Researchers from Chalmers University of Technology in Sweden and the University of Maryland in the USA have developed a new quantum refrigerator. This refrigerator can cool superconducting qubits to very low temperatures independently, a big step toward making quantum computers more practical.
Quantum computers need to be extremely cold to work well. A significant problem has been cooling qubits close to absolute zero. The new quantum refrigerator can cool qubits to about 22 millikelvin by itself.
This improves their performance by reducing errors that previously limited their efficiency. Quantum computers, which use qubits instead of bits like regular computers, could change important areas like medicine, energy, encryption, artificial intelligence, and logistics.
Qubits can be 0 and 1 at the same time. This lets them do many calculations at once, giving them amazing computing power. But they are easily affected by their surroundings.
Even weak electromagnetic interference can cause mistakes. Current quantum computers use dilution refrigerators to cool qubits to about 50 millikelvin above absolute zero. The new quantum refrigerator fits on a small chip.
It cools qubits further to around 22 millikelvin. This greatly improves how reliably they work. With this method, we were able to increase the qubit’s probability to be in the ground state before computation to 99.97 percent,” said Aamir Ali, a research specialist at Chalmers University.
The refrigerator uses interactions between the qubit being cooled and two other qubits used for cooling. It is powered by heat from the environment. This makes it work on its own once started.
“Energy from the thermal environment pumps heat from the target qubit into the quantum refrigerator’s cold qubit, which then transfers the heat to a cold environment,” explained Nicole Yunger Halpern, Adjunct Assistant Professor of Physics at the University of Maryland. This is the first time a quantum thermal machine has worked on its own to do something useful. According to Simone Gasparinetti, Associate Professor at Chalmers University, it works better than existing methods.
This pioneering development opens the way for quantum computations that are more reliable and have fewer errors.
Quantum cooling breakthrough for qubits
It could lead to quantum computing technology being used more widely.
The quantum refrigerator is made up of three qudits, which are quantum systems. Each qudit is connected to a waveguide that acts as a heat bath. The target qubit is accidentally coupled to an uncontrolled bath in its environment.
This bath excites the target to a higher temperature. The qudits next to each other are coupled together to create a three-body interaction. This is a key part of a quantum absorption refrigerator.
The interaction is set up so that one excitation in one qudit and one in another are swapped at the same time with a double excitation in the third qudit. As the heat baths drive the resetting, the system works on its own as a quantum absorption refrigerator. Heat flows from a hot bath coupled to one qudit into a medium-temperature bath coupled to another.
A net heat current enters the bath coupled to the target qubit, effectively cooling and resetting it. This practical autonomous quantum machine uses less control and thermodynamic work than non-autonomous ones. It is a promising step toward practical quantum information-processing systems.
The researchers built two qubits and one “qutrit” from tiny superconducting circuits. The qutrit and one qubit formed a fridge for the second target qubit, which could later be used for computation. They carefully designed the interactions between the three parts.
When the target qubit had too much energy, heat automatically flowed out of it and into the other two elements, lowering its temperature and resetting. Since this process was autonomous, the qubit-and-qutrit fridge could fix errors without any outside control.
This method of resetting the qubit required less new hardware and worked better. Without major changes to the quantum computer or new wires, the qubit’s starting state was correct 99.97 percent of the time, whereas other reset methods usually achieve only 99.8 percent.
This shows how thermodynamic principles about heat, energy, and temperature can be useful in the quantum world. The researchers are already investigating ways to build on their experiment. They might create autonomous quantum engines or design a quantum computer with other functions automatically driven by temperature differences.
April Isaacs is a news contributor for DevX.com She is long-term, self-proclaimed nerd. She loves all things tech and computers and still has her first Dreamcast system. It is lovingly named Joni, after Joni Mitchell.
