World’s tiniest fridge

Staying cool, even in the quantum world

Scientists like to break records: The fastest this, the slowest that; the most ever seen, the least possible. In fact, much of science may seem like a quest for the hottest, coldest, closest, farthest, biggest, or smallest something — or to know how the laws of nature work for the biggest or smallest things.

But here’s one new record you might not have expected to see: A team of scientists has come up with a plan for the smallest possible refrigerator. If it is ever built, it would be much smaller than the kitchen kinds.

It would be so small, we couldn’t see it — even with ordinary microscopes. That means it would be useless for cooling things like chocolate milk and frozen pizzas, but its designers say it may have other uses — like, say, creating faster computers.

The scientists weren’t thinking about how the mini fridge would be used. They wanted to know if such tiny objects transfer heat the same way larger objects, such as refrigerators, do. The area of physics in which scientists study heat and other types of energy is called thermodynamics.

Sandu Popescu, Paul Skrzypczyk and Noah Linden are three scientists who worked on the project. “We believe this is the smallest possible thing you can call a fridge,” Linden told Science News. All three researchers work at the University of Bristol in the United Kingdom.

For objects we can see, like baseballs, physics follows rules. If you hit the ball, it moves away from you. If you have two baseballs, and hit one, the other one doesn’t move. If you hit the second one, the first one doesn’t move. And baseballs certainly don’t talk to each other.

For the extremely small particles studied in a field of research called quantum mechanics, nature gets strange. Sometimes, tiny particles behave as though they’re talking to each other — even if they’re not close together. If a scientist stops one of the particles and measures it, for example, the other particle may act like it’s also been stopped to get measured.

When particles “talk” like this, they’re said to be “correlated.”

The tiny refrigerator works by using three correlated particles. These particles are called “qubits.” To understand how it works, imagine you’re playing a game, called “Qubit Refrigerator,” with two friends. Let’s call them Alice and Sam. The object of the game is to make you cold.

At the beginning, each of you is wearing a few coats. The game has three rules: 1. If Alice gets hot, she gives a coat to Sam. 2. If Alice gives a coat to Sam, you have to give a coat to Sam at the same time. 3. If Sam gets hot, he throws his extra coats on the floor.

Here’s how it works. Someone (who is not playing the game) starts giving coats to Alice. Alice puts them on. She keeps getting coats and putting them on, until she’s hot — and then she starts giving her coats to Sam. When she gives a coat to Sam, you also have to give a coat to Sam. Sam gets hot and throws these extra coats on the floor.

Alice is given more coats to wear. The more extra coats Alice is given, the more she gives to Sam. This means you have to give more coats to Sam. And as you lose coats, you get colder. The more coats Alice is given, the more she gives to Sam, and the more you give to Sam — and the colder you get. The game acts like a machine that makes you colder!

This may be the strangest science game you’ve ever played, but it’s similar to the a property of thermodynamics that explains how your refrigerator at home works. Your refrigerator has to be cooler than the air around it. But usually, heat moves from a warmer place to a cooler place — so heat should be warming up your refrigerator all the time. But if you — or your refrigerator — does work on the air outside, it’s possible to keep the air inside cold. This idea — that you can keep something cold by doing work — also helps explain the scientists’ tiny refrigerator. In the case of the “Qubit Refrigerator,” work is done when someone gives coats to Alice.

Here’s how the game connects to the scientists’ work: Instead of you and two friends, the “players” in the tiny refrigerator are tiny particles called qubits. And instead of giving coats, two qubits (you and Alice) are actually giving heat to the third qubit (Sam). The hotter Alice gets, the colder you get.

The researchers found that as the hottest qubit (Alice) gets hotter, the refrigerator gets colder. And as long as the hot qubit stays hot, this quantum fridge continues to work.

“Once you set it up, it just sits there, gently cooling away,” Linden told Science News.

Right now, the tiny fridge is just an idea, and the scientists have only devised the device. They haven’t actually built one. “We don’t want to claim that we know of a place where this happens,” Linden told Science News.

POWER WORDS (adapted from the Yahoo! Kids Dictionary, NASA Science and Science News for Kids)

qubit A quantum bit; a unit of information in quantum mechanics.

refrigerator An appliance for storing substances at a low temperature.

physics The science of matter and energy and of interactions between the two, grouped in traditional fields such as acoustics, optics, mechanics, thermodynamics and electromagnetism, as well as in modern fields including atomic and nuclear physics, solid-state physics, particle physics and plasma physics.

particle A very small piece or part; a tiny portion or speck.

Stephen Ornes lives in Nashville, Tenn., and his family has two rabbits, six chickens and a cat. He has written for Science News Explores since 2008 on topics including lightning, feral pigs, big bubbles and space junk.

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