Quantum computing represents a turning point in technological evolution, promising to radically change the way we process complex data. However, a major barrier remains: the need to cool qubits to extremely low temperatures, close to absolute zero. A team of researchers may have found a solution to this technical problem with a new type of quantum refrigerator.
Quantum computers require extreme cooling to perform reliable calculations. Qubits, their fundamental elements, must be maintained at temperatures close to absolute zero to avoid calculation errors. This need for intense cooling was a major obstacle to the integration of these machines into society. Researchers from Chalmers University of Technology in Sweden and the University of Maryland in the United States have designed an innovative refrigerator capable of autonomously cooling superconducting qubits to record temperatures.
The advantages of quantum computers
Quantum computers have immense potential to transform diverse sectors such as medicine, energy, encryption, artificial intelligence and logistics. Unlike bits in classical computers which record either 0 or 1, qubits can represent both values simultaneously thanks to the superposition. This feature allows quantum computers to perform calculations in parallel, increasing their processing power exponentially. However, the length of time a quantum computer can perform calculations is limited by the time it takes to correct errors.
« Qubits, constituents of the quantum computer, are extremely sensitive to their environment. Even very weak electromagnetic interference can randomly change the value of the qubit, leading to errors and hindering quantum calculation. “, explained Aamir Ali, a quantum technology research specialist at Chalmers University of Technology.
Refrigeration at record temperatures
Currently, many quantum computers use superconducting electrical circuits to minimize resistance and retain information. To operate without errors for extended periods of time, the qubits must be cooled to approximately minus 273.15 degrees Celsius, or zero Kelvin, the lowest temperature theoretically possible. Current cooling systems, called dilution refrigerators, bring qubits to about 50 millikelvin above absolute zero. However, according to the laws of thermodynamics, reaching absolute zero is impossible by a finite process.
Researchers have developed a new type of quantum refrigerator that complements dilution refrigerators and allows superconducting qubits to be cooled to even lower temperatures, 22 millikelvin, autonomously.
« The quantum refrigerator relies on superconducting circuits and is powered by heat from the environment. It can cool the target qubit to 22 millikelvin, without external control. This paves the way for more reliable and less error-prone quantum calculations “, added Aamir Ali, lead author of the study.
How the quantum refrigerator works
The refrigerator uses the interaction between different qubits, in particular between the qubit to be cooled and two other qubits used for cooling. A hot environment is created next to one of the qubits, serving as a hot thermal bath. This thermal bath transfers energy to one of the qubits in the quantum refrigerator, thereby activating the cooling system.
« Energy from the thermal environment, channeled through one of the quantum refrigerator’s qubits, extracts heat from the target qubit to the refrigerator’s second qubit, which is cold. This final qubit is then cooled by a cold environment, where the heat from the target qubit is ultimately dissipated “, said Nicole Yunger Halpern, physicist at NIST and assistant professor at the University of Maryland.
The system operates autonomously, without requiring external control, powered by the temperature difference between two thermal baths.
« Our work is likely the first demonstration of an autonomous quantum thermal machine accomplishing a useful task. Initially designed as a proof of concept, we were pleasantly surprised by the performance of the machine, which outperforms all existing reset protocols to cool the qubit to record temperatures “, concluded Simone Gasparinetti, associate professor at Chalmers University of Technology and co-lead author of the study.
Illustration caption: The new quantum refrigerator – the square object at the center of the qubit in the image – is based on superconducting circuits and powered by heat from the environment. It can autonomously cool qubits to extremely low temperatures, paving the way for more reliable quantum computers. The device was fabricated in the Myfab nanofabrication laboratory at Chalmers University of Technology, Sweden. Credit:Chalmers University of Technology | Lovisa Håkansson
Article : ‘Thermally driven quantum refrigerator autonomously resets a superconducting qubit’ / ( 10.1038/s41567-024-02708-5 ) – Chalmers University of Technology – Publication dans la revue Nature Physics
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