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LSU physicists and material scientists are building a quantum simulator with a gold chip to discover better materials for solar cells. Ideal materials must introduce some amount of mayhem, the researchers say, to get more light to go through.

Light consists of photons, and they move in mysterious ways. Because photons are what physicists call 鈥渄iscrete particles,鈥� and thus subject to quantum effects, they don鈥檛 abide by what we鈥檇 consider normal rules. Traveling from the sun, photons from sunlight have a mindboggling number of options open to them. Indeed, it鈥檚 difficult to predict where even a single photon will go since it can be in two places at the same time (a quantum effect called superposition). Superposition is why that relatively cute cat that belonged to a man named Schr枚dinger (or in truth, probably to his grant-aunt) could be described as simultaneously dead and alive. And to put this quantum lawlessness into yet another context, you could consider Newton鈥檚 notorious-yet-unconfirmed incident under the apple tree鈥攊f the apple that supposedly fell and hit him on the head (helping him discover gravity) had been a photon instead, it might have struck a nearby church bell while also serving as a delightful snack for Newton鈥檚 horse.

鈥淪unlight is chaotic, crazy light,鈥� said Omar Maga帽a-Loaiza, assistant professor and head of the quantum photonics lab in the LSU Department of Physics & Astronomy. 鈥淪unlight will be transported differently from other kinds of light, and this is a major challenge in the design of solar cells.鈥�

鈥� A photon that becomes lodged inside a material usually gets absorbed鈥攏othing comes out. 鈥�

Most solar cells on the market today contain a crystalline silicon to help transport photons in the conversion of light energy into electricity. But even on a sunny day, the harvest can be uneven. Apart from being unpredictable, photons are subject to so-called localization effects due to interferences on the atomic level. Localization can cause photons to get 鈥渟tuck鈥� in seemingly random patterns and cluster in odd places. In solar cells, this is a big problem. A photon that becomes lodged inside a material usually gets absorbed鈥攏othing comes out. What the LSU researchers are trying to do is break localization effects to make solar cells more efficient. With a recent grant from the U.S. Department of Energy, they are building a quantum simulator to study the transport of light and are now figuring out how to manipulate it in just the right way to be able to harvest more photons faster.

鈥淢aterials are basically networks,鈥� Maga帽a-Loaiza said. 鈥淣etworks of atoms. And if you order the atoms in a certain way you can slow down or speed up the photons as they travel through the material, and with just the right amount of disorder in the network, you can break the localization effects.鈥�

By changing different parameters in their simulator, Maga帽a-Loaiza and You can mimic the transport of light in multiple crystalline materials that could be used to fabricate solar cells. Their goal is to be able to tell the materials scientists what materials to make鈥攚hich properties would be required.

鈥淚n a nice and strong material, such as a well-organized and well-behaved crystal, you can get localization,鈥� Maga帽a-Loaiza explained. 鈥淪o, we have to add randomness in a clever way to boost the transport. Just the right kind of disturbance can get the photons from the entrance to the exit. The material cannot allow certain interference conditions, and we鈥檙e figuring out which ones.鈥�

Because of the unfathomable number of possibilities for how photons can move through space, you could not run this kind of simulation on a regular computer鈥攏ot even a supercomputer. A quantum leap is required. While the late LSU physicist Jon Dowling, a world-renowned expert in quantum information science, addressed many of these transport problems in quantum random walk networks, Maga帽a-Loaiza and You are building a physical device to mimic the 鈥渃haotic, crazy鈥� paths that not just individual photons but multiple photons can take. And to simulate more options more quickly, they鈥檝e built their apparatus around a sliver of gold with slits cut at precise intervals (manufactured at the LSU Shared Instrumentation Facility, or SIF) to upgrade the capacity of their calculator by working with not just photons but plasmons鈥攅lectromagnetic waves around the surfaces of metals, such as gold. Photons can excite plasmons, which move in even more mysterious ways. To understand what plasmons add to Maga帽a-Loaiza鈥檚 and You鈥檚 research, you could imagine a highway toll booth with an inquisitive attendant. The attendant asks everyone who passes through about their final destination, but instead of manning a single-passenger lane, the attendant works the HOV lane. For every vehicle that passes through, the attendant learns the final destination of several passengers at once鈥攎ultiple drop-off points. The vans and buses are the plasmons, while Maga帽a-Loaiza and You take turns as the attendant.

