Physicists Observe ‘Unobservable’ Quantum Phase Transition

In 2019, Noel and her colleagues began collaborating with two theorists who had come up with an easier way of doing the experiment. They had worked out a way of setting aside one qubit that, like a canary in a coal mine, could serve as a bellwether for the state of the entire chain.

The group also used a second trick to reduce the number of repetitions — a technical procedure that amounted to digitally simulating the experiment in parallel with actually doing it. That way, they knew what to expect. It was like being told in advance how the loaded die was weighted, and it reduced the number of experimental runs needed to work out the invisible entanglement structure.

With those two tricks, they could detect the entanglement transition in chains that were 13 qubits long, and they posted their results in the summer of 2021.

“We were amazed,” Nahum said. “Certainly I didn’t think it would have happened so soon.”

Unbeknownst to Nahum or Noel, a full execution of the original, exponentially more difficult version of the experiment — with no tricks or caveats — was already underway.

Around that time, IBM had just upgraded its quantum computers, giving them the ability to make relatively quick and reliable measurements of qubits on the fly. And Jin Ming Koh, an undergraduate student at the time at the California Institute of Technology, had given an internal presentation to IBM researchers and convinced them to assist with a project that would push the new feature to its limits. Under the supervision of Austin Minnich, an applied physicist at Caltech, the team set out to directly detect the phase transition in an effort Skinner calls “heroic.”

After consulting Noel’s team for advice, the group simply rolled the metaphorical dice enough times to determine the entanglement structure of every possible measurement history for chains of up to 14 qubits. They found that when measurements were rare, entanglement entropy doubled when they doubled the number of qubits — a clear signature of entanglement filling the chain. The longest chains (which involved more measurements) required more than 1.5 million runs on IBM’s devices, and altogether, the company’s processors ran for seven months. It was one of the most computationally intensive tasks ever completed using quantum computers.

Minnich’s group posted their realization of the two phases in March 2022, and it quieted any lingering doubts that the phenomenon was measurable.

“They really just brute-force did this thing,” Noel said, and proved that “for small system sizes, it’s doable.”

Recently, a team of physicists collaborated with Google to go even bigger, studying the equivalent of a chain almost twice as long as the previous two. Vedika Khemani of Stanford University and Matteo Ippoliti, now at the University of Texas, Austin, had already used Google’s quantum processor in 2021 to create a time crystal, which, like phases of spreading entanglement, is an exotic phase existing in a changing system.

Working with a large team of researchers, the pair took the two tricks developed by Noel’s group and added one new ingredient: time. The Schrödinger equation links a particle’s past with its future, but measurement severs that connection. Or, as Khemani put it, “once you put in measurements into a system, this arrow of time is completely destroyed.”

Without a clear arrow of time, the group was able to re-orient Nahum’s chain-link fence to let them access different qubits at different moments, which they used in advantageous ways. Among other results, they found a phase transition in a system equivalent to a chain of about 24 qubits, which they described in a March preprint.

Measurement Power

Skinner and Nahum’s debate over pudding, along with the work of Fisher and Smith, has spawned a new subfield among physicists interested in measurement, information and entanglement. At the heart of the various lines of investigation is a growing appreciation that measurements do more than just gather information. They are physical events that can generate genuinely new phenomena.

“Measurements are not something condensed matter physicists have thought about historically,” Fisher said. We make measurements to gather information at the end of an experiment, he continued, but not to actually manipulate a system.

In particular, measurements can produce unusual outcomes because they can have the same sort of everywhere-all-at-once flavor that once troubled Einstein. At the instant of measurement, the alternative possibilities contained in the quantum state vanish, never to be realized, including those that involve far-off spots in the universe. While the nonlocality of quantum mechanics doesn’t allow for faster-than-light transmissions in the way Einstein feared, it does enable other surprising feats.

“People are intrigued by what kind of new collective phenomena can be induced by these nonlocal effects of measurements,” Altman said.

Entangling a collection of many particles, for instance, has long been thought to require at least as many steps as the number of particles you hope to entangle. But last winter theorists detailed a way to pull it off in far fewer steps by using judicious measurements. Earlier this year, the same group put the idea into practice and fashioned a tapestry of entanglement hosting fabled particles that remember their pasts. Other teams are looking into other ways measurement could be used to supercharge entangled states of quantum matter.

The explosion of interest has come as a complete surprise to Skinner, who recently traveled to Beijing to receive an award for his work in the Great Hall of the People in Tiananmen Square. (Fisher’s team was also honored.) Skinner had initially believed that Nahum’s question was purely a mental exercise, but these days he isn’t so sure where it’s all heading.

“I thought it was just a fun game we were playing,” he said, “but I’d no longer be willing to put money on the idea that it’s not useful.”

Editor’s note: Jedediah Pixley receives funding from the Simons Foundation, which also funds this editorially independent magazine. Simons Foundation funding decisions have no influence on our coverage. More details are available here.