Our quantum refrigerator experiment is now published in Nature Physics

Link: Thermally driven quantum refrigerator autonomously resets a superconducting qubit

Our experiment is arguably the first demonstration of a quantum thermal machine performing a useful task for quantum technology. Our device can cool down the target superconducting qubit to 22 milliKelvin, breaking previous records. It does so autonomously without the need for external control.

Why Quantum Refrigeration Matters 

Quantum thermodynamics should be more useful. While classical thermal machines power industries and modern living, their quantum counterparts have yet to prove useful.

On the other hand, in quantum computing, qubits must be initialized in their ground (lowest energy) state before they can perform accurate calculations. However, even with advanced dilution refrigerators, qubits can only be cooled to temperatures between 45 to 70 milliKelvin, leaving a small chance they remain in excited states instead of the ground state. This reduces the effectiveness of computations. The newly developed quantum absorption refrigerator can autonomously cool qubits to much lower temperatures, reaching 22 milliKelvin, making the qubits more reliable for computing tasks. 

How it works

This refrigerator is based on superconducting circuits and exploits a 3-body interaction between the target qubit and two auxiliary qubits. The auxiliary qubits are connected to thermal baths—sources of thermal energy—and form the quantum refrigerator. By transferring heat from the qubit into these baths, the device cools the qubit more efficiently than conventional thermalization processes. The system is autonomous, meaning once set in motion, it operates without external intervention, drawing on naturally occurring heat from the thermal gradient between different temperature baths. The researchers achieved an excited-state population of the qubit as low as 0.03%—significantly better than current techniques, where populations range between 0.08% and 0.2%. This results in much cooler qubits, critical for improving the accuracy and efficiency of quantum computations. 

Significance of Autonomous machines

This quantum absorption refrigerator exemplifies a broader class of autonomous quantum machines, which can run entirely on heat from the environment, requiring no additional resources for control. This simplifies the process of cooling qubits and makes the system more energy-efficient. The refrigerator’s autonomy is important for quantum computing because it can reduce the complexity and energy demands of current qubit cooling methods. 

Implications for Quantum Technologies

This work demonstrates that quantum thermal machines can be more than theoretical. The refrigerator not only cools qubits effectively but also offers a practical tool for quantum information processing. While challenges remain in making quantum thermal technologies mainstream, this experiment proves they can be integrated into existing quantum systems, potentially revolutionizing future quantum devices. 

In short, the quantum absorption refrigerator marks a significant step toward the practical use of quantum thermal machines in quantum computing, providing a reliable, autonomous cooling method for qubits. 

Image credit: Chalmers University of Technology and Boid AB, Sweden