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  • richardmitnick 12:26 pm on May 24, 2019 Permalink | Reply
    Tags: , , , , Quantum dots, , The Geometry of an Electron Has Been Mapped"   

    From Science Alert: “For The First Time, The Geometry of an Electron Has Been Mapped” 

    ScienceAlert

    From Science Alert

    24 MAY 2019
    MICHELLE STARR

    1
    (Camenzind et al., PRL, 2019)

    If you’ve ever opened a science textbook, you’ve probably seen a picture of an atom, with a cluster of protons and neutrons making up its nucleus, around which whirls a swarm of electrons. But you also probably know that all these particles aren’t shaped like neat little spheres, as usually depicted.

    As far as we know, electrons don’t actually have a ‘shape’ per se – rather, they are either point particles or they are behaving like a wave, which changes shape depending on its energy. Now, for the first time, physicists have revealed the mapping of a single electron in an artificial atom.

    The technique involves the use of quantum dots, tiny semiconducting crystals on nanometre scales. You may have heard of quantum dot display technology, such as QLED televisions, but they’re useful for a lot more than watching Avengers in high definition.

    They are also referred to as artificial atoms because they can basically trap electrons and confine their movement in three dimensions, holding them in place with electric fields. These trapped electrons behave like electrons bound to an atom, and remain in specific locations.

    Using a spectroscope, the researchers were able to determine the energy levels in a quantum dot, observing how they behave in magnetic fields of varying strength and orientation.

    This in turn allowed the team to calculate the shape of an electron’s wave function within the quantum dot, down to scales even smaller than a nanometre.

    “To put it simply, we can use this method to show what an electron looks like for the first time,” said physicist Daniel Loss of the University of Basel.

    But that wasn’t all they did. By tuning the electric field, they were able to change the shape of the electron movement, controlling their spins in a highly targeted and precise manner.

    This has tremendous implications for future research and technology. It could play a role in quantum entanglement research, since successful entanglement requires the wave functions of two electrons to be oriented along the same plane. Being able to control the shape of an electron’s wave function could be vastly beneficial.

    As for technology, the spin rate of an electron is a candidate for use as a qubit, the smallest unit of information in a quantum computer, but only if the spin can be brought under control.

    Since this spin is partially dependent on the geometry of an electron, this is one potential method for achieving that control.

    The research has been published in two papers in Physical Review Letters and Physical Review B.

    See the full article here .


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  • richardmitnick 4:57 pm on April 2, 2019 Permalink | Reply
    Tags: , , Quantum dots, Semiconductor research,   

    From Stanford University: “Stanford researchers measure near-perfect performance in low-cost semiconductors” 

    Stanford University Name
    From Stanford University

    March 15, 2019
    Taylor Kubota

    Stanford researchers redefine what it means for low-cost semiconductors, called quantum dots, to be near-perfect and find that quantum dots meet quality standards set by more expensive alternatives.

    1
    A close-up artist’s rendering of quantum dots emitting light they’ve absorbed. (Image credit: Ella Marushchenko)

    Tiny, easy-to-produce particles, called quantum dots, may soon take the place of more expensive single crystal semiconductors in advanced electronics found in solar panels, camera sensors and medical imaging tools. Although quantum dots have begun to break into the consumer market – in the form of quantum dot TVs – they have been hampered by long-standing uncertainties about their quality. Now, a new measurement technique developed by researchers at Stanford University may finally dissolve those doubts.

    “Traditional semiconductors are single crystals, grown in vacuum under special conditions. These we can make in large numbers, in flask, in a lab and we’ve shown they are as good as the best single crystals,” said David Hanifi, graduate student in chemistry at Stanford and co-lead author of the paper written about this work, published March 15 in Science.

    The researchers focused on how efficiently quantum dots reemit the light they absorb, one telltale measure of semiconductor quality. While previous attempts to figure out quantum dot efficiency hinted at high performance, this is the first measurement method to confidently show they could compete with single crystals.

    This work is the result of a collaboration between the labs of Alberto Salleo, professor of materials science and engineering at Stanford, and Paul Alivisatos, the Samsung Distinguished Professor of Nanoscience and Nanotechnology at the University of California, Berkeley, who is a pioneer in quantum dot research and co-senior author of the paper. Alivisatos emphasized how the measurement technique could lead to the development of new technologies and materials that require knowing the efficiency of our semiconductors to a painstaking degree.

    “These materials are so efficient that existing measurements were not capable of quantifying just how good they are. This is a giant leap forward,” said Alivisatos. “It may someday enable applications that require materials with luminescence efficiency well above 99 percent, most of which haven’t been invented yet.”

    Between 99 and 100

    Being able to forego the need for pricey fabrication equipment isn’t the only advantage of quantum dots. Even prior to this work, there were signs that quantum dots could approach or surpass the performance of some of the best crystals. They are also highly customizable. Changing their size changes the wavelength of light they emit, a useful feature for color-based applications such as tagging biological samples, TVs or computer monitors.

    Despite these positive qualities, the small size of quantum dots means that it may take billions of them to do the work of one large, perfect single crystal. Making so many of these quantum dots means more chances for something to grow incorrectly, more chances for a defect that can hamper performance. Techniques that measure the quality of other semiconductors previously suggested quantum dots emit over 99 percent of the light they absorb but that was not enough to answer questions about their potential for defects. To do this, the researchers needed a measurement technique better suited to precisely evaluating these particles.

