A Rydberg atom has an electron that is far away from the nucleus. Credit: TU Wien
Researchers have created an extremely exotic state of matter. The atoms have a diameter that is a hundred times larger than normal.
Time crystals, originally 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 in space like traditional crystals, but in time, spontaneously displaying periodic rhythms without an external stimulus, a phenomenon known as spontaneous symmetry breaking.
A crystal is a system 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 posed 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 particles is completely independent of time?
For years, Frank Wilczek’s idea has caused much controversy. Some considered time crystals 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 hundreds of times larger than normal. The results have now been published in the journal Physics.
Spontaneous symmetry breaking
The ticking of a clock is also an example of a temporally periodic motion. However, it does not happen by itself: someone must have wound up the clock and started it at a certain time. This starting time then determined the timing of the ticks. With a time crystal it is different: according to Wilczek’s idea, a periodicity should arise spontaneously, although in reality there is no physical difference between different times.
“The ticking frequency is predetermined by the physical properties of the system, but the moments at which the tick occurs are completely random; this is known as spontaneous symmetry breaking,” explains Prof. Thomas Pohl from the Institute for Theoretical Physics at TU Wien.
A static system with a continuous light input leads to time-dependent periodic 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 was shone 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 actually a static experiment where 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, the intensity arriving at the other end of the glass cell oscillates in very regular patterns.”
Giant atoms
The key to the experiment was to prepare the atoms in a special way: the electrons of a atom 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, its distance from the nucleus can become very large. In extreme cases, it can be hundreds of times further from the nucleus than normal. In this way, atoms with a gigantic electron shell are created – so-called Rydberg atoms.
“If the atoms in our glass container are prepared in such Rydberg states and their diameter becomes enormous, 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 laser light such that it can excite two different Rydberg states in each atom simultaneously, then a feedback loop is created that causes spontaneous oscillations between the two atomic states. This in turn also leads to oscillatory light absorption.” On their own, the giant atoms stumble into a regular beat, and this beat is translated into the rhythm of the light intensity arriving at the end of the glass container.
“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 Pohl. “Precise, self-sustaining 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: “Dissipative time 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, July 2, 2024, Physics.
DOI file: 10.1038/s41567-024-02542-9