Quantum refrigerator achieves record-low qubit temperatures

Boosts reliability of superconducting qubits

Researchers claim to have created a quantum refrigerator capable of autonomously cooling superconducting qubits to record-low temperatures.

Quantum computers work fundamentally differently to classic computers, using qubits (quantum bits) as the basic building blocks of computing. Instead of the binary 0 or 1 of a standard bit, qubits can take both states at the same time.

The primary challenge with present-day quantum computers is their inability to generate meaningful outcomes due to qubits’ extreme sensitivity.

Even minor interferences, such as stray light, can trigger calculation errors, and the issue worsens as quantum computers expand in size, where errors can make the devices practically unusable.

But researchers from Sweden's Chalmers University of Technology and the University of Maryland, USA believe they have found a workaround.

Many of today’s quantum computers rely on superconducting electrical circuits, which exhibit zero resistance, allowing them to preserve information with exceptional efficiency.

However, these systems require temperatures close to absolute zero (–273.15°C) to function effectively. These frigid conditions minimise errors and extend the qubits’ coherence time, allowing for more complex and accurate quantum calculations.

Current cooling methods, such as dilution refrigerators, can cool qubits to approximately 50 millikelvin (mK) above absolute zero. However, further cooling is increasingly challenging as the system approaches absolute zero, due to the fundamental laws of thermodynamics.

The latest innovation lies in a novel quantum refrigerator, detailed in the journal Nature Physics.

This quantum refrigerator, also based on superconducting circuits, operates autonomously, leveraging heat from the environment to cool the target qubit.

It functions by harnessing interactions between the target qubit and two auxiliary qubits. One of the auxiliary qubits interacts with a warm environment, acting as a heat source. This energy is then transferred through the system, effectively cooling the target qubit.

"Energy from the thermal environment, channelled through one of the quantum refrigerator's two qubits, pumps heat from the target qubit into the quantum refrigerator's second qubit, which is cold," explains Nicole Yunger Halpern, NIST Physicist and Adjunct Assistant Professor of Physics and IPST at the University of Maryland.

"That cold qubit is thermalised to a cold environment, into which the target qubit's heat is ultimately dumped."

In experimental tests, the quantum refrigerator achieved a steady-state temperature of 23.5 mK above absolute zero in 1.6 microseconds.

By pre-cooling the qubit to such low temperatures, researchers say they increased the probability of the qubit being in its ground state before computation to 99.97%.

While this may seem like a modest improvement compared to previous methods, which achieved probabilities between 99.8% and 99.92%, the difference becomes significant in long computations, immensely reducing error rates.

Aamir Ali, a quantum researcher at Chalmers and the lead author of the study, said the new quantum refrigerator can cool the target qubit to 22 mK without external control.

"This paves the way for more reliable and error-free quantum computations that require less hardware overload," he said.

The system operates autonomously, running without external control once initiated and drawing its power from the heat generated by the natural temperature difference between two thermal baths.

"Our work is arguably the first demonstration of an autonomous quantum thermal machine executing a practically useful task," said Simone Gasparinetti, Associate Professor at Chalmers University of Technology and lead author of the study.

The new research opens new avenues in quantum information processing and experimental studies of quantum thermodynamics.

The device was fabricated at Myfab, a nanofabrication laboratory at Chalmers University of Technology.