    “We want to measure emission efficiencies in the realm of 99.9 to 99.999 percent because, if semiconductors are able to reemit as light every photon they absorb, you can do really fun science and make devices that haven’t existed before,” said Hanifi.

    The researchers’ technique involved checking for excess heat produced by energized quantum dots, rather than only assessing light emission because excess heat is a signature of inefficient emission. This technique, commonly used for other materials, had never been applied to measure quantum dots in this way and it was 100 times more precise than what others have used in the past. They found that groups of quantum dots reliably emitted about 99.6 percent of the light they absorbed (with a potential error of 0.2 percent in either direction), which is comparable to the best single-crystal emissions.

    “It was surprising that a film with many potential defects is as good as the most perfect semiconductor you can make,” said Salleo, who is co-senior author of the paper.

    Contrary to concerns, the results suggest that the quantum dots are strikingly defect-tolerant. The measurement technique is also the first to firmly resolve how different quantum dot structures compare to each other – quantum dots with precisely eight atomic layers of a special coating material emitted light the fastest, an indicator of superior quality. The shape of those dots should guide the design for new light-emitting materials, said Alivisatos.

    Entirely new technologies

    This research is part of a collection of projects within a Department of Energy-funded Energy Frontier Research Center, called Photonics at Thermodynamic Limits. Led by Jennifer Dionne, associate professor of materials science and engineering at Stanford, the center’s goal is to create optical materials – materials that affect the flow of light – with the highest possible efficiencies.

    A next step in this project is developing even more precise measurements. If the researchers can determine that these materials reach efficiencies at or above 99.999 percent, that opens up the possibility for technologies we’ve never seen before. These could include new glowing dyes to enhance our ability to look at biology at the atomic scale, luminescent cooling and luminescent solar concentrators, which allow a relatively small set of solar cells to take in energy from a large area of solar radiation. All this being said, the measurements they’ve already established are a milestone of their own, likely to encourage a more immediate boost in quantum dot research and applications.

    “People working on these quantum dot materials have thought for more than a decade that dots could be as efficient as single crystal materials,” said Hanifi,” and now we finally have proof.”

    See the full article here .


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    Stanford University campus. No image credit

    Stanford University

    Leland and Jane Stanford founded the University to “promote the public welfare by exercising an influence on behalf of humanity and civilization.” Stanford opened its doors in 1891, and more than a century later, it remains dedicated to finding solutions to the great challenges of the day and to preparing our students for leadership in today’s complex world. Stanford, is an American private research university located in Stanford, California on an 8,180-acre (3,310 ha) campus near Palo Alto. Since 1952, more than 54 Stanford faculty, staff, and alumni have won the Nobel Prize, including 19 current faculty members

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  • richardmitnick 3:16 pm on February 13, 2019 Permalink | Reply
    Tags: “Pump” light, , Optics and photonics research, Quantum dot lasers, Quantum dots,   

    From University of Utah: “New phenomenon discovered that fixes a common problem in lasers: Wavelength splitting” 

    From University of Utah

    Feb 7, 2019
    Lisa PotterScience writer,
    University Marketing and Communications
    Office: 801-585-3093
    Mobile: 949-533-7899
    lisa.potter@utah.edu

    A team led by University of Utah physicists has discovered how to fix a major problem that occurs in lasers made from a new type of material called quantum dots. The never-before-seen phenomenon will be important for an emerging field of photonics research, including one day making micro-chips that code information using light instead of electrons.

    Laser wave length spliting U Utah

    The study published on Feb. 4, 2019, in the journal Nature Communications.

    Lasers are devices that amplify light, often producing a single, narrow beam of light. The strength of the beam depends on the material with which the laser was built; light passes through the material, which produces a beam made of light waves all with similar wavelengths, concentrating a lot of energy into a small area. This material property to be able to amplify the beam’s energy is called “gain.”

    Many scientists are building lasers with quantum dots. Quantum dots are tiny crystals of semiconductor materials grown to sizes of only about 100-atoms across. The size of the crystals determines the light beam’s wavelength, from blue light to red light and even into the infrared.

    People are interested in quantum dot lasers because they can tune properties simply by growing the crystals in different sizes by using different semiconducting materials and choosing different shapes and sizes of the lasers. The downside is that quantum dot lasers often contain miniscule defects that split the light into multiple wavelengths, which distributes the beam’s energy and makes it less powerful. Ideally, you want the laser to concentrate the power into one wavelength.

    The new study sought to correct this defect. First, collaborators from the Georgia Institute of Technology made 50 microscopic disk-shaped quantum dot lasers out of cadmium selenide. The U team then showed that that almost all of the individual lasers had defects that split the wavelengths of beams.

    The researchers then coupled two lasers together to correct the wavelength splitting. They put one laser at full gain, which describes the maximum amount of energy possible. To achieve full gain, the scientists shined a green light, called the pump light, onto the first laser. The quantum dot material absorbed the light and re-emitted a more powerful beam of red light. The stronger the green light they shined on the laser, the higher the gain in energy. When the second laser had no gain, the difference between the two lasers prevented any interaction, and splitting still occurred. However, when the team shined a green light onto the second laser, its gain increased, closing the gain difference between the two lasers. Once the gain in the two lasers became similar the interaction between the two lasers corrected the splitting and focused the energy into a single wavelength. This is the first time anyone has observed this phenomenon.

