Quantum computers need extremely low temperatures to function correctly. One major hurdle keeping quantum computers from widespread use is the challenge of cooling qubits to levels near absolute zero. Recently, researchers from Chalmers University of Technology in Sweden and the University of Maryland in the USA have developed a new refrigerator that can automatically bring superconducting qubits to record-low temperatures, enhancing the reliability of quantum computing.
Quantum computers need extremely low temperatures to function correctly. One major hurdle keeping quantum computers from widespread use is the challenge of cooling qubits to levels near absolute zero. Recently, researchers from Chalmers University of Technology in Sweden and the University of Maryland in the USA have developed a new refrigerator that can automatically bring superconducting qubits to record-low temperatures, enhancing the reliability of quantum computing.
Quantum computers hold the promise of transforming various essential technologies across sectors such as healthcare, energy, cybersecurity, artificial intelligence, and supply chain logistics. Unlike classical computers that use bits which can be either 0 or 1, quantum computers utilize qubits that can represent 0 and 1 at the same time due to a phenomenon known as superposition. This trait allows quantum computers to perform numerous calculations simultaneously, offering vast computational capabilities. However, the effective calculation time for a quantum computer is still significantly limited due to the need for continual error correction.
“Qubits, the essential components of a quantum computer, are extremely sensitive to their surroundings. Even minuscule electromagnetic interference can unpredictably change a qubit’s value, leading to errors that disrupt quantum computation,” explains Aamir Ali, a research expert in quantum technology at Chalmers University of Technology.
Achieving Record Low Temperatures
Many current quantum computers rely on superconducting electrical circuits that exhibit zero resistance and effectively maintain information. To enable qubits to function accurately over extended periods, they must be cooled to temperatures close to absolute zero, which is minus 273.15 degrees Celsius or 0 Kelvin. Reaching these extreme conditions allows the qubits to settle into their minimal energy state, known as the ground state, which represents the value 0 and is necessary for beginning calculations.
The existing cooling systems, referred to as dilution refrigerators, can lower the temperature of qubits to around 50 millikelvin above absolute zero. As the temperature approaches absolute zero, further cooling becomes increasingly complex. According to thermodynamic principles, it’s impossible to cool any system down to absolute zero using a finite process. However, researchers at Chalmers University of Technology and the University of Maryland have developed a novel type of quantum refrigerator that works alongside the dilution refrigerator to autonomously cool superconducting qubits to unprecedented low temperatures. The details of this quantum refrigerator are published in the journal Nature Physics.
“The quantum refrigerator relies on superconducting circuits and harnesses environmental heat for its operation. It can cool the target qubit down to 22 millikelvin without external intervention. This advancement could lead to more dependable and error-free quantum computations that require less extra hardware,” states Aamir Ali, the lead author of the research, who further notes:
“With this new method, we enhanced the qubit’s likelihood of being in the ground state before computation to an impressive 99.97 percent, which is significantly better compared to previous methods that achieved 99.8 to 99.92 percent. Although this might seem like a minor improvement, its effects multiply during multiple computations, leading to a significant increase in the efficiency of quantum computers.”
Naturally Powered by Environmental Heat
The refrigerator operates using the interactions between various qubits, particularly between the qubit being cooled and two additional qubits employed for the cooling process. A warm environment is created adjacent to one of the qubits, providing a hot thermal bath that inputs energy into one of the superconducting qubits, driving the quantum refrigerator’s cooling capabilities.
“The thermal energy from the warm environment flows through one of the quantum refrigerator’s two qubits, transferring heat from the target qubit into the second, colder qubit, which is then thermally connected to a very cold environment where the heat from the target qubit is ultimately discarded,” explains Nicole Yunger Halpern, a physicist at NIST and an Adjunct Assistant Professor at the University of Maryland.
Once activated, the system operates independently and is fueled by the natural temperature difference between the two thermal baths.
“Our work represents a pioneering demonstration of an autonomous quantum thermal machine accomplishing a practically significant task. Initially intended as a proof of concept, we were pleasantly surprised to find that the machine outperformed all existing protocols for resetting and cooling the qubits to unprecedented temperatures,” shares Simone Gasparinetti, an Associate Professor at Chalmers University of Technology and one of the study’s lead authors.
