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Friday, April 10th, 2020

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    12:00a
    First sighting of mysterious Majorana fermion on a common metal

    Physicists at MIT and elsewhere have observed evidence of Majorana fermions — particles that are theorized to also be their own antiparticle — on the surface of a common metal: gold. This is the first sighting of Majorana fermions on a platform that can potentially be scaled up. The results, published in the Proceedings of the National Academy of Sciences, are a major step toward isolating the particles as stable, error-proof qubits for quantum computing.

    In particle physics, fermions are a class of elementary particles that includes electrons, protons, neutrons, and quarks, all of which make up the building blocks of matter. For the most part, these particles are considered Dirac fermions, after the English physicist Paul Dirac, who first predicted that all fermionic fundamental particles should have a counterpart, somewhere in the universe, in the form of an antiparticle — essentially, an identical twin of opposite charge.

    In 1937, the Italian theoretical physicist Ettore Majorana extended Dirac’s theory, predicting that among fermions, there should be some particles, since named Majorana fermions, that are indistinguishable from their antiparticles. Mysteriously, the physicist disappeared during a ferry trip off the Italian coast just a year after making his prediction. Scientists have been looking for Majorana’s enigmatic  particle ever since. It has been suggested, but not proven, that the neutrino may be a Majorana particle. On the other hand, theorists have predicted that Majorana fermions may also exist in solids under special conditions.

    Now the MIT-led team has observed evidence of Majorana fermions in a material system they designed and fabricated, which consists of nanowires of gold grown atop a superconducting material, vanadium, and dotted with small, ferromagnetic “islands” of europium sulfide. When the researchers scanned the surface near the islands,  they saw signature signal spikes near zero energy on the very top surface of gold that, according to theory, should only be generated by pairs of Majorana fermions.

    “Majorana ferminons are these exotic things, that have long been a dream to see, and we now see them in a very simple material — gold,” says Jagadeesh Moodera, a senior research scientist in MIT’s Department of Physics. “We’ve shown they are there, and stable, and easily scalable.”

    “The next push will be to take these objects and make them into qubits, which would be huge progress toward practical quantum computing,” adds co-author Patrick Lee, the William and Emma Rogers Professor of Physics at MIT.

    Lee and Moodera’s coauthors include former MIT postdoc and first author Sujit Manna (currently on the faculty at the Indian Institute of Technology at Delhi), and former MIT postdoc Peng Wei of University of California at Riverside, along with Yingming Xie and Kam Tuen Law of the Hong Kong University of Science and Technology.

    High risk

    If they could be harnessed, Majorana fermions would be ideal as qubits, or individual computational units for quantum computers. The idea is that a qubit would be made of combinations of pairs of Majorana fermions, each of which would be separated from its partner. If noise errors affect one member of the pair, the other should remain unaffected, thereby preserving the integrity of the qubit and enabling it to correctly carry out a computation.

    Scientists have looked for Majorana fermions in semiconductors, the materials used in conventional, transistor-based computing. In their experiments, researchers have combined semiconductors with superconductors — materials through which electrons can travel without resistance. This combination imparts superconductive properties to conventional semiconductors, which physicists believe should induce particles in the semiconductor to split , forming the pair of  Majorana fermions.

    “There are several material platforms where people believe they’ve seen Majorana particles,” Lee says. “The evidence is stronger and stronger, but it’s still not 100 percent proven.”

    What’s more, the semiconductor-based setups to date have been difficult to scale up to produce the thousands or millions of qubits needed for a practical quantum computer, because they require growing very precise crystals of semiconducting material and it is very challenging to turn these into high-quality superconductors.

    About a decade ago, Lee, working with his graduate student Andrew Potter, had an idea: Perhaps physicists might be able to observe Majorana fermions in metal, a material that readily becomes superconductive in proximity with a superconductor. Scientists routinely make metals, including gold, into superconductors. Lee’s idea was to see if gold’s surface state — its very top layer of atoms — could be made to be superconductive. If this could be achieved, then gold could serve as a clean, atomically precise system in which researchers could observe Majorana fermions.

    Lee proposed, based on Moodera’s prior work with ferromagnetic insulators, that if it  were placed atop a superconductive surface state of gold, then researchers should have a good chance of clearly seeing signatures of Majorana fermions.

