Scientists successfully create a time crystal made of giant atoms

A crystal is a system of atoms that repeats itself in space at regular intervals: at every point, the crystal looks exactly the same. In 2012, Nobel Prize winner Frank Wilczek asked the question: Could there also be a time crystal, an object that repeats itself not in space but in time? And could it be possible for a periodic rhythm to emerge, even though no specific rhythm is imposed on the system and the interaction between particles is completely independent of time?

For years, Frank Wilczek’s idea caused much controversy. Some considered time crystals impossible in principle, while others tried to find loopholes and realize time crystals under certain special conditions.

Now, a particularly spectacular type of time crystal has been successfully created at Tsinghua University in China, with support from the TU Wien in Austria.

The team used laser light and special types of atoms, called Rydberg atoms, with a diameter hundreds of times larger than normal. The results were published in the journal Physics.

Spontaneous symmetry breaking

The ticking of a clock is also an example of a temporal periodic motion. However, it does not happen by itself: someone has to have wound up the clock and started it at a certain time. This starting time then determined the timing of the ticks. With a time crystal this is different:

According to Wilczek, periodicity should arise spontaneously, even though there is in fact no physical difference between different points in time.

“The ticking frequency is predetermined by the physical properties of the system, but the moments at which the tick occurs are completely random; this is known as spontaneous symmetry breaking,” explains Prof. Thomas Pohl from the Institute for Theoretical Physics at TU Vienna.

Pohl was responsible for the theoretical part of the research work that has now led to the discovery of a time crystal at Tsinghua University in China: laser light was shone into a glass container filled with a gas of rubidium atoms. The strength of the light signal arriving at the other end of the container was measured.

“This is actually a static experiment where no specific rhythm is imposed on the system,” says Pohl. “The interactions between light and atoms are always the same, the laser beam has a constant intensity. But surprisingly, it turns out that the intensity arriving on the other side of the glass cell starts to oscillate in very regular patterns.”

Giant atoms

The key to the experiment was to prepare the atoms in a special way: The electrons of an atom can orbit the nucleus in different ways, depending on how much energy they have. If energy is added to the outermost electron of an atom, the distance to the nucleus can become very large.

In extreme cases it can be hundreds of times further from the nucleus than normal. In this way atoms with a gigantic electron shell are created, the so-called Rydberg atoms.

“If the atoms in our glass container are brought into such Rydberg states and their diameter becomes enormous, then the forces between these atoms also become very large,” Pohl explains.

“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 simultaneously, then you create a feedback loop that causes spontaneous oscillations between the two atomic states. This in turn also leads to oscillatory light absorption.”

The giant atoms begin to form a regular rhythm on their own, and this rhythm is translated into the rhythm of the light intensity reaching the end of the glass container.

“We have created a new system here that provides a powerful platform to deepen our understanding of the time crystal phenomenon in a way that comes very close to Frank Wilczek’s original idea,” Pohl said.

“Precise, self-sustained oscillations can be used for sensors, for example. Giant atoms with Rydberg states have already been successfully used for such techniques in other contexts.”