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  • richardmitnick 8:56 am on May 10, 2017 Permalink | Reply
    Tags: , Quantum Computing,   

    From Stanford: “Stanford team brings quantum computing closer to reality with new materials” 

    Stanford University Name
    Stanford University

    May 9, 2017
    Tom Abate

    Researchers are developing quantum computers based on light rather than electricity. At Stanford, new materials could be the key to progress in this field. (Image credit: iStock/Pobytov)

    For 60 years computers have become smaller, faster and cheaper. But engineers are approaching the limits of how small they can make silicon transistors and how quickly they can push electricity through devices to create digital ones and zeros.

    That limitation is why Stanford electrical engineering Professor Jelena Vuckovic is looking to quantum computing, which is based on light rather than electricity. Quantum computers work by isolating spinning electrons inside a new type of semiconductor material. When a laser strikes the electron, it reveals which way it is spinning by emitting one or more quanta, or particles, of light. Those spin states replace the ones and zeros of traditional computing.

    Vuckovic, who is one of the world’s leading researchers in the field, said quantum computing is ideal for studying biological systems, doing cryptography or data mining – in fact, solving any problem with many variables.

    “When people talk about finding a needle in a haystack, that’s where quantum computing comes in,” she said.

    Marina Radulaski, a postdoctoral fellow in Vuckovic’s lab, said the problem-solving potential of quantum computers stems from the complexity of the laser-electron interactions at the core of the concept.

    “With electronics you have zeros and ones,” Radulaski said. “But when the laser hits the electron in a quantum system, it creates many possible spin states, and that greater range of possibilities forms the basis for more complex computing.”

    Capturing electrons

    Harnessing information based on the interactions of light and electrons is easier said than done. Some of the world’s leading technology companies are trying to build massive quantum computers that rely on materials super-cooled to near absolute zero, the theoretical temperature at which atoms would cease to move.

    In her own studies of nearly 20 years, Vuckovic has focused on one aspect of the challenge: creating new types of quantum computer chips that would become the building blocks of future systems.

    “To fully realize the promise of quantum computing we will have to develop technologies that can operate in normal environments,” she said. “The materials we are exploring bring us closer toward finding tomorrow’s quantum processor.”

    The challenge for Vuckovic’s team is developing materials that can trap a single, isolated electron. Working with collaborators worldwide, they have recently tested three different approaches to the problem, one of which can operate at room temperature – a critical step if quantum computing is going to become a practical tool.

    In all three cases the group started with semiconductor crystals, material with a regular atomic lattice like the girders of a skyscraper. By slightly altering this lattice, they sought to create a structure in which the atomic forces exerted by the material could confine a spinning electron.

    “We are trying to develop the basic working unit of a quantum chip, the equivalent of the transistor on a silicon chip,” Vuckovic said.

    Quantum dots

    One way to create this laser-electron interaction chamber is through a structure known as a quantum dot. Physically, the quantum dot is a small amount of indium arsenide inside a crystal of gallium arsenide. The atomic properties of the two materials are known to trap a spinning electron.

    In a recent paper in Nature Physics, Kevin Fischer, a graduate student in the Vuckovic lab, describes how the laser-electron processes can be exploited within such a quantum dot to control the input and output of light. By sending more laser power to the quantum dot, the researchers could force it to emit exactly two photons rather than one. They say the quantum dot has practical advantages over other leading quantum computing platforms but still requires cryogenic cooling, so it may not be useful for general-purpose computing. However, it could have applications in creating tamper-proof communications networks.

    Color centers

    In two other papers Vuckovic took a different approach to electron capture, by modifying a single crystal to trap light in what is called a color center.

    In a recent paper published in NanoLetters, her team focused on color centers in diamond. In nature the crystalline lattice of a diamond consists of carbon atoms. Jingyuan Linda Zhang, a graduate student in Vuckovic’s lab, described how a 16-member research team replaced some of those carbon atoms with silicon atoms. This one alteration created color centers that effectively trapped spinning electrons in the diamond lattice.

    But like the quantum dot, most diamond color center experiments require cryogenic cooling. Though that is an improvement over other approaches that required even more elaborate cooling, Vuckovic wanted to do better.

    So she worked with another global team to experiment with a third material, silicon carbide. Commonly known as carborundum, silicon carbide is a hard, transparent crystal used to make clutch plates, brake pads and bulletproof vests. Prior research had shown that silicon carbide could be modified to create color centers at room temperature. But this potential had not yet been made efficient enough to yield a quantum chip.

    Vuckovic’s team knocked certain silicon atoms out of the silicon carbide lattice in a way that created highly efficient color centers. They also fabricated nanowire structures around the color centers to improve the extraction of photons. Radulaski was the first author on that experiment, which is described in another NanoLetters paper. She said the net results – an efficient color center, operating at room temperature, in a material familiar to industry – were huge pluses.

