In an experiment seemingly defying the laws of physics, scientists have created the first-ever time crystal two-body system – and it may have some incredible implications for the future of quantum computing.
In a paper published today in the journal Nature Communications, researchers from the UK, Russia, and Finland described how they created two time crystals inside a superfluid – in this case, a rare isotope of helium, cooled to about one ten-thousandth of a degree from absolute zero – and brought them together to touch each other, creating a coupled system that relies not on classical physics, but quantum rules.
“It turns out putting two of them together works beautifully,” explained EPSRC Fellow Dr Samuli Autti, lead author of the study. “Even if time crystals should not exist in the first place.”
“Time crystals” might sound like something out of an Indiana Jones movie, but they’re actually much more awesome than that. They’re one of those strange quantum phenomena that slightly baffle scientists – their existence was only proposed in 2012, and for a long time they were assumed to be purely theoretical.
Imagine the scientific community’s collective surprise, then, when two separate research teams announced the discovery of some real-life time crystals back in 2017. Since then, the mysterious little objects have turned up everywhere – from state-of-the-art quantum computers to an everyday children’s toy.
But what exactly are time crystals? Depending on how you think about it, they’re either exactly what they sound like, or nothing like that at all. See, a normal, non-time crystal – something like an emerald or a snowflake – is defined by its regular, repeating atomic structure. A diamond, for instance, looks like this under the microscope:
It’s extremely symmetrical – no matter where you are in the space of the structure, the pattern will be identical. And time crystals are the same – except that the structure doesn’t repeat in space, but in time.
This is the way of understanding time crystals where their name makes perfect sense: they’re the time analog of a normal crystal. The slightly more confusing aspect comes when you try to imagine what that actually looks like.
“Let’s say you took pictures of a planet and its orbiting moon every time it finishes its orbit over a period of time with the Hubble Telescope,” explained Google Quantum AI research scientists Pedram Roushan and Kostyantyn Kechedzhi, who were not involved in the research. “These pictures would all look the same with the moon repeating its orbit over and over again.”
But “what if there was a system of a planet and many moons where the moons could periodically repeat their orbits, without ever increasing entropy?” they continue. “This configuration — evidently hard to achieve — would be considered a time crystal.”
In other words, a time crystal isn’t really a crystal at all – at least, not how we’re used to thinking of them. It’s a new phase of matter, simultaneously stable and in constant evolution at the same time, and always periodically coming back to the same pattern.
And that … shouldn’t make sense. “Everybody knows that perpetual motion machines are impossible,” Autti said. “However, in quantum physics perpetual motion is okay as long as we keep our eyes closed.”
“By sneaking through this crack, we can make time crystals,” he explained.
But the creation of a time crystal two-body system is more than just a way to cheat the laws of physics. The basic building block of a quantum computer – widely considered the next big leap in computation – is something called a “two level system”: a quantum system that exists in a superposition of two independent quantum states. And that’s exactly what the researchers have constructed: “In our experiments, two coupled time crystals consisting of spin-wave quasiparticles … form a macroscopic two-level system,” the paper explains.
“The two levels evolve in time as determined intrinsically by a nonlinear feedback, allowing us to construct spontaneous two-level dynamics,” the authors continue. “[The] magnon time crystals allow access to every aspect and detail of quantum-coherent interactions in a single run of the experiment.”
And that opens up some exciting possibilities for the future. Generally speaking, quantum computers rely on extremely cold temperatures – the one at Google, for example, is kept below 50 millikelvin, which is literally colder than the coldest place in the universe.
But “we already know [time crystals] also exist at room temperature,” said Autti – and so the discovery of this two-body system may provide a way to make quantum computers that can work without the supercooling.
And that … would be extremely exciting.