    The findings have implications for a new field, called optics and photonics research. In the past 30 years, researchers have been experimenting with using light to carry information, rather than electrons used in traditional electronics. For example, rather than putting lots of electrons on a microchip to make a computer run, some envision using light instead. Lasers would be a big part of that and the to correct wavelength splitting can provide a significant benefit to controlling information through light. It could also be a major advantage to use materials such as quantum dots in this field.

    “It’s not impossible that someone could make a defect-free laser with quantum dots, but it would be expensive and time-consuming. In comparison, coupling is a quicker, more flexible, cost-effective way to correct the problem,” said Evan Lafalce, research assistant professor of physics and astronomy at the U and lead author of the study. “This is a trick so that we don’t have to make perfect quantum dot lasers.”

    Authors who contributed to the study include Qingji Zeng and Valy Z. Vardeny from the Department of Physics & Astronomy at the University of Utah and Chun Hao Lin, Marcus J. Smith, Sidney T. Malak, Jaehan Jung, Young Jun Yoon, Zhiqun Lin and Vladmir V. Tsukruk from the School of Materials Science and Engineering at the Georgia Institute of Technology. Smith also holds a position at the Air Force Research Laboratory at Wright-Patterson Air Force Base and Jung holds a position at Hongik University.

    See the full article here .

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    The University of Utah (also referred to as the U, the U of U, or Utah) is a public coeducational space-grant research university in Salt Lake City, Utah, United States. As the state’s flagship university, the university offers more than 100 undergraduate majors and more than 92 graduate degree programs. The university is classified in the highest ranking: “R-1: Doctoral Universities – Highest Research Activity” by the Carnegie Classification of Institutions of Higher Education. The Carnegie Classification also considers the university as “selective”, which is its second most selective admissions category. Graduate studies include the S.J. Quinney College of Law and the School of Medicine, Utah’s only medical school. As of Fall 2015, there are 23,909 undergraduate students and 7,764 graduate students, for an enrollment total of 31,673.

    The university was established in 1850 as the University of Deseret (Listeni/dɛz.əˈrɛt./[12]) by the General Assembly of the provisional State of Deseret, making it Utah’s oldest institution of higher education.It received its current name in 1892, four years before Utah attained statehood, and moved to its current location in 1900.

    The university ranks among the top 50 U.S. universities by total research expenditures with over $486 million spent in 2014. 22 Rhodes Scholars,[14] three Nobel Prize winners, two Turing Award winners, three MacArthur Fellows, various Pulitzer Prize winners, two astronauts, Gates Cambridge Scholars, and Churchill Scholars have been affiliated with the university as students, researchers, or faculty members in its history. In addition, the university’s Honors College has been reviewed among 50 leading national Honors Colleges in the U.S. The university has also been ranked the 12th most ideologically diverse university in the country.

     
  • richardmitnick 3:23 pm on August 3, 2018 Permalink | Reply
    Tags: Boson sampling, Complexity test offers new perspective on small quantum computers, , Quantum dots, Sampling complexity   

    From Joint Quantum Institute: “Complexity test offers new perspective on small quantum computers” 

    JQI bloc

    From Joint Quantum Institute

    August 2, 2018
    Chris Cesare

    1
    Simulating the behavior of quantum particles hopping around on a grid may be one of the first problems tackled by early quantum computers. (Credit: E. Edwards/JQI)

    State-of-the-art quantum devices are not yet large enough to be called full-scale computers. The biggest comprise just a few dozen qubits—a meager count compared to the billions of bits in an ordinary computer’s memory. But steady progress means that these machines now routinely string together 10 or 20 qubits and may soon hold sway over 100 or more.

    In the meantime, researchers are busy dreaming up uses for small quantum computers and mapping out the landscape of problems they’ll be suited to solving. A paper by researchers from the Joint Quantum Institute (JQI) and the Joint Center for Quantum Information and Computer Science (QuICS), published recently in Physical Review Letters, argues that a novel non-quantum perspective may help sketch the boundaries of this landscape (link is external) and potentially even reveal new physics in future experiments.

    The new perspective involves a mathematical tool—a standard measure of computational difficulty known as sampling complexity—that gauges how easy or hard it is for an ordinary computer to simulate the outcome of a quantum experiment. Because the predictions of quantum physics are probabilistic, a single experiment could never verify that these predictions are accurate. You would need to perform many experiments, just like you would need to flip a coin many times to convince yourself that you’re holding an everyday, unbiased nickel.

    If an ordinary computer takes a reasonable amount of time to mimic one run of a quantum experiment—by producing samples with approximately the same probabilities as the real thing—the sampling complexity is low; if it takes a long time, the sampling complexity is high.

    Few expect that quantum computers wielding lots of qubits will have low sampling complexity—after all, quantum computers are expected to be more powerful than ordinary computers, so simulating them on your laptop should be hard. But while the power of quantum computers remains unproven, exploring the crossover from low complexity to high complexity could offer fresh insights about the capabilities of early quantum devices, says Alexey Gorshkov, a JQI and QuICS Fellow who is a co-author of the new paper.

    “Sampling complexity has remained an underappreciated tool,” Gorshkov says, largely because small quantum devices have only recently become reliable. “These devices are now essentially doing quantum sampling, and simulating this is at the heart of our entire field.”

    To demonstrate the utility of this approach, Gorshkov and several collaborators proved that sampling complexity tracks the easy-to-hard transition of a task that small- and medium-sized quantum computers are expected to perform faster than ordinary computers: boson sampling.

