New Evidence Shows Heat Destroys Quantum Entanglement

The original version of this story appeared in Quanta Magazine.

Nearly a century ago, the physicist Erwin Schrödinger called attention to a quirk of the quantum world that has fascinated and vexed researchers ever since. When quantum particles such as atoms interact, they shed their individual identities in favor of a collective state that’s greater, and weirder, than the sum of its parts. This phenomenon is called entanglement.

Researchers have a firm understanding of how entanglement works in idealized systems containing just a few particles. But the real world is more complicated. In large arrays of atoms, like the ones that make up the stuff we see and touch, the laws of quantum physics compete with the laws of thermodynamics, and things get messy.

At very low temperatures, entanglement can spread over long distances, enveloping many atoms and giving rise to strange phenomena such as superconductivity. Crank up the heat, though, and atoms jitter about, disrupting the fragile links that bind entangled particles.

Physicists have long struggled to pin down the details of this process. Now, a team of four researchers has proved that entanglement doesn’t just weaken as temperature increases. Rather, in mathematical models of quantum systems such as the arrays of atoms in physical materials, there’s always a specific temperature above which it vanishes completely. “It’s not just that it’s exponentially small,” said Ankur Moitra of the Massachusetts Institute of Technology, one of the authors of the new result. “It’s zero.”

Researchers had previously observed hints of this behavior and dubbed it the “sudden death” of entanglement. But their evidence was mostly indirect. The new finding establishes a much stronger limit on entanglement in a mathematically rigorous way.

Curiously, the four researchers behind the new result aren’t even physicists, and they didn’t set out to prove anything about entanglement. They’re computer scientists who stumbled on the proof accidentally while developing a new algorithm.

Regardless of their intent, the results have excited researchers in the area. “It’s a very, very strong statement,” said Soonwon Choi, a physicist at MIT. “I was very impressed.”

Finding Equilibrium

The team made their discovery while exploring the theoretical capabilities of future quantum computers—machines that will exploit quantum behavior, including entanglement and superposition, to perform certain calculations far faster than the conventional computers we know today.

One of the most promising applications of quantum computing is in the study of quantum physics itself. Let’s say you want to understand the behavior of a quantum system. Researchers need to first develop specific procedures, or algorithms, that quantum computers can use to answer your questions.