Scientists Held an Extreme Atomic Experiment—and Challenged the Notions of What’s Possible

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• Scientists have observed “bending” atoms using a crystal grating—an experiment once believed impossible at the high energies required.

• The authors of a new, non-peer reviewed study detail how the a graphene sheet used as a crystal grating withstood 100 hours of irradiation from an atom beam without damage, and distinctive circular patters showed atomic diffraction.

• Current atomic interferometers rely on nanomechanical membranes, but increased momentum transfer via crystal gratings could make interferometers even more sensitive.

On September 14, 2015, for the first time ever, the Laser-Interferometer Gravitational-Wave Observatory (LIGO) detected a ripple in spacetime. This breakthrough moment—one Albert Einstein once thought nearly impossible—was made possible by the observatory’s ability to measure minute changes in the distance that a laser beam travels, which betray a warping of spacetime.

However, if scientists could somehow develop atomic interferometers—ones that captured the wave-like nature of atoms—gravitational wave detectors could be many times more sensitive and discover different types of gravitational waves, including any that could potentially be produced by some advanced alien civilization’s warp engines.

Developing such a tool is a big “if,” but scientists from the Institute of Quantum Technologies at the German Aerospace Center and the University of Vienna think they might be onto something. In a new paper published on the preprint server arXiv, Christian Brand from the German Aerospace Center and his colleagues developed an experiment to essentially “bend” atoms. To do this, the team diffracted high-energy hydrogen and helium atoms through an atom-thick graphene sheet, and saw distinctive ring patterns—a tell-tale sign of diffraction.

This was particularly surprising because this kind of diffraction has only ever been reported in subatomic particles—not entire atoms. But even in electrons, this was incredibly useful. As New Scientist notes, in 1927, physicist George Paget Thomson demonstrated how diffracted electrons produced specific patterns when passing through gaps in a crystal structure, which scientists call “gratings.” In fact, this was so useful that it led to the development of electron microscopes (and also earned Thomas a Nobel Prize).

Diffraction patterns were discovered for atoms, but they used gratings much larger than what could be found in crystals, as it was believed the high energy of these atoms would damage crystal gratings before diffraction could take place. However, using an atom-thick layer of carbon atoms known as graphene, experts found that even after 100 hours under irradiation of an atom beam, the grating showed no damage and diffraction still took place.

“Due to its outstanding electronic and mechanical properties, single-layer graphene is the perfect candidate to act as a grating for atoms in transmission,” the authors wrote in the study. “Further, it can be routinely prepared as free-standing material on suitable support structures.”

According to the laws of quantum mechanics, this diffraction is only possible because the energy exchange between graphene and the hydrogen/helium atoms is undetectable. Speaking with New Scientist, Bill Allison from the University of Cambridge describes this like opening and closing doors. “If I open the door and then close it deftly without losing or gaining energy, then no one, including me, knows which door I used and therefore there will be diffraction.”

As the authors note, the sensitivity of interferometers “scales with the momentum imparted by the grating to the matter wave,” but the size of grating periods are currently limited by the manufacturing process to just 100 nanometers. This demonstration of atomic diffraction through a crystalline lattice could open doors to more sensitive instruments, which will be vital for probing some of the universe’s biggest mysteries.

“Combining crystalline transmission gratings into interferometers might give rise to new quantum-based sensors,” the authors wrote. “Fast atoms have advantages for detecting gravitational waves compared to cold-atom experiments and might give rise to new multi-dimensional interferometers.”

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