Human eye spots single photons

Human eyes are capable of detecting a single photon — the tiniest possible speck of light — new research suggests.

The result, published July 19 in Nature Communications, may settle the debate on the ultimate limit of the sensitivity of the human visual system, a puzzle scientists have pondered for decades. Scientists are now anticipating possibilities for using the human eye to test quantum mechanics with single photons.

Researchers also found that the human eye is more sensitive to single photons shortly after it has seen another photon. This was “an unexpected phenomenon that we just discovered when we analyzed the data,” says physicist Alipasha Vaziri of Rockefeller University in New York City.

Previous experiments have indicated that humans can see blips of light made up of just a few photons. But there hasn’t been a surefire test of single photons, which are challenging to produce reliably. Vaziri and colleagues used a quantum optics technique called spontaneous parametric down-conversion. In this process, a high-energy photon converts into two low-energy photons inside of a crystal. One of the resulting photons is sent to someone’s eye, and one to a detector, which confirms that the photons were produced.

During the experiment, subjects watched for the dim flash of a photon, which arrived at one of two times, with both times indicated by a beep. Subjects then chose which beep they thought was associated with a photon, and how confident they were in their decision.

In 2,420 trials, participants fared just slightly better than chance overall. That seemingly unimpressive success rate is expected. Because most photons don’t make it all the way through the eye to the retina where they can be seen, in most trials, the subject wouldn’t be able to see a photon associated with either beep. But in trials where the participants indicated they were most certain of their choice, they were correct 60 percent of the time. Such a success rate would be unlikely if humans were unable to see photons — the chance of such a fluke is 0.1 percent.

“It’s not surprising that the correctness of the result might rely on the confidence,” says physicist Paul Kwiat of the University of Illinois at Urbana-Champaign, who was not involved with the research. The high-confidence trials may represent photons that made it through to the retina, Kwiat suggests.

Additionally, the data indicate that single photons may be able to prime the brain to detect more dim flashes that follow. When participants had seen another photon in the preceding 10 seconds, they had better luck picking out the photon.

Scientists hope to use the technique to test whether humans can directly observe quantum weirdness. Photons can be in two places at once, a state known as a quantum superposition. The technique could be adapted to send such quantum states to a subject’s eye. But, says Leonid Krivitsky, a physicist at the Agency for Science, Technology and Research in Singapore, “I’m pretty skeptical about this idea of observing quantumness in the brain.” The signals, he suggests, will have lost their quantum properties by the time they reach the brain.

Whether humans can see individual photons may seem to be a purely academic question. But, Vaziri says, “If you are somewhere outside of a city in nature and on a moonless light and you have only stars to navigate, on average the number of photons that get into your eye is approaching the single photon regime.” So, he says, having eyes sensitive enough to see single photons may have some evolutionary advantage.

Source: https://www.sciencenews.org/

How dinosaurs hopped across an ocean

Two land bridges may have allowed dinosaurs to saunter between Europe and North America around 150 million years ago.

The bridges would explain how dinosaurs, mammals and other animals were able to hop from one continent to the other after the Atlantic Ocean formed during the breakup of the Pangaea supercontinent. Some species of Stegosaurus, for instance, appear in the fossil record on both sides of the Atlantic.

Leonidas Brikiatis, an independent biogeographer in Palaio Faliro, Greece, proposes that two strips of land bridged North America and Europe during the late Jurassic and early Cretaceous periods. One bridge spanned from eastern Canada to the Iberian Peninsula, where Spain is today, and lasted from around 154 million to 151 million years ago. The other linked North America and Scandinavia from around 131 million to 129 million years ago, Brikiatis reports in the August Earth-Science Reviews.

The routes allowed dinosaurs to “foil plate tectonics’ plan to break up the world,” says Paul Sereno, a vertebrate paleontologist at the University of Chicago who was not involved in the study, which reviewed recent studies of vertebrates in the fossil record appearing on opposite sides of the Atlantic. “A continent can’t contain a dinosaur; they’ll escape. This work highlights two of the routes they took.”