    “When we first proposed this, I couldn’t convince a lot of experimentalists to try it, because the technology was daunting,” says Lee who eventually partnered with Moodera’s experimental group to to secure crucial funding from the Templeton Foundation to realize the design. “Jagadeesh and Peng really had to reinvent the wheel. It was extremely courageous to jump into this, because it’s really a high-risk, but we think a high-payoff, thing.”

    “Finding Majorana”

    Over the last few years, the researchers have characterized gold’s surface state and proved that it could work as a platform for observing Majorana fermions, after which the group began fabricating the setup that Lee envisioned years ago.

    They first grew a sheet of superconducting vanadium, on top of which they overlaid nanowires of gold layer, measuring about 4 nanometers thick. They tested the conductivity of gold’s very top layer, and found that it did, in fact, become superconductive in proximity with the vanadium. They then deposited over the gold nanowires “islands” of europium sulfide, a ferromagnetic material that is able to provide the needed internal magnetic fields to create the Majorana fermions.

    The team then applied a tiny voltage and used scanning tunneling microscopy, a specialized technique that enabled the researchers to scan the energy spectrum around each island on gold’s surface.

    Moodera and his colleagues then looked for a very specific energy signature that only Majorana fermions should produce, if they exist. In any superconducting material, electrons travel through at certain energy ranges. There is however a desert, or “energy gap” where there should be no electrons. If there is a spike inside this gap, it is very likely a signature of Majorana fermions.

    Looking through their data, the researchers observed spikes inside this energy gap  on opposite ends of several islands along the the direction of the magnetic field, that were clear signatures of pairs of Majorana fermions.

    “We only see this spike on opposite sides of the island, as theory predicted,” Moodera says. “Anywhere else, you don’t see it.”

    “In my talks, I like to say that we are finding Majorana, on an island in a sea of gold,” Lee adds.

    Moodera says the team’s setup, requiring just three layers — gold sandwiched between a ferromagnet and a superconductor — is an “easily achievable, stable system” that should also be economically scalable compared to conventional, semiconductor-based approaches to generate qubits.

    “Seeing a pair of Majorana fermions is an important step toward making a qubit,” Wei says. “The next step is to make a qubit from these particles, and we now have some ideas for how to go about doing this.”

    This research was funded, in part, by the John Templeton Foundation, the U.S.  Office of Naval Research, the National Science Foundation, and the U.S.  Department of Energy.

    1:55p
    Gamma radiation found ineffective in sterilizing N95 masks

    The research described in this article has been published on a preprint server but has not yet been peer-reviewed by scientific or medical experts.

    In mid-March, members of the Department of Nuclear Science and Engineering (NSE) joined forces with colleagues in Boston’s medical community to answer a question of critical importance during the Covid-19 pandemic: Can gamma irradiation sterilize disposable N95 masks without diminishing the masks’ effectiveness?

    This type of personal protective equipment (PPE), which offers protection against infectious particles like coronavirus-laden aerosols, is in desperately short supply worldwide, and medical professionals in Covid-19 hotspots are already rationing the masks. Gamma radiation is commonly used to sterilize hospital foods and equipment surfaces, as well as much of the public’s food supply, and there has been significant interest in determining if it could allow N95 masks to be reused and address the expanding scarcity.

    In a study uploaded on March 28 to medRχiv, the preprint server for health sciences, researchers announced their results: N95 masks subjected to cobalt-60 gamma irradiation for sterilization pass a qualitative fit test but lose a significant degree of filtration efficiency. This form of sterilization compromises the masks’ ability to protect medical providers from Covid-19.

    The study, NSE’s first research effort related to the pandemic, also drew on the expertise of MIT’s Office of Environment, Health, and Safety.

    “One of our students thought gamma irradiation might be a cool solution to a big problem, and I really wanted it to work,” says Michael Short, the Class of ’42 Associate Professor of Nuclear Science and Engineering, one of the study’s coauthors. “But we quickly recognized that the data went against the hypothesis.”

