Harvard physicists have created and observed a new state of matter that has been predicted and pursued for 50 years but has never been observed before.

A quantum spin liquid is a bizarre state of matter that, despite its name, has nothing to do with ordinary liquids like water.

It’s all about non-freezing magnets and their electrons spinning. In ordinary magnets, when the temperature drops below a specific point, the electrons solidify and create a magnetic solid. Quantum spin liquid is one of the most entangled quantum states yet created, with electrons constantly altering and fluctuating (like a liquid).

Quantum spin liquids contain a range of features that could be exploited to develop quantum technologies like high-temperature superconductors and quantum computers. The issue with this state of matter, however, has been its very existence. No one had ever seen it before – at least, no one had seen it in over 50 years.

A group of physicists led by Harvard announced today that they have now experimentally documented this long-sought exotic state of matter.

“It is a very special moment in the field,” says Mikhail Lukin, the George Vasmer Leverett Professor of Physics, one of the senior authors of the study.

“You can really touch, poke, and prod at this exotic state and manipulate it to understand its properties. …It’s a new state of matter that people have never been able to observe.”

The findings of this scientific study may one day aid in the development of superior quantum materials and technology. Quantum spin liquids, in particular, may hold the key to developing more robust quantum bits, known as topological qubits, that are believed to be immune to noise and external interference.

“That is a dream in quantum computation,” says Giulia Semeghini, lead author of the study.

“Learning how to create and use such topological qubits would represent a major step toward the realization of reliable quantum computers.”

The study team used the programmable quantum simulator developed by the lab in 2017 to examine this liquid-like state of matter. The simulator is a type of quantum computer that allows researchers to construct alternative interactions and entanglements amongst ultracold atoms by creating programmable configurations such as squares, honeycombs, or triangular lattices. It’s utilized to investigate a wide range of sophisticated quantum processes.

The goal of using the quantum simulator is to be able to simulate the same tiny physics that can be found in condensed matter systems, especially given the system’s programmability.

“You can move the atoms apart as far as you want, you can change the frequency of the laser light, you can really change the parameters of nature in a way that you couldn’t in the material where these things are studied earlier,” adds study co-author Subir Sachdev.

“Here, you can look at each atom and see what it’s doing.”

Electron spins in conventional magnets point up or down in a predictable fashion. The spins on a common refrigerator magnet, for example, all point in the same direction. This occurs because the spins normally follow a checkerboard pattern and can pair up to point in the same direction or alternate directions while maintaining a specific order.

There is no magnetic order in quantum spin liquids. This occurs because a third spin is added, thereby changing the checker box pattern into a triangular pattern. In a triangle, the third spin will always be the odd electron out, but a pair can always stabilize in one direction or the other. This results in a “frustrated” magnet, in which the electron spins are unable to settle in one direction.

“Essentially, they’re in different configurations at the same time with certain probability,” adds Semeghini. “This is the basis for quantum superposition.”

The Harvard researchers used the simulator to design their own frustrated lattice pattern, arranging the atoms in such a way that they would interact and entangle. After the entire structure entangled, the researchers were able to measure and analyze the strings that connected the atoms. The appearance and analysis of those topological strings indicated that quantum correlations were taking place and that the quantum spin liquid form of matter had evolved.

The research draws on earlier theoretical predictions by Sachdev and his graduate student Rhine Samajdar, as well as a specific proposal by Harvard professor of physics Ashvin Vishwanah and HQI postdoctoral scholar Ruben Verresen. The experiment was carried out in partnership with scientists from the University of Innsbruck and QuEra Computing in Boston, as well as Markus Griener, co-director of the Max Planck-Harvard Research Center for Quantum Optics and George Vasmer Leverett Professor of Physics.

“The back-and-forth between theory and experiment is extremely stimulating,” says Verresen.

“It was a beautiful moment when the snapshot of the atoms was taken and the anticipated dimer configuration stared us in the face. It is safe to say that we did not expect our proposal to be realized in a matter of months.”

After verifying the existence of quantum spin liquids, the researchers looked into how this state of matter may be used to create durable qubits. They used the simulator to execute a proof-of-concept test that indicated it would be possible to manufacture these quantum bits in the future by arranging quantum spin liquids in a particular geometrical array.

The researchers intend to use the programmable quantum simulator to continue their study on quantum spin liquids and how they might be used to make more reliable qubits. After all, qubits are quantum computers’ essential building components and the source of their tremendous computing capacity.

“We show the very first steps on how to create this topological qubit, but we still need to demonstrate how you can actually encode it and manipulate it,” says Semeghini.

“There’s now a lot more to explore.”

Source: 10.1126/science.abi8794

Image Credit: Kris Snibbe/Harvard Staff Photographer

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