Quantum computers only work when they are kept extremely cold. The problem is that today’s cooling systems also create noise, which can interfere with the fragile quantum information they are supposed to protect. Researchers at Chalmers University of Technology in Sweden have now introduced a new type of minimal quantum “refrigerator” that turns this challenge into an advantage. Instead of fighting noise, the device partially relies on it to operate. The result is highly precise control over heat and energy flow, which could help make large scale quantum technology possible.
Quantum technology is widely expected to reshape major areas of society. Potential applications include drug discovery, artificial intelligence, logistics optimization, and secure communications. Despite this promise, serious technical barriers still stand in the way of real world use. One of the most difficult challenges is maintaining and controlling the delicate quantum states that make these systems work.
Why Quantum Computers Must Be Near Absolute Zero
Quantum computers built with superconducting circuits must be cooled to temperatures very close to absolute zero (around — 273 °C). At these temperatures, materials become superconducting, allowing electrons to move without resistance. Only under these extreme conditions can stable quantum states form inside qubits, the basic units of quantum information.
These quantum states are extremely sensitive. Small changes in temperature, electromagnetic interference, or background noise can quickly erase stored information. This sensitivity makes quantum systems difficult to operate and even harder to expand.
As researchers attempt to scale up quantum computers to solve practical problems, heat and noise become harder to control. Larger and more complex systems create more opportunities for unwanted energy to spread and disrupt fragile 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 as a Cooling Tool
In a study published in Nature Communications, the Chalmers team describes a fundamentally different kind of quantum refrigerator. Instead of trying to eliminate noise, the system uses it as the driving force behind cooling.
“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 core of the refrigerator is a superconducting artificial molecule created in Chalmers’ nanofabrication laboratory. It behaves much like a natural molecule, but instead of atoms, it is built from tiny superconducting electrical circuits.
The artificial molecule is connected to multiple microwave channels. By adding carefully controlled microwave noise in the form of random signal fluctuations within a narrow frequency range, the researchers can guide how heat and energy move through the system with remarkable precision.
“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 Paths Toward Scalable Quantum Technology
By carefully adjusting reservoir temperatures and tracking minuscule heat flows, the quantum refrigerator can operate in multiple ways. Depending on conditions, it can function as a refrigerator, act as a heat engine, or amplify thermal transport.
This level of control is especially important in larger quantum systems, where heat is produced locally during qubit operation and measurement. Managing that heat directly inside quantum circuits could improve stability and performance in ways conventional cooling systems cannot.
“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.
More Information
The study Quantum refrigeration powered by noise in a superconducting circuit was published in the scientific journal Nature Communications. The authors are Simon Sundelin, Mohammed Ali Aamir, Vyom Manish Kulkarni, Claudia Castillo-Moreno, and Simone Gasparinetti from the Department of Microtechnology and Nanoscience at Chalmers University of Technology.
The quantum refrigerator was fabricated at the Nanofabrication Laboratory, Myfab, at Chalmers University of Technology.
Funding for the research was provided by 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.