On the Braamfontein campus of the University of the Witwatersrand (Wits University) in Johannesburg, South Africa, at high noon with sunlight beating down on the asphalt, a laser beam traveling between a mirror mounted on a rooftop and a laboratory below was constantly exposed to atmospheric turbulence generated by heat shimmer.
In free-space optical (FSO) communication on the ground, as well as in laser links between satellites and the Earth, atmospheric turbulence is the greatest obstacle. Vortices of air created by temperature differences irregularly distort the amplitude, phase, and polarization structure of light waves. Conventional approaches have relied on adaptive optics—using rapidly deforming deformable mirrors—or sophisticated post-reception computation to measure wavefront distortion and dynamically correct it, recovering the signal. This inevitably demands expensive, bulky hardware and substantial power consumption.
A collaborative research team from Wits University's School of Physics and School of Electrical and Information Engineering—including Cade Peters and Andrew Forbes—together with colleagues from the Laboratoire Ondes et Matière d'Aquitaine (LOMA) at the University of Bordeaux and CNRS (France's National Centre for Scientific Research), approached this long-standing challenge from a different angle. Rather than physically correcting the wavefront, they encoded information into a mathematical property that is inherently immune to distortion. The team confirmed that in a light beam transmitted over a 270-meter outdoor atmospheric path, the topological properties of a special light structure called an optical skyrmion were preserved—without any prior measurement of the medium or wavefront correction whatsoever. The findings were published in the journal Science Advances in August 2026 (DOI: 10.1126/sciadv.aee2671).
Donuts and Coffee Mugs: What Optical Skyrmions' "Invariants" Actually Are
Topology is a branch of mathematics dealing with geometric properties that remain unchanged under continuous deformation. As Professor Forbes likes to illustrate with a classic analogy, a coffee cup and a donut look dramatically different, yet both share the topological feature of having exactly one hole passing through them. Whether you stretch the cup's handle or squash it like clay, as long as you don't tear it or punch a new hole, this invariant—the number of holes, or the topological number—remains unchanged.
What the research team applied to light is precisely this mathematical robustness. In conventional optical communication, bits are assigned to a single physical quantity—the brightness (amplitude) of light, the timing of wave peaks and troughs (phase), or the direction of oscillation (polarization). These are easily modulated by atmospheric turbulence and appear as noise at the receiving end.
By contrast, the optical skyrmion used by the research team is a specially engineered vector light field in which the polarization vectors distributed across the light's cross-section wrap completely and an integer number of times around the entire surface of the Poincaré sphere—a sphere representing all possible polarization states. This "number of wraps" is expressed as an integer value called the skyrmion number (winding number $N$). No matter how much the local polarization direction or intensity is disturbed by atmospheric fluctuations, as long as the deformation remains continuous in space, the integer value $N$—calculated by integrating over the entire cross-section—remains, in principle, unchanged. Rather than armoring individual physical parameters against disturbance, meaning is embedded in the mathematical knot structure itself.
A prior theoretical and experimental study (Guo, Peters et al., Nature Communications, 2026) had shown that atmospheric turbulence effectively acts on light as a "one-sided channel." Because mode conversions capable of destroying topology tend to occur only in the far field—after long-distance propagation through the atmosphere—topological structures were thought to possess inherent resistance to local wavefront disturbances. This outdoor experiment set out to test whether that theoretical prediction, which had previously been confirmed only in simulated laboratory environments, would hold up in real, natural atmospheric conditions.
How Polarization Was Measured Across a 270-Meter Outdoor Campus Link
The demonstration took place between two buildings on the Wits University campus. A green continuous-wave laser at a wavelength of was expanded in beam diameter and sent to a transmissive/reflective spatial light modulator (HoloEye PLUTO-VIS), which shaped it into a light field carrying a specific skyrmion number via complex modulation of amplitude and phase.
The shaped beam was expanded to a beam waist of through two sets of 3x telescope optics, then launched into the open air toward a flat mirror mounted on the roof of the opposite building. The design had the light travel roughly 135 meters one way—270 meters round trip—through open air before returning to the same laboratory from which it was sent.
