Refrigeration breakthrough for superconducting quantum qubits

Researchers in Sweden and the US have developed a new type of refrigerator that can autonomously cool superconducting qubits to record low temperatures, paving the way for more reliable quantum computers.

However, the time a quantum computer can work on a calculation is still significantly constrained, because it spends a lot of time correcting errors that come from the thermal excitation of the qubits.

The dilution refrigerators used today bring the qubits to about 50 millikelvin above absolute zero. The closer a system approaches to absolute zero, the more difficult it is to achieve further cooling.

The researchers at Chalmers University of Technology and University of Maryland h constructed a new type of quantum refrigerator that can complement the dilution refrigerator and autonomously cool superconducting qubits to record-low temperatures.

“Our work is arguably the first demonstration of an autonomous quantum thermal machine executing a practically useful task. We originally intended this experiment as a proof of concept, so we were pleasantly surprised when we found out that the performance of the machine surpasses all existing reset protocols in cooling down the qubit to record-low temperatures,” says Simone Gasparinetti, Associate Professor at Chalmers University of Technology.

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The process is driven by a thermal gradient and is autonomous — requires no external control. The refrigerator exploits an engineered three-body interaction between the target qubit and two auxiliary qubits coupled to thermal environments. These additional qubits, called qudits, have multiple quantum states in Hilbert space, rather than 1 or 0.

The environments consist of microwave waveguides populated with synthesized thermal photons that act as hot thermal baths. These give energy to one of the quantum refrigerator’s superconducting qudits and power the quantum refrigerator to reduce the temperature of the target qubit.

When fully excited, the target qubit reaches a steady-state temperature of 23.5mK in about 1.6μs. This shows how quantum thermal machines can be used for quantum information-processing tasks and provides a path to experimental studies of quantum thermodynamics with superconducting circuits coupled to propagating thermal microwave fields.

The system is autonomous in that once it is started, it operates without external control and is powered by the heat that naturally arises from the temperature difference between two thermal baths.

“The quantum refrigerator is based 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 error-free quantum computations that require less hardware overload,” says Aamir Ali, a quantum researcher at Chalmers. “With this method, we were able to increase the qubit’s probability to be in the ground state before computation to 99.97 per cent, which is significantly better than what previous techniques could achieve, that is, between 99.8 and 99.92 per cent. This might seem like a small difference, but when performing multiple computations, it compounds into a major performance boost in the efficiency of quantum computers.”

“Energy from the thermal environment, channeled 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. That cold qubit is thermalised to a cold environment, into which the target qubit’s heat is ultimately dumped,” says Nicole Yunger Halpern, NIST Physicist and Adjunct Assistant Professor of Physics and IPST at the University of Maryland.