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Thursday, March 8th, 2018
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| 10:00a |
Scientists gain new visibility into quantum information transfer When we talk about “information technology,” we generally mean the technology part, like computers, networks, and software. But information itself, and its behavior in quantum systems, is a central focus for MIT’s interdisciplinary Quantum Engineering Group (QEG) as it seeks to develop quantum computing and other applications of quantum technology.
A QEG team has provided unprecedented visibility into the spread of information in large quantum mechanical systems, via a novel measurement methodology and metric described in a new article in Physics Review Letters. The team has been able, for the first time, to measure the spread of correlations among quantum spins in fluorapatite crystal, using an adaptation of room-temperature solid-state nuclear magnetic resonance (NMR) techniques.
Researchers increasingly believe that a clearer understanding of information spreading is not only essential to understanding the workings of the quantum realm, where classical laws of physics often do not apply, but could also help engineer the internal “wiring” of quantum computers, sensors, and other devices.
One key quantum phenomenon is nonclassical correlation, or entanglement, in which pairs or groups of particles interact such that their physical properties cannot be described independently, even when the particles are widely separated.
That relationship is central to a rapidly advancing field in physics, quantum information theory. It posits a new thermodynamic perspective in which information and energy are linked — in other words, that information is physical, and that quantum-level sharing of information underlies the universal tendency toward entropy and thermal equilibrium, known in quantum systems as thermalization.
QEG head Paola Cappellaro, the Esther and Harold E. Edgerton Associate Professor of Nuclear Science and Engineering, co-authored the new paper with physics graduate student Ken Xuan Wei and longtime collaborator Chandrasekhar Ramanathan of Dartmouth College.
Cappellaro explains that a primary aim of the research was measuring the quantum-level struggle between two states of matter: thermalization and localization, a state in which information transfer is restricted and the tendency toward higher entropy is somehow resisted through disorder. The QEG team’s work centered on the complex problem of many-body localization (MBL) where the role of spin-spin interactions is critical.
The ability to gather this data experimentally in a lab is a breakthrough, in part because simulation of quantum systems and localization-thermalization transitions is extremely difficult even for today’s most powerful computers. “The size of the problem becomes intractable very quickly, when you have interactions,” says Cappellaro. “You can simulate perhaps 12 spins using brute force but that’s about it — far fewer than the experimental system is capable of exploring.”
NMR techniques can reveal the existence of correlations among spins, as correlated spins rotate faster under applied magnetic fields than isolated spins. However, traditional NMR experiments can only extract partial information about correlations. The QEG researchers combined those techniques with their knowledge of the spin dynamics in their crystal, whose geometry approximately confines the evolution to linear spin chains.
“That approach allowed us to figure out a metric, average correlation length, for how many spins are connected to each other in a chain,” says Cappellaro. “If the correlation is growing, it tells you that interaction is winning against the disorder that’s causing localization. If the correlation length stops growing, disorder is winning and keeping the system in a more quantum localized state.”
In addition to being able to distinguish between different types of localization (such as MBL and the simpler Anderson localization), the method also represents a possible advance toward the ability to control of these systems through the introduction of disorder, which promotes localization, Cappellaro adds. Because MBL preserves information and prevents it from becoming scrambled, it has potential for memory applications.
The research’s focus “addresses a very fundamental question about the foundation of thermodynamics, the question of why systems thermalize and even why the notion of temperature exists at all,” says former MIT postdoc Iman Marvian, who is now an assistant professor in Duke University’s departments of Physics and Electrical and Computer Engineering. “Over the last 10 years or so there’s been mounting evidence, from analytical arguments to numerical simulations, that even though different parts of the system are interacting with each other, in the MBL phase systems don’t thermalize. And it is very exciting that we can now observe this in an actual experiment."
“People have proposed different ways to detect this phase of matter, but they’re difficult to measure in a lab,” Marvian explains. “Paola’s group studied it from a new point of view and introduced quantities that can be measured. I’m really impressed at how they’ve been able to extract useful information about MBL from these NMR experiments. It’s great progress, because it makes it possible to experiment with MBL on a natural crystal.”