    Team members believe these negative results nevertheless contribute to the larger effort to combat the pandemic. “There has never been a time when negative results are more significant,” notes study lead and co-author Avilash Cramer SM ’18, a fifth-year doctoral candidate in the Harvard-MIT Program in Health Sciences and Technology studying radiation physics. “Publishing as quickly as we can means that others working on the same problem can direct their energies in different directions.”

    Fast-track research

    While they may not have produced the desired outcome, the researchers nevertheless pulled off a study remarkable for its speed and multidisciplinary cooperation — a process inspired and shaped by the immediate threat of the Covid-19 pandemic. “The study took nine days from start to finish,” says Short. “It was the fastest I’ve ever done anything, by orders of magnitude.”

    The dire reality of an N95 shortage in the United States sparked widespread concerns early in March. “It had already hit New York, and was on its way to Massachusetts, and President [L. Rafel] Reif wanted to know if we could do something to masks to permit their reuse,” recounts Short. “We looked into different methods, and noticed the idea of using gamma radiation was popping up in a lot of places.”

    Cramer was losing sleep worrying about his classmates, medical residents at Boston-area hospitals already in the thick of treating Covid-19 patients. “After reading the literature, it was clear there wasn't a lot of good research out there regarding reusing masks,” he says. “The sky was falling in hospitals with equipment shortages everywhere, and while others had shown gamma rays could inactivate viruses, I wanted to demonstrate one way or the other if they damage the masks themselves.”

    N95 masks are manufactured through a variety of proprietary processes using wool, glass particles, and plastics, with 1-2 percent copper and/or zinc. Viewed under a scanning electron microscope, these masks reveal a matrix of fibers with openings of approximately 1 micron. Because the filtering occurs through an electrostatic, rather than mechanical, process, a mask can repel or trap smaller incoming particles. This includes at least 95 percent of airborne particles 0.3 microns or larger in size, such as the airborne droplets that can convey the Covid-19 virus.

    A call for multidisciplinary action

    On March 11, Cramer emailed several contacts in the radiation physics community in search of a gamma irradiation source. Among the group was Short, who has some experience, among many things, in irradiating plastics. Cramer had worked with Short on previous research ventures, and was familiar with NSE from his time serving as a teaching assistant for an NSE class, Radiation Biophysics (22.055), taught by his PhD advisor, Rajiv Gupta, a physician at Massachusetts General Hospital and an associate professor of radiology at Harvard Medical School.

    Short instantly responded to Cramer, offering the campus Cobalt-60 irradiation facility, a source of gamma radiation. “I had an exemption to work on campus and thought, let’s just do it: irradiate and sterilize the masks, then see if they can be used again,” says Short.

    With support and guidance from Gupta, also a study co-author, Cramer paused his doctoral work (on low-cost radiology solutions for rural areas), and began writing up a research protocol and drafting additional researchers.

    The experiment began on Saturday, March 14, and the first results emerged the next Thursday.

    Short gathered the masks from his and a collaborator’s laboratory, keeping a handful for this study before donating the rest (a few hundred) to Beverly Hospital. In Building 6, Short and Mitchell Galanek of MIT Environmental Health and Safety placed the masks into the shielded ring of Cobalt-60, subjecting one group of masks to 10 kilograys (kGy) and another to 50 kGy of gamma radiation (A kilogray is a unit of ionizing radiation). One control group of masks was left unirradiated.

    Short then biked the masks to Brigham and Women’s Hospital. There, resident and study co-author Sherry H. Yu, who had signed onto the study after receiving a single emailed invitation, carried out a series of qualitative fit tests. These tests, designed by the U.S. Occupational Safety and Health Administration, establish whether a mask fits securely to someone’s face and screens out potentially harmful aerosolized particles. Yu’s N95 mask-wearing guinea pig was Short himself.

    “I spent three hours in a back room at the Brigham in the midst of Covid craziness trying to taste a nebulized sugar solution,” says Short. For this test, saccharin vapor is sprayed into a hood and collar assembly fitted over the head of a subject wearing an N95 mask. By moving their face from side to side and reading a passage, the subject simulates facial movements that might displace or detach the mask and render it less effective. If, after all these motions, a subject cannot taste the sweet mist, the N95 passes. All of Short’s gamma irradiated masks passed the qualitative fit test.

    “We thought, Awesome, we’ve done it,” recalls Short. “But colleagues from the Greater Boston biomedical community told us the fit test wasn’t good enough — we needed to assess filter efficiency as well.”

