyperfine structure

Hyperfine structure is defined by tiny energy shifts caused by an atom’s nuclear spin coupling with its electron’s angular momentum.

Until recently, those shifts were often overlooked when investigating collision rates. Regal and D’Incao’s team identified that hyperfine structure can be crucial for determining exactly how collisions occur and how much energy is released.

“This energy is imparted to the colliding atoms, which can be considered bad as they are large enough to cause atoms to escape from typical traps. But these collisions can also be useful when that energy can be controlled,” said Regal.

Optical tweezers for precision

The researchers used optical tweezers, which are tightly focused laser beams that can hold individual atoms in place.

The experts arranged two rubidium atoms in separate tweezers, then carefully merged them to see how quickly they would collide when a second laser beam was introduced at different frequencies.

By controlling the laser’s frequency relative to atomic transitions, the team could measure how variations in the collisional light influenced the atoms’ escape. This approach offered a level of control that was hard to achieve in larger, cloud-based setups.

Refining collision rates

“We set the laser at a certain frequency, then varied the duration of the collisional light to see how many atoms remained in the trap, from this, we could determine how quickly the atoms collided and gained enough energy to escape,” explained Steven Pampel, the study’s first author.

Different hyperfine states led to distinct collision rates. Those observations prompted the team to build a refined model that linked these collision outcomes to the underlying potential energy curves connecting atoms under the influence of laser light.

A new way to detect atomic collisions

Standard imaging systems can complicate data collection in these experiments. Shining light to see if atoms are still trapped might change their energy status and nudge them out prematurely.

In response, Pampel and colleagues developed a way to pinpoint whether one or both atoms had been ejected without disturbing the system.

This finer detection method ensured the collisions themselves were accurately counted, leading to more dependable results.

Future uses of controlled atomic collisions

D’Incao noted that visualizing the molecular states formed during these collisions helped confirm the impact of hyperfine interactions on collision outcomes. 

“Mapping out the potential energy curves for two colliding atoms, in the presence of light and the hyperfine interaction, required more complex analysis than previous works that had only taken into account the atomic fine structure,” explained D’Incao.

Regal’s group hopes that this deeper understanding of small energy splittings will inspire fresh ways to manipulate atoms in optical tweezer arrays. Fine-tuning collisions can be useful in quantum computing and other research areas that rely on controlling how atoms move and interact.

Significance of the study

Researchers in this field believe these light-assisted collisions will continue to be a powerful tool in preparing and shaping trapped neutral atoms for advanced experiments.

There is also a growing interest in extending these techniques to other atomic species, potentially leading to better control in molecular quantum science.

The entire set of findings could give scientists new ideas for building next-generation devices that rely on carefully choreographed atom interactions. Better collision management may pave the way for more reliable measurements and calculations in a range of fundamental studies.

The study is published in Physical Review Letters.

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