The research was able to leverage NMR-related capabilities developed under a previous grant from the US Air Force, says Cappellaro, and some additional funding from the National Science Foundation. Prospects for this research area are promising, she adds. “For a long time, most many-body quantum research was focused on equilibrium properties. Now, because we can do many more experiments and would like to engineer quantum systems, there’s much more interest in dynamics, and new programs devoted to this general area. So hopefully we can get more funding and continue the work.” | | 1:59p |
Study: On Twitter, false news travels faster than true stories A new study by three MIT scholars has found that false news spreads more rapidly on the social network Twitter than real news does — and by a substantial margin.
“We found that falsehood defuses significantly farther, faster, deeper, and more broadly than the truth, in all categories of information, and in many cases by an order of magnitude,” says Sinan Aral, a professor at the MIT Sloan School of Management and co-author of a new paper detailing the findings.
“These findings shed new light on fundamental aspects of our online communication ecosystem,” says Deb Roy, an associate professor of media arts and sciences at the MIT Media Lab and director of the Media Lab’s Laboratory for Social Machines (LSM), who is also a co-author of the study. Roy adds that the researchers were “somewhere between surprised and stunned” at the different trajectories of true and false news on Twitter.
Moreover, the scholars found, the spread of false information is essentially not due to bots that are programmed to disseminate inaccurate stories. Instead, false news speeds faster around Twitter due to people retweeting inaccurate news items.
“When we removed all of the bots in our dataset, [the] differences between the spread of false and true news stood,”says Soroush Vosoughi, a co-author of the new paper and a postdoc at LSM whose PhD research helped give rise to the current study.
The study provides a variety of ways of quantifying this phenomenon: For instance, false news stories are 70 percent more likely to be retweeted than true stories are. It also takes true stories about six times as long to reach 1,500 people as it does for false stories to reach the same number of people. When it comes to Twitter’s “cascades,” or unbroken retweet chains, falsehoods reach a cascade depth of 10 about 20 times faster than facts. And falsehoods are retweeted by unique users more broadly than true statements at every depth of cascade.
The paper, “The Spread of True and False News Online,” is published today in Science.
Why novelty may drive the spread of falsity
The genesis of the study involves the 2013 Boston Marathon bombings and subsequent casualties, which received massive attention on Twitter.
“Twitter became our main source of news,” Vosoughi says. But in the aftermath of the tragic events, he adds, “I realized that … a good chunk of what I was reading on social media was rumors; it was false news.” Subsequently, Vosoughi and Roy — Vosoughi’s graduate advisor at the time — decided to pivot Vosoughi’s PhD focus to develop a model that could predict the veracity of rumors on Twitter.
Subsequently, after consultation with Aral — another of Vosoughi’s graduate advisors, who has studied social networks extensively — the three researchers decided to try the approach used in the new study: objectively identifying news stories as true or false, and charting their Twitter trajectories. Twitter provided support for the research and granted the MIT team full access to its historical archives. Roy served as Twitter’s chief media scientist from 2013 to 2017.
To conduct the study, the researchers tracked roughly 126,000 cascades of news stories spreading on Twitter, which were cumulatively tweeted over 4.5 million times by about 3 million people, from the years 2006 to 2017.
To determine whether stories were true or false, the team used the assessments of six fact-checking organizations (factcheck.org, hoax-slayer.com, politifact.com, snopes.org, truthorfiction.com, and urbanlegends.about.com), and found that their judgments overlapped more than 95 percent of the time.
Of the 126,000 cascades, politics comprised the biggest news category, with about 45,000, followed by urban legends, business, terrorism, science, entertainment, and natural disasters. The spread of false stories was more pronounced for political news than for news in the other categories.
The researchers also settled on the term “false news” as their object of study, as distinct from the now-ubiquitous term “fake news,” which involves multiple broad meanings.
The bottom-line findings produce a basic question: Why do falsehoods spread more quickly than the truth, on Twitter? Aral, Roy, and Vosoughi suggest the answer may reside in human psychology: We like new things.
