New evidence suggests that heat destroys quantum entanglement

But not all questions about quantum systems are easy to answer using quantum algorithms. Some questions are equally easy for classical algorithms, which run on ordinary computers, while other questions are hard for both classical and quantum algorithms.

To understand where quantum algorithms and the computers that run them might offer advantages, researchers often analyze mathematical models called spin systems, which capture the basic behavior of arrays of interacting atoms. Then they can ask: What will a spin system do when you leave it alone at a given temperature? The state in which it is stable, called its thermal equilibrium state, determines many of its other properties, so researchers have long tried to develop algorithms for finding equilibrium states.

Whether those algorithms actually benefit from the quantum nature depends on the temperature of the spin system in question. At very high temperatures, known classical algorithms can work easily. The problem becomes more difficult as the temperature decreases and quantum phenomena dominate; in some systems it becomes too difficult even for quantum computers to solve in any reasonable time. But the details of all this are still unclear.

“When do you get to a place where you need quantum, and when do you get to a place where quantum doesn’t even help you?” Evin Tang“There’s not much information out there about this,” says Dr. , a researcher at the University of California, Berkeley, and one of the authors of the new results.

In February, Tang and Moitra began thinking about the thermal equilibrium problem with two other MIT computer scientists: a postdoctoral researcher named anish bakshi and Moitra’s graduate student Alan LiuIn 2023, they will all cooperate An unprecedented quantum algorithm for a different task involving spin systems, and he was looking for a new challenge.

“When we work together, things just fall into place,” Bakshi said. “It’s been great.”

Before that 2023 breakthrough, the three MIT researchers had never worked on quantum algorithms. Their backgrounds were in learning theory, a subfield of computer science that focuses on algorithms for statistical analysis. But like ambitious newcomers everywhere, they saw their relative naivete as an advantage, a way to look at a problem from a new perspective. “One of our strengths is that we don’t know much quantum,” Moitra said. “We only know the quantum that Evin taught us.”

The team decided to focus on relatively high temperatures, where researchers suspected faster quantum algorithms would exist, even though no one had been able to prove it. Soon, they found a way to turn an old technique from learning theory into a new faster algorithm. But while they were writing their paper, another team came up with a new method. Similar results: there is evidence that Promising algorithm These machines, developed last year, will work well at high temperatures. They were scooped up.

sudden death rebirth

Somewhat disappointed at coming in second, Tang and his colleagues began correspondence. Alvaro Alhambraphysicist at the Institute of Theoretical Physics in Madrid and one of the authors of the rival paper. They wanted to explore the differences between the results obtained independently. But when Alhambra read an initial draft of the four researchers’ proof, he was surprised to learn that they had proved something else in an intermediate step: In any spin system in thermal equilibrium, entanglement disappears completely above a certain temperature. “I told them, ‘Oh, this is very, very important,'” Alhambra said.