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A new state of light: Physicists observe new phase in Bose-Einstein condensate of light particles
A single “super photon” made up of many thousands of individual light particles: About ten years ago, researchers at the University of Bonn produced such an extreme aggregate state for the first time and presented a completely new light source. The state is called optical Bose-Einstein condensate and has captivated many physicists ever since, because this exotic world of light particles is home to its very own physical phenomena.
The Bose-Einstein condensate is an extreme physical state that usually only occurs at very low temperatures. What’s special: The particles in this system are no longer distinguishable and are predominantly in the same quantum mechanical state, in other words they behave like a single giant “superparticle.” The state can therefore be described by a single wave function.
In 2010, researchers led by Martin Weitz succeeded for the first time in creating a Bose-Einstein condensate from light particles (photons). Their special system is still in use today: Physicists trap light particles in a resonator made of two curved mirrors spaced just over a micrometer apart that reflect a rapidly reciprocating beam of light. The space is filled with a liquid dye solution, which serves to cool down the photons. This is done by the dye molecules “swallowing” the photons and then spitting them out again, which brings the light particles to the temperature of the dye solution — equivalent to room temperature. Background: The system makes it possible to cool light particles in the first place, because their natural characteristic is to dissolve when cooled.
Clear separation of two phases
Phase transition is what physicists call the transition between water and ice during freezing. But how does the particular phase transition occur within the system of trapped light particles? The scientists explain it this way: The somewhat translucent mirrors cause photons to be lost and replaced, creating a non-equilibrium that results in the system not assuming a definite temperature and being set into oscillation. This creates a transition between this oscillating phase and a damped phase. Damped means that the amplitude of the vibration decreases.
“The overdamped phase we observed corresponds to a new state of the light field, so to speak,” says lead author Fahri Emre Öztürk, a doctoral student at the Institute for Applied Physics at the University of Bonn. The special characteristic is that the effect of the laser is usually not separated from that of Bose-Einstein condensate by a phase transition, and there is no sharply defined boundary between the two states. This means that physicists can continually move back and forth between effects.
“However, in our experiment, the overdamped state of the optical Bose-Einstein condensate is separated by a phase transition from both the oscillating state and a standard laser,” says study leader Prof. Dr. Martin Weitz. “This shows that there is a Bose-Einstein condensate, which is really a different state than the standard laser. “In other words, we are dealing with two separate phases of the optical Bose-Einstein condensate,” he emphasizes.
The researchers plan to use their findings as a basis for further studies to search for new states of the light field in multiple coupled light condensates, which can also occur in the system. “If suitable quantum mechanically entangled states occur in coupled light condensates, this may be interesting for transmitting quantum-encrypted messages between multiple participants,” says Fahri Emre Öztürk.
Funding
The study received funding from the Collaborative Research Center TR 185 “OSCAR — Control of Atomic and Photonic Quantum Matter by Tailored Coupling to Reservoirs” of the Universities of Kaiserslautern and Bonn and the Cluster of Excellence ML4Q of the Universities of Cologne, Aachen, Bonn and the Research Center Jülich, funded by the German Research Foundation. The Cluster of Excellence is embedded in the Transdisciplinary Research Area (TRA) “Building Blocks of Matter and Fundamental Interactions” of the University of Bonn. In addition, the study was funded by the European Union within the project “PhoQuS — Photons for Quantum Simulation” and the German Aerospace Center with funding from the Federal Ministry for Economic Affairs and Energy.
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Human activities have an intense impact on Earth’s deep subsurface fluid flow
The impact of human activities — such as greenhouse gas emissions and deforestation — on Earth’s surface have been well-studied. Now, hydrology researchers from the University of Arizona have investigated how humans impact Earth’s deep subsurface, a zone that lies hundreds of meters to several kilometers beneath the planet’s surface.
“The deep subsurface is out of sight and out of mind for most people, and we thought it was important to provide some context to these proposed activities, especially when it comes to our environmental impacts,” said lead study author Grant Ferguson, an adjunct professor in the UArizona Department of Hydrology and Atmospheric Sciences and a professor in the University of Saskatchewan’s School of Environment and Sustainability.
In the future, these human-induced fluid fluxes are projected to increase with strategies that are proposed as solutions for climate change, according the study. Such strategies include: geologic carbon sequestration, which is capturing and storing atmospheric carbon dioxide in underground porous rocks; geothermal energy production, which involves circulating water through hot rocks for generating electricity; and lithium extraction from underground mineral-rich brine for powering electric vehicles. The study was done in collaboration with researchers from the University of Saskatchewan in Canada, Harvard University, Northwestern University, the Korea Institute of Geosciences and Mineral Resources, and Linnaeus University in Sweden.
“Responsible management of the subsurface is central to any hope for a green transition, sustainable future and keeping warming below a few degrees,” said Peter Reiners, a professor in the UArizona Department of Geosciences and a co-author of the study.
With oil and natural gas production, there is always some amount of water, typically saline, that comes from the deep subsurface, McIntosh said. The underground water is often millions of years old and acquires its salinity either from evaporation of ancient seawater or from reaction with rocks and minerals. For more efficient oil recovery, more water from near-surface sources is added to the salt water to make up for the amount of oil removed and to maintain reservoir pressures. The blended saline water then gets reinjected into the subsurface. This becomes a cycle of producing fluid and reinjecting it to the deep subsurface.