    “We think we’ve demonstrated a practical approach to making a quantum chip,” Radulaski said.

    But the field is still in its early days and electron tapping is no simple feat. Even the researchers aren’t sure which method or methods will win out.

    “We don’t know yet which approach is best, so we continue to experiment,” Vuckovic said.

    The diamond research team included Stanford faculty members Zhi-Xun Shen, the Paul Pigott Professor in Physical Sciences, professor of photon science, of physics and of applied physics, and a senior fellow at the Precourt Institute for Energy; Nicholas Melosh, an associate professor of materials science and engineering and of photon science; and Steven Chu, the William R. Kenan Jr. Professor, professor of physics and of molecular and cellular physiology, and member of Stanford Bio-X and the Stanford Neurosciences Institute.

    See the full article here .

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  • richardmitnick 8:27 am on May 3, 2017 Permalink | Reply
    Tags: Europe’s billion-euro quantum project takes shape, , Quantum Computing   

    From Nature: “Europe’s billion-euro quantum project takes shape” 

    Nature Mag

    03 May 2017
    Elizabeth Gibney

    Quantum computers are Europe’s next big project. Ion Quantum Technology Group, Univ. Sussex

    Scientists offer more detail on flagship programme to harness quantum effects in devices.

    As China and the United States threaten to corner the market on quantum technologies, Europe is slowly waking up to the opportunity with investment of its own. A year ago, the European Commission announced that it would create a €1-billion (US$1.1-billion) research effort in the field, and it should start to invite grant applications later this year. But scientists coordinating the project say that they are already concerned because industry partners seem reluctant to invest.

    Members of an advisory group steering the Quantum Technology Flagship, as the project is called, gave details of how it will work at a meeting on 7 April at the Russian Centre of Science and Culture in London. The project aims to exploit the bizarre behaviour shown by quantum systems to develop new technologies, such as super-secure communication systems and miniature, ultra-accurate sensors.

    Quantum computers are Europe’s next big project.

    As China and the United States threaten to corner the market on quantum technologies, Europe is slowly waking up to the opportunity with investment of its own. A year ago, the European Commission announced that it would create a €1-billion (US$1.1-billion) research effort in the field, and it should start to invite grant applications later this year. But scientists coordinating the project say that they are already concerned because industry partners seem reluctant to invest.

    Members of an advisory group steering the Quantum Technology Flagship, as the project is called, gave details of how it will work at a meeting on 7 April at the Russian Centre of Science and Culture in London. The project aims to exploit the bizarre behaviour shown by quantum systems to develop new technologies, such as super-secure communication systems and miniature, ultra-accurate sensors.

    But the programme is playing catch-up. Many labs in rival regions are already developing quantum technologies, including at large firms such as Google and Microsoft.

    “Europe cannot afford to miss this train,” says Vladimir Buzek, a member of the advisory group and a physicist at the Research Center for Quantum Information of the Slovak Academy of Sciences in Bratislava. “The industry here, to my taste, is really waiting too long,” he said at the meeting.

    Launched in April 2016 as part of an apparently unrelated initiative in cloud-computing, the quantum project is the European Commission’s latest decade-long, billion-euro initiative. Yet, the two previous EU mega-projects — the Graphene Flagship and the Human Brain Project, both announced in 2013 — have yet to fully prove their value. The latter has been plagued by disputes over its leadership. And both have had difficulty drumming up complementary investment from member states, says Tommaso Calarco, a physicist at the Centre for Integrated Quantum Science and Technology at the Universities of Ulm and Stuttgart in Germany, and another adviser on the steering committeee.

    The Quantum Technology Flagship will work differently, he says. Rather than run largely as a closed consortium selected at the project’s outset, it will operate with open calls throughout. He says that this should ensure high levels of competition, and offer the flexibility to fund the best researchers throughout. And he hopes that it will encourage member states to invest nationally to make stronger bids for funding.

    Some European countries show signs of supporting the project. Hungary, Austria and Germany have all announced their own national quantum-technology programmes since the flagship’s launch. The German initiative, called QUTEGA, is currently in a pilot form, but is likely to be worth around €300 million over 10 years. Initial projects include miniaturized magnetic sensors, which pick up tiny electric currents and could be used to monitor the brain during surgery, as well as small, transportable, high-precision atomic clocks, says Gerd Leuchs, a physicist at the Max Planck Institute for the Science of Light, Erlangen, and coordinator of the project.

    Product potential

    The European flagship will focus on four quantum technologies: communication, computing, sensing and simulation. It will also incorporate basic science. Although Europe produces some of the best research in these fields, other regions file more patents, says Martino Travagnin, who, along with his colleagues at the European Commission’s Joint Research Centre in Ispra, Italy, has analysed patenting in quantum technologies.

