The Smallest Fridge In The World: A Single Atom

A single barium atom can act as either a heat engine or refrigerator, a property that could be used to cool small devices and nanomachines in the future.

AsianScientist (Jun. 9, 2020) – Researchers in Singapore have shown that a single atom can function as either an engine or a fridge. These findings, published in npj Quantum Information, pave the way for future computers and fuel cells to control energy flows.

Engines and refrigerators are both machines described by thermodynamics, a branch of science that tells us how energy moves within a system and how we can extract useful work. A classical engine turns energy into useful work. A refrigerator does work to transfer heat, reducing the local temperature. They are, in some sense, opposites.

While previous research has shown that a single atom can act as a heat engine, the present study shows that single atoms can also act as mini refrigerators, converting energy in the opposite direction.

“Think about how your computer or laptop has a lot of things inside it that heat up. Today you cool that with a fan that blows air. In nanomachines or quantum computers, small devices that do cooling could be something useful,” said Associate Professor Dario Poletti from the Singapore University of Technology and Design, Singapore.

Using a single barium atom, the researchers used lasers to move one of the atom’s electrons between two energy levels as part of a cycle, causing some energy to be pushed into the atom’s vibrations. Just as a car engine consumes petrol to both move pistons and charge up its battery, the atom uses energy from lasers as fuel to increase its vibrating motion. The atom’s vibrations thus act like a battery, storing energy that can be extracted later.

By rearranging the cycle, the researchers were able to make the atom act like a fridge, removing energy from the vibrations. In either mode of operation, quantum effects showed up in correlations between the atom’s electronic states and vibrations.

“At this scale, the energy transfer between the engine and the load is a bit fuzzy. It is no longer possible to simply do work on the load, you are bound to transfer some heat,” said Poletti.

To get around the problem, the team used a barium atom from which an electron (a negative charge) is removed. This makes the atom positively charged, so it can be more easily held still inside a metal chamber by electrical fields. The atom was then zapped with lasers to move it through a four-stage cycle.

After applying several cycles, the researchers measured the atom’s vibration to determine how much vibrational energy was present at the end. They could see the vibrational energy increasing when the atom was zapped with an engine cycle, and decreasing when the zaps followed the fridge cycle.

“Loosely speaking, we’ve designed a little machine that creates entropy as it is filled up with free energy, much like kids when they are given too much sugar,” said study first author, PhD student Noah Van Horne.

The article can be found at: Van Horne et al. (2020) Single-atom Energy-conversion Device with a Quantum Load.


Source: National University of Singapore.
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