Over the last decade or so, the concept of time crystals has gone from an intriguing hypothesis to an experimental reality. These peculiar states have been observed in a variety of systems, including a children’s toy. Now, scientists have reported that they have been able to construct one using a very peculiar state of matter: Rydberg atoms.
First of all, what is a time crystal? Regular crystals show regularity in space; every bit of them is repeated across the space of the crystal. Could there be something that repeats itself regularly on time? This was the question that Nobel Prize-winning physicist Frank Wilczek put forward in 2012. There’s another caveat: this periodic, rhythmic behavior cannot be imposed from the outside. It needs to be spontaneous. Therefore, a clock is not a time crystal.
Enter another new addition to the extreme physics world, Rydberg atoms. By cooling atoms to almost absolute zero and then shining a laser on them, it is possible to push the external electron much further out, creating an atom hundreds of times the size of the original atom. In this particular experiment, the team used rubidium atoms and a single laser to move two electrons.
A very schematic view of how a Rudberg atom is created. A very cold atom is hit by a laser, sending an electron to orbit at a larger distance from the nucleus.
Image Credit: TU Wien
“If the atoms in our glass container are prepared in such Rydberg states and their diameter becomes huge, then the forces between these atoms also become very large,” co-author Thomas Pohl, from TU Wien in Austria, said in a statement.
“And that in turn changes the way they interact with the laser. If you choose laser light in such a way that it can excite two different Rydberg states in each atom at the same time, then a feedback loop is generated that causes spontaneous oscillations between the two atomic states. This, in turn, also leads to oscillating light absorption.”
The fact that a rhythm would emerge, let alone something like a time crystal, was not at all obvious.
“This is actually a static experiment in which no specific rhythm is imposed on the system,” Pohl continued. “The interactions between light and atoms are always the same, the laser beam has a constant intensity. But surprisingly, it turned out that the intensity that arrives at the other end of the glass cell begins to oscillate in highly regular patterns.”
The giant atoms naturally turn into a time crystal. This discovery is not just intriguing for the study of the Rydberg atoms, which have been known for less than a decade; it is also a completely new way to study continuous time crystals.
“We have created a new system here that provides a powerful platform for deepening our understanding of the time crystal phenomenon in a way that comes very close to Frank Wilczek’s original idea,” explained Pohl. “Precise, self-sustained oscillations could be used for sensors, for example. Giant atoms with Rydberg states have already been successfully used for such techniques in other contexts.”
The study is published in the journal Nature Physics.
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