US researchers have developed a way to control and measure atoms that are so close together they are impossible to distinguish by optical means.
When they get cosy – that is, within a few billionths of a metre of each other – they exhibit interesting quantum mechanical behaviour. At this scale, their spins begin to exert an influence on each other, and two or more can become entangled: a strange quantum phenomenon where the atoms will thereafter mirror each other’s properties instantly, even if they are kilometres or light-years apart.
Entanglement is key for future technologies like quantum computing – but first, scientists must observe and understand these tightly-packed atoms. Conventional microscopes are unable to distinguish between atoms that are just nanometers apart, just as our eyes are often unable to spatially resolves two distant stars that are close together in the night sky.
Researchers from Princeton University have now demonstrated a technique to resolve such atoms. In a paper published in the journal Science, they describe using a finely tuned laser to excite closely spaced erbium atoms in a crystal.
Each atom responds slightly differently to different wavelengths, re-emitting the light at unique frequencies that subtly change according to an atom’s spin state.
“By tuning the laser carefully to the frequency of one or the frequency of the other, we can address them, even though we have no ability to spatially resolve them,” explains lead author Jeff Thompson, an electrical engineer at Princeton. “Each atom sees all of the light, but they only listen to the frequency they’re tuned to.”
Thompson and team exploited this fact to observe and control the erbium atoms, laying the groundwork to study the intriguing spin interactions with unprecedented clarity.
“We always wonder, at the most fundamental level – inside solids, inside crystals – what do atoms actually do? How do they interact?” says physicist Andrei Faraon from the California Institute of Technology, who was not involved in the research. “This [paper] opens the window to study atoms that are in very, very close proximity.”
Other techniques have been developed to solve this observation problem, but this new method is unique in its ability to observe hundreds of atoms at a time. This means researchers can gather large volumes of data and begin to unravel the mysteries of the quantum world.
The next step will be to figure out how to arrange the erbium atoms to form quantum logic gates, which can then be used to encode and process information in a quantum circuit.
Related reading: Atoms in close-up
Lauren Fuge is a science journalist at The Royal Institution of Australia.
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