At the receiving end, the light that had passed through the turbulence was captured with a polarization high-speed camera (Allied Vision Mako G-508B POL). This camera is equipped with a micro-polarizer array oriented in four different directions on a per-pixel basis, allowing it to simultaneously measure all four Stokes parameters of light () from a single shot while preserving spatial resolution.
| Experimental Parameter | Setting / Equipment Used |
|---|---|
| Laser wavelength | (green light) |
| Beam waist | |
| Transmission distance | ~135 m one way (270 m round trip) |
| Spatial light modulator | HoloEye PLUTO-VIS (complex amplitude modulation) |
| Detection device | Allied Vision Mako G-508B POL (single-shot polarization camera) |
| Measurement conditions | Early morning (calm, low temperature), noon (strong turbulence, high temperature), and evening—three conditions tested |
| Correction technology | No adaptive optics, wavefront correction hardware, or pre-calibration used |
Atmospheric turbulence varies significantly depending on the time of day. The research team comprehensively collected data across a range of atmospheric conditions—from the calm, low-temperature, clear-air conditions of early morning, through the extreme turbulence of noon when heat rising from the ground produces intense shimmer, to the conditions of evening. In none of these conditions was any deformable mirror for wavefront correction or medium probing (advance measurement of the transmission path) incorporated.
What the Data Reveal: 98% Fidelity Alongside a 40% Drop in Polarization Degree
The experiment showed that the spatial amplitude, phase, and local polarization vector distribution of the light beam arriving at the receiver were disturbed so severely as to be unrecognizable from their original form. The raw intensity profile captured by the camera had collapsed into disordered speckle patterns.
However, when the topological winding number was calculated from the spatial polarization distribution, the intended integer value was confirmed to be preserved under every measurement condition. Across most atmospheric conditions, the recovery fidelity of the topological information reached 98% or higher, and even under the extreme turbulence of noon, a fidelity of 86% was maintained.
A notable physical phenomenon was observed here. When exposure was averaged over a duration longer than the atmospheric coherence time (the timescale over which phase remains stable), the degree of polarization (DOP) of the light dropped by roughly 40% across the field (the published paper does not disclose the specific absolute values before and after this drop, but substantial depolarization clearly occurred). In conventional polarization-multiplexed communication, such a substantial drop in polarization degree would be fatal, causing severe crosstalk and elevated error rates. Yet the topological number $N$—obtained by integrating over the entire spatial field—remained perfectly intact. This data confirms that even as local order is lost, the comprehensive connectivity of the field as a whole is preserved.
It is important, however, to correctly understand what this "fidelity" metric actually means. The fidelity reported in the paper refers to the probability that a transmitted topological number (for example, $N=1$ or $N=2$) was correctly classified and identified through polarization analysis at the receiving end. It is not a measurement of the throughput—data rate in gigabits per second—or the bit error rate (BER) of a transmitted bit stream. The paper contains no measured data on modulation speed or communication latency.
It should also be noted that the topological number was not simply read directly from raw data. In the experiment, stable topological numbers were extracted only after a series of digital post-processing steps applied to the polarization data captured by the camera: 300 rotational averages on the Poincaré sphere to reduce noise, removal of high-frequency components via a 2D Gaussian low-pass filter, and intensity thresholding (set at 2% to 3.5% of maximum intensity, depending on the time of day) to exclude ambient light (unpolarized background light).
A "Decoupling of Topology and Texture" Emerges When Compared With Prior Research
To clarify where this study stands, it's useful to organize the differences from related prior research.
