We humans experience spacetime in four familiar dimensions: Three spatial (depth, width, and length), plus time. While most models of the universe assume that all matter exists within these parameters, many theories speculate there could be all sorts of lurking higher dimensions that are hidden beyond our comprehension.
While we have not been able to break free of our 4D experience, scientists are getting pretty good at simulating extra dimensions through the creation of so-called “synthetic dimensions.” These trippy experimental concepts provide a way to mimic the kinds of higher-dimensional concepts explored in some far-out models of nature using lower-dimensional materials that exist in the real world.
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Now, in a first-of-its-kind experiment, scientists led by Armandas Balčytis, a research fellow at the Royal Melbourne Institute of Technology, have created a synthetic dimension, a tiny photonic device known as a silicon ring resonator. In addition to demonstrating a new method of producing synthetic dimensions, the results will enable further experiments “that could model phenomena beyond three dimensions,” according to a study published on Friday in Science Advances.
“Mathematicians have it easy in that they can make assumptions or construct models (either more or less related to objects in the real world) and extend them to higher dimensions as far as their imagination allows,” said Balčytis in an email. “These can sometimes have useful or at least intriguing implications for experimentalists to check. Experimental science is much more limited however, since the materials we have or devices we can create, operate within the boundaries of our three-dimensional space (our team works with integrated photonic microchips, arranged on a flat surface, so we only have two dimensions from the outset).”
To create a synthetic dimension, Balčytis explained, scientists experimentally model an extra plane using some other variable, like a frequency.
“The key to synthetic dimensions is that it is possible to use some other variable of the system that is not generally thought of as spatial (frequency of light waves, polarization, delay between pulses etc.) as if it represented an additional coordinate,” he continued. “In this way you can have a single device (like the ring in our study) stand in for a linear chain of rings. By extension, a chain of synthetic dimension devices can act like a 2D array and so forth. By combining multiple different synthetic dimension variables it is also possible to emulate models beyond 3D.”
Balčytis notes that synthetic dimensions have become an “intense focus” in many fields of physics, especially with regards to topology, a branch of geometry that describes objects in continuous deformations, such as a twisted Möbius strip. As an example, synthetic dimensions can be used to investigate light’s behavior in exotic topological contexts, which can help to answer fundamental questions in optics and photonics while also opening up practical innovations in telecommunications, computing, and other applied fields.
“There are many predicted effects that can be either explored to satiate curiosity or harnessed for creating innovative on-chip devices,” Balčytis said. “So, while not quite probing into the nature of reality on a cosmic scale,” he added, these studies can “help answer fundamental questions about nature through the growing role topology has in explaining it.”
Ring resonators are devices that split light into a variety of complicated patterns and forms for scientific purposes, often optics or photonics. In the new experiment led by Balčytis, a ring resonator divided light up into a comb-like pattern that enabled the researchers to arrange photons along a one-dimensional lattice and control the movement of those photons.
“We injected photons into the system at one mode and made them fan out along the frequency scale as if they were a bunch of marbles overflowing from one container into nearest-neighboring ones,” which replicated “expected real-space behavior using a synthetic dimension relating to photons (which are quite unlike particles of typical matter we deal with every day),” Balčytis explained.
“We were also able through manipulation of how the ring is modulated exert control over the movement of photons along this synthetic lattice—making them respond as if affected by either an electric or a magnetic field force,” he continued.
The central innovation of the team’s approach was miniaturization: Whereas previous comparable experiments used fiber-based platforms that stretched some 10 meters in circumference, the new study’s ring resonator is several millimeters across.
“This brings with it not merely an advantage in convenience, but also robustness—as large setups are inherently far more susceptible to fluctuations than microscale chips where everything is packed close together,” Balčytis said. “This will help to greatly increase the complexity of models that can be explored (in simple terms one can chain together many more rings and have them be synchronized on a chip than in other ways).”
“That the device was demonstrated using silicon photonics is likewise important, since this is the most mature integrated platform, and researchers have access to numerous foundry services that are already equipped and ready to create sophisticated devices,” he added. “We hope our work will serve to energize the investigations into more complex synthetic dimension devices.”
To that point, the team is already looking ahead to further optimization of their experiment, including working with two-dimensional materials, as opposed to the 1D lattice from this study. Stacking ring resonators, for instance, could create more complex simulations of dimensions beyond three spatial dimensions, which could both enable advanced new photonic technologies as well as a broader understanding of the fundamental physics that govern our reality.
“For us now the next stage is improving the devices and creating more elaborate arrangements of them to explore a wider range of effects,” Balčytis said. “We are eager to push into two dimensions and beyond. Finding how higher dimensional phenomena can be employed to power new functionalities in quantum photonics, optical isolation on a chip, or optical information processing is an intriguing challenge to optical scientists and engineers”.
“Ideally, we’d like to include more and more functionalities onto our tiny integrated photonic chips, so we can shrink expensive, bulky pieces of equipment, and miniaturize them onto tiny, more robust, powerful, integrated photonic chips,” he concluded.