“False news is more novel, and people are more likely to share novel information,” says Aral, who is the David Austin Professor of Management. And on social networks, people can gain attention by being the first to share previously unknown (but possibly false) information. Thus, as Aral puts it, “people who share novel information are seen as being in the know.”
The MIT scholars examined this “novelty hypothesis” in their research by taking a random subsample of Twitter users who propagated false stories, and analyzing the content of the reactions to those stories.
The result? “We saw a different emotional profile for false news and true news,” Vosoughi says. “People respond to false news more with surprise and disgust,” he notes, whereas true stories produced replies more generally characterized by sadness, anticipation, and trust.
So while the researchers “cannot claim that novelty causes retweets” by itself, as they state in the paper, the surprise people register when they see false news fits with the idea that the novelty of falsehoods may be an important part of their propagation.
Directions for further research
While the three researchers all think the magnitude of the effect they found is highly significant, their views on its civic implications vary slightly. Aral says the result is “very scary” in civic terms, while Roy is a bit more sanguine. But the scholars agree it is important to think about ways to limit the spread of misinformation, and they hope their result will encourage more research on the subject.
On the first count, Aral notes, the recognition that humans, not bots, spread false news more quickly suggests a general approach to the problem.
“Now behavioral interventions become even more important in our fight to stop the spread of false news,” Aral says. “Whereas if it were just bots, we would need a technological solution.”
Vosoughi, for his part, suggests that if some people are deliberately spreading false news while others are doing so unwittingly, then the phenomenon is a two-part problem that may require multiple tactics in response. And Roy says the findings may help create “measurements or indicators that could become benchmarks” for social networks, advertisers, and other parties.
The MIT scholars say it is possible that the same phenomenon occurs on other social media platforms, including Facebook, but they emphasize that careful studies are needed on that and other related questions.
In that vein, Aral says, “science needs to have more support, both from industry and government, in order to do more studies.”
For now, Roy says, even well-meaning Twitter users might reflect on a simple idea: “Think before you retweet.” | | 11:59p |
Evading in-flight lightning strikes Aviation experts estimate that every commercial airplane in the world is struck by lightning at least once per year. Around 90 percent of these strikes are likely triggered by the aircraft itself: In thunderstorm environments, a plane’s electrically conductive exterior can act as a lightning rod, sparking a strike that could potentially damage the plane’s outer structures and compromise its onboard electronics.
To avoid lightning strikes, flights are typically rerouted around stormy regions of the sky. Now, MIT engineers are proposing a new way to reduce a plane’s lightning risk, with an onboard system that would protect a plane by electrically charging it. The proposal may seem counterintuitive, but the team found that if a plane were charged to just the right level, its likelihood of being struck by lighting would be significantly reduced.
The idea stems from the fact that, when a plane flies through an ambient electric field, its external electrical state, normally in balance, shifts. As an external electric field polarizes the aircraft, one end of the plane becomes more positively charged, while the other end swings towards a more negative charge. As the plane becomes increasingly polarized, it can set off a highly conductive flow of plasma, called a positive leader — the preceding stage to a lightning strike.
In such a precarious scenario, the researchers propose temporarily charging a plane to a negative level to dampen the more highly charged positive end, thus preventing that end from reaching a critical level and initiating a lightning strike.
The researchers have shown through modeling that such a method would work, at least conceptually. They report their results in the American Institute of Aeronautics and Astronautics Journal.
The team, which includes Emeritus Professor Manuel Martinez-Sanchez and Assistant Professor Carmen Guerra-Garcia, envisions outfitting a plane with an automated control system consisting of sensors and actuators fitted with small power supplies. The sensors would monitor the surrounding electric field for signs of possible leader formation, in response to which the actuators would emit a current to charge the aircraft in the appropriate direction. The researchers say such charging would require power levels lower than that for a standard lightbulb.
