糖心视频

Relaying a message from point A to B can be as simple as flashing a thumbs-up at a stranger in an intersection, signaling them to proceed鈥攏onverbal, clear, and universally understood. But light-based communication is rarely that straightforward.

Photons, tiny particles of light, are fragile and unpredictable. Unlike electrons, which must be conserved in circuits, photons can scatter, split, merge into different colors, or be absorbed, meaning that the number of photons in a system isn't fixed, even while the energy they carry remains the same. This makes guiding them through fiber optics or photonic chips鈥攐ptical mazes鈥攆ar trickier than steering electrons through copper wires, because light signals can scatter into dead ends or vanish before reaching their destination.

Engineers often respond by obsessively refining every imperfection, polishing the maze to perfection. However, this approach can be exhausting and never fully addresses these limitations.

"What if, instead of fighting the flaws, you could change the rules of the maze itself鈥攔eshaping the landscape on the fly so light has no choice but to keep going forward," asks Bo Zhen, a physicist at Penn's School of Arts & Sciences.

Now, in a published in Nature 糖心视频, Zhen and his colleagues have built a "secret tunnel" for photons that guarantees a beam can make it from A to B without getting stuck in traffic or lost along the way.

Their approach paves the way for chips that route light one-way around defects, optical isolators that don't need bulky magnets, and lasers that emit light only in a forward direction, immune to destabilizing reflections. Robust photonic highways like these could reshape telecommunications, sensing, and quantum technologies鈥攆ewer wrong turns, fewer detours, and far less baggage.

The project started in 2018, when first author Li He arrived in Zhen's lab to begin postdoctoral training. Working with Zhen and Penn physicist Eugene Mele鈥攂est known for helping discover electronic topological insulators鈥擧e proposed using polarized light on a photonic crystal, a semiconductor punctured with a regular array of holes, to create a stable topological state.

The aim was to reshape how light moves inside, forming a one-way path along the edges where light travels only travels forward, explains He. This "secret tunnel," or chiral edge states, retains light's one-way character despite small bumps or defects.

Their 2019 theoretical proposal published in Nature Communications, showed that photons could be shepherded much like electrons in exotic quantum materials.

"Back then, it was just equations; the math told us it was possible but making it real meant designing the right material and driving it in just the right way," says He. "And theory doesn't always agree with practice."

The experiment demanded two ultrafast lasers鈥攐ne to drive the crystal and one to probe it鈥攚hich Zhen ordered from Europe at the start of 2020, just as COVID-19 closures tightened borders and throttled supply chains.

"For a while, it felt like everything that could go wrong did," Zhen laughs. "Our fancy new laser showed up in 12 boxes and the delivery truck only brought us six. Then the person who could make it work was stuck in Lithuania. We were sitting here with half a laser and no instruction manual."

With help from collaborators at University of California Santa Barbara, they had a stable device by 2022.

Their work showed that with linear polarization, the crystal stayed gapless, its internal bands crossing like indifferent pedestrians. With circular polarization, the bands twisted open a full gap, marked by the unmistakable topological signature: a Chern number of one (C=1), a distinct characteristic indicating an open one-way channel.

From their laser spectra鈥攄etailed energy footprints鈥攖he team reconstructed the band structure, confirming they had pushed the system such that the crystal was rhythmically driven in time, creating new protected routes within the energy gaps.

"It was a Eureka moment鈥攖he plots on our screen matched the topological band diagram we had dreamed about," says He. "Suddenly, this wasn't just a route on a map; we were walking the road."

In nonlinear optical material, two photons of one color can merge into a single photon of a different color, or a single photon can split into several. "Pump in one blue photon, watch two red photons come out鈥攃onnections and transfers that electronics simply don't allow," Zhen says. "That freedom means photonic systems aren't bound by the same constraints as electronics, and this new topological phase of light hands over a fresh rulebook for device design."

Zhen says the team is already marching ahead, pushing the concept into three-dimensional crystals, scaling it to microwave frequencies where components are larger and easier to handle, and exploring whether the effect can shelter fragile quantum states of light.

"We've shown it's possible," Zhen says. "Now the goal is to see whether this approach can protect quantum information or create new classes of optical devices."