Kicked off campus by the pandemic, Giulia Semeghini and her team members came up with a quantum spin liquid
Elle Bernbaum
The pandemic inspired all sorts of ingenuity for people making lemonade from lemons, but Giulia Semeghini took the metaphor to another level when she created a new state of matter from the comfort of her own couch.
Semeghini is a postdoctoral fellow in the Lukin Group at the Max Planck-Harvard Research Center for Quantum Optics. She’s also the lead author of a study which produced the first-ever observed quantum spin liquid (QSL), and she has impressive adaptability under quarantine, including transforming her home into a lab.
Speaking to Truly Curious, she explained the science behind this exotic state of matter, and she discussed its implications for quantum computing.
But first, a little context.
A surprise success
In 1973, Phil Anderson proposed a model for a state of matter which had characteristics of a liquid but would not freeze, even at absolute zero (-459.67 degrees Fahrenheit; -273.16 degrees Celsius). The model arranged and manipulated atoms to take advantage of the laws of quantum physics, which differ from the laws of classical physics at very small scales. It especially utilized the quantum attribute of a particle that’s called spin. Thus, Anderson named this theoretical matter a quantum spin liquid.
For nearly 50 years after Anderson’s theory emerged, physicists tried and failed to conclusively make a QSL. Previous experiments mostly used condensed matter systems, where physicists searched for QSLs by manipulating solids that already existed. While some groups claimed success, they could never show with a high degree of certainty that they had created a quantum spin liquid. (Condensed matter systems aren’t easy to probe.)
The Lukin Group tried something new. Theorists and experimentalists in atomic physics partnered to create a QSL by collecting individual atoms and applying specific conditions to them – sort of like making custom matter instead of seasoning the store-bought stuff. This approach made their matter much easier to test and manipulate.
The theorists had to come up with a model that the experimentalists could implement – a tall order, considering how intricate the model had to be. The experimentalists had to troubleshoot brand new procedures and techniques to match the theory.
“All of the steps needed to be invented,” explained Semeghini, an experimentalist.
In July 2020, during the early months of the pandemic, Semeghini and her team launched their experiment, operating the lab equipment from home.
About a year later, they had created the world’s first QSL.
“We were able to do this [thing] that we never, two years ago, never thought was possible,” Semeghini said of creating the new state of matter. “It was an amazing collaboration between theorists and experiment … In all parts of it, it was necessary to have two different worlds meet.”
The rocky road to quantum physics
Semeghini hasn’t always known she wanted to be a physicist. Although her contributions to physics have been so significant that she’s made history, she often wondered if she was on the right career track. Still, her indecision may have instilled the patience, flexibility, and tenacity she needed.
She was born in Italy, and before physics came along, she was an avid writer. Semeghini considered both journalism and law in high school, when most Italian students essentially choose their majors.
The path to journalism seemed unclear though, and, once her older brother became a lawyer, that option lost its appeal – as other younger siblings might understand.
Semeghini became fascinated with physics after seeing a graduate student’s exhibit at a local cultural event and went on to pursue her bachelor’s in the subject at the University of Milan. There, she took her first course in quantum physics and was hooked.
Unsure of her capabilities and still interested in other disciplines, however, she questioned her career prospects. Thoughts like “Maybe I’m not that good. Maybe I’m not good enough to do it” drove Semeghini to advisors’ offices. Their optimism and encouragement inspired the patience and tenacity that Semeghini used to plunge through uncertainty.
“You really need to get to know yourself,” Semeghini said. “Everybody has their own qualities and defects.”
For her PhD, Semeghini worked with Giovanni Modugno at the University of Florence’s European Laboratory for Nonlinear Spectroscopy. She did research on ultracold atoms called Bose-Einstein condensates, doing research similar to the quantum spin liquid project that she would later work on at Harvard.
She discovered that she loves a hands-on experiment, one that’ll get her out from behind a computer and into a collaborative space where she can exercise her scientific creativity.
“You could actually see quantum physics at play in a very clean and a very surprising and beautiful way,” Semeghini said. “You can actually touch the atoms.”
As a postdoc in Harvard’s Lukin Group, she enjoys working in a small team that has direct control over the experimental plan. It’s freeing. But it also means that the researchers set their own limits; group ambitions can catalyze intense agendas.
Recalling the start of the quantum spin liquid experiment, Semeghini elucidated, “At the time, I think we were doing four projects in parallel, dividing the week in two days each or something like that… It was challenging.”