    Bosons are one of the two families of fundamental particles (the other being fermions). In general two bosons can interact with one another, but that’s not the case for the boson sampling problem. “Even though they are non-interacting in this problem, bosons are sort of just interesting enough to make boson sampling worth studying,” says Abhinav Deshpande, a graduate student at JQI and QuICS and the lead author of the paper.

    In the boson sampling problem, a fixed number of identical particles are allowed to hop around on a grid, spreading out into quantum superpositions over many grid sites. Solving the problem means sampling from this smeared-out quantum probability cloud, something a quantum computer would have no trouble doing.

    Deshpande, Gorshkov and their colleagues proved that there is a sharp transition between how easy and hard it is to simulate boson sampling on an ordinary computer. If you start with a few well-separated bosons and only let them hop around briefly, the sampling complexity remains low and the problem is easy to simulate. But if you wait longer, an ordinary computer has no chance of capturing the quantum behavior, and the problem becomes hard to simulate.

    The result is intuitive, Deshpande says, since at short times the bosons are still relatively close to their starting positions and not much of their “quantumness” has emerged. For longer times, though, there’s an explosion of possibilities for where any given boson can end up. And because it’s impossible to tell two identical bosons apart from one another, the longer you let them hop around, the more likely they are to quietly swap places and further complicate the quantum probabilities. In this way, the dramatic shift in the sampling complexity is related to a change in the physics: Things don’t get too hard until bosons hop far enough to switch places.

    Gorshkov says that looking for changes like this in sampling complexity may help uncover physical transitions in other quantum tasks or experiments. Conversely, a lack of ramping up in complexity may rule out a quantum advantage for devices that are too error-prone. Either way, Gorshkov says, future results arising from this perspective shift should be interesting. “A deeper look into the use of sampling complexity theory from computer science to study quantum many-body physics is bound to teach us something new and exciting about both fields,” he says.

    See the full article here .

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    We are on the verge of a new technological revolution as the strange and unique properties of quantum physics become relevant and exploitable in the context of information science and technology.

    The Joint Quantum Institute (JQI) is pursuing that goal through the work of leading quantum scientists from the Department of Physics of the University of Maryland (UMD), the National Institute of Standards and Technology (NIST) and the Laboratory for Physical Sciences (LPS). Each institution brings to JQI major experimental and theoretical research programs that are dedicated to the goals of controlling and exploiting quantum systems.

     
  • richardmitnick 10:30 am on January 11, 2018 Permalink | Reply
    Tags: , , , Extremely bright and fast light emission, , , Quantum dots   

    From ETH: “Extremely bright and fast light emission” 

    ETH Zürich bloc

    ETH Zürich

    11.01.2018
    Fabio Bergamin

    A type of quantum dot that has been intensively studied in recent years can reproduce light in every colour and is very bright. An international research team that includes scientists from ETH Zürich has now discovered why this is the case. The quantum dots could someday be used in light-emitting diodes.

    1
    A caesium lead bromide nanocrystal under the electron microscope (crystal width: 14 nanometres). Individual atoms are visible as points. (Photograph: ETH Zürich / Empa / Maksym Kovalenko)

    An international team of researchers from ETH Zürich, IBM Research Zürich, Empa and four American research institutions have found the explanation for why a class of nanocrystals that has been intensively studied in recent years shines in such incredibly bright colours. The nanocrystals contain caesium lead halide compounds that are arranged in a perovskite lattice structure.

    Three years ago, Maksym Kovalenko, a professor at ETH Zürich and Empa, succeeded in creating nanocrystals – or quantum dots, as they are also known – from this semiconductor material. “These tiny crystals have proved to be extremely bright and fast emitting light sources, brighter and faster than any other type of quantum dot studied so far,” says Kovalenko. By varying the composition of the chemical elements and the size of the nanoparticles, he also succeeded in producing a variety of nanocrystals that light up in the colours of the whole visible spectrum. These quantum dots are thus also being treated as components for future light-emitting diodes and displays.

    In a study published in the most recent edition of the scientific journal Nature, the international research team examined these nanocrystals individually and in great detail. The scientists were able to confirm that the nanocrystals emit light extremely quickly. Previously-studied quantum dots typically emit light around 20 nanoseconds after being excited when at room temperature, which is already very quick. “However, caesium lead halide quantum dots emit light at room temperature after just one nanosecond,” explains Michael Becker, first author of the study. He is a doctoral student at ETH Zürich and is carrying out his doctoral project at IBM Research.

    2
    A sample with several green glowing perovskite quantum dots excited by a blue laser. (Photograph: IBM Research / Thilo Stöferle)

    Electron-hole pair in an excited energy state

    Understanding why caesium lead halide quantum dots are not only fast but also very bright entails diving into the world of individual atoms, light particles (photons) and electrons. “You can use a photon to excite semiconductor nanocrystals so that an electron leaves its original place in the crystal lattice, leaving behind a hole,” explains David Norris, Professor of Materials Engineering at ETH Zürich. The result is an electron-hole pair in an excited energy state. If the electron-hole pair reverts to its energy ground state, light is emitted.

    Under certain conditions, different excited energy states are possible; in many materials, the most likely of these states is called a dark one. “In such a dark state, the electron hole pair cannot revert to its energy ground state immediately and therefore the light emission is suppressed and occurs delayed. This limits the brightness”, says Rainer Mahrt, a scientist at IBM Research.