    Flawed filtering

    Fortunately, the right kind of experimental setup existed just next door at MIT — in the laboratory of Ju Li, Battelle Energy Alliance Professor of Nuclear Science and Engineering and professor of materials science and engineering. Li and doctoral student Enze Tian (both study coauthors) signed on to shepherd the next phase of the study, using an apparatus that shoots sodium chloride particles of different sizes into the N95 masks. The device, normally used to test the protective properties of the Li lab’s masks against tiny metal fragments and nanoparticles, revealed the disappointing results.

    “The sterilized masks lost two-thirds of their filtering efficiency, essentially turning N95 into N30 masks,” says Cramer. But why the deterioration?

    “Our hypothesis is that ionizing radiation of whatever kind likely decharges the electrostatic filtration of the mask,” says Gupta. “The mechanical filtration of gauze can trap some particles, but radiation interferes with the electrostatic filter’s ability to repel or capture particles of 0.3 microns.”

    Gupta is nevertheless pleased by the study’s results. “Even with lowered efficiency, these N95 masks are much better than the surgical masks we use,” he says. “Instead of throwing out N95 masks, they could be sterilized and used as N30 masks for the kind of procedures I do all day long.”

    Cramer, who is continuing to explore other N95 mask sterilization methods, believes the study’s results serve a larger purpose: “Adding one more data point to the global understanding of how to clean devices is important — it’s the purest example of the scientific method I’ve ever had the fortune to be part of.”

    “Every piece of our hastily assembled machine worked perfectly,” says Short. “We demonstrated that when a crisis hits, scientists can come together for the greater good and do what needs to happen.”

    1:59p
    Researchers achieve remote control of hormone release

    Abnormal levels of stress hormones such as adrenaline and cortisol are linked to a variety of mental health disorders, including depression and posttraumatic stress disorder (PTSD). MIT researchers have now devised a way to remotely control the release of these hormones from the adrenal gland, using magnetic nanoparticles.

    This approach could help scientists to learn more about how hormone release influences mental health, and could eventually offer a new way to treat hormone-linked disorders, the researchers say.

    “We’re looking how can we study and eventually treat stress disorders by modulating peripheral organ function, rather than doing something highly invasive in the central nervous system,” says Polina Anikeeva, an MIT professor of materials science and engineering and of brain and cognitive sciences.

    To achieve control over hormone release, Dekel Rosenfeld, an MIT-Technion postdoc in Anikeeva’s group, has developed specialized magnetic nanoparticles that can be injected into the adrenal gland. When exposed to a weak magnetic field, the particles heat up slightly, activating heat-responsive channels that trigger hormone release. This technique can be used to stimulate an organ deep in the body with minimal invasiveness.

    Anikeeva and Alik Widge, an assistant professor of psychiatry at the University of Minnesota and a former research fellow at MIT’s Picower Institute for Learning and Memory, are the senior authors of the study. Rosenfeld is the lead author of the paper, which appears today in Science Advances.

    Controlling hormones

    Anikeeva’s lab has previously devised several novel magnetic nanomaterials, including particles that can release drugs at precise times in specific locations in the body.

    In the new study, the research team wanted to explore the idea of treating disorders of the brain by manipulating organs that are outside the central nervous system but influence it through hormone release. One well-known example is the hypothalamic-pituitary-adrenal (HPA) axis, which regulates stress response in mammals. Hormones secreted by the adrenal gland, including cortisol and adrenaline, play important roles in depression, stress, and anxiety.

    “Some disorders that we consider neurological may be treatable from the periphery, if we can learn to modulate those local circuits rather than going back to the global circuits in the central nervous system,” says Anikeeva, who is a member of MIT’s Research Laboratory of Electronics and McGovern Institute for Brain Research.

    As a target to stimulate hormone release, the researchers decided on ion channels that control the flow of calcium into adrenal cells. Those ion channels can be activated by a variety of stimuli, including heat. When calcium flows through the open channels into adrenal cells, the cells begin pumping out hormones. “If we want to modulate the release of those hormones, we need to be able to essentially modulate the influx of calcium into adrenal cells,” Rosenfeld says.