And as architects of their experiments, the pressure was also on.
“It gives you this, this flexibility and this freedom to actually be there and decide what to do, which is also scary,” Semeghini said. “You need to have ideas.”
Harvard’s pandemic-driven campus closure hung a cloud over the quantum spin liquid experiment. Semeghini’s determination wavered.
“We were kicked out of Harvard,” Semeghini recollected, and there was no way of knowing when they could go back. “That was a moment where I thought, OK, maybe I should give up this career and go do something else.”
She decided that once her postdoctoral studies were done, she would try something outside of physics, but in the meantime, she would see her project through to the end.
The research devoured the researchers’ personal lives. They transformed their studios into remote labs, and Semeghini stopped her habitual running and slept little. She found herself working morning to night for days on end to create a state of matter that seemed impossible to make.
“When there is no separation from life at home [and] work, it becomes very absorbing… very challenging,” Semeghini said. “My bed was very close to my desk with my computer, so even at night, if I woke up, I could see my laptop in the background.”
To make matters worse, the collaborative community that would usually make her intense work easier had fractured under the pressure of the pandemic.
“We were pretty isolated, and that was certainly a pretty tough thing for all of us,” she said. “Having good friends – having someone to call and ask, ‘Did you sleep?’” was what made things bearable.
Then the lockdown ended.
“We were able to get back to the lab in person,” Semeghini said. “It was… life-changing.”
Their determination had paid off; they had made unprecedented progress under quarantine, and they were closer than ever to the proof that they were chasing.
Experiment, meet theory
A quantum spin liquid isn’t quite like the classical liquid that you put in your coffee cup or water bottle.
It’s difficult to imagine what a cup of a quantum spin liquid might look like, since we can’t see the matter without the help of technology. The experiment that the Lukin Group runs only uses 219 rubidium atoms set in a pattern of triangles (see image below). It takes millions for a solid or a liquid to be visible to the naked eye, and many septillions more to slosh around.
But, since the quantum spin liquid walks the line between a liquid and a solid at the atomic level, it probably wouldn’t look like a classical liquid.
Recall that solids don’t have much energy, move little, and are composed of rigidly structured particles, while liquids have a little more energy and have particles that move around without much structure.
Atoms in a quantum spin liquid exist in a “frustrated” system with solid-like order, but they fluctuate within that structure as if they comprised a liquid. How? It’s all quantum.
The Lukin Group creates special conditions by forcing atoms into a specific triangular pattern that exploits their spins to create fluctuation. The group also entangles the atoms so that they are physically stable but look as though they have fluctuating, semi-liquid disorder.
Semeghini was candid: “There were moments where some things were not clear, and even the results we were getting were a little bit counterintuitive…, moments where you’re like, ‘Okay, I have no idea [what is going on].’
“It took a while and a lot of discussions with a lot of people to put things together.”
The triangular lattice
Consider “spin.” In the world we can see, a spinning object has angular momentum, the curving version of the momentum that you fight when you hurriedly brake at a red light. An electron also has intrinsic spin, but it doesn’t actually rotate. Electron spin is more like an affinity for a specific orientation, and only two orientations are possible: “up” and “down.” These categories are based on how the electron would look from a bird’s eye view if it did rotate. A clockwise rotation correlates with spin down, and an anticlockwise rotation correlates with spin up. Electrons with opposite spins like to align to balance each other, kind of like magnets, where positive poles are attracted to negative charges.
An atom’s spin is the sum of the spins of the particles that comprise it – like electrons. The atoms that the Lukin Group uses also have spin states of up and down.
Normally, atoms’ spins would play a role in determining how atoms would situate themselves, but when the Lukin Group initially forces a triangular structure on their atoms, the atoms can’t form stable pairs. In one triangle of the lattice, an atom with spin up pairs with an atom with spin down, but the third atom can’t align with both at the same time, so it flips back and forth between them. It is “frustrated.” Together, these frustrated atoms seem to create a disorder typical of a classical liquid.
The team uses high-intensity lasers called “optical tweezers” to capture and place atoms in the triangular lattice. (Sometimes, they break from the experiment and use a little artistic license to arrange those atoms into pictures, emojis, anything they want. The Lukin Group knows how to party.)
“It’s always a nice thing when someone visits the lab,” Semeghini laughed. “We write down their name [using atoms].”
But when work time rolls around, the researchers sensibly keep the atoms in a triangular lattice.
