Prof. Mikhail Lukin and Giulia Semeghini have never observed a state of matter predicted and hunted before. They use lasers inside the LISE building. Kris Snibbe is a Staff Photographer at Harvard. Kris Snibbe is a Staff Photographer at Harvard.
The existence of a new state of matter that has been a major focus of the field, especially in the race for quantum computers, has been attributed to physicist Philip W. Anderson.
The quantum spin liquid has nothing to do with everyday liquids like water. Magnets that spin and never freeze are what it's all about. When the temperature drops below a certain temperature, the electrons form a solid piece of matter with magnetic properties. In quantum spin liquid, the electrons don't form into a solid, but are constantly changing and fluctuating, and are one of the most entangled quantum states ever conceived.
The properties of quantum spin liquids can be used to advance quantum technologies such as high-temperature superconductors and quantum computers. The problem with this state of matter is its existence. For almost 50 years, no one had ever seen it.
A team of Harvard-led physicists said today that they have documented an exotic state of matter. The work described in the journal Science is a big step towards being able to produce this elusive state on demand and to gain a novel understanding of its mysterious nature.
"This is a very special moment in the field of quantum physics, and it's very exciting," said Mikhail Lukin, one of the senior authors of the study. It's a new state of matter that people have never been able to observe.
One day, the learnings from this science research could lead to better quantum materials and technology. More specifically, the exotic properties of quantum spin liquids could hold the key to creating more robust quantum bits that are resistant to noise and interference.
"That is a dream in quantum computation," said Giulia Semeghini, the lead author of the study. Learning how to create and use such qubits would be a major step towards the realization of reliable quantum computers.
The research team set out to observe this liquid-like state of matter using a quantum simulator. The simulator is a special kind of quantum computer that allows researchers to create shapes like squares, honeycombs, or triangular lattices to engineer different interactions and entanglements between ultracold atoms. It is used to study quantum processes.
The idea of using the quantum simulator is to be able to reproduce the same physics found in Condensed Matter systems, with the freedom that the programmability of the system allows.
"You can change the parameters of nature in a way that you couldn't in the material where these things are studied earlier," said study co-author Subir Sachdev. You can look at each atom and see what it is doing.
In conventional magnets, electron spins point up or down. The spins all point in the same direction. The spins 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.
None of that magnetic order is displayed in quantum spin liquids. The checker box pattern is turned to a triangular pattern by adding a third spin. The third spin of a pair is always the odd electron out. The electron spins can't be stable in a single direction.
They are in different configurations at the same time. This is the basis for quantum superposition.
The scientists at Harvard used a simulation to create their own lattice pattern. The strings that connected the atoms were measured and analyzed by the researchers. The presence and analysis of those strings, which are called topological strings, indicates that quantum correlations were happening and that the quantum spin liquid state of matter had emerged.
The work builds on predictions made by Sachdev and his graduate student, Rhine Samajdar, as well as a specific proposal by Ashvin Vishwanah, a Harvard professor of physics. The experiment was done with the help of scientists from the University of Innsbruck and QuEra Computing in Boston.
The back-and-forth between theory and experiment is very stimulating. When the snapshot of the atoms was taken, the anticipated configuration stared us in the face. We did not expect our proposal to be realized in a matter of months.
The researchers looked at the possible application of the state of matter to create qubits. A proof-of-concept test showed that it is possible to create quantum bits by putting quantum spin liquids in a special geometrical array.
The researchers plan to use the quantum simulator to investigate quantum spin liquids and how they can be used to create more robust qubits. Qubits are the building blocks of quantum computers and the source of their massive processing power.
"We show how to create a topological qubit, but we still need to demonstrate how you can actually decode it and manipulate it," he said. There's more to be explored.
Giulia Semeghini and her team are probing topological spin liquid on a quantum simulation. www.science.org/doi/10.1126/science.abi8794
Science journal information.
Scientists document the presence of quantum spin liquids, a never-before-seen state of matter, on December 2, 2021.
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