“Everything is born out of nothing. All you do is shine a light, and this whole world of time crystals emerges,” said University of Colorado Boulder physicist Ivan Smalyukh. In a field where most advances are invisible in quantum states or relegated to the mathematical realm, his group has pushed one of physics’ oddest phases of matter out into the light.
Originally proposed in 2012 by Nobel laureate Frank Wilczek, time crystals are a state of matter that violate time-translation symmetry. In crystals like diamond or quartz, atoms arrange in a repeating lattice in space. In a time crystal, the order appears in the time dimension: the microscopic structure of the system oscillates indefinitely, returning to its initial configuration at regular times without expending energy in the usual sense. Early sightings whether in chains of ytterbium ions or within quantum computers were imperceptible to the human eye, measurable only by indirect observation of quantum oscillations.
UC Boulder researchers, under the direction of graduate student Hanqing Zhao and Smalyukh, found visibility by leveraging the quirky physics of liquid crystals, the very rod-like molecules that provide LCD displays with their image stability. Such molecules are in a state intermediate between liquid and solid and possess long-range orientational order, but position fluidity. The scientists placed a thin layer of liquid crystal between two coated glass plates, with the coating consisting of a photoresponsive dye. When exposed to polarized light, the dye molecules reoriented and applied mechanical stress to the liquid crystal layer.
That stress created topological defects kinks in molecular alignment that performed like quasi-particles. These kinks engaged with each other in a complicated, recurring dance, generating psychedelic, tiger-stripe patterns that rippled through the sample for hours. The oscillations continued even when the team changed light intensity and temperature, a sign of the kind of robustness one might expect of a time crystal. With the proper conditions, the patterns were visible not only with a microscope but to the naked eye, a first in the field.
The phenomenon has its roots in the same symmetry-breaking processes that govern most condensed matter systems. In space crystals, translational symmetry is broken when atoms get pinned into a periodic lattice. In time crystals, the symmetry of continuous time flow is broken by a system’s preference for discrete temporal periods. This is not eternal motion energy from the light powers the system but the oscillations take place at a constant multiple of the driving frequency, a “subharmonic response” that characterizes the time crystal phase.
Liquid crystals are especially well-suited to casting this behavior in visible form because their optical properties respond to molecular orientation. As the kinks travel, they alter the way that the material responds to polarized light, translating molecular-scale motion into macroscopic visual patterns. This optical amplification is comparable in principle to the manner in which photoresponsive liquid crystal elastomers can translate nanoscale molecular rotation into large-scale mechanical bending.
Simplicity of the UC Boulder device is remarkable: glass plates, a dye coat, and a layer of liquid crystals micrometers thin. Yet the collective behavior is complex enough to imply many applications. By stacking layers of time crystals with varying periods of oscillation or in spatial patterns, engineers might encode multidimensional, complex information. This could lead to novel data storage architectures in which information is embedded not just in spatial arrangements but in temporal sequences of patterns.
Another possible application is anti-counterfeiting. A banknote with a “time watermark” could display a special, dynamic stripe pattern when light from a portable light source falls on it no microscope or electronics needed. Since the pattern is both spatially and temporally encoded, it would be practically impossible to reproduce it without the same material structure. Zhao and Smalyukh also identify telecommunications and photonic device possibilities, where the manipulation of light in space and time might provide new means of signal processing.
The research highlights the way that principles of quantum condensed matter can translate into physical, even beautiful, manifestations. It also opens an experimental door: a system in which the dynamics of time crystals can be examined directly, not just with abstracted information. As Smalyukh explained, “You just create some conditions that aren’t that special. You shine a light, and the whole thing happens.”
