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Monday, 16 December 2019

Discovery: thermal energy can flow through absolute vacuum through a quantum phenomenon

In a new study at the University of California at Berkeley, researchers have shown that thermal energy can travel through absolute vacuum thanks to quantum fluctuations. To achieve this, the team placed two gold-coated silicon nitride membranes, placed a few hundred nanometers apart in a vacuum chamber. When they heated one of the membranes, the other also warmed up when there was no contact between the two membranes and the electromagnetic radiation was negligible.

To understand the extent of the discovery, we should give a telling example: you probably know that if you could, placing a vacuum thermos (complete absence of air) to keep your coffee warm would be a great way to keep more heat, the vacuum being a very good insulator because thermal energy has difficulty moving there. The vibrations of atoms or molecules, which carry thermal energy, simply cannot travel if there are no atoms or molecules around.

But researchers at the University of California at Berkeley have successfully shown how the strangeness of quantum mechanics can overturn this basic principle of classical physics.

The study, published this week in the journal Nature , shows that thermal energy can travel through a few hundred nanometers of absolute vacuum, thanks to a phenomenon of quantum mechanics known as the Casimir effect.

Casimir effect and quantum vacuum fluctuations

Although this effect is only significant at very small scales, it could have profound implications for the design of computer chips and other electronic components at the nanoscale, where heat dissipation is essential. It also upsets what many of us have learned about heat transfer in physics.

" Heat is generally conducted in a solid through the vibrations of atoms or molecules, or so-called phonons - but in vacuum there is no physical medium. So, for many years, textbooks have told us that phonons cannot travel through the void, ”said Xiang Zhang, professor of mechanical engineering at UC Berkeley, who led the study. " What we have discovered, surprisingly, is that phonons can indeed be transferred across the void by invisible quantum fluctuations ."

In the experiment, Zhang's team placed two gold-coated silicon nitride membranes a few hundred nanometers apart, placed inside a vacuum chamber. When they heated one of the membranes, the other also warmed up.

To carry out the experiment, the team designed extremely thin silicon nitride membranes, made in a clean room, and then used optical and electronic components to precisely control and monitor the temperature of the membranes when they were locked in the vacuum chamber. Credits: Violet Carter / UC Berkeley

This discovery of a new heat transfer mechanism offers unprecedented opportunities for thermal management at the nanoscale, which is important for high-speed computing and data storage ," says Hao-Kun Li, a former doctoral student in Zhang's group and lead co-author of the study. " Now we can design the quantum vacuum to extract heat from integrated circuits ."

In quantum physics, absolute vacuum does not exist

The feat at first glance impossible to move molecular vibrations through vacuum can be accomplished here because, according to quantum mechanics, absolute vacuum does not exist, said King Yan Fong, postdoctoral researcher at UC Berkeley and second lead co-author of the study.

" Even if you get 'empty' space according to classical physics - no matter, no light - quantum mechanics says it can't be really empty. There are still some fluctuations in the quantum field in a vacuum, ”said Fong. " These fluctuations give rise to a force which can link two objects together, and this is what we call the 'Casimir effect'. So when an object heats up and begins to vibrate and oscillate, this movement can be transmitted to the other object through vacuum, through quantum fluctuations in vacuum . ”

Although theorists have long speculated that the Casimir effect could help molecular vibrations travel through vacuum, proving it experimentally has been a major challenge.

In particular, the researchers discovered that by carefully choosing the size and structure of the membranes, they could transfer thermal energy over a few hundred nanometers of vacuum. This distance was far enough that other possible modes of heat transfer were negligible, such as the energy transported by electromagnetic radiation.

Sound could also travel through the void

Because molecular vibrations are also the basis of the sounds we perceive, this discovery suggests that sound could also travel through vacuum.

" Twenty-five years ago, during my doctoral qualification exam at Berkeley, a professor asked me, 'Why can you hear my voice?'. I replied: 'It is because your sound moves by vibrating molecules in the air'. Then he asked, 'What if we suck all the air molecules out of this room? Would you still hear me? ' I said, 'No, because there would be no more space to vibrate,' ”said Zhang.

" Today, what we have discovered is a surprising new mode of heat conduction through a vacuum without medium, which is reached by the intriguing fluctuations of the quantum vacuum. So, I made a mistake during my 1994 exam! Now you can shout through the void, ”he concludes.


Phonon heat transfer across a vacuum through quantum fluctuations

King Yan Fong, Hao-Kun Li, Rongkuo Zhao, Sui Yang, Yuan Wang & Xiang Zhang

Nature volume 576, pages243–247(2019)


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