First theorized in 1973 by physicist Philip W. Anderson, quantum spin liquids are exotic phases of matter with topological order. They feature long-range quantum entanglement that can potentially be exploited to realize robust quantum computation. But the problem about this exotic state of matter has been its very existence: no one had ever seen it — at least, that had been the case for almost five decades. “A quantum spin liquid has nothing to do with everyday liquids like water,” said Harvard University’s Professor Mikhail Lukin and colleagues.

“Instead, it’s all about magnets that never freeze and the way electrons in them spin.”

“In regular magnets, when the temperature drops below a certain temperature, the electrons stabilize and form a solid piece of matter with magnetic properties. In quantum spin liquid, the electrons don’t stabilize when cooled, don’t form into a solid, and are constantly changing and fluctuating (like a liquid) in one of the most entangled quantum states ever conceived.”

Professor Lukin and co-authors set out to observe a quantum spin liquid using the programmable quantum simulator.

The simulator is a special kind of quantum computer that allows the researchers to create programmable shapes like squares, honeycombs, or triangular lattices to engineer different interactions and entanglements between ultracold atoms.

The idea of using the quantum simulator is to be able to reproduce the same microscopic physics found in condensed matter systems, especially with the freedom that the programmability of the system allows.

“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,” said Harvard University’s Professor Subir Sachdev.

In conventional magnets, electron spins point up or down in some regular pattern.

In the everyday refrigerator magnet, for example, the spins all point toward the same direction. This happens because the spins usually work in a checker box pattern and can pair so that they can point in the same direction or alternating ones, keeping a certain order.

Quantum spin liquids display none of that magnetic order. This happens because, essentially, there is a third spin added, turning the checker box pattern to a triangular pattern.

While a pair can always stabilize in one direction or another, in a triangle, the third spin will always be the odd electron out.

This makes for a frustrated magnet where the electron spins can’t stabilize in a single direction.

“Essentially, they’re in different configurations at the same time with certain probability. This is the basis for quantum superposition,” said Dr. Giulia Semeghini, a postdoctoral researcher at the Harvard-Max Planck Quantum Optics Center.

The authors used the simulator to create their own frustrated lattice pattern, placing the atoms there to interact and entangle.

They were then able to measure and analyze the strings that connected the atoms after the whole structure entangled.

The presence and analysis of those strings, which are called topological strings, signified that quantum correlations were happening and that the quantum spin liquid state of matter had emerged.

“The back-and-forth between theory and experiment is extremely stimulating,” said Dr. Ruben Verresen, a postdoctoral researcher at Harvard University.

“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 confirming the presence of quantum spin liquids, the scientists turned to the possible application of this state of matter to creating the robust qubits.

They performed a proof-of-concept test that showed it may one day be possible to create these quantum bits by putting the quantum spin liquids in a special geometrical array using the simulator.

The researchers plan to use the programmable quantum simulator to continue to investigate quantum spin liquids and how they can be used to create the more robust qubits.

Qubits, after all, are the fundamental building blocks on which quantum computers run and the source of their massive processing power.

“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. There’s now a lot more to explore,” Dr. Semeghini said.

Source: journal Science.