| Method/Study | Test Environment | Transmission Distance | Object of Verification | Key Result | Key Limitation/Challenge |
|---|---|---|---|---|---|
| Conventional FSO (adaptive optics) | Real outdoor atmosphere / practical links | Hundreds of meters to tens of thousands of km | Intensity/phase-modulated data streams | High-speed transmission achievable (Gbps to Tbps class) | Expensive deformable mirrors, wavefront sensors, high power consumption and computational latency |
| Guo et al. (Nat. Commun. 2026) | Laboratory (simulated turbulence plates, SLM) | Tens of cm to a few meters (equivalent distance) | Topology of quantum/classical skyrmions | Proposed one-sided channel theory, theoretical demonstration of robustness | Controlled indoor environment only; quantum entanglement itself degrades rapidly under turbulence |
| Peters et al. (Sci. Adv. 2026, this study) | Real outdoor atmosphere (Johannesburg) | 270 m (round trip) | Winding number of classical optical skyrmions | 86–98% fidelity maintained without wavefront correction | Limited to static state identification demonstration; communication speed and BER unmeasured |
This outdoor demonstration goes a step beyond earlier indoor simulation experiments, providing the first example showing that topological invariants remain intact under conditions of real wind and thermal gradients. Lead author Cade Peters said in a statement, "Small-scale laboratory experiments had suggested that topology could serve as a robust means of information transfer, but this is the first time that robustness has been demonstrated in a real optical link under natural atmospheric conditions."
At the same time, related research has emerged showing that topological protection is not an all-purpose shield. A separately published theoretical study (arXiv:2603.01856) points to a phenomenon called the "decoupling of topology and texture." As light propagates through atmospheric turbulence, the global topological number $N$ is indeed conserved, but the fine local polarization pattern (texture) within the beam's cross-section itself collapses rapidly. Methods that attempt to embed high-density information directly in the fine structure of the texture cannot directly benefit from topological protection.
Furthermore, a companion study in Nature Communications dealing with "quantum skyrmions" (topological states composed of non-local photon pairs) found that even when the topological structure itself remains preserved, the quantum entanglement between photons is rapidly lost due to atmospheric turbulence. A clear gap still remains between the preservation of topology and the maintenance of quantum coherence required for quantum cryptographic communication.
Three Hurdles Standing Between This Result and Practical Communication Technology
The results of this experiment carry enormous physical significance as a proof of concept for protecting information using a degree of freedom of light. However, it cannot immediately be regarded as "the completion of next-generation optical communication technology that eliminates the need for atmospheric correction." Clear challenges remain to be overcome, in both physics and engineering, before this can become a practical communication system.
The first hurdle involves modulation speed and hardware constraints. Currently, liquid-crystal-based spatial light modulators (SLMs) are widely used to generate optical skyrmions, but SLM response speeds typically range from milliseconds (tens to hundreds of hertz) up to, at best, the kilohertz range. This falls far short of the gigahertz-range high-speed modulation demanded by modern optical communication. Research into ultra-high-speed skyrmion generation devices using integrated photonics—such as optical phased arrays based on silicon micro-ring resonators (e.g., arXiv:2605.10283)—is underway, but has not yet reached a stage where it can be incorporated into communication systems.
Second, there are theoretical limits to channel capacity and bit error rate (BER). According to a simulation study on skyrmion-number modulation for free-space optical communication presented at IEEE ICC Workshops (2026), even a four-level ($M=4$) discrete topological state modulation scheme yields a bit error rate exceeding $10^{-2}$ under strong turbulence conditions (refractive index structure constant $C_n^2 = 1 \times 10^{-13}$). Channel capacity has also been reported to saturate at around 1.5 bits per channel use. Between the ability to distinguish global topology and the construction of a high-throughput, low-error-rate digital communication channel lies a substantial design challenge involving coding schemes and error correction.
Third, there are questions of distance scaling and real-time detection. The experiment was conducted over a 270-meter round-trip link, and it remains unknown how the approach would behave at the practical distances of inter-building communication (several kilometers) or space-based optical communication linking satellites to the ground (hundreds to thousands of kilometers). Moreover, as noted above, the multiple digital image processing steps performed at the receiving end (Poincaré sphere rotational averaging and Gaussian filtering) are currently offline analyses; no real-time receiver circuit architecture capable of instantly demodulating pulses as they strike the photodetector has yet been established.
As the research team itself states in the paper's conclusion, the essence of this study is not the presentation of a completed communication system, but solid experimental evidence that "topological properties of light can serve as a promising foundation for overcoming real-world disturbances." Topology—once an abstract mathematical concept—has now passed through atmospheric chaos and traversed 270 meters of open ground-level space. Building on that fact, the focus going forward will be on how the engineering foundations of high-speed modulation devices and real-time demodulation systems can be constructed.