Scientists have developed an incredibly exotic state of matter. Its atoms are a hundred times wider than usual.
A 2012 proposal by Nobel laureate Frank Wilczek, time crystals were realized with Rydberg atoms and laser light at Tsinghua University in China; theoretical support was from TU Wien in Austria. Unlike other crystals, which repeat in space, this new state of matter does that in time, and, displaying a spontaneous periodic rhythm without ne any external stimulus, it does spontaneous symmetry breaking.
A crystal is a structure of atoms that repeats at regular distances in space: it looks the same at every point. Back in 2012, Nobel-prize winner Frank Wilczek asked a provocative question: could there be such a thing as a time crystal – something that, yes, repeats itself eternally in time? And is it actually imaginable that a period rhythm sets in, although no specific rhythm is imposed on the system and the interaction of the particles with each other is completely time-symemtric?
For years, Frank Wilczek’s proposal was hotly controversial. Some considered time crystals to be fundamentally impossible others looked for loopholes to make time crystals possible under particular conditions. Now, with support from TU Wien in Austria, a particularly spectacular kind of time crystal has been successfully realized at Tsinghua University in China. Here, one employed laser light and very special types of atoms-giant clusters of so-called Rydberg atoms. These atoms have a diameter that is several hundred times bigger than normal. The results have now been been published in the journal Nature Physics.
Spontaneous Symmetry Breaking
Also, the ticking of a clock constitutes a temporally periodic movement. This does not take place out of the blue on its own: Someone actually wound the clock and started it ticking at some moment of time. This then fixes the timing of the ticks. With a time crystal, in contrast, periodicity should arise spontaneously according to Wilczek’s idea; although there is no physical difference between different points in time.
“The tick frequency is set by the physical properties of the system, but the times when the tick happens is completely random; that’s spontaneous symmetry breaking,” adds Prof Thomas Pohl from the Institute of Theoretical Physics at TU Wien.
Time-Dependent Periodic Signals
A none-driven static system with continuous input of light results in time-dependent periodic signals. Credit: TU Wien
The theoretical work forming the basis of the research, now resulting in the discovery of a time crystal at Tsinghua University in China, was under the supervision of Thomas Pohl. Laser light is sent into a glass container filled with a gas of rubidium atoms; they measured the strength of the light signal reaching the other end of the container.
“This is actually a static experiment in which no specific rhythm is imposed on the system,” explains 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 that arrives at the other end of the glass cell begins to oscillate in highly regular patterns.”
Giant Atoms
However, the key to the experiment is a special preparation of the atoms: an atom’s electrons can orbit around the nucleus on a different number of paths depending on how much energy they possess. When the energy of an atom’s outermost electron is increased, the radius of the orbit around the nucleus can become very large. That means, in extreme cases, several hundred times further away from the core than usual. This is how atoms with a giant electron shell are created – so-called Rydberg atoms.
“If the atoms in our glass container are prepared in such Rydberg states and their diameter is huge, then the forces between these atoms also get very large,” says 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 in each atom at the same time two different Rydberg states, then it generates feedback creating a loop that has spontaneous oscillations between the two atomic states.”. This again, in turn, results in oscillating light absorption. Of their own accord, the giant atoms stumble into a regular beat, and this beat is translated into the rhythm of the light intensity that arrives at the end 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, 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.”
The ability to experimentally construct time crystals from Rydberg atoms and laser light represents a milestone in experimental physics that takes it to what used to be seen as the theoretical fancy. Now, scientists at Tsinghua University in China, in collaboration with TU Wien in Austria, have found that these time crystals can exist. This discovery questions all the old notions and creates new paths for research into unconventional states of matter.
During the process of this experiment, the Rydberg atoms, with their particularly large orbitals of electrons, played an important role. The scientists excited the atoms with laser light and measured spontaneous oscillations of periodic rhythms in intensity that were related to light passed through the gas of rubidium atoms. Such creation of phenomena, named spontaneous symmetry breaking, does not involve any rhythmic external input and gives evidence on the intrinsic complexity and potential of time crystals.
The implications of this very recent discovery for basic physics are broader. Time crystals may lead to technological revolutions in fields ranging from sensors to, well, basically everything. The exact, stable oscillations that are observed in this work may be implemented into extremely precise instruments that follow changes in physical parameters at an unbelievable level of accuracy. This application motivates the real relevance of studying exotic states of matter and makes it possible that from theoretical concepts these states can be turned into real instrumental improvements and much else, for that matter.
The discovery of time crystals, therefore, opens new possibilities toward understanding quantum dynamics and complex systems from a completely different perspective. It provides a concrete platform through which the characterization of how quantum phenomena are manifest in macroscopic systems will become clear and how such an understanding will be generalized within science and technology. As scientists investigate the properties and behavior of time crystals, much more new room for discovery and innovation will likely be opened.