Nanofridges, thermometers and qubits – working in the quantum world

4 Apr 2018

Throughout his career, Academy Professor Jukka Pekola – who is also a professor at Aalto University – has researched the vicinity of absolute-zero temperatures, the home of many peculiarities. While laws of nature won’t fly out the window there, they do work differently than in our everyday lives.

In such a low temperature, some materials become superconductors – they allow electricity to flow without resistance. What’s more, quantum mechanical phenomena start to emerge. For example, systems can be simultaneously in two states, and separate quantum systems can become entangled with each other.

By way of analogy, think of a nano-sized car that is simultaneously red and blue, and turning the steering wheel in one car would cause another car 100 kilometres away to turn a corner.

“In the past, we were merely investigating these phenomena, but we’re now at the second phase where we can utilise these genuine quantum mechanical phenomena,” Pekola says, predicting that many breakthroughs will occur in the field soon – also in Finland, but more about this later.

Finland is positively cryogenic

Finland has a long tradition of researching extremely cold temperatures and the phenomena that occurs therein. Professor Olli Lounasmaa, one of the luminaries of Finnish physics, succeeded in making the Low Temperature Laboratory of Helsinki University of Technology one of the top laboratories in its field. The laboratory accomplished several world records in low temperatures by cooling their devices extremely close to absolute zero.

Absolute zero at −273.15°C is the coldest temperature possible, because at that point, the movement of atoms stops entirely. Absolute zero is also the starting point of the Kelvin scale used in physics – in it, absolute zero is expressed as zero Kelvin (0 K).

While the university is now known as Aalto University, the laboratory still holds the world record. In 2000, the laboratory reached a temperature that was only 0.0000000001° above absolute zero.

However, at that time, Pekola was a member of another research group in cryogenic physics in Jyväskylä.

“I studied here in Otaniemi and started my career here in Olli Lounasmaa’s group,” Pekola says. “I started as a thesis worker in the early 1980s and later worked as a doctoral candidate. The topic of my research was ultralow temperatures. To attain them, we used liquid helium-3 and researched superconductivity at those temperatures. It was a hot research topic at that time, and one that resulted in the Nobel Prize in Physics being awarded in 1996 to three American researchers in the field.”

In the early 1990s, the interest shifted to a broader approach for studying extremely low temperatures. In these temperatures, thermal motion decreases and many exciting quantum phenomena start to emerge, which researchers realised will open up many opportunities for new microscale and nanoscale manufacturing technologies.

“The Finnish researcher Mikko Paalanen had been studying these phenomena in the famous Bell Labs in the US, and was returning to Finland. He was appointed as professor at the University of Jyväskylä, which wanted to invest in cryogenics and nano research. He recruited me to his research group, so I set off for Jyväskylä.”

Pekola recounts those times with a smile on his face, telling stories about how their group was the first in Finland to research one-electron phenomena, coolers, thermometers... The group became the pioneers of nanoelectronics and nanophysics in Finland.

“Developing a thermometer was my first great passion. Back then, we also tried fervently to create one-electron transistors – you know, the kind that are now made routinely in our lab. We didn’t quite succeed in it in our simple lab at Jyväskylä at that time, but we did succeed in making the thermometer. It’s like a poor man’s transistor with only one electron. Thermometers like it are now used in many applications, since it requires no calibration whatsoever.”

In 2003, Pekola, who by then had become a professor, was offered an opportunity to set up his own research team at Aalto University, so he decided to return to Otaniemi. He is now in charge of a research group called Pico, which continues to study electrons and extremely low temperatures. In addition to basic research, the group also wishes to turn cryogenic temperatures and phenomena therein into ordinary everyday technology.

One of the projects is a nanofridge. It’s nowhere near a consumer application yet, but the fridge and the technology behind it might have several other practical applications. For example, quantum computing – currently a hot research topic – requires a super-cold environment, so if we wish to develop everyday quantum computing applications, we must be able to reach temperatures close to absolute zero everywhere, not just in research institutes.

Tunnelling tricks

“The nanofridge started as an outgrowth of basic research. We were interested in energy transport in minuscule nanostructures. Heat transfer is extremely important even in ordinary microchips that must dissipate the heat generated in their circuits. This is particularly important in quantum devices, where heat must be transferred from one location to another to achieve precision refrigeration.

To put it simply, a nanofridge operates by transferring the hottest electrons away from an object, thereby cooling down the object.

The principle is the same as cooling hot coffee by blowing on it: the airflow removes steam from the cup, which means that the hottest gas molecules on top of the coffee fly away while cooler molecules are left behind.

When asked for further details, Pekola describes the manufacturing process of a nanofridge (no, homebrewing one probably won’t work).