Tangled up in entanglement
The Lukin Group creates physical stability that looks visibly disordered by entangling frustrated atoms, putting them in states of superposition. Entanglement and superposition are principles that exist on a very small scale – in quantum physics. We can’t see them in our everyday world.
When a frustrated atom is entangled with another frustrated atom, instead of flipping between two different orientations of up and down, its spin exists in a state of superposition of both orientations. It’s a composite state, a mix of both “up” and “down,” allowing it to pair with both neighboring spins simultaneously, creating stability.
Clever experimentation can prove that an atom is in a state of superposition, but when we look at an atom in superposition, we can only see spin up or spin down. By observing the atom, we force it into either up or down, even if in the instant before we look, the atom’s spin is a mix of both. Probability dictates which state we observe. So, when the Lukin Group looks at their lattice with so many atoms in states of superposition, all they see is the disorder that characterizes a liquid.
The procedure for entangling atoms looks like this:
The Lukin team first zaps their array of atoms with energy from a laser. The resulting excited state is called a Rydberg state, named after the presumably ebullient Johannes Rydberg.
When the atoms are excited, two things happen.
First, they take up about a thousand times as much space as they did previously. This forces them to interact with each other.
Second, this interaction shifts energy between atoms, leaving only one atom in an area “frustrated.” The phenomenon is called a Rydberg blockade. The Lukin team controls the size of the affected area by choosing the amount of energy to apply with the laser. By limiting the area affected, the team creates a pattern of entanglement.
At this point, the Lukin team has carefully entangled their atoms.
“They could be on two different planets,” Semeghini said, “but they’re still somehow connected to each other.”
But their work isn’t finished. The Lukin Group has to make sure that what they have is actually a QSL. That was pretty tricky when the experiment first got going.
“We had an idea of a measurement that we wanted to do, but we couldn’t figure out how to actually do it in the experiment,” Semeghini explained. “We had this exciting thing going on, but there was this little piece missing… Without that, it didn’t quite work as well.”
The Lukin group had to invent a way to take this measurement. They developed three ideas that they could use to test the model, and after a lot of trial and error, one of their ideas worked.
“We started to get these really amazing results, and we understood that we had managed to get to the bottom of it,” Semeghini said.
“It was really a great moment,” she remembered. “That started, really, a new chapter in that project.”
A powerful tool
“This thing that we found was so intriguing to observe,” Semeghini remarked. She’s fascinated by its very existence; for her, the great effort that the Lukin Group expended was worth it, “even if it’s never useful [for] anything.”
That said, when she first came across this sort of work with ultracold atoms and optical tweezers, she knew its immense significance: “I thought, ‘this is the future.’”
The Lukin Group’s QSL has resilient properties that open up a new field of study for potentially powerful tools in quantum information.
Today’s methods in quantum computing utilize bits that aren’t very durable–that’s a big reason why you don’t have a quantum computer on your desk right now.
But, as long as the atoms in the QSL remain deeply entangled, even if one atom is damaged or changed, the QSL still maintains the structure and useful properties it had before that damage. This characteristic is called topological order. Semeghini likens it to a donut: A donut is still a donut if you take just a little crumble from it.
“If you are able to use the topological properties [in] the objects in which you encode information, like the zeros and ones of the computer,” Semeghini goes on, “those would be very resistant to external noise or perturbations – unlike usual quantum bits… Those are usually very fragile.”
Of course, the Lukin Group’s discovery doesn’t mean you’ll start seeing quantum computers in stores tomorrow – it’s just the beginning.
“It’s the starting point for – in a sense – a new field,” Semeghini says. “What can we do next?”
Onward
Semeghini’s success hasn’t muted her restlessness.
“I always think there are so many things I would like to do with my life,” she said. But for now, she’s content. She sees physics in her future, and a professorship on her horizon.
“Sometimes,” she half-joked, “I wish I had a nine-to-five work[day].” But with job offers from across Europe and the US flooding her inbox, she’ll almost certainly become a professor.
Semeghini is happy to continue building her community of collaborators. That’s one of her favorite parts of her job: “You keep learning from other people – any people, from students, from professors – and that is beautiful to me.”
To researchers considering their next step, Semeghini suggested they “look for good people to work with.”
That can make all the difference.
Elle Bernbaum is a freelance writer with a bachelor’s in physics from the University of Washington. She loves this stuff.
For a deeper dive into this material, check out Semeghini’s lecture on her QSL project here, or read her original paper here.