The same process happens in lithium extraction, geothermal energy production and geologic carbon sequestration, the operations of which involve leftover saline water from the underground that is reinjected.
“We show that the fluid injection rates or recharge rates from those oil and gas activities is greater than what naturally occurs,” McIntosh said.
Using existing data from various sources, including measurements of fluid movements related to oil and gas extraction and water injections for geothermal energy, the team found that the current fluid movement rates induced by human activities are higher compared to how fluids moved before human intervention.
As human activities like carbon capture and sequestration and lithium extraction ramp up, the researchers also predicted how these activities might be recorded in the geological record, which is the history of Earth as recorded in the rocks that make up its crust.
Human activities have the potential to alter not just the deep subsurface fluids but also the microbes that live down there, McIntosh said. As fluids move around, microbial environments may be altered by changes in water chemistry or by bringing new microbial communities from Earth’s surface to the underground.
For example, with hydraulic fracturing, a technique that is used to break underground rocks with pressurized liquids for extracting oil and gas, a deep rock formation that previously didn’t have any detectable number of microbes might have a sudden bloom of microbial activity.
There remain a lot of unknowns about Earth’s deep subsurface and how it is impacted by human activities, and it’s important to continue working on those questions, McIntosh said.
“We need to use the deep subsurface as part of the solution for the climate crisis,” McIntosh said. “Yet, we know more about the surface of Mars than we do about water, rocks and life deep beneath our feet.”
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Holographic displays offer a glimpse into an immersive future
Setting the stage for a new era of immersive displays, researchers are one step closer to mixing the real and virtual worlds in an ordinary pair of eyeglasses using high-definition 3D holographic images, according to a study led by Princeton University researchers.
The result is a virtual and augmented reality display that has the potential to be truly immersive, the kind where you can move your head normally and never lose the holographic images from view. “To get a similar experience using a monitor, you would need to sit right in front of a cinema screen,” said Felix Heide, assistant professor of computer science and senior author on a paper published April 22 in Nature Communications.
And you wouldn’t need to wear a screen in front of your eyes to get this immersive experience. Optical elements required to create these images are tiny and could potentially fit on a regular pair of glasses. Virtual reality displays that use a monitor, as current displays do, require a full headset. And they tend to be bulky because they need to accommodate a screen and the hardware necessary to operate it.
“Holography could make virtual and augmented reality displays easily usable, wearable and ultrathin,” said Heide. They could transform how we interact with our environments, everything from getting directions while driving, to monitoring a patient during surgery, to accessing plumbing instructions while doing a home repair.
One of the most important challenges is quality. Holographic images are created by a small chip-like device called a spatial light modulator. Until now, these modulators could only create images that are either small and clear or large and fuzzy. This tradeoff between image size and clarity results in a narrow field of view, too narrow to give the user an immersive experience. “If you look towards the corners of the display, the whole image may disappear,” said Nathan Matsuda, research scientist at Meta and co-author on the paper.
Heide, Matsuda and Ethan Tseng, doctoral student in computer science, have created a device to improve image quality and potentially solve this problem. Along with their collaborators, they built a second optical element to work in tandem with the spatial light modulator. Their device filters the light from the spatial light modulator to expand the field of view while preserving the stability and fidelity of the image. It creates a larger image with only a minimal drop in quality.
Image quality has been a core challenge preventing the practical applications of holographic displays, said Matsuda. “The research brings us one step closer to resolving this challenge,” he said.
The new optical element is like a very small custom-built piece of frosted glass, said Heide. The pattern etched into the frosted glass is the key. Designed using AI and optical techniques, the etched surface scatters light created by the spatial light modulator in a very precise way, pushing some elements of an image into frequency bands that are not easily perceived by the human eye. This improves the quality of the holographic image and expands the field of view.
Still, hurdles to making a working holographic display remain. The image quality isn’t yet perfect, said Heide, and the fabrication process for the optical elements needs to be improved. “A lot of technology has to come together to make this feasible,” said Heide. “But this research shows a path forward.”
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This salt battery harvests osmotic energy where the river meets the sea
Estuaries — where freshwater rivers meet the salty sea — are great locations for birdwatching and kayaking. In these areas, waters containing different salt concentrations mix and may be sources of sustainable, “blue” osmotic energy. Researchers in ACS Energy Letters report creating a semipermeable membrane that harvests osmotic energy from salt gradients and converts it to electricity. The new design had an output power density more than two times higher than commercial membranes in lab demonstrations.
The researchers’ RED membrane prototype contained separate (i.e., decoupled) channels for ion transport and electron transport. They created this by sandwiching a negatively charged cellulose hydrogel (for ion transport) between layers of an organic, electrically conductive polymer called polyaniline (for electron transport). Initial tests confirmed their theory that decoupled transport channels resulted in higher ion conductivity and lower resistivity compared to homogenous membranes made from the same materials. In a water tank that simulated an estuary environment, their prototype achieved an output power density 2.34 times higher than a commercial RED membrane and maintained performance during 16 days of non-stop operation, demonstrating its long-term, stable performance underwater. In a final test, the team created a salt battery array from 20 of their RED membranes and generated enough electricity to individually power a calculator, LED light and stopwatch.
Ye, Qin and their team members say their findings expand the range of ecological materials that could be used to make RED membranes and improve osmotic energy-harvesting performance, making these systems more feasible for real-world use.
The authors acknowledge funding from the National Natural Science Foundation of China.
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