“We create multilayered nanoscale metal structures by means of lithography in a cleanroom. That is, we use an electron microscope to draw patterns on a plastic surface and then use the patterned surface as a mask when evaporated metal is deposited on the surface. This results in structures with a size of approximately 10 nanometres, a billionth of a metre. When this is repeated several times, depositing layers different metals and finally removing the plastic mask, we can produce the desired three-dimensional nanostructure.”

Layers of insulators can also be deposited between the metal layers, and electricity can only flow through these insulating layers by tunnelling.

Tunnelling means that a particle can penetrate a potential wall also in cases where classical physics says the particle lacks the energy to penetrate the wall. An electrostatic potential wall is created by a change in material or its properties.

“Some of the very ordinary, well-known materials, such as aluminium or copper, become superconductors at extremely low temperatures, while others don’t. Aluminium is a superconductor at temperatures of about one Kelvin and colder.

Superconducting materials not only conduct electricity without resistance, but also contain so-called energy holes. They are forbidden states for electrons in the material, like energy holes in semiconductors.

“Particles that happen to hit an energy hole cannot tunnel, but the ones above the hole can. This results in a transfer of energy from the ordinary metal to the superconductor side.”

Refrigerators are fairly basic technology nowadays, which is why it is more interesting to develop variants and applications of them. One of these is heat transfer in quantum computers, which must be solved before the computers can be used in practical applications.

“For this purpose, we have this wild idea of using qubits, the computation units of a quantum computer, as a heat engine. We could control the state of the qubits from the outside and thus create multistage coolers, which is an approach used in traditional coolers as well. However, we are still far from productisation, even though cooling is an important part of our research.

Results give rise to everyday technology

“People commonly think that cryogenic physics and research into low temperatures is an exotic activity, but these fields have already become industry in Finland,” Pekola explains.

He mentions that the Finnish company BlueFors Cryogenics manufactures extremely easy-to-use coolers for the millikelvin range.

Traditionally, cryogenic physics laboratories have required large numbers of liquid helium flasks, since low temperatures are achieved with liquid helium. However, this is no longer necessary. The BlueFors device contains a compressor that recycles the helium used for creating the basic operating temperature.

“This represents a major change not only for research, but also for practical applications, since helium is expensive, handling the flasks is cumbersome and using liquid helium always requires special arrangements. Achieving supercold temperatures by just plugging in the device has been a small revolution. These devices are now commonly used everywhere, with Finland and the Low Temperature Laboratory as pioneers.”

“Next, we want to apply the same technology to extremely challenging and small targets. For example, space applications need cooling equipment that are electrically powered and don’t need large liquid helium flasks. This equipment should also be able to tolerate the conditions of launch and space, while being lightweight and easy to use.”

Quantum computing is not the only field that needs small coolers that work by simply switching the power on.

A promising area of application are quantum mechanical sensors. Several types of sensors are already being manufactured in Finland, and when this industrial-scale manufacturing is combined with expertise in superconductivity, cryogenic know-how and a good infrastructure for making difficult structures, Finland might become a major player in the manufacture of quantum mechanic sensors.

Attracted to the laboratory bench

With slightly more than a dozen members, Pekola’s Pico group is not big. The group was bigger in the past, but Pekola says it was difficult to manage. At present, the group consists of two senior researchers, a few PhD researchers and half a dozen postgraduates.

“Work like this touches several areas of expertise. We need theoreticians and people who are good at building things. It’s great to be a group leader and see different kinds of people with different skills and to promote their career also in the future.”

“I want to participate in the work at the lab, and I suppose that the students also benefit from seeing me in the lab from time to time.”

However, Pekola often emphasises the importance of combining practical experiments with theory.

“It’s useless to tinker with experiments if you don’t understand exactly what’s happening. Explaining things theoretically is also important, and it has always fascinated me.

One of these combinations of theory and practice – and one that Pekola has given considerable thought to – is the development of the standard of electric current. The idea is to move electrons one at a time through similar structures while controlling the movement of the electrons with external voltage.

“Approximately ten years ago, I had an idea how this process could be made extremely precise, but we haven’t yet progressed up to the level required for an approved current standard in metrology. However, we’re already very close, and new avenues of research keep cropping up as a result of this research.

The quantum world would also have many other interesting things to study, such as the development of entire quantum processors. But Finland simply lacks the resources for that, Pekola says.

“Lounasmaa taught me that there’s no point in working on a field in which huge investments are made abroad and in which massive research teams work. It’s better to find your own special niche where even a small group can leave a mark. I think we’ve succeeded fairly well in this.”

As our chat draws to a close, Pekola also praises his Academy Professorship, which enables him to concentrate on leading his research group and also carrying out research.

“This is important, since I’m still very excited about physics. It’s an endless treasure trove for research!”

Original text in Finnish and photo by Jari Mäkinen

Last modified 4 Apr 2018
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