A LINK BETWEEN WORLDS A land route may have connected eastern Canada and the Iberian Peninsula around 150 million years ago. That bridge would have allowed animals to hop between Europe and North America

L. BRIKIATIS/EARTH-SCIENCE REVIEWS 2016

Dinosaurs, including species of Supersaurusand Allosaurus,probably made the transatlantic trek alongside turtles, lizards and early mammals. While the Atlantic Ocean was narrower back then, it was probably too wide to swim across. Brikiatis used the dates of the relocations to establish a potential window of time when the bridges existed and considered potential crossings that might have existed at the time. The best contenders are patches of relatively shallow water called ocean shelves. Tectonic activity could have lifted these shelves above sea level, creating narrow strips of land around 80 to 160 kilometers across, Brikiatis says. Over time, the bridges may have sunk back below the sea.

Those land routes would have been somewhat similar to other ocean crossings, such as the Bering land bridge humans traversed around 23,000 years ago between Asia and North America (SN: 8/22/15, p. 6) and the modern Isthmus of Panama that links North and South America (SN: 5/2/15, p. 10).

The ancient bridge connecting North America and Scandinavia may have coexisted with another land route that connected Europe and what would later become Russia, allowing migrations across much of the world, Brikiatis proposes.

While the routes proposed in the work are plausible, the dates might be off, says Octávio Mateus, a paleontologist at the Universidade Nova de Lisboa in Caparica, Portugal. Species may have migrated earlier than evidenced in the fossil record, he says. “Just because you find them then doesn’t mean they came then. They could have come millions of years before, but just didn’t leave fossils.”

The bridges may also have been more like stepping stones than an unbroken migration highway, says vertebrate paleontologist Anne Schulp of the Naturalis Biodiversity Center in Leiden, the Netherlands. “A narrow body of water is not impenetrable,” he says. “You don’t need a full bridge.”

Source: https://www.sciencenews.org/

Scientists unlock 'green' energy from garden grass

Garden grass could become a source of cheap and clean renewable energy, scientists have claimed.

A team of UK researchers, including experts from Cardiff University's Cardiff Catalysis Institute, have shown that significant amounts of hydrogen can be unlocked from fescue grass with the help of sunlight and a cheap catalyst.

It is the first time that this method has been demonstrated and could potentially lead to a sustainable way of producing hydrogen, which has enormous potential in the renewable energy industry due to its high energy content and the fact that it does not release toxic or greenhouse gases when it is burnt.

Co-author of the study Professor Michael Bowker, from the Cardiff Catalysis Institute, said: "This really is a green source of energy.

"Hydrogen is seen as an important future energy carrier as the world moves from fossil fuels to renewable feedstocks, and our research has shown that even garden grass could be a good way of getting hold of it."

The team, which also includes researchers from Queen's University Belfast, have published their findings in the Royal Society journal Proceedings A.

Hydrogen is contained in enormous quantities all over in the world in water, hydrocarbons and other organic matter.

Up until now, the challenge for researchers has been devising ways of unlocking hydrogen from these sources in a cheap, efficient and sustainable way.

A promising source of hydrogen is the organic compound cellulose, which is a key component of plants and the most abundant biopolymer on Earth.

In their study, the team investigated the possibility of converting cellulose into hydrogen using sunlight and a simple catalyst -- a substance which speeds up a chemical reaction without getting used up.

This process is called photoreforming or photocatalysis and involves the sunlight activating the catalyst which then gets to work on converting cellulose and water into hydrogen.

The researchers studied the effectiveness of three metal-based catalysts -- Palladium, Gold and Nickel.

Nickel was of particular interest to the researchers, from a practical point of view, as it is a much more earth-abundant metal than the precious metals, and is more economical.

In the first round of experiments, the researchers combined the three catalysts with cellulose in a round bottom flask and subjected the mixture to light from a desk lamp. At 30 minutes intervals the researchers collected gas samples from the mixture and analysed it to see how much hydrogen was being produced.

To test the practical applications of this reaction, the researchers repeated the experiment with fescue grass, which was obtained from a domestic garden.

Professor Michael Bowker continued: "Up until recently, the production of hydrogen from cellulose by means of photocatalysis has not been extensively studied.

"Our results show that significant amounts of hydrogen can be produced using this method with the help of a bit of sunlight and a cheap catalyst.

"Furthermore, we've demonstrated the effectiveness of the process using real grass taken from a garden. To the best of our knowledge, this is the first time that this kind of raw biomass has been used to produce hydrogen in this way. This is significant as it avoids the need to separate and purify cellulose from a sample, which can be both arduous and costly."

