A new state of matter! Quantum Spin Liquids

Theorised in 1973 and has now been made a reality. It's time to warp your brain around Quantum Spin Liquids...
14 December 2021

Interview with 

Giulia Semeghini, Harvard University

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There has been a huge find in the world of quantum physics. Real life laboratory based observation of a long theorised form of matter called Quantum Spin Liquid. Contrary to the name it has absolutely nothing to do with everyday liquids like water. Strap on your mental seat belt and settle in as Harry Lewis speaks to Guilia Semeghini from Harvard University to hear what this matter is and why it might be useful...

Guilia - Quantum spin liquids have been theorized about 50 years ago by Philip Anderson, who was a pretty big guy in the condensed matter community. He was trying to find a solution for why certain types of superconductors, which are called high temperature superconductors, exist

Harry - High temperature superconductors have long baffled physicists as they work at higher temperatures than normal superconductors still very cold mind you we're talking about -196 Celsius.

Guilia - So normally when you have a solid, you have the atoms are ordered in some crystal structure and the electrons are also somehow ordered.

Harry - And when you say crystalline, that means basically like a very uniform structure doesn't it? Something we can predict?

Guilia - They are in a certain structure. It can be a square lattice or a triangular lattice. You can think of something like that.

Harry - In most observed matter, electrons exist as either 'spin up' or 'spin down' and they like to pair up in opposites. Whenever you have a moving electron that generates a magnetic field, think of the ''up and 'down' a little bit like the north and south pole of a magnet. When electrons do pair up, they cancel out one another's magnetic fields.

Guilia - It's a magnetic equilibrium. Let's put it like this. If you instead have a triangular lattice for example, what happens is that the third one doesn't know which way to orient itself.

Harry - It's a third wheel.

Guilia - Yeah exactly. So this is what it's called 'frustration'. The third electron is frustrated and so his idea was that the electrons would choose to do something different.

Harry - It chooses to do everything, nothing, one thing and that other thing all at the same time. Get your head around that. It's a concept known as 'quantum superposition'.

Guilia - This is a quantum concept this idea of superposition. Where instead of choosing an orientation, they would create this state that is called the quantum spin liquid.

Harry - In regular magnets, when the temperature drops low enough, electrons stabilize and form solid matter with magnetic properties. Because the electrons are stuck in either a 'spin up' state or a 'spin down' state. In quantum spin liquid, the constant ambiguity of the electron state prevents freezing. It therefore has a range of unusual and novel properties such as, here's another one for you, long ranged quantum electron entanglement.

Guilia - Which simply means that basically the electrons that are on opposite side of the same piece of material, they're somehow tied in their state with each other.

Harry - That, my non physicist friends, is freaky. It's action at a distance. Altering the state of one electron on one side of the material would affect its partner all the way across on the other side of the material. Almost like when one twin in the UK gets hurt, and the other twin residing in Australia gets the shivers. After 50 years of searching Guilia and her team have observed for the first time, quantum spin liquid, sat there right in front of them on the lab bench.

Guilia - What you would see is this vacuum chamber, with a vacuum setup and a bunch of mirrors and lenses and lasers that occupy the quite big table, what you would see if you looked into the vacuum chamber with this microscope objective that we use, you would see these array of atoms. You really take photographs of the atoms and how they look like and they're actually very nice.

Harry - How did you identify this quantum spin liquid? What was it that proved that it was there for you?

Guilia - What you need to do is you need to prove that there is this entanglement that I was mentioning before. If they had specific properties, if you see these correlations, it means that you have a quantum spin liquid. We measured these and we found that they were there.

Harry - But why is there so much hype over this hard to wrap your head around quantum matter? What Guilia says is another step towards building the elusive quantum computer. If only I actually knew what that meant. We hear a lot about quantum computers. How do we define what quantum means?

Guilia - The basic idea of a quantum computer is this. When you have a computer you have bits, and the bits are the systems in which you encode the information. It's a system that can have a 'zero' or 'one' state. If you have a quantum computer, these bits are quantum, called 'quantum bits' or 'cubits'. The main thing is that being quantum means that they can exist in a superposition state, which is what I was mentioning before. A bit can only be 'zero' or 'one' while a quantum bit can be in a super position of these two states and anything in between. Which means it's not in 'zero' or 'one', it's in a superposition of these states. The second thing is that there is this entanglement property. If you have more cubits, they can be in these entangled states where their states are really connected to each other. You cannot treat them individually. You really need to consider their combined state.

Harry - And this could be really useful for future uses.

Guilia - The idea is that the quantum computers could do things that classical computers cannot do because, thanks to these qualities, you can basically multiply exponentially the calculation capability of this quantum computer.

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