A Rydberg atom has an electron that is far from the nucleus. Credit: TU Wien
Researchers have created an extremely exotic state of matter. Its atoms are a hundred times larger in diameter than usual.
Time crystals, first 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 TU Wien in Austria. This new state of matter does not repeat itself in space like traditional crystals, but in time, exhibiting spontaneous periodic rhythms without an external stimulus, a phenomenon known as spontaneous symmetry breaking.
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 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 could it be 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 Wilczek’s idea has caused a lot of controversy. Some considered time crystals to be 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 the support of TU Wien in Austria. The team used laser light and very special types of atoms, namely Rydberg atoms, with a diameter that is several hundred times larger than normal. The results have now been published in the journal Nature Physics.
Spontaneous breaking of symmetry
The rotation of a clock is also an example of a periodic movement of time. However, this does not happen by itself: Someone must have wound the clock and started it at a certain time. This start time then determined the tick time. It is different with a time crystal: according to Wilczek’s idea, a periodicity should arise spontaneously, although in fact there is no physical difference between different points in time.
“The frequency of ticks 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,” explains Prof Thomas Pohl from the Institute of Theoretical Physics at TU Wien.
A static system with a continuous light input leads to periodic time-dependent signals. Credit: TU Wien
Thomas 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 shone on a glass container filled with a gas of rubidium atoms. The strength of the light signal that arrived at the other end of the container was measured.
“This is actually a static experiment in which no specific rhythm is imposed on the system,” says Thomas Pohl. “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 reaching the other end of the glass cell starts to oscillate in very regular patterns.”
Giant atoms
The key to the experiment was preparing the atoms in a special way: The electrons of one atom they 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 – so-called Rydberg atoms – are created.
“If the atoms in our glass container are prepared in such Rydberg states and their diameter becomes large, then the forces between these atoms also become very large,” explains Thomas Pohl. “And that 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, then a feedback loop is generated that causes spontaneous oscillations between the two atomic states. This in turn also leads to the absorption of oscillating light. The giant atoms themselves stumble into a regular beat, and this beat translates into the rhythm of the light intensity arriving at the bottom of the glass container.
“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,” says Thomas Pohl. “Precise and self-sustaining oscillations can be used for sensors, for example. Giant atoms with Rydberg states have already been used successfully for such techniques in other contexts.
Reference: “Time-dispersive crystal in a strongly interacting Rydberg gas” by Xiaoling Wu, Zhuqing Wang, Fan Yang, Ruochen Gao, Chao Liang, Meng Khoon Tey, Xiangliang Li, Thomas Pohl, and Li You, 2 July 2024, Nature Physics.
DOI: 10.1038/s41567-024-02542-9