ɫɫ��

Text: Sedeer el-Showk

Strictly speaking, ‘quantum’ roughly means the smallest countable unit of something, but that doesn’t really explain why quantum computing or quantum materials hold such exciting promise. The word ‘quantum’ refers to quantum mechanics, the study of the smallest units of things like light or electricity. Things behave differently at that scale. They don’t follow the familiar rules of the everyday world, and that shift is the cornerstone of quantum technologies. 

‘There’s quantum mechanics, and then there’s our everyday world, where we don’t observe quantum effects. You don’t see entanglement between two cats, for example,’ says Mika A. Sillanpää, a physics professor at Aalto University. Entanglement means that separate objects somehow have their properties linked, so they exist together in an undetermined state. With two cats, you could imagine that neither could be awake or asleep on its own. They would be somehow connected—regardless of how far apart they are—so that being awake or asleep is a property of the pair together. And it’s a property that stays undecided—until you check on one of them. Then you’d see whether it’s awake or asleep, and you could immediately tell whether the other one was. 

It sounds like the stuff of fairy tales, but it’s perfectly normal at the quantum scale.   

Entanglement and other quantum phenomena easily get washed out by the energy of interactions between particles. That’s why quantum researchers usually work with extremely tiny, extremely cold systems—fewer particles means fewer interactions, and lower temperatures make the interactions less energetic. Sillanpää wanted to find out how big those systems could get. ‘Could you see phenomena like entanglement if you do things just right?’ 

Even ‘large’ things can be entangled 

In 2018, Sillanpää’s research group pushed up the size boundary for quantum research. They set a record by observing entanglement between relatively large objects—though in this case, ‘large’ means tens of millionths of a metre. The experiment also had to be done at just a few thousandths of a degree above absolute zero, a temperature made possible through . Nobody had managed to entangle objects at such a large scale before, and Sillanpää says the accomplishment excited the quantum research community. 

But now Sillanpää isn’t continuing to try to push the scale any higher. He says  the experiments would be very challenging but wouldn’t really offering any new insights. ‘I don’t think there’s really any boundary. It just gets more and and more difficult, and at some point, when you make things big enough, you don’t observe quantum effects anymore. What do we learn from that? Actually, not much.’ 

Instead, Sillanpää has pivoted to another challenge. Having pushed up the scale at which we can see quantum phenomena, he’s now trying to pull down the scale at which we can measure another fundamental physical phenomenon: gravity. 

Can we measure the gravity of tiny objects? 

Quantum mechanics and gravity are both extremely well-tested theories. The problem is that the theories don’t work together, and nobody has figured out how to combine them—a theory that works at tiny scales for massive objects. It’s like having two maps that show an area at different scales but contradict each other. Physicists have many ideas to reconcile the two theories, but they can’t be tested with today’s technology.  Researchers are still decades away from being able to measure quantum effects and gravity at the same time. 

Sillanpää’s team are laying the groundwork that could one day be the basis for quantum gravity research. In a new project, they’re trying to bring down the scale at which gravity can be measured—a feat that few others could even attempt. ‘The first step is to observe gravitational forces between relatively small masses,’ he says. ‘There’s absolutely nothing quantum going on there, but it’s already very, very difficult.’ 

Even if the team succeed, it would only be the first step in a long path. These experiments use objects about 10 million times more massive than in the entanglement experiments, which is a huge gap to bridge. ‘We’re really right at the beginning. This is a pathfinder towards the goal of actually experimentally studying quantum gravity, which is beyond experimental reach at the moment. Somebody might do something related to that in 10 or 20 years, but not now,’ says Sillanpää. 

Long time scales are the norm in fundamental research like Sillanpää’s. Some of the technologies used in his experiments go back to research done at Aalto in the 1970s, research which also fuelled the ecosystem driving quantum tech and quantum computing at Aalto. ‘Studying something which hasn’t been done before and which is difficult is kind of its own motivation. Sometimes, people like to study things which don’t have any immediate applications,’ Sillanpää says. 

A helium droplet can model other universes  

Vladimir Eltsov’s team are using drops of liquid helium to probe some of the deepest questions in physics. They work with helium-3 superfluids, which have the remarkable property of flowing without friction, like a whirlpool that will never stop. ‘That’s only one of the interesting properties that comes from the fact that it’s a macroscopic quantum system,’ says Eltsov, a senior scientist at Aalto.

The enormous number of atoms in these centimetre-sized droplets are spontaneously entangled, offering Eltsov’s team a tool where quantum effects exist at human scales. ‘We can use these systems to understand nature on a global scale, including the behaviour of our Universe.’ 

Eltsov explains how helium-3 superfluids, in some ways, mirror the structure of the Universe because ‘many theoreticians believe the universe is a macroscopic quantum system and that it’s more fluid-like than solid-like.’ Eltsov’s team uses a unique instrument at Aalto to control the superfluid at temperatures a tiny fraction of a degree above absolute zero. By manipulating the structure and confinement of the superfluid, the team can create pockets with strange particle-like structures that seem to follow unfamiliar rules—effectively, electromagnetic and gravitational fields that emerge in the model universe. ‘Within these systems, we we can experimentally study and understand physics from the smallest to the largest relevant scales. Some theoreticians believe that this knowledge would help to resolve quantum gravity problem also in our Universe,’ says Eltsov. ‘In a sense, you can create universes with different physical laws.’  

Helium-3 superfluids also turn out to be one of the best tools to make time crystals. Ordinary crystals are structured patterns in space, like a shape that’s been stamped over and over. Time crystals are the same thing but in time. They’re processes with a rhythm that repeats forever, like a clock that started on its own and keeps ticking without needing a battery. 

‘People have been thinking that this long-term coherence of time crystals could be exploited to make quantum memory elements, for example. But time crystals are very fragile things,’ says Eltsov. Fragile or not, his team have managed to engineer time crystals and hook them up to external systems, a first step towards using them in quantum sensing and computing. 

Figuring out how to make bigger, more stable quantum systems is vital for turning them into useful new technologies, while learning how and why they behave the way they do could help answer some of the biggest questions about our universe.  

‘Our motivation is the hope to open new frontiers in knowledge,’ says Eltsov.