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The above post is reprinted from materials provided by Cardiff University.Note: Materials may be edited for content and length.

New light harvesting potentials uncovered

Researchers for the first time have found a quantum-confined bandgap narrowing mechanism where UV absorption of the grapheme quantum dots and TiO2 nanoparticles can easily be extended into the visible light range.

Such a mechanism may allow the design of a new class of composite materials for light harvesting and optoelectronics.

Dr Qin Li, Associate Professor in the Environmental Engineering & Queensland Micro- and Nanotechnology Centre, says real life application of this would be high efficiency paintable solar cells and water purification using sun light.

"Wherever there is abundant sun we can brush on this nanomaterial to harvest solar energy to create clean water," she says.

"This mechanism can be extremely significant for light harvesting. What's more important is we've come up with an easy way to achieve that, to make a UV absorbing material to become a visible light absorber by narrowing the bandgap."

Visible light makes up 43 per cent of solar energy compared to only 5 per cent possessed by UV light.

Major efforts have been made to improve titania's absorption of visible light or develop visible-light sensitive materials in general.

Methods used for titania, including metal ion doping, carbon doping, nitrogen doping and hydrogenation usually require stringent conditions to obtain the modified TiO2 such as elevated temperature or high pressure.

In their innovative paper published in Chemical Communications, a Royal Society of Chemistry journal, the researchers observed that when TiO2 particles are mixed with graphene quantum dots, the resulting composite absorbs visible light by a quantum-confined bandgap narrowing mechanism.

"We were really excited to discover this: when two UV absorbing materials, namely TiO2 and graphene quantum dots, were mixed together, they started to absorb in the visible range, more significantly, the bandgap can be tuned by the size of graphene quantum dots," says Dr Li.

"We named the phenomenon 'quantum-confined bandgap narrowing' and this mechanism may be applicable to all semiconductors, when they are linked with graphene quantum dots. Flexible tuning of bandgap is extremely desirable in semiconductor-based devices."

This work has been selected to feature in the front inside cover of Chemical Communications. The team's work on the fluorescence mechanism of graphene quantum dots recently has also been featured in Nanoscale.

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The above post is reprinted from materials provided by Griffith University.Note: Materials may be edited for content and length.

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Journal Reference:

1. Shujun Wang, Ivan S. Cole, Qin Li. Quantum-confined bandgap narrowing of TiO2nanoparticles by graphene quantum dots for visible-light-driven applications. Chem. Commun., 2016; 52 (59): 9208 DOI: 10.1039/C6CC03302D

Happy cows make more nutritious milk

Daily infusions with a chemical commonly associated with feelings of happiness were shown to increase calcium levels in the blood of Holstein cows and the milk of Jersey cows that had just given birth. The results, published in the Journal of Endocrinology, could lead to a better understanding of how to improve the health of dairy cows, and keep the milk flowing.

Demand is high for milk rich in calcium: there is more calcium in the human body than any other mineral, and in the West dairy products such as milk, cheese and yoghurt are primary sources of calcium. But this demand can take its toll on milk-producing cows: roughly 5-10% of the North American dairy cow population suffers from hypocalcaemia -- in which calcium levels are low. The risk of this disease is particularly high immediately before and after cows give birth.

Hypocalcaemia is considered a major health event in the life of a cow. It is associated with immunological and digestive problems, decreased pregnancy rates and longer intervals between pregnancies. These all pose a problem for dairy farmers, whose profitability depends upon regular pregnancies and a high-yield of calcium-rich milk.

Whilst there has been research into the treatment of hypocalcaemia, little research has focused on prevention. In rodents it has been shown that serotonin (a naturally-occurring chemical commonly associated with feelings of happiness) plays a role in maintaining calcium levels; based on this, a team from the University of Wisconsin-Madison, led by Dr Laura Hernandez, investigated the potential for serotonin to increase calcium levels in both the milk and blood of dairy cows. The team infused a chemical that converts to serotonin into 24 dairy cows, in the run up to giving birth. Half the cows were Jersey and half were Holstein -- two of the most common breeds. Calcium levels in both the milk and circulating blood were measured throughout the experiment.

