More than 100 charged calcium atoms chilled to extremely low temperatures have been arranged into a two-dimensional crystal, which could be used for studying quantum materials or building quantum computations.
The largest two-dimensional crystal of extremely cold, charged atoms ever created could be used to study poorly understood quantum materials, as well as for building quantum computers.
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| Calcium ions are released through the triangular funnel and caught by electric potentials on top of the metallic chip IQOQI/D. Jordan |
Some crystals exhibit intriguing quantum behaviours like unusual magnetism or perfect conduction of electricity, but understanding what each atom is doing to make that happen can be difficult to determine experimentally and impossible to simulate on a computer. So, since the 1990s, researchers have been assembling model crystals from extremely cold atoms, which can be manipulated and imaged precisely. In this way, researchers can put their theories about natural crystals to the test by forcing ultracold atoms to behave in a certain way and seeing whether the effect matches what they have previously seen in nature.
Helene Hainzer at the University of Innsbruck in Austria and her colleagues have now devised a way to make these crystals larger than before, meaning that they can test more complex theories and study more complicated crystal behaviours.
Hainzer and her colleagues created the ultracold crystal out of charged atoms, or ions, of calcium inside of a small, airless metal-and-glass container by using lasers and a metallic chip. To make the ions very cold they hit them with lasers, which made them lose enough energy to become almost as cold as absolute zero.
To force the ions into the crystal shape, the researchers relied on electric forces exerted on the ions by the chip and also by each ion on its neighbours. By using these forces, they pushed 105 ions into an evenly spaced grid within a flat, two-dimensional pancake shape, like a perfect array of chocolate chips.
Researchers have previously created similar two-dimensional crystals but never from more than about two dozen ions. Hainzer says that this is because past setups for “trapping” ions used electric voltages that had the side effect of making some ions slightly jiggle and shake out of place. Her team designed its experiment so that all such jiggling is restricted to one direction, then hit the ions with an extra laser beam perpendicular to that direction to reduce the motion.
“This experiment really upped the bar,” says Philip Richerme at Indiana University, who is also working on creating ultracold ionic crystals. He says that it is important that the new crystal is not only stable but also two-dimensional because that makes it a lot like existing ultra-thin quantum materials that it may be used to simulate.
John Bollinger at the National Institute of Standards and Technology in Colorado says that each ion in the crystal could also be used as a qubit, or a quantum bit, for quantum computation. Here, however, the team would have to be able to precisely change how a single pair of ions interact with each other while leaving all others undisturbed. The team is already working on implementing this by tweaking the lasers and the electromagnetic properties of the chip, says Hainzer.
Reference:
PRX Quantum, forthcoming
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