Editorial Feature

What Are Quantum Spin Liquids?

A team of researchers has created a sample of an exotic state of matter that could have important implications for quantum computing.

quantum, electrons, quantum spin liquids, matter, states, quantum computer

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Quantum spin liquids are bizarre states of matter that have nothing to do with ordinary everyday liquids like water, as the name may suggest. Instead, the key properties of this state of matter are magnetic in nature.

The magnets in quantum spin liquids differ from regular magnets because the electrons in this state of matter do not stabilize and form a solid with magnetic properties when the temperature drops below a certain point, as is the case with traditional magnetic materials. 

In a quantum spin liquid, the electrons do not stabilize when cooled; they also do not form into a solid. Instead, the electrons in a quantum spin liquid are constantly changing and fluctuating — just like they do in a conventional liquid.

Whereas with a conventional magnet the spins of electrons tend to point up and down in a regular arrangement, quantum spin liquids are not so well ordered.

This is because a third spin direction is added, meaning that rather than spin arrangements taking a “chessboard” like arrangement where an up spin electron is neighboured by a down spin counterpart, spins in quantum spin liquids form a triangular-like arrangement.

This leads to a situation where every third electron is an “odd electron out”, creating what is known as a frustrated magnet — this can, in turn, lead to a quantum spin liquid.

This exotic state of matter — highly debated by physicists since it was first suggested in 1973 by Philip W. Anderson —  exists in one of the most entangled quantum states ever conceived. This could make quantum spin liquids incredibly important for quantum computing and other applications such as high-temperature supercomputers. 

That is, of course, depending on physicists being able to create such a state. In the fifty years since it was first suggested, physicists have struggled to create this state of matter. 

In December 2021, researchers at Havard University announced that they may have achieved such a feat. 

Creating A Quantum Spin Liquid

In a paper published in the journal Science, the authors document how they were able to create this rare and exotic state of matter. The team was also able to take an important step in understanding the nature of quantum spin liquids.

In order to observe this exotic state of matter, the team used a programmable quantum simulator Harvard-Max Planck Quantum Optics Center had originally developed in 2017. A unique type of quantum computer, the simulator allowed the team to model programmable shapes like squares, honeycombs, or triangular lattices. 

From here they could engineer different interactions and entanglements between ultracold atoms, reproducing the quantum behavior observed in condensed matter, and use this to study a host of complex quantum processes.

The team found that they could move the atoms apart as they needed as well as change the frequency of the laser light, thus changing the parameters of nature in a way not possible in the materials that had been previously used to study quantum spin liquids.

The quantum simulator allowed the team to look at each individual atom and observe how it was behaving. This enabled the researchers to closely study the more complex magnetic behavior of quantum spin liquids.

The Harvard researchers then used the simulator to create their own frustrated magnet lattice pattern. Within this, they placed the atoms allowing them to interact and entangle. 

The team could then analyze the connections between these atoms — known as topological strings — after the whole structure had become entangled. These “strings” pointed towards the fact that quantum correlations were occurring and that a quantum spin liquid state of matter had emerged.

The question is: why is this highly entangled state an important one for quantum computers?

Entanglement and Quantum Computing

Thanks to quantum phenomena like entanglement, quantum computers are much more than just more powerful versions of classical computers. The bits that form the foundational unit of classic computers can take a value of either “0” or “1”. The foundational unit of quantum computers, the qubit, on the other hand, can exist in a superposition of entangled states. 

A superposition of states is the idea that a quantum system can occupy two or more different states at the same time, even if these states are contradictory. 

The thought experiment of Schrodinger’s cat is perhaps the most famous example of a superposition. The experiment describes a system in which a cat is placed in a sealed box with a diabolical poison device. The poison is released by particle decay — a quantum process that is completely random. This system acts as an analogy for a quantum system.

Treating the box as a quantum system, the cat exists in a superposition of states. Namely, it is both dead and alive at the same time. When the researcher opens the box a phenomenon called “decoherence” occurs and the wave function that describes the system “collapses.” Thus, the cat’s state — dead or alive — is revealed.

In an entangled version of Schrodinger’s cat image two boxes, both containing this setup. If the poison is released in one box, however, it must not have been released in the other. Thus, if cat A is dead, it is instantly revealed that cat B is alive.

Qubits in a quantum computer exist in such a superposition of states. That means that qubits can both take a range of states or a superposition of these states rather than just 0 or 1. 

However, creating a large system of entangled qubits is no easy feat. Quantum decoherence occurs very easily, meaning quantum entanglement is easily destroyed. To prevent decoherence quantum computers have to be kept at extremely cold temperatures and protected from environmental effects.

Quantum spin liquids could assist in this regard. 

A Special Moment In Physics

Mikhail Lukin, the George Vasmer Leverett Professor of Physics, co-director of the Harvard Quantum Initiative (HQI), and one of the senior authors of the Science study, described the team’s achievement as a “special moment” in physics. 

This is because it represents the ability of researchers to “touch” and investigate this rare and exotic state of matter, and manipulate it to understand its properties.

Information gathered this way from this research could potentially provide advancements that lead to the better design of quantum materials and technology. 

In particular, the unique properties of quantum spin liquids could be the key to creating more robust qubits — known as topological qubits —expected to be more resistant to environmental interference. 

Giulia Semeghini, a postdoctoral fellow in the Harvard-Max Planck Quantum Optics Center and lead author of the Science study says this could represent a “major step toward the realization of reliable quantum computers.”

More from AZoQuantum: What is the Quantum Anomalous Hall Effect in Bilayer Graphene?

Source

Semeghini. G., Levine. H., Keesling. A., et al, [2021], ‘Probing topological spin liquids on a programmable quantum simulator,’ Science, [https://www.science.org/doi/10.1126/science.abi8794]

Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.

Robert Lea

Written by

Robert Lea

Robert is a Freelance Science Journalist with a STEM BSc. He specializes in Physics, Space, Astronomy, Astrophysics, Quantum Physics, and SciComm. Robert is an ABSW member, and aWCSJ 2019 and IOP Fellow.

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