    Unlike previous research in Anikeeva’s group, in this study magnetothermal stimulation was applied to modulate the function of cells without artificially introducing any genes.

    To stimulate these heat-sensitive channels, which naturally occur in adrenal cells, the researchers designed nanoparticles made of magnetite, a type of iron oxide that forms tiny magnetic crystals about 1/5000 the thickness of a human hair. In rats, they found these particles could be injected directly into the adrenal glands and remain there for at least six months. When the rats were exposed to a weak magnetic field — about 50 millitesla, 100 times weaker than the fields used for magnetic resonance imaging (MRI) — the particles heated up by about 6 degrees Celsius, enough to trigger the calcium channels to open without damaging any surrounding tissue.

    The heat-sensitive channel that they targeted, known as TRPV1, is found in many sensory neurons throughout the body, including pain receptors. TRPV1 channels can be activated by capsaicin, the organic compound that gives chili peppers their heat, as well as by temperature. They are found across mammalian species, and belong to a family of many other channels that are also sensitive to heat.

    This stimulation triggered a hormone rush — doubling cortisol production and boosting noradrenaline by about 25 percent. That led to a measurable increase in the animals’ heart rates.

    Treating stress and pain

    The researchers now plan to use this approach to study how hormone release affects PTSD and other disorders, and they say that eventually it could be adapted for treating such disorders. This method would offer a much less invasive alternative to potential treatments that involve implanting a medical device to electrically stimulate hormone release, which is not feasible in organs such as the adrenal glands that are soft and highly vascularized, the researchers say.

    Another area where this strategy could hold promise is in the treatment of pain, because heat-sensitive ion channels are often found in pain receptors.

    “Being able to modulate pain receptors with this technique potentially will allow us to study pain, control pain, and have some clinical applications in the future, which hopefully may offer an alternative to medications or implants for chronic pain,” Anikeeva says. With further investigation of the existence of TRPV1 in other organs, the technique can potentially be extended to other peripheral organs such as the digestive system and the pancreas.

    The research was funded by the U.S. Defense Advance Research Projects Agency ElectRx Program, a Bose Research Grant, the National Institutes of Health BRAIN Initiative, and a MIT-Technion fellowship.

    2:05p
    Safe Paths: A privacy-first approach to contact tracing

    The research described in this article has been published on a preprint server but has not yet been peer-reviewed by scientific or medical experts.

    Fast containment is key to halting the progression of pandemics, and rapid determination of a diagnosed patient’s locations and contact history is a vital step for communities and cities. This process is labor-intensive, susceptible to human memory errors, and fraught with privacy concerns.

    Smartphones can aid in this process, though any type of mass surveillance network and analytics can lead to — or be misused by — a surveillance state.

    Early contact-tracing tools deployed in certain countries against the current Covid-19 pandemic have indeed helped slow the spread, but have done so at the expense of the privacy of citizens and businesses, exposing even the most private details about individuals.

    To help address this urgent challenge, a team led by MIT Media Lab Associate Professor Ramesh Raskar is designing and developing Safe Paths, a citizen-centric, open source, privacy-first set of digital tools and platforms to help stem the spread of Covid-19.  

    The Safe Paths project is a multi-faculty, cross-MIT effort, with input and expertise from institutes including Harvard University, Stanford University, and the State University of New York at Buffalo; clinical input from Mayo Clinic and Massachusetts General Hospital; and mentors from the World Health Organization, the U.S. Department of Health and Human Services, and the Graduate Institute of International and Development Studies.

    A number of leaders and personnel from the global company EY are volunteering their time across many disciplines, including strategy and inclusion on the core initiative leadership team. Numerous additional companies are also participating in this way, including TripleBlind, Public Consulting Group, and Earned Media Consultants.

    Experts from government agencies and academic institutes in Canada, Germany, India, Italy, the United Kingdom, and Vietnam are also helping to guide the platform’s development. 