Whilst serotonin improved the overall calcium status in both breeds, this was brought about in opposite ways. Treated Holstein cows had higher levels of calcium in their blood, but lower calcium in their milk (compared to controls). The reverse was true in treated Jersey cows and the higher milk calcium levels were particularly obvious in Jerseys at day 30 of lactation -- suggesting a role for serotonin in maintaining levels throughout lactation.

"By studying two breeds we were able to see that regulation of calcium levels is different between the two," says Laura Hernandez. "Serotonin raised blood calcium in the Holsteins, and milk calcium in the Jerseys. We should also note that serotonin treatment had no effect on milk yield, feed intake or on levels of hormones required for lactation."

The next steps are to investigate the molecular mechanism by which serotonin regulates calcium levels, and how this varies between breeds.

"We would also like to work on the possibility of using serotonin as a preventative measure for hypocalcaemia in dairy cows," continues Laura Hernandez, "That would allow dairy farmers to maintain the profitability of their businesses, whilst making sure their cows stay healthy and produce nutritious milk."

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The above post is reprinted from materials provided by European Society of Endocrinology. Note: Materials may be edited for content and length.

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Journal Reference:

1. Samantha R Weaver, Austin P Prichard, Elizabeth L Endres, Stefanie A Newhouse, Tonia L Peters, Peter M Crump, Matthew S Akins, Thomas D Crenshaw, Rupert M Bruckmaier, Laura L Hernandez. Elevation of circulating serotonin improves calcium dynamics in the peripartum dairy cow. Journal of Endocrinology, 2016; 230 (1): 105 DOI:10.1530/JOE-16-0038

Solar panels study reveals impact on Earth

Researchers have produced the first detailed study of the impact of solar parks on the environment, opening the door to smarter forms of farming and better land management.

Environmental Scientists at Lancaster University and the Centre for Ecology and Hydrology monitored a large solar park, near Swindon, for a year.

They found that solar parks altered the local climate, measuring cooling of as much as 5 degrees Centigrade under the panels during the summer but the effects varied depending on the time of year and the time of day.

As climate controls biological processes, such as plant growth rates, this is really important information and can help understand how best to manage solar parks so they have environmental benefits in addition to supplying low carbon energy.

Their paper 'Solar park microclimate and vegetation management effects on grassland carbon cycling' is published in the Journal Environmental Research Letters.

Increasing energy demands and the drive towards low carbon energy sources have prompted a rapid increase in ground-mounted solar parks across the world.

This means a significant land use change on a global scale and has prompted urgent calls for a detailed understanding of the impacts of solar parks on the fields beneath them.

Dr Alona Armstrong, of Lancaster University, said the new study raises some key questions for the future.

She said: "Solar parks are appearing in our landscapes but we are uncertain how they will affect the local environment."

"This is particularly important as solar parks take up more space per unit of power generated compared with traditional sources. This has implications for ecosystems and the provision of goods, for example crops, and services, such as soil carbon storage. But until this study we didn't understand how solar parks impacted climate and ecosystems."

"With policies in dominant economies supporting solar energy, it is important that we understand the environmental impacts to ensure we get more than just low carbon energy from the land they occupy."

The authors of the study say understanding the climate effects of solar parks will give farmers and land managers the knowledge they need to choose which crops to grow and how best to manage the land; there is potential to maximise biodiversity and improve yields.

Dr Armstrong added: "This understanding becomes even more compelling when applied to areas that are very sunny that may also suffer water shortages. The shade under the panels may allow crops to be grown that can't survive in full sun. Also, water losses may be reduced and water could be collected from the large surfaces of the solar panels and used for crop irrigation."

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The above post is reprinted from materials provided by Lancaster University. Note: Materials may be edited for content and length.

Analysis of ant colonies could improve network algorithms

Ants, it turns out, are extremely good at estimating the concentration of other ants in their vicinity. This ability appears to play a role in several communal activities, particularly in the voting procedure whereby an ant colony selects a new nest.

Biologists have long suspected that ants base their population-density estimates on the frequency with which they -- literally -- bump into other ants while randomly exploring their environments.

That theory gets new support from a theoretical paper that researchers from MIT's Computer Science and Artificial Intelligence Laboratory will present at the Association for Computing Machinery's Symposium on Principles of Distributed Computing conference later this month. The paper shows that observations from random exploration of the environment converge very quickly on an accurate estimate of population density. Indeed, they converge about as quickly as is theoretically possible.

