The crystal that can bend time

A Rydberg atom has an electron far from the nucleus. Image credit: Vienna University of Technology

Researchers have succeeded in creating an extremely strange state of matter, in which the diameter of its atoms is a hundred times larger than their natural diameter.

Time crystals, proposed by Nobel laureate Frank Wilczek in 2012, have now been successfully created using Rydberg atoms and laser light at Tsinghua University in China, with theoretical support from the Vienna University of Technology in Austria. This new state of matter does not repeat in space like conventional crystals, but rather in time, where periodic rhythms emerge spontaneously without an external stimulus, a phenomenon known as spontaneous symmetry breaking.

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

For years, Frank Wilczek’s idea has caused much controversy. Some have argued that time crystals are impossible in principle, while others have tried to find loopholes and achieve time crystals under certain special conditions. Now, a particularly amazing type of time crystal has been successfully created at Tsinghua University in China, with support from the Vienna University of Technology in Austria. The team used laser light and very special types of atoms, called Rydberg atoms, with a diameter several hundred times larger than usual. The results are now published in the journal Physics of nature.

automatic symmetry breaking

The ticking of a clock is also an example of a periodic movement. However, it does not happen by itself: someone must have wound the clock and started it at a certain time. This starting time then determines the timing of the ticking. It is different with a time crystal: according to Wilczek’s idea, the periodicity must arise spontaneously, even though there is no physical difference between different points in time.

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

A stationary system based on a continuous light input results in time-dependent periodic signals. Copyright: Vienna University of Technology

Thomas Paul was responsible for the theoretical part of the research that has now led to the discovery of a time crystal at Tsinghua University in China: Laser light was shone on a glass container filled with a gas of rubidium atoms. The strength of the light signal that reached the other end of the container was measured.

“This is actually a static experiment where no specific rhythm is imposed on the system,” says Thomas Paul. “The interactions between the light and the atoms are always the same, with the laser beam having a constant intensity. But surprisingly, the intensity that reaches the other end 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 were arranged in corn Atoms can orbit the nucleus in different paths, depending on how much energy they have. If energy is added to the outermost electron of an atom, the distance between it and the atomic nucleus can become very large. In extreme cases, the distance between it and the nucleus can be several hundred times greater than usual. In this way, atoms with a giant electron shell are created – so-called Rydberg atoms.

“If the atoms in our glass vessel are prepared in such Rydberg states and their diameter becomes huge, the forces between these atoms also become very large,” explains Thomas Paul. “This in turn changes the way they interact with the laser. If you choose the laser light in such a way that it can excite two different Rydberg states in each atom at the same time, a feedback loop is created that causes spontaneous oscillations between the two atomic states. This in turn also leads to the absorption of the oscillating light.” By themselves, the giant atoms stumble into a regular rhythm, and this rhythm is translated into the rhythm of the light intensity that reaches the end of the glass vessel.

“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,” says Thomas Paul. “These 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.”

Reference: “Time-dissipative crystallization in a strongly interacting Rydberg gas” by Xiaoling Wu, Chuqing Wang, Fan Yang, Ruochen Gao, Zhao Liang, Meng Jun Ti, Xiangliang Li, Thomas Paul and Li Yu, July 2, 2024, Physics of nature.
DOI: 10.1038/s41567-024-02542-9