Sunlight transmitted through an optical fibre could be used to help cool a single charged atom to a temperature only a few millionths of a degree above absolute zero.
Sunlight could be used to cool a charged atom to its lowest temperature allowed by the laws of physics.
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| Sunlight coming out of an optical fibre Amanda Younes |
In the 1990s, several people won Nobel prizes for working out how to make atoms extremely cold with precisely controlled laser light. Now, Amanda Younes and Wesley Campbell at the University of California, Los Angeles, have found that some parts of the cooling process could be done with light straight from the sun.
The researchers calculated how a single positively charged barium atom, or ion, kept in a device called an ion trap where electric fields prevent it from flying away, can be cooled in two steps.
First, it would be hit with a laser to slow it down, reducing its energy and therefore its temperature. This step is standard when making extremely cold atoms, but it doesn’t make them as cold as possible because it doesn’t change their entropy.
Entropy is a measure of disorder in a system, and temperature is affected by both energy and entropy. To reduce the entropy, normally researchers follow up the first step with laser light of a slightly different colour, but Younes and Campbell calculated that just getting some sunlight onto the ion would work too.
“You could probably just do this outdoors, but it would be tricky to set up an ion trap outside,” says Younes.
Instead, in the pair’s scheme, sunlight would be transmitted onto the ion through an optical fibre. One end of the fibre is put on the roof of the lab and is capped by a glass lens for focusing sunlight, and the other end goes into the ion trap.
The researchers calculated that electrons inside the ion would absorb so much energy from sunlight that they would become unstable. At this point, they would have to release some energy as light and because this emission process would decrease their entropy, they would end up stuck in their lowest possible energy state, only about a millionth of a degree above absolute zero.
Nils Engelsen at the Swiss Federal Institute of Technology in Lausanne says that in the new scheme, even though lasers still have to be used, sunlight does the most important part of cooling because it lowers the ion’s entropy. This is interesting because it shows that something as commonplace and simple as the sun can replace sophisticated laser technology in exacting and challenging experiments, he says.
This experiment also combines classical and quantum physics, says Campbell. “This is an example of creating something very quantum, like this low energy state of the ion, with something as disgustingly classical as the sun. In this way, we are exploring a connection between things that usually just do not connect,” he says.
Now that calculations have suggested the process will work, the next step is to test it. Younes says she has already built the fibre, tested that it does in fact transmit sunlight from the lab’s roof and is expecting to shine it onto an actual barium ion soon.
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