A crystal is an arrangement of atoms that repeat themselves in space at equal intervals of time: at every point the crystal looks exactly the same. In 2012, Nobel Prize winner Frank Wilczek asked: Could there also be a time crystal, an object that repeats itself in time rather than in space? But is it possible for a periodic rhythm to emerge even if no specific rhythm is imposed on the system and the interactions between particles are completely independent of time?

Over the years, Frank Wilczek’s idea has caused much controversy. Some believed that time crystals were fundamentally impossible, while others tried to find loopholes and implement time crystals under certain special conditions. Now, a particularly impressive type of time crystal has been successfully created at Tsinghua University in China, with support from TU Wien in Austria. The team used laser light and a special type of atoms, called Rydberg atoms, whose diameter is several hundred times larger than normal ones. The results are published in the journal Nature Physics.

Spontaneous symmetry violation

A ticking clock is also an example of periodic motion. But this does not happen automatically; someone has to start the clock and set it to a certain time. This starting time then determines the countdown time. In a time crystal, everything is different:

According to Wilchek’s idea, periodicity should arise spontaneously even though there is no physical difference between different moments of time in reality.

“The frequency of the clocks is determined by the physical properties of the system, but the time at which the clock occurs is completely random; this is known as spontaneous symmetry breaking,” explains Professor Thomas Paul from the Institute for Theoretical Physics at the University of Vienna.

Paul led the theoretical part of the research work at Tsinghua University in China that led to the discovery of the time crystal: Laser light was directed 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 essentially a static experiment, where no rhythm is imposed on the system,” Paul says. “The interaction between the light and the atoms is always the same, the intensity of the laser beam is constant. But surprisingly, it turns out that once you reach the other end of the glass cell, the intensity starts to oscillate in very regular patterns.”

Giant atoms

The key to the experiment was to prepare the atoms in a special way: An atom’s electrons can orbit the nucleus in different ways, depending on how much energy they have. If you add energy to an atom’s outermost electron, its distance from the atom’s nucleus can be very large.

In extreme cases, it can be several hundred times farther from the nucleus than normal, creating atoms with a giant electron shell called Rydberg atoms.

“If the atoms in our glass container are prepared in these Rydberg states and their diameters are very large, then the forces between those atoms also become very large,” Paul explains.

“And that changes the way they interact with the laser. If you choose the laser light so that it can excite two different Rydberg states in each atom simultaneously, a feedback loop is created that causes spontaneous oscillations between the two atomic states, which leads to oscillatory absorption of light.”

The giant atoms spontaneously establish a regular rhythm, and this rhythm becomes the rhythm of the intensity of light reaching the tip of the glass container.

“Here we have created a new system that provides a powerful platform to advance our understanding of time crystal phenomena in a way that is very close to Frank Wilczek’s original idea,” says Paul. “For example, precise, self-sustained oscillations can be used for sensors. Giant atoms with Rydberg states have been used successfully for such methods in other contexts.”