Creation of a Highly Exotic State of Matter

According to a study published in Nature Physics, a particularly spectacular type of time crystal was successfully constructed at Tsinghua University in China, with cooperation from TU Wien in Austria.

A crystal is an arrangement of atoms that repeats itself in space at regular intervals. The crystal seems to be identical at all points. In 2012, Nobel Prize laureate Frank Wilczek posed the question: Could there be a time crystal, an object that repeats itself in time rather than space? And is it feasible for a periodic rhythm to form even when no specific rhythm is placed on the system and particle interaction is entirely time-independent?

For years, Frank Wilczek’s theory has sparked intense discussion. Some believed time crystals to be impossible in principle, while others attempted to identify loopholes and create time crystals under certain situations.

The researchers employed laser light and particularly unique atoms, known as Rydberg atoms, which have a diameter several hundred times larger than normal.

Spontaneous Symmetry Breaking

The ticking of a clock is another example of temporally periodic movement. However, it does not occur by chance: someone must have wound the clock and started it at a certain moment, which dictates the timing of the ticks. In contrast to a time crystal, Wilczek’s theory states that a periodicity should emerge spontaneously, despite the fact that there is no physical difference between distinct places in time.

The tick frequency is predetermined by the physical properties of the system, but the times at which the tick occurs are completely random; this is known as spontaneous symmetry breaking.

Thomas Pohl, Professor, Institute of Theoretical Physics, Technische Universität Wien

At Tsinghua University in China, Thomas Pohl oversaw the theoretical part of the study that resulted in the discovery of a time crystal. The light from a laser was directed into a glass container containing a gas of rubidium atoms. The intensity of the light signal received at the opposite end of the container was measured.

Pohl added, “This is actually a static experiment in which no specific rhythm is imposed on the system. 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.”

Giant Atoms

The key to the experiment was to arrange the atoms in a specific way: an atom’s electrons can orbit the nucleus on different pathways depending on their energy level. If energy is provided to an atom’s outermost electron, its distance from the atomic nucleus can increase significantly.

In severe circumstances, it might be hundreds of times farther out from the nucleus than normal. This process produces atoms with a massive electron shell, known as Rydberg atoms.

“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. 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,” Pohl further added.

The giant atoms, on their own, stumble into a regular beat, which is translated into the rhythm of the light intensity that reaches the end of the glass container.

Pohl concluded, “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. 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.”

Journal Reference:

Wu, X., et. al. (2024) Dissipative time crystal in a strongly interacting Rydberg gas. Nature Physics. doi:10.1038/s41567-024-02542-9