    No dark state

    The researchers were able to show that the caesium lead halide quantum dots differ from other quantum dots: their most likely excited energy state is not a dark state. Excited electron-hole pairs are much more likely to find themselves in a state in which they can emit light immediately. “This is the reason that they shine so brightly,” says Norris.

    The researchers came to this conclusion using their new experimental data and with the help of theoretical work led by Alexander Efros, a theoretical physicist at the Naval Research Laboratory in Washington. He is a pioneer in quantum dot research and, 35 years ago, was among the first scientists to explain how traditional semiconductor quantum dots function.

    Great news for data transmission

    As the examined caesium lead halide quantum dots are not only bright but also inexpensive to produce they could be applied in television displays, with efforts being undertaken by several companies, in Switzerland and world-wide. “Also, as these quantum dots can rapidly emit photons, they are of particular interest for use in optical communication within data centres and supercomputers, where fast, small and efficient components are central,” says Mahrt. Another future application could be the optical simulation of quantum systems which is of great importance to fundamental research and materials science.

    ETH professor Norris is also interested in using the new knowledge for the development of new materials. “As we now understand why these quantum dots are so bright, we can also think about engineering other materials with similar or even better properties,” he says.

    Science team:
    Becker MA, Vaxenburg R, Nedelcu G, Sercel PC, Shabaev A, Mehl MJ, Michopoulos JG, Lambrakos SG, Bernstein N, Lyons JL, Stöferle T, Mahrt RF, Kovalenko MV, Norris DJ, Rainò G, Efros AL.

    See the full article here .

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    ETH Zürich campus
    ETH Zürich is one of the leading international universities for technology and the natural sciences. It is well known for its excellent education, ground-breaking fundamental research and for implementing its results directly into practice.

    Founded in 1855, ETH Zürich today has more than 18,500 students from over 110 countries, including 4,000 doctoral students. To researchers, it offers an inspiring working environment, to students, a comprehensive education.

    Twenty-one Nobel Laureates have studied, taught or conducted research at ETH Zürich, underlining the excellent reputation of the university.

     
  • richardmitnick 4:45 pm on November 7, 2017 Permalink | Reply
    Tags: , , , Quantum dots, Quantum photonic circuits, Waveguides   

    From NIST: “Hybrid Circuit Combines Single-Photon Generator and Efficient Waveguides on One Chip” 


    NIST

    November 07, 2017

    Ben Stein
    benjamin.stein@nist.gov
    (301) 975-2763

    New architecture could prove essential for high-performance quantum photonic circuits.

    1
    The architecture of this hybrid quantum photonic circuit is among the first to combine on a single chip a reliable generator of individual photons—a quantum dot (red dot), here embedded in gallium arsenide (yellow)—with passive elements such as a low-loss waveguide (purple) that transports the photons. Credit: NIST

    Scientists at the National Institute of Standards and Technology (NIST) and their collaborators have taken a new step forward in the quest to build quantum photonic circuits—chip-based devices that rely on the quantum properties of light to process and communicate information rapidly and securely.

    The quantum circuit architecture devised by the team is among the first to combine two different types of optical devices, made from different materials, on a single chip—a semiconductor source that efficiently generates single particles of light (photons) on demand, and a network of “waveguides” that transports those photons across the circuit with low loss. Maximizing the number of photons, ideally having identical properties, is critical to enabling applications such as secure communication, precision measurement, sensing and computation, with potentially greater performance than that of existing technologies.

    The architecture, developed by Marcelo Davanco and other NIST researchers along with collaborators from China and the U.K., employs a nanometer-scale semiconductor structure called a quantum dot—made from indium arsenide—to generate individual photons on the same chip as the optical waveguides—made from silicon nitride. Combining these two materials requires special processing techniques. Such hybrid circuit architectures could become building blocks for more complex systems.

    Previously, quantum integrated photonic circuits typically consisted of only passive devices such as waveguides and beam splitters, which let photons through or allowed them to coalesce. The photons themselves still had to be produced outside the chip, and getting them onto the chip resulted in losses, which significantly degraded the performance of the circuit. Circuit architectures that did include quantum light generation on a chip either incorporated sources that only produced photons randomly and at low rates—which limits performance—or had sources in which one photon was not necessarily identical with the next. In addition, the fabrication processes supporting these previous architectures made it difficult to scale up the number, size and complexity of the photonic circuits.

    In contrast, the new architecture and the fabrication processes the team developed should enable researchers to reliably build larger circuits, which could perform more complex computations or simulations and translate into higher measurement precision and detection sensitivity in other applications.

    The quantum dot employed by the team is a well-studied nanometer-scale structure: an island of the semiconductor indium arsenide surrounded by gallium arsenide. The indium arsenide/gallium arsenide nanostructure acts as a quantum system with two energy levels—a ground state (lower energy level) and an excited state (higher energy level). When an electron in the excited state loses energy by dropping down to the ground state, it emits a single photon.

    Unlike most types of two-level emitters that exist in the solid state, these quantum dots have been shown to generate—reliably, on demand, and at large rates—the single photons needed for quantum applications. In addition, researchers have been able to place them inside nanoscale, light-confining spaces that allow a large speedup of the single-photon emission rate, and in principle, could also allow the quantum dot to be excited by a single photon. This enables the quantum dots to directly assist with the processing of information rather than simply produce streams of photons.