鈥� Because of the unfathomable number of possibilities for how photons can move through space, you could not run this kind of simulation on a regular computer鈥攏ot even a supercomputer. A quantum leap is required. 鈥�

鈥淚ncluding plasmons in our work allows more options for where the light can go,鈥� Maga帽a-Loaiza said. 鈥淲e use plasmons to simulate complex physical effects and this more sophisticated system allows us to handle all possibilities, especially since plasmons are 鈥榣ossy鈥� fields and we then can study the losses and try to use them in our favor. You can鈥檛 avoid losses; you have to live with them. So, working with plasmons helps us understand and utilize the loss instead of trying to avoid its adversary effects鈥攖hat鈥檚 a particular thing for our platform.鈥�

Plasmons help the researchers mimic more disorder. Typically, you wouldn鈥檛 want disorder in a material or in a lab, but in this case, Maga帽a-Loaiza and You need disorder to simulate some of the crazy things that happen with light.

鈥淟ots of companies are trying to work on improving the efficiency of solar cells, but it鈥檚 very difficult because it is challenging to simulate the underlying physics that define light transport,鈥� Maga帽a-Loaiza continued. 鈥淧hotons don鈥檛 necessarily move in ordered patterns, like straight through a crystal. Instead, they can loop around and induce complex superpositions.鈥�

While 鈥渜uantum supremacy鈥� has been tossed around quite a bit lately after Google claimed achieving it in 2019, Maga帽a-Loaiza is someone lukewarm on the term, which simply means that you鈥檝e been able to do something with a quantum device that you couldn鈥檛 do without it. This holds true for Maga帽a-Loaiza鈥檚 and You鈥檚 simulator.

鈥淛on Dowling performed research devoted to the development of photonic circuits for information processing and identified specific conditions that cannot be simulated on a classical computer,鈥� said Maga帽a-Loaiza about the late LSU scientist who encouraged him to join his team at LSU. 鈥淗e also identified specific multiparticle events that have enormous potential for quantum computers. I was blown away when I read his work in grad school. Now, our solution, a quantum-enhanced simulator with a chip, is like a specialized quantum computer鈥攚e will use it to help solve the particular problem our simulator was built to solve.鈥�

Maga帽a-Loaiza points out that it would be possible to build similar quantum simulators for other specific tasks, such as how to design drugs or predict traffic jams.

鈥淥nce we鈥檙e done with this project, we鈥檒l be able to say, 鈥楿se this design to build your solar cell and localization won鈥檛 happen,鈥� or, 鈥楳ake this change to speed up the photons as they go through,鈥欌�� he said. 鈥淏ut you could build a similar setup to help calculate answers to a lot of different questions.鈥�

鈥淲e鈥檙e merging two research fields together here,鈥� Maga帽a-Loaiza continued. 鈥淨uantum optics, which is what we normally work on, and plasmonics, which belongs more in the classical, non-quantum, realm. We鈥檙e not experts on materials and we鈥檙e not making crystals or building solar cells鈥攚e鈥檙e simply building and using our platform to simulate something that cannot be simulated on regular computers. We鈥檙e simulating what otherwise cannot be simulated, but our platform is very complex鈥攊t has to be. You can鈥檛 just put some light in there and click 鈥榩lay.鈥� We have to study one parameter at a time, and we鈥檙e doing this with a single photon as well as entangled multiphoton packets where we can control the interaction. We can鈥檛 simulate the whole thing at once, but we can do it in pieces and figure out how much of the light is reflected, transmitted, and absorbed.鈥�

鈥滾SU鈥檚 exciting research on materials and light for improved solar cells could have positive implications for some of our customers鈥� resilience and efficiency efforts.鈥�

Stephen Toups, Turner Industries

Baton Rouge-based Turner Industries is a global company that provides turnkey services and solutions for the heavy industrial sector, supporting clients who produce fuel, energy, and essential products. The company鈥檚 president, Stephen Toups, is interested in the frontier work on solar cells the LSU quantum photonics group is doing; he understands that meeting demand for a growing world population will require an array of efficient energy sources.

鈥淎s one of the nation鈥檚 leading heavy industrial contractors, our company is committed to championing innovative solutions to help our customers secure and sustain operations in often challenging environments,鈥� Toups said. 鈥淟SU鈥檚 exciting research on materials and light for improved solar cells could have positive implications for some of our customers鈥� resilience and efficiency efforts.鈥