    The Safe Paths platform, currently in beta, comprises both a smartphone application, PrivateKit, and a web application, Safe Places. The PrivateKit app will enable users to match the personal diary of location data on their smartphone with anonymized, redacted, and blurred location history of infected patients. The digital contact tracing uses overlapped GPS and Bluetooth trails that allow an individual to check if they have crossed paths with someone who was later diagnosed positive for the virus. The PACT Bluetooth protocol, announced earlier by MIT, will be available through Safe Paths. The design of the PACT system has benefited from Safe Paths early work in this area. Through Safe Places, public health officials are equipped to redact location trails of diagnosed carriers and thus broadcast location information with privacy protection for both diagnosed patients and for local businesses.

    The platform takes a fundamentally different approach to app-based epidemic analytics, and in the future will use techniques based on Split Learning, research that Raskar’s Camera Culture group at the Media Lab has been developing for the past several years, and which enables distributed deep learning without the sharing of raw data. Safe Paths uses either on-device calculation or encrypted trail match. The Safe Paths platform provides users information on whether they have experienced a near-contact with a diagnosed individual, while maintaining the privacy of both the user and the diagnosed patient. Users long their location history privately on their own phone and remain in control of their data. Diagnosed patients can opt to provide their location history to health officials (providing similar, yet much more accurate, information to the current healthcare intake interviews).

    Safe Places also provides a secure tool for public health officials to make infected patient contact history much more efficient, and enables anonymized and safe sharing of patient location history. In the future, this data will also be encrypted.  

    In the white paper, "Apps Gone Rogue: Maintaining Personal Privacy in an Epidemic," the research team describes the application of contact tracing to slow the spread of epidemics; outlines the current landscape of interventions; and details challenges and risks to data security and privacy protection. Ongoing and collaborative research designed to further explore critical aspects of contact tracing, and to test increasingly robust privacy protection methodologies. Findings will be continuously shared and published.

    “We are dedicated to privacy-first solutions — user location and contact history should never leave a user’s phone without direct consent,” Raskar says. “We strongly believe that all users should be in control of their own data, and that we should never need to sacrifice consent for Covid-19 safety.”

    Zelalem Temesgen, an infectious disease specialist at Mayo Clinic who has contributed clinical expertise to the project, emphasizes the vital role of contact tracing in stemming an epidemic.

    “In conjunction with rapid diagnosis and isolation of suspected or confirmed cases, contact tracing is a critical intervention for controlling outbreaks of infectious diseases,” Temesgen states. “In the best of times, contact tracing is a laborious and difficult task; it becomes even more challenging in situations where individuals without symptoms are able to transmit infection to others.”

    Temesgen notes that having tools to enhance contact tracing capabilities by more efficiently, and accurately identifying locations where transmission may have occurred will empower public health officials to intervene expeditiously and offer testing to those who need it, and initiate other measures such as isolation and treatment.  

    “In situations like we are facing now, where our knowledge about this new infection is incomplete and continuously evolving, having accurate and comprehensive contact tracing capability can also provide crucial information about how the infection is spread,” he adds.

    According to Ronald Rivest, Institute Professor at MIT and inventor of the RSA public-key cryptosystem, contact tracing has proven to be an important and effective tool in fighting pandemics. “It’s fortunately possible to achieve good contact tracing using smartphones, which can detect the presence of other nearby smartphones,” Rivest notes. “Furthermore, such contact tracing can be done quite simply in a privacy-preserving manner — one doesn’t need to implement ‘big brother’ systems that hand over everyone’s location history to a big database somewhere. I believe that we can see such systems implemented and fielded quickly.”

    MIT Assistant Professor Kevin Esvelt, an evolutionary engineer who specializes in mitigating global catastrophic bio-risks, notes that automated contact tracing becomes increasingly effective as more people adopt it. “Safe Paths uses anonymized GPS, which improves upon traditional contact tracing for everyone using it, as well as Bluetooth, which can only anonymously log an interaction if both people have it. In the long run, it would be best to integrate these capabilities into the operating system of every smartphone as a defense against all pandemics, with each user freely deciding whether or not to share their local data when they learn they’re infected.”  

    “Until that day,” Esvelt adds, “a statewide emergency message with a download link — or prominent placement by the big tech companies — is likely the best we can do.”

    The initial phases of the PrivateKit mobile application and Safe Places web application rollout will emphasize rapid iteration and deployment of solutions for privacy-first, pandemic contact tracing. In the later phases, the goal is the building of encrypted computational methods that can be useful in future types of outbreaks.

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