Researchers Create Tiny Refrigerator Just Three Atoms in Size

Scientists in Singapore have developed a refrigerator with a size equivalent to just three atoms.

Although this quantum fridge may not keep drinks cold, it is cool evidence of physics working at the smallest scales. The study has been reported in a paper published in Nature Communications on January14^th, 2019.

Although scientists had previously designed tiny “heat engines”, quantum fridges prevailed only in theory until the researchers at the Centre for Quantum Technologies at the National University of Singapore developed their atomic refrigerator.

The device is an “absorption refrigerator” that operates without the need for moving parts, by using heat to urge a cooling process.

Launched in the 1850s, the first absorption refrigerators cycled the evaporation and absorption of a liquid, where cooling occurred during the evaporation stage. Until the 20^th century, they were largely used to chill food and make ice. Albert Einstein was even having a patent on an optimized design.

The existing air conditioners and fridges commonly use a compressor; however, absorption refrigerators still find their applications, including science experiments.

Our device is the first implementation of the absorption refrigeration cycle on the nanoscale.

Stefan Nimmrichter, Study Co-Author, Centre for Quantum Technologies at the National University of Singapore

Exquisite control was needed for developing an absorption fridge with only three atoms.

As an experimental scientist, it’s a pure joy to be able to manipulate individual atoms.

Gleb Maslennikov, Study First Author, Centre for Quantum Technologies at the National University of Singapore

Firstly, the scientists captured and retained three atoms of the element Ytterbium within a metal chamber from which all the air was removed. They also removed one electron from each of the atoms to leave them with a positive charge.

Then, the charged atoms—known as ions—can be held in place by applying electric fields. At the same time, the scientists nudged and zapped the ions using lasers to transfer them to their lowest energy state of motion. The outcome: the ions are suspended nearly standstill, strung out in a line.

Through another laser zap, some heat is again injected, inducing the ions to wiggle about. Due to their like charges, the ions interact with each other. The outcome is three wiggle patterns—stretching and squishing along the line (like a slinky), rocking similar to a seesaw that pivots about the central atom, and zig-zagging out from the line similar to a waving skipping rope.

The energy carried by several “phonons” quantizes the energy in each wiggling mode. The researchers tune the wiggling frequencies to set up conditions for refrigeration—rendering it such that a phonon traveling from the see-saw mode to the slinky one will pull a phonon from the zig-zag mode along with it. Thus, the zig-zag mode loses energy and there is a drop in its temperature. At its coldest, its temperature is within 40 μK of absolute zero (−273 °C), the coldest temperature possible. Each round of ion preparation the and counting phonons took nearly 70 ms, with cooling taking place for about 1ms. This process was repeated thousands of times.

Analyzing such small devices is vital to observe how thermodynamics—best ever insights into heat flows—may require adjustment to reflect more fundamental laws. The principles of thermodynamics are hinged on the average behaviors of huge systems. They do not take quantum effects into consideration, which is a subject of concern for researchers developing quantum devices and nanomachines.

In order to test quantum thermodynamics, The scientists performed cautious measurements of the way phonons spread through the modes over time.

Specifically, the scientists investigated whether a quantum effect called “squeezing” would enhance the performance of the quantum fridge.

The term squeezing suggests the researchers fixed the position of the ions in a more precise way. Due to the quantum uncertainty principle, that increases the variations in momentum. In turn, this enhances the average number of phonons in the see-saw mode that urges the cooling.

The researchers were astonished to observe that squeezing was not helpful to the fridge. “If you have a finite amount of energy to spend, it’s better to turn it directly into heat than use it preparing a squeezed state,” stated Dzmitry Matsukevich, who headed the experimental study.

However, they found that the maximum cooling, realized using a method called “single shot,” was in excess of that predicted by classical equilibrium thermodynamics. In this strategy, the researchers terminate the refrigeration effect through de-tuning of the wiggling modes before it attains its natural endpoint. The cooling surpasses the equilibrium.

Physicist Valerio Scarani, another researcher in the team, anticipates to moving things ahead. “The next question is, can you cool what you want with it? So far, we have the engine of the fridge, but not the box for the beer,” he stated.

This study is supported by the Singapore Ministry of Education through the Academic Research Fund Tier 2 (Grant No. MOE2016-T2-1-141) and Tier 3 (Grant No. MOE2012-T3-1-009); by the National Research Foundation, Prime Minister’s Office, Singapore, through the Competitive Research Programme (Award No. NRF-CRP12-2013-03); and by both above-mentioned sources, under the Research Centres of Excellence programme.