Quantum technology is expected to transform multiple fundamental technologies in society, with applications ranging from drug development and artificial intelligence to logistics and secure communication. Yet, before quantum technology can be put to practical use, various major technical challenges remain. One of the most critical is protecting and controlling the delicate quantum states upon which this technology relies.
For a quantum computer based on superconducting circuits to operate, it must be cooled to extremely low temperatures, close to absolute zero (around - 273 °C). At these temperatures, the system becomes superconducting and electrons can move freely without resistance. Only under these conditions can the desired quantum states emerge in the fundamental information units of a quantum computer, qubits. But these quantum states are fragile. Even the slightest temperature fluctuation, electromagnetic disturbance, or ambient noise can rapidly destroy any information stored in the system.
To use quantum computers to solve real-world problems, they must be scaled up. But as quantum systems grow larger and more complex, it becomes increasingly difficult to prevent heat and noise from spreading and destroying the quantum states.
“Many quantum devices are ultimately limited by how energy is transported and dissipated. Understanding these pathways and being able to measure them allows us to design quantum devices in which heat flows are predictable, controllable and even useful,” says Simon Sundelin, doctoral student of quantum technology at Chalmers University of Technology and the study’s lead author.
Using noise for cooling
In a study published in Nature Communications, Chalmers researchers have now developed a completely new kind of quantum refrigerator which, paradoxically, uses noise itself as the driving force for cooling, rather than attempting to eliminate it.
“Physicists have long speculated about a phenomenon called Brownian refrigeration; the idea that random thermal fluctuations could be harnessed to produce a cooling effect. Our work represents the closest realisation of this concept to date,” says Simone Gasparinetti, associate professor at Chalmers and senior author of the study.
At the heart of the refrigerator is a superconducting artificial molecule, engineered in Chalmers' nanofabrication laboratory. In many respects, it behaves like a naturally occurring molecule but instead of comprising real atoms, it has tiny superconducting electrical circuits. By coupling this artificial molecule to different microwave channels and introducing controlled microwave noise in the form of random signal fluctuations across a narrow frequency band, the researchers can precisely steer and regulate how heat and energy flow through the system.
“The two microwave channels serve as hot and cold reservoirs, but the key point is that they are only effectively connected when we inject controlled noise through a third port. This injected noise enables and drives heat transport between the reservoirs via the artificial molecule. We were able to measure extremely small heat currents, down to powers in the order of attowatts, or 10-18 watt. If such a small heat flow were used to warm a drop of water, it would take the age of the universe to see its temperature rise one degree Celsius,” explains Sundelin.
New opportunities for future quantum technology
By adjusting the temperatures of the reservoirs and measuring extremely small heat flows, the researchers’ refrigerator can operate in several different modes – as a refrigerator, a heat engine or a thermal transport amplifier. The ability to control and steer energy with such high precision is particularly important in larger quantum systems, where heat is generated locally during the control and measurement of qubits.
“We see this as an important step towards controlling heat directly inside quantum circuits, at a scale that conventional cooling systems can’t reach. Being able to remove or redirect heat at this tiny scale opens the door to more reliable and robust quantum technologies,” says Aamir Ali, a researcher in quantum technology at Chalmers and co-author of the study.
Caption: Schematic illustration of the quantum refrigerator in a superconducting quantum circuit. Two microwave channels act as hot and cold heat reservoirs, highlighted by a reddish and a bluish glow, respectively. The heat reservoirs are coupled to an artificial molecule consisting of two qubits. Controlled microwave noise (white zigzag arrows) is injected through the side ports to drive and regulate heat transport. The wide arrow shows the heat flow from hot to cold. Credit: Chalmers University of Technology / Simon Sundelin.
More information:
The study Quantum refrigeration powered by noise in a superconducting circuit has been published in the scientific journal Nature Communications. The authors of the study are Simon Sundelin, Mohammed Ali Aamir, Vyom Manish Kulkarni, Claudia Castillo-Moreno, and Simone Gasparinetti, all working at the Department of Microtechnology and Nanoscience at Chalmers University of Technology.
The researchers’ quantum refrigerator was fabricated at the Nanofabrication Laboratory, Myfab, at Chalmers University of Technology.
The research project has received funding from: the Swedish Research Council; the Knut and Alice Wallenberg Foundation through the Wallenberg Centre for Quantum Technology (WACQT); the European Research Council; and the European Union.