Beyond offering support for biologists' suppositions, this theoretical framework also applies to the analysis of social networks, of collective decision making among robot swarms, and of communication in ad hoc networks, such as networks of low-cost sensors scattered in forbidding environments.

"It's intuitive that if a bunch of people are randomly walking around an area, the number of times they bump into each other will be a surrogate of the population density," says Cameron Musco, an MIT graduate student in electrical engineering and computer science and a co-author on the new paper. "What we're doing is giving a rigorous analysis behind that intuition, and also saying that the estimate is a very good estimate, rather than some coarse estimate. As a function of time, it gets more and more accurate, and it goes nearly as fast as you would expect you could ever do."

Random walks

Musco and his coauthors -- his advisor, NEC Professor of Software Science and Engineering Nancy Lynch, and Hsin-Hao Su, a postdoc in Lynch's group -- characterize an ant's environment as a grid, with some number of other ants scattered randomly across it. The ant of interest -- call it the explorer -- starts at some cell of the grid and, with equal probability, moves to one of the adjacent cells. Then, with equal probability, it moves to one of the cells adjacent to that one, and so on. In statistics, this is referred to as a "random walk." The explorer counts the number of other ants inhabiting every cell it visits.

In their paper, the researchers compare the random walk to random sampling, in which cells are selected from the grid at random and the number of ants counted. The accuracy of both approaches improves with each additional sample, but remarkably, the random walk converges on the true population density virtually as quickly as random sampling does.

That's important because in many practical cases, random sampling isn't an option. Suppose, for instance, that you want to write an algorithm to analyze an online social network -- say, to estimate what fraction of the network self-describes as Republican. There's no publicly available list of the network's members; the only way to explore it is to pick an individual member and start tracing connections.

Similarly, in ad hoc networks, a given device knows only the locations of the devices in its immediate vicinity; it doesn't know the layout of the network as a whole. An algorithm that uses random walks to aggregate information from multiple devices would be much easier to implement than one that has to characterize the network as a whole.

Repeat encounters

The researchers' result is surprising because, at every step of a random walk, the explorer has a significant likelihood of returning to a cell that it has already visited. An estimate derived from random walks thus has a much higher chance of oversampling particular cells than one based on random sampling does.

Initially, Musco says, he and his colleagues assumed that this was a liability that an algorithm for estimating population density would have to overcome. But their attempts to filter out oversampled data seemed to worsen their algorithm's performance rather than improve it. Ultimately, the were able to explain why, theoretically.

"If you're randomly walking around a grid, you're not going to bump into everybody, because you're not going to cross the whole grid," Musco says. "So there's somebody on the far side of the grid that I have pretty much a zero percent chance of bumping into. But while I'll bump into those guys less, I'll bump into local guys more. I need to count all my interactions with the local guys to make up for the fact that there are these faraway guys that I'm never going to bump into. It sort of perfectly balances out. It's really easy to prove that, but it's not very intuitive, so it took us a while to realize this."

Generalizations

The grid that the researchers used to model the ants' environment is just a special instance of a data structure called a graph. A graph consists of nodes, typically represented by circles, and edges, typically represented as line segments connecting nodes. In the grid, each cell is a node, and it shares edges only with those cells immediately adjacent to it.

The researchers' analytic techniques, however, apply to any graph, such as one describing which members of a social network are connected, or which devices in an ad hoc network are within communication range of each other.

If the graph is not very well connected -- if, for instance, it's just a chain of nodes, each connected only to the two nodes adjacent to it -- then oversampling can become a problem. In a chain of, say, 100 nodes, an explorer taking a random walk could get stuck traversing the same five or six nodes over and over again.

But as long as two random walks starting from the same node are likely to branch out in different directions, as is often the case in graphs describing communication networks, random walks remain virtually as good as random sampling.

Moreover, in the new paper, the researchers analyze random walks executed by a single explorer. Pooling observations from many explorers would converge on an accurate estimate more quickly. "If they were robots instead of ants, they could get gains by talking to each other and saying, 'Oh, this is my estimate,'" Musco says.

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The above post is reprinted from materials provided by Massachusetts Institute of Technology. Note: Materials may be edited for content and length.