    The other part of the team’s hybrid circuit architecture consists of passive waveguides made of silicon nitride, known for their ability to transmit photons across a chip’s surface with very low photon loss. This allows quantum-dot-generated photons to efficiently coalesce with other photons at a beam splitter, or interact with other circuit elements such as modulators and detectors.

    “We’re getting the best of both worlds, with each behaving really well together on a single circuit,” said Davanco. In fact, the hybrid architecture keeps the high performance achieved in devices made exclusively of each of the two materials, with little degradation when they are put together. He and his colleagues described the work (link is external) in a recent issue of Nature Communications.

    To make the hybrid devices, Davanco and his colleagues first bonded two wafers together—one containing the quantum dots, the other containing the silicon nitride waveguide material. They used a variation of a process that had originally been developed for making hybrid photonic lasers, which combined silicon for waveguides and compound semiconductors for classical light emission. Once the bonding was finished, the two materials were then sculpted with nanometer-scale resolution into their final geometries through state-of-the-art semiconductor device patterning and etching techniques.

    Although this wafer bonding technique was developed more than a decade ago by other researchers, the team is the first to apply it towards making integrated quantum photonic devices.

    “Since we have expertise in both fabrication and quantum photonics, it seemed clear that we could borrow and adapt this process to create this new architecture,” notes Davanco.

    This work was performed in part at NIST’s Center for Nanoscale Science and Technology (CNST), a shared-use facility available to researchers from industry, academia and government, and also included researchers from NIST’s Physical Measurement Laboratory.

    Science paper:
    Heterogeneous integration for on-chip quantum photonic circuits with single quantum dot devices. Nature Communications.

    See the full article here.

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    NIST Campus, Gaitherberg, MD, USA

    NIST Mission, Vision, Core Competencies, and Core Values

    NIST’s mission

    To promote U.S. innovation and industrial competitiveness by advancing measurement science, standards, and technology in ways that enhance economic security and improve our quality of life.
    NIST’s vision

    NIST will be the world’s leader in creating critical measurement solutions and promoting equitable standards. Our efforts stimulate innovation, foster industrial competitiveness, and improve the quality of life.
    NIST’s core competencies

    Measurement science
    Rigorous traceability
    Development and use of standards

    NIST’s core values

    NIST is an organization with strong values, reflected both in our history and our current work. NIST leadership and staff will uphold these values to ensure a high performing environment that is safe and respectful of all.

    Perseverance: We take the long view, planning the future with scientific knowledge and imagination to ensure continued impact and relevance for our stakeholders.
    Integrity: We are ethical, honest, independent, and provide an objective perspective.
    Inclusivity: We work collaboratively to harness the diversity of people and ideas, both inside and outside of NIST, to attain the best solutions to multidisciplinary challenges.
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  • richardmitnick 1:45 pm on October 10, 2017 Permalink | Reply
    Tags: , Antibiotics spiked with quantum dots fought off bacteria as effectively as 1000 times as much antibiotic alone, , , , Quantum dots, , Various superbugs are evolving too rapidly to be counteracted by traditional drugs   

    From ScienceNews: “Superbugs may meet their match in these nanoparticles” 

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    Science News

    October 9, 2017
    Maria Temming

    ‘Quantum dots’ mess with bacteria’s defenses, allowing antibiotics to work.

    1
    ARMED AND DANGEROUS By producing a chemical that makes bacteria more vulnerable to antibiotic attack, quantum dots could help reboot medications that have lost their edge against hard-to-kill microbes. Kateryna Kon/Shutterstock

    Antibiotics may have a new teammate in the fight against drug-resistant infections.

    Researchers have engineered nanoparticles to produce chemicals that render bacteria more vulnerable to antibiotics. These quantum dots, described online October 4 in Science Advances, could help combat pathogens that have developed resistance to antibiotics (SN: 10/15/16, p. 11).

    “Various superbugs are evolving too rapidly to be counteracted by traditional drugs,” says Zhengtao Deng, a chemist at Nanjing University in China not involved in the research. “Drug resistant infections will kill an extra 10 million people a year worldwide by 2050 unless action is taken.”

    In the study, antibiotics spiked with quantum dots fought off bacteria as effectively as 1,000 times as much antibiotic alone. That’s “really impressive,” says Chao Zhong, a materials scientist at ShanghaiTech University who was not involved in the study. “This is a really comprehensive study.”

    Quantum dots, previously investigated as a tool to trace drug delivery throughout the body or to take snapshots of cells, are made of semiconductors — the same kind of material in such electronics as laptops and cellphones (SN: 7/11/15, p. 22). The new quantum dots are tiny chunks of cadmium telluride just 3 nanometers across, or about as wide as a strand of DNA.

    When illuminated by a specific frequency of green light, the nanoparticles’ electrons can pop off and glom onto nearby oxygen molecules — which are dissolved in water throughout the body — to create a chemical called superoxide. When a bacterial cell absorbs this superoxide, it throws the microbe’s internal chemistry so off-balance that the pathogen can’t defend itself against antibiotics, explains study coauthor Anushree Chatterjee, a chemical engineer at the University of Colorado Boulder.

    Chatterjee and colleagues mixed various amounts of quantum dots into different concentrations of each of five antibiotics, and then added these concoctions to samples of five drug-resistant bacterial strains, such as Salmonella and methicillin-resistant Staphylococcus aureus, or MRSA. In more than 75 percent of 480 tests of different antibiotic combinations on different bacteria, the researchers found that lower doses of antibiotics were required to kill or curb the growth of bacteria when the medicine was combined with quantum dots.

