Novel ‘quantum refrigerator’ is great at erasing quantum computer’s chalkboard

(Press-News.org) If you’d like to solve a math problem on a good old-fashioned chalkboard, you want the board clean and free of any previous markings so that you have space to work. Quantum computers have a similar need for a clean workspace, and a team including scientists at the National Institute of Standards and Technology (NIST) has found an innovative and effective way to create and maintain it. 

The research effort, a collaboration with physicists at Sweden’s Chalmers University of Technology, could address one of the main issues confronting quantum computer designers: the need to keep the bits in a superconducting quantum processor free of errors and ready to perform calculations whenever necessary. These “qubits” are notoriously sensitive to heat and radiation, which can spoil their calculations just as stray chalk marks might make the numeral 1 look like a 7. 

Erasing these qubits after a calculation involves cooling them to a fraction of a degree above absolute zero and then keeping them there. The team’s method is not only more effective than other state-of-the-art methods for erasing the qubit chalkboard because of the lower temperatures it achieves, but it also achieves them in a novel way — powering the eraser using heat flowing between two parts of the refrigerator that keeps the computer cold. This approach could prove itself useful in other ways. 

“The technique in this paper could benefit quantum computers,” said the team’s Nicole Yunger Halpern, a physicist at NIST and the University of Maryland’s Joint Center for Quantum Information and Computer Science (QuICS). “It could address one of the problems in quantum computer design, and it also shows that we can siphon heat from one part of the computer’s refrigerator and convert the heat into work. It could introduce technological capabilities we haven’t even thought of yet.” 

The team’s proof-of-principle demonstration of the method appears [Jan. 9, 2025] in the journal Nature Physics.  

Although quantum computers are far from reaching maturity, they remain the object of intense research because they offer the potential to perform certain tasks that conventional computers cannot do easily, including simulating complex molecular structures that are important in drug design. These projected capabilities derive from a difference between qubits and the bits in a conventional computer: While a conventional bit can exist in two states, 1 or 0, a qubit can have both values simultaneously, nominally allowing a quantum computer to sift through vast numbers of potential solutions at once. 

A promising way to make qubits is to build them from superconducting circuits, which are the type the team used in its study. Superconducting qubits bring advantages including tunability: Experimentalists can change the properties of the qubits as desired. However, qubits — even those that superconduct — can develop errors very quickly, which can ruin calculations.  

Erasing a superconducting qubit means resetting it to its lowest energy state, which has proved to be tricky. An effective way to reset the qubit would be to make it as cold as possible, down in the tens of millikelvins (mK), or thousandths of a degree above absolute zero. Until now, the best reset methods have brought qubits to a range of 40-49 mK. While those numbers might sound good, they aren’t good enough, said co-author and quantum physicist Aamir Ali of Chalmers University of Technology, where the team’s experimental work was conducted, supervised by principal investigator Simone Gasparinetti.  

“In a quantum computer, initial errors can compound as the calculation proceeds,” Ali said. “The more you can get rid of them at the outset, the more effort you will save later.” 

The team’s method can cool the qubit to 22 mK. The improvement would erase the board more completely, reducing the likelihood of initial errors causing trouble down the line.  

“If you didn’t cool the qubit to that low a temperature, you wouldn’t be able to erase the board as thoroughly,” Yunger Halpern said. 

The team has achieved these performance numbers using a “quantum refrigeration” technique that has never been harnessed in a practical machine before. A refrigerator cools objects by using some sort of energy to draw heat away from the fridge’s interior. In a conventional kitchen fridge, the energy source is electricity, but the quantum refrigerator would use heat from elsewhere in the computer to do the job.  

The team’s fridge uses two other quantum bits as its components. One qubit, which would be connected to a warmer part of the computer, would serve as the energy supply. The second quantum bit would serve as a heat sink into which the computational qubit’s undesired extra heat could flow. In an actual quantum computer, if the computational qubit — the chalkboard — got too warm, the fridge’s first qubit would pump heat from the computational qubit into the heat sink, which would carry the heat away, returning the computational qubit to nearly its ground state and erasing the board.  

The process works autonomously, requiring minimal external control or additional resources to maintain the computational qubit’s ability to calculate.  

“We think this approach will pave the way for more reliable quantum computing,” Ali said. “It’s hard to manage errors in quantum computers right now. Beginning closer to the ground state will compound into fewer errors you’d need to correct down the line, reducing errors before they occur.”  

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