“Everything is born out of nothing,” said Ivan Smalyukh, a physicist at the University of Colorado Boulder. “All you do is shine a light, and this whole world of time crystals emerges.”.
In a paper in Nature Materials, scientists have developed the first visible time crystals, their quivering, stripe-patterns visible with a microscope and, in certain instances, to the naked eye. The achievement unites two previously distinct domains of physics: the unusual, symmetry-breaking structure of time crystals and the tunable optical properties of liquid crystals, the same substances used in LCD monitors.
Time crystals, originally conceived in 2012 by Nobel winner Frank Wilczek, are states of matter whose internal structure periodically repeats in time and not in space. While in regular crystals, atoms are grouped in periodic lattices in space; in time crystals, the state of the system repeats indefinitely between configurations, violating time-translation symmetry but staying in its ground state. This temporal stability is strong: the oscillations continue with no energy added or removed, sustained here by photons and not by mechanical or thermal input.
So far, time crystals have been achieved in quantum systems, including trapped ions, superconducting qubits, and nitrogen-vacancy centers in diamond, but only their behavior could be gleaned from indirect observations. The Colorado researchers, on the other hand, designed a macroscopic, classical continuous space-time crystal (CSTC) that meets the strict requirements for true time crystallinity spontaneous symmetry breaking, phase randomness, and perturbation resistance and it does so in a directly observable way.
The device is made up of a narrow cell only 2 to 4 micrometers thick of nematic liquid crystal between photoresponsive azobenzene-coated glass plates. The dye molecules on the surfaces, under irradiation with linearly polarized blue light, reorient, changing the alignment of rod-shaped liquid crystal molecules in the bulk. This induces the creation of topological solitons localized molecular orientation twists that organize into a periodic space–time lattice. The interplay of optical torque, surface anchoring forces, and the liquid crystal elastic constants, modeled by the Frank–Oseen free energy, supports the oscillations.
Under polarized light microscopy, the soliton arrays are seen as alternating colored stripes that change rhythmically with a temporal period of approximately 4.6 seconds. Fourier analysis of image intensity demonstrates a strong peak in frequency, and correlation functions exhibit a quasi-long-range order in time, similar to the positional order of smectic liquid crystals. Numerical simulations, including the medium’s birefringence and the feedback between light polarization and molecular orientation, replicate the experimentally observed patterns and validate the function of many-body interactions among solitons as the means of preserving order.
The visible character of these CSTCs provides an avenue to useful applications. Since their spatial and temporal structures are reproducible and steady, they might provide “time watermarks” for anti-counterfeiting placed in high-denomination bills or tamper-evident documents, authenticated by illuminating them and observing the typical oscillations. By piling multiple CSTCs with varying temporal intervals, scientists can construct intricate, high-capacity “2+1D” barcodes that utilize the additional temporal dimension for encoding, possibly storing more than 100,000 bits per second with inherent error correction from the robustness of the crystal.
The photonic consequences are no less dramatic. Periodic modulation of molecular orientation confers a dynamic Pancharatnam–Berry phase to transmitted light and hints at applications in tunable diffractive optics, space-time photonic crystals, and telecommunications. Since the driving light wavelength (~450 nm) can be different from the modulated signal wavelength (e.g., >850 nm for fiber optics), CSTCs may be able to control polarization-encoded data streams independently of the control beam.
The research also overlaps with developments in liquid-crystal-based nonlinear optics. Experiments on ferroelectric nematic liquid crystals in recent times have demonstrated that they are capable of producing entangled photon pairs through spontaneous parametric down-conversion with efficiencies comparable to solid crystals but with electrical tunability of polarization entanglement. The same molecular sensitivity that makes such quantum light sources possible is the basis for the reconfigurability of CSTCs, suggesting hybrid devices incorporating time-crystalline order with quantum photonics.
Smalyukh and lead author Hanqing Zhao highlight the versatility of the platform. Modulating cell geometry, dye chemistry, or illumination parameters might alter oscillation frequencies, soliton configurations, or optical properties. “We don’t want to put a limit on the applications right now,” Smalyukh said. “I think there are opportunities to push this technology in all sorts of directions.”