    One limitation of this treatment is that the green light that activates the nanoparticles can shine through only a few millimeters of flesh, says coauthor Prashant Nagpal, a chemical engineer also at the University of Colorado Boulder. So these quantum dots could probably be used only to treat skin or accessible wound infections.

    The researchers are now designing nanoparticles that absorb infrared light, which can pass through the body. “That could be really effective in deep tissue and bone infections,” Nagpal says.

    See the full article here .

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  • richardmitnick 10:26 am on September 28, 2017 Permalink | Reply
    Tags: 2-D superlattice, , , Band Gaps Made to Order, Could usher a new generation of light-emitting devices for photonics applications, Each quantum dot acts as a quantum well where electron-hole activity occurs, In the quantum realm precision is even more important, , , Quantum dots, The quantum dot is theoretically an artificial “atom.”,   

    From UCSB: “Band Gaps, Made to Order” 

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    UC Santa Barbara

    September 25, 2017
    James Badham

    1
    This artist’s representation shows an electron beam (in purple) being used to create a 2D superlattice made up of quantum dots having extraordinary atomic-scale precision and placement.
    Photo Credit: PETER ALLEN

    Control is a constant challenge for materials scientists, who are always seeking the perfect material — and the perfect way of treating it — to induce exactly the right electronic or optical activity required for a given application.

    One key challenge to modulating activity in a semiconductor is controlling its band gap. When a material is excited with energy, say, a light pulse, the wider its band gap, the shorter the wavelength of the light it emits. The narrower the band gap, the longer the wavelength.

    As electronics and the devices that incorporate them — smartphones, laptops and the like — have become smaller and smaller, the semiconductor transistors that power them have shrunk to the point of being not much larger than an atom. They can’t get much smaller. To overcome this limitation, researchers are seeking ways to harness the unique characteristics of nanoscale atomic cluster arrays — known as quantum dot superlattices — for building next generation electronics such as large-scale quantum information systems. In the quantum realm, precision is even more important.

    New research conducted by UC Santa Barbara’s Department of Electrical and Computer Engineering reveals a major advance in precision superlattices materials. The findings by Professor Kaustav Banerjee, his Ph.D. students Xuejun Xie, Jiahao Kang and Wei Cao, postdoctoral fellow Jae Hwan Chu and collaborators at Rice University appear in the journal Nature Scientific Reports.

    Their team’s research uses a focused electron beam to fabricate a large-scale quantum dot superlattice on which each quantum dot has a specific pre-determined size positioned at a precise location on an atomically thin sheet of two-dimensional (2-D) semiconductor molybdenum disulphide (MoS2). When the focused electron beam interacts with the MoS2 monolayer, it turns that area — which is on the order of a nanometer in diameter — from semiconducting to metallic. The quantum dots can be placed less than four nanometers apart, so that they become an artificial crystal — essentially a new 2-D material where the band gap can be specified to order, from 1.8 to 1.4 electron volts (eV).

    This is the first time that scientists have created a large-area 2-D superlattice — nanoscale atomic clusters in an ordered grid — on an atomically thin material on which both the size and location of quantum dots are precisely controlled. The process not only creates several quantum dots, but can also be applied directly to large-scale fabrication of 2-D quantum dot superlattices. “We can, therefore, change the overall properties of the 2-D crystal,” Banerjee said.

    Each quantum dot acts as a quantum well, where electron-hole activity occurs, and all of the dots in the grid are close enough to each other to ensure interactions. The researchers can vary the spacing and size of the dots to vary the band gap, which determines the wavelength of light it emits.

    “Using this technique, we can engineer the band gap to match the application,” Banerjee said. Quantum dot superlattices have been widely investigated for creating materials with tunable band gaps but all were made using “bottom-up” methods in which atoms naturally and spontaneously combine to form a macro-object. But those methods make it inherently difficult to design the lattice structure as desired and, thus, to achieve optimal performance.

    As an example, depending on conditions, combining carbon atoms yields only two results in the bulk (or 3-D) form: graphite or diamond. These cannot be ‘tuned’ and so cannot make anything in between. But when atoms can be precisely positioned, the material can be designed with desired characteristics.

    “Our approach overcomes the problems of randomness and proximity, enabling control of the band gap and all the other characteristics you might want the material to have — with a high level of precision,” Xie said. “This is a new way to make materials, and it will have many uses, particularly in quantum computing and communication applications. The dots on the superlattice are so close to each other that the electrons are coupled, an important requirement for quantum computing.”

    The quantum dot is theoretically an artificial “atom.” The developed technique makes such design and “tuning” possible by enabling top-down control of the size and the position of the artificial atoms at large scale.

    To demonstrate the level of control achieved, the authors produced an image of “UCSB” spelled out in a grid of quantum dots. By using different doses from the electron beam, they were able to cause different areas of the university’s initials to light up at different wavelengths.

    “When you change the dose of the electron beam, you can change the size of the quantum dot in the local region, and once you do that, you can control the band gap of the 2-D material,” Banerjee explained. “If you say you want a band gap of 1.6 eV, I can give it to you. If you want 1.5 eV, I can do that, too, starting with the same material.”

    This demonstration of tunable direct band gap could usher a new generation of light-emitting devices for photonics applications.

    See the full article here .