“We’re trying to make the aircraft as invisible to lightning as possible,” says co-author Jaime Peraire, head of MIT’s Department of Aeronautics and Astronautics and the H.N. Slater Professor of Aeronautics and Astronautics. “Aside from this technological solution, we are working on modeling the physics behind the process. This is a field where there was little understanding, and this is really an attempt at creating some understanding of aircraft-triggered lightning strikes, from the ground up.”
The paper’s other co-author is Ngoc Cuong Nguyen, a research scientist in the aeronautics and astronautics department.
Lightning flourishing
To be clear, lightning itself poses very little danger to passengers inside an aircraft, as a plane’s cabin is well-insulated against any external electrical activity. In most cases, passengers may only see a bright flash or hear a loud bang. Nevertheless, an aircraft that has been hit by lightning often requires follow-up inspections and safety checks that may delay its next flight. If there is physical damage to the plane, it may be taken out of service — something the airlines would rather avoid.
What’s more, newer aircraft made partly from nonmetallic composite structures such as carbon fiber may be more vulnerable to lightning-related damage, compared with their older, all-metal counterparts. That’s because charge may accumulate on poorly conducting panels and create potential diffences from panel to panel, which may cause certain regions of a panel to spark. A standard protective measure is to cover the outside of the aircraft with a light metallic mesh.
“Modern aircraft are about 50 percent composites, which changes the picture very significantly,” Guerra-Garcia says. “Lightning-related damage is very different, and repairs are much more costly for composite versus metallic aircraft. This is why research on lightning strikes is flourishing now.”
Following the leader
Guerra-Garcia and her colleagues looked at whether electrically charging an airplane would bring down its risk of lightning strikes — an idea that was initially suggested to them by collaborators at Boeing, the research sponsor.
“They are very eager to reduce the incidence of these things, partly because there are large cost expenses related to lightning protection,” Martinez-Sanchez says.
To see whether the charging idea held up, the MIT team first developed a simple model of an aircraft-triggered lightning strike. As a plane flies through a thunderstorm or other electrically charged environment, the outside of the plane begins to be polarized, forming “leaders,” or channels of highly conductive plasma, flowing from opposite ends of the plane and eventually out toward oppositely charged regions of the atmosphere.
“Imagine two channels of plasma propagating very quickly, and when they reach the cloud and the ground, they form a circuit, and current flows through,” Guerra-Garcia says.
“These leaders carry current, but not very much,” Martinez-Sanchez adds. “But in the worst cases, once they establish a circuit, you can get 100,000 amps, and that is when damage happens.”
The researchers developed a mathematical model to describe the electric field conditions under which leaders would develop, and how they would evolve to trigger a lightning strike. They applied this model to a representative aircraft geometry and looked to see whether changing the aircraft’s potential (charging it negatively) would prevent the leaders from forming and triggering a lightning strike.
Their results show that, averaging over field directions and intensities, the charged scenario required a 50 percent higher ambient electric field to initiate a leader, compared with an uncharged scenario. In other words, by charging a plane to an optimal level, its risk of being struck by lightning would be significantly reduced.
“Numerically, one can see that if you could implement this charge strategy, you would have a significant reduction in the incidents of lightning strikes,” Martinez-Sanchez says. “There’s a big if: Can you implement it? And that’s where we’re working now.”
Graduate student Theodore Mouratidis is performing preliminary experiments in MIT’s Wright Brothers Wind Tunnel, testing the feasibility of charging on a simple, metallic sphere. The researchers also hope to carry out experiments in more realistic environments, for instance by flying drones through a thunderstorm.
To make the charging system practical, Martinez-Sanchez says researchers will have to work to speed up its response time. Based on their modeling, he and his colleagues have found that such a system could charge and protect a plane within fractions of a second, but this will not be enough to protect against some forms of triggered lightning.
“The scenario we can take care of is flying into an area where there are storm clouds, and the storm clouds produce an intensification of the electric field in the atmosphere,” Martinez-Sanchez says. “That can be sensed and measured on board, and we can claim that for such relatively slow-developing events, you can charge a plane and adapt in real time. That is quite feasible.”
This research was sponsored by the Boeing Company. |
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