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A crystal is an arrangement 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 raised the question: Could there be a time crystal, an object that repeats itself not in space, but in time? And is it possible for a periodic rhythm to emerge even though no specific rhythm is imposed on the system and the interaction between the particles is completely independent of time?

For years, Frank Vilcek’s idea has caused a lot of controversy. Some believe that time crystals are basically impossible, while others try to find loopholes and realize time crystals under certain special conditions.

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

The team used laser light and special types of atoms, namely Rydberg atoms, with a diameter that is several hundred times larger than normal. The results are published in the journal Natural physics.

Spontaneous breaking of symmetry

A ticking clock is also an example of temporal periodic motion. However, this does not happen by itself: someone must have wound the clock and started it at a certain time. This start time then determines the timing of the ticks. It’s different with the time crystal:

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

“The frequency of the tics is predetermined by the physical properties of the system, but the times at which the tics occur are completely random; this is known as spontaneous symmetry breaking,” explains Prof. Thomas Paul from the Institute of Theoretical Physics at TU Wien.

Paul 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 directed at 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 Paul. “The interactions between the light and the atoms are always the same, the laser beam has a constant intensity. But surprisingly, it turned out that the intensity arriving at the other end of the glass cell began 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 energy is added to an atom’s outermost electron, its distance from the atomic nucleus can become very large.

In extreme cases, it can be several hundred times farther from the nucleus than usual. In this way, atoms with a giant electron shell – the so-called Rydberg atoms – are created.

“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,” Paul 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 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 absorption of light.”

All by themselves, the giant atoms stumble into a proper rhythm, and this rhythm translates into the rhythm of the intensity of the light that arrives at the end of the glass container.

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

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