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    The University of California, Santa Barbara (commonly referred to as UC Santa Barbara or UCSB) is a public research university and one of the 10 general campuses of the University of California system. Founded in 1891 as an independent teachers’ college, UCSB joined the University of California system in 1944 and is the third-oldest general-education campus in the system. The university is a comprehensive doctoral university and is organized into five colleges offering 87 undergraduate degrees and 55 graduate degrees. In 2012, UCSB was ranked 41st among “National Universities” and 10th among public universities by U.S. News & World Report. UCSB houses twelve national research centers, including the renowned Kavli Institute for Theoretical Physics.

     
  • richardmitnick 10:07 am on July 13, 2017 Permalink | Reply
    Tags: , Electron valley states, , , Quantum dots, ,   

    From UCLA: “Technique for measuring and controlling electron state is a breakthrough in quantum computing” 

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    UCLA

    July 06, 2017
    Meghan Steele Horan

    1
    UCLA professor HongWen Jiang (center) and graduate students Blake Freeman and Joshua Schoenfield affixing a quantum dot device to the gold plate of a cooling chamber. Nick Penthorn.

    During their research for a new paper on quantum computing, HongWen Jiang, a UCLA professor of physics, and Joshua Schoenfield, a graduate student in his lab, ran into a recurring problem: They were so excited about the progress they were making that when they logged in from home to their UCLA desktop — which allows only one user at a time — the two scientists repeatedly knocked each other off of the remote connection.

    The reason for their enthusiasm: Jiang and his team created a way to measure and control the energy differences of electron valley states in silicon quantum dots, which are a key component of quantum computing research. The technique could bring quantum computing one step closer to reality.

    “It’s so exciting,” said Jiang, a member of the California NanoSystems Institute. “We didn’t want to wait until the next day to find out the outcome.”

    Quantum computing could enable more complex information to be encoded on much smaller computer chips, and it holds promise for faster, more secure problem-solving and communications than today’s computers allow.

    In standard computers, the fundamental components are switches called bits, which use 0s and 1s to indicate that they are off or on. The building blocks of quantum computers, on the other hand, are quantum bits, or qubits.

    The UCLA researchers’ breakthrough was being able to measure and control a specific state of a silicon quantum dot, known as a valley state, an essential property of qubits. The research was published in Nature Communications.

    “An individual qubit can exist in a complex wave-like mixture of the state 0 and the state 1 at the same time,” said Schoenfield, the paper’s first author. “To solve problems, qubits must interfere with each other like ripples in a pond. So controlling every aspect of their wave-like nature is essential.”

    Silicon quantum dots are small, electrically confined regions of silicon, only tens of nanometers across, that can trap electrons. They’re being studied by Jiang’s lab — and by researchers around the world — for their possible use in quantum computing because they enable scientists to manipulate electrons’ spin and charge.

    Besides electrons’ spin and charge, another of their most important properties is their “valley state,” which specifies where an electron will settle in the non-flat energy landscape of silicon’s crystalline structure. The valley state represents a location in the electron’s momentum, as opposed to an actual physical location.

    Scientists have realized only recently that controlling valley states is critical for encoding and analyzing silicon-based qubits, because even the tiniest imperfections in a silicon crystal can alter valley energies in unpredictable ways.

    “Imagine standing on top of a mountain and looking down to your left and right, noticing that the valleys on either side appear to be the same but knowing that one valley was just 1 centimeter deeper than the other,” said Blake Freeman, a UCLA graduate student and co-author of the study. “In quantum physics, even that small of a difference is extremely important for our ability to control electrons’ spin and charge states.”

    At normal temperatures, electrons bounce around, making it difficult for them to rest in the lowest energy point in the valley. So to measure the tiny energy difference between two valley states, the UCLA researchers placed silicon quantum dots inside a cooling chamber at a temperature near absolute zero, which allowed the electrons to settle down. By shooting fast electrical pulses of voltage through them, the scientists were able to move single electrons in and out of the valleys. The tiny difference in energy between the valleys was determined by observing the speed of the electron’s rapid switching between valley states.

    After manipulating the electrons, the researchers ran a nanowire sensor very close to the electrons. Measuring the wire’s resistance allowed them to gauge the distance between an electron and the wire, which in turn enabled them to determine which valley the electron occupied.

    The technique also enabled the scientists, for the first time, to measure the extremely small energy difference between the two valleys — which had been impossible using any other existing method.

    In the future, the researchers hope to use more sophisticated voltage pulses and device designs to achieve full control over multiple interacting valley-based qubits.

    “The dream is to have an array of hundreds or thousands of qubits all working together to solve a difficult problem,” Schoenfield said. “This work is an important step toward realizing that dream.”

    The research was supported by the U.S. Army Research Office.

    See the full article here .

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    UC LA Campus

    For nearly 100 years, UCLA has been a pioneer, persevering through impossibility, turning the futile into the attainable.

    We doubt the critics, reject the status quo and see opportunity in dissatisfaction. Our campus, faculty and students are driven by optimism. It is not naïve; it is essential. And it has fueled every accomplishment, allowing us to redefine what’s possible, time after time.

    This can-do perspective has brought us 12 Nobel Prizes, 12 Rhodes Scholarships, more NCAA titles than any university and more Olympic medals than most nations. Our faculty and alumni helped create the Internet and pioneered reverse osmosis. And more than 100 companies have been created based on technology developed at UCLA.

     
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