The Quantum Engine That Simultaneously Heats and Cools

It’s easy to forget that Franco Nori is a not a poet or philosopher. Take the way the quantum physicist, based at RIKEN in Saitama, Japan, speaks about his old hobby astronomy. "It’s like a form of meditation or sensory deprivation: no internet, no music—just the wind and the immense night sky," he say. "Sometimes it feels like some sort of religious experience to be in the dark outdoors, looking at star clusters or nebulae."

It’s somewhat ironic, then, that Nori’s current research on quantum devices is attempting to illuminate a path to a bright, technologically advanced future. In 2019, alongside Jukka Pekola from Aalto University, in Finland, Nori was awarded close to US$1 million from FQXi to explore the fundamental limits set by thermodynamics in the quantum world. One project involved building an ’analog quantum heat engine’—a "very very tiny electrical analog of the older and much larger mechanical engine," he explains. The team has now made such a machine, which, in one configuration, can operate in both heating and cooling mode simultaneously. The device could one day have applications in quantum electronics, helping for instance to cool quantum chips. It may also enable physicists to explore the fundamental limits set by the laws of thermodynamics—which govern heat and energy transfer—in the quantum world.

Grounded in the Lab

Nori has always found physics captivating. "Physics provides stunning, powerful knowledge to better understand the world around us, from the microscopic all the way to the large-scale structure of the universe," he says. Though he is a theoretical physicist, his quest is firmly grounded in the lab. "I’m particularly interested in foundational questions that can be tested experimentally—either with current technologies or ones that would be available in the very near future." Otherwise, Nori warns, it’s closer to philosophy. It’s an interesting distinction given his penchant for the profound. Here he cites two influential books, both by FQXi members: Lee Smolin’s The Trouble with Physics and Lost in Math by Sabine Hossenfelder. They both tell a similar cautionary tale—physicists can sometimes lose their way, ending up down a mathematical rabbit-hole of unfalsifiable theories. Nori is keen not to fall into that trap.

Along with colleagues in Japan and Ukraine, Nori is now working on analogs of a quantum heat engine—a futuristic sounding device that can trace its heritage right back to the 19th-century industrial revolution. Then, the drive to build more efficient engines led to the development of classical thermodynamics. "Today heat engines can be found everywhere—such as the combustion engine in automobiles and buses—and have played a central role in everyday life for well over a century," Nori says.

There are some important differences between the operation of his analog quantum heat engine and a classical heat engine or refrigerator (which is essentially a heat engine operating in reverse). In the classical devices, heat is exchanged between two heat baths, also called ’thermal leads.’ The team’s quantum device acts as an electric analog of this macroscale thermal system, Nori explains: "Our quantum device is connected to higher and lower voltage leads."

The device studied by Nori and his collaborators takes the form of a silicon transistor deliberately riddled with impurities. When a voltage is applied, an electron can tunnel from one impurity close to the surface of the transistor to another much deeper within—transitioning from a lower energy to a higher, ’excited’ energy level. The time it takes for the system to calm down from its excited state is called the ’relaxation time.’ Nori and collaborators found that they could adjust the gap between the energy levels by modulating the frequency and amplitude of the device’s magnetic fields. This allowed them to drive the machine, similar to the way in which energy is pumped between two heat baths in a macroscale engine or fridge. "Depending on whether the system was driven to the excited state when the gap between energy levels was large and relaxed when it was smaller, or the converse, it operated analogously to a heat engine or refrigerator," Nori says.

Hot Regime

Nori’s is not the only team working to produce such nanoscale devices, which might have applications in cooling quantum computing chips, in the future. But while many other similar quantum engines often require frigid, milliKelvin temperatures—a whisker above absolute zero—a big advantage of Nori’s is that can operate at a few Kelvin—a regime they whimsically describe as "hot" despite being three times colder than any naturally occurring place on Earth and even colder than the dwarf planet Pluto. "It’s less technologically difficult," Nori says. However, he cautions that there is a trade-off: hot systems are a bit more susceptible to disruption by thermal noise. But so far, the team has managed to control this quite well.

Natalia Ares, a quantum physicist at the University of Oxford, UK, who is not part of Nori’s team, describes the research as a promising development. "This work is a significant step towards demonstrating the potential of semiconductor technology for quantum thermodynamic experiments," she says. "It highlights the high-level control over quantum states that semiconductor devices can offer and will open up new paths—from fundamental aspects to applications."

Ares is particularly excited about applying such an engine to on-chip refrigeration. The world has been transformed as we’ve packed more and more transistors onto ever smaller chips. However, that increases heat, which can cause processing errors. A chip with its own system of cooling would be a real shot in the arm for the continued march of computing power. Currently that waste heat is lost, but further developments could come as we put that energy to good use. Ares sees this ’energy harvesting’ as another potential application of Nori’s work.

But the benefits of the mini machine do not end with the potential for practical computing applications. The device also opens a window on some fundamental questions. Things get really interesting when the relaxation period and the period of the driving voltage matched. In this regime, another famous quantum phenomenon comes into play. According to quantum physics, a system can be in a ’superposition,’ which means that it can be in more than one state at the same time—a particle can be in two places at once, and Schrödinger’s famed cat (at least in theory) can be both alive and dead. Nori’s team monitored which state their device was in—whether it was operating as a tiny heat engine or as a mini fridge—through the microwaves it produced. In one particular regime, they noticed something bizarre. The microwaves produced a characteristic interference pattern, implying that the device must be acting as both an engine and a fridge at the same time (Phys. Rev. Lett. 125, 166802 (2020)).

"The experiments show that the device function can be in a superposition," Nori says (see images on left).

We’re unlikely to ever want to heat and refrigerate something simultaneously in the macroworld, but understanding how this is possible in the quantum arena could lead to a deeper understanding of fundamental questions about thermodynamics on the atomic scale. Still, further investigations of this superposition will require delicate handling, says Jonathan Oppenheim, a quantum physicist at University College London, UK, who is not involved in the study.

"In order to maintain the superposition in this case, we would need to keep not only the device in a superposition, but also the fridge, energy source and other components," Oppenheim says. This fragile superposition could easily snap. If the system in one state affects the environment in a sufficiently different way compared with when it is in the other state, it will no longer be in superposition. Take Schrödinger’s cat-in-a-box as an example. A living cat leads to less oxygen in the box than a dead cat—a distinct giveaway that there’s no longer a superposition. "If we could tell that the fridge was being cooled, the superposition will have collapsed," Oppenheim says. Still he posits that it will one day be possible to isolate and engineer a fridge which could be in a superposition of two different temperatures. "What we could do with this, I’m not sure, but it is fun to think about," Oppenheim says.

Nori muses that there may even be practical applications from harnessing the superposition, leading to "new quantum electronic functionalities." It could help switch quickly between the two functions on a quantum chip, for example.

To further illustrate his point, Nori recounts a perhaps apocryphal story of when a British politician asked Michael Faraday—one of the godfather’s of electromagnetism—what his work was useful for. Faraday’s response? "I do not know, but I am sure that one day you will tax it."

"Faraday proved to be right," says Nori. Likely the same will prove true of his mini engine.