    China currently dominates in quantum communication, which uses quantum properties of particles to develop shared secret keys for encryption. The country holds the most patents in the field and is already trialling both a quantum-communication satellite and a 2,000-kilometre secure ground-based link. And the United States leads on patents in quantum computing and ultra-sensitive sensors.

    Companies are involved with the EU project, Buzek told the meeting, with 12 representatives on the expert group. “But industry seems like it’s just waiting for what the academy is going to produce, and then at some point, it’s willing to take the result,” he said. Although EU companies might lack the cash to dive into quantum technologies, as their US counterparts have done, smaller companies could invest in producing crucial components, he said.

    Brexit problems

    One problem facing the quantum-flagship scheme is the possible loss of the United Kingdom, one of Europe’s strongest research communities in quantum technology. (Following the Brexit vote, the United Kingdom is scheduled to leave the European Union in 2019, the year in which the first projects kick off.) The United Kingdom is one of the few nations to involve relevant companies in the research, Calarco points out, through its £350-million (US$450-million) UK National Quantum Technologies Programme. He hopes that the United Kingdom will be able to continue in some capacity — either by paying into the European funding pot, as Switzerland does, or through a match-funding model.

    The timing of the project should also play in its favour, he notes. A UK government commitment to underwrite funding for existing EU projects means that the early years of investment will be guaranteed. The next round should start sufficiently long enough after the Brexit negotiation for a solution to have emerged. “Given the circumstances, this is the best timing we could imagine,” he says.

    See the full article here .

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  • richardmitnick 5:20 pm on March 30, 2017 Permalink | Reply
    Tags: , , Quantum computers use quantum bits (or qubits), Quantum Computing, Rigetti Computing   

    From Futurism: “This Startup Plans to Revolutionize Quantum Computing Technology Faster Than Ever” 



    Dom Galeon

    Investor Interest

    Since Rigetti Computing launched three years ago, the Berekely and Fremont-based startup has attracted a host of investors — including private American venture capital firm, Andreessen Horowitz (also known as A16Z). As of this week, Rigetting Computing has raised a total of $64 million after successfully hosting a Series A and Series B round of funding.


    The startup is attracting investors primarily because it promises to revolutionize quantum computing technology: “Rigetti has assembled an impressive team of scientists and engineers building the combination of hardware and software that has the potential to finally unlock quantum computing for computational chemistry, machine learning and much more,” Vijay Pande, a general partner at A16Z, said when the fundraising was announced.

    Quantum Problem Solving

    Quantum computers are expected to change computing forever in large part due to their speed and processing power. Instead of processing information the way existing systems do — relying on bits of 0s and 1s operating on miniature transistors — quantum computers use quantum bits (or qubits) that can both be a 0 or a 1 at the same time. This is thanks to a quantum phenomenon called superposition. In existing versions of quantum computers, this has been achieved using individual photons.

    “Quantum computing will enable people to tackle a whole new set of problems that were previously unsolvable,” said Chad Rigetti, the startup’s founder and CEO. “This is the next generation of advanced computing technology. The potential to make a positive impact on humanity is enormous.” This translates to computing system that are capable of handling problems deemed too difficult for today’s computers. Such applications could be found everywhere from advanced medical research to even improved encryption and cybersecurity.

    How is Rigetti Computing planning to revolutionize the technology? For starters, they’re building a quantum computing platform for artificial intelligence and computational chemistry. This can help overcome the logistical challenges that currently plague quantum computer development. They also have an API for quantum computing in the cloud, called Forest, that’s recently opened up private beta testing.

    Rigetti expects it will be at least two more years before their technology can be applied to real world problems. But for interested investors, investing in such a technological game-changer sooner rather than later makes good business sense.

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  • richardmitnick 8:48 am on March 20, 2017 Permalink | Reply
    Tags: Anderson limit, , How small can superconductors be?, , P.W. Anderson, Parity effect, , Quantum Computing, Richardson-Gaudin models   

    From phys.org: “How small can superconductors be?” 


    March 20, 2017
    Lisa Zyga

    Topographic image of a lead nanocrystal used in the study. Scale bar: 10 nm. Credit: Vlaic et al. Nature Communications

    For the first time, physicists have experimentally validated a 1959 conjecture that places limits on how small superconductors can be. Understanding superconductivity (or the lack thereof) on the nanoscale is expected to be important for designing future quantum computers, among other applications.

    In 1959, physicist P.W. Anderson conjectured that superconductivity can exist only in objects that are large enough to meet certain criteria. Namely, the object’s superconducting gap energy must be larger than its electronic energy level spacing—and this spacing increases as size decreases. The cutoff point (where the two values are equal) corresponds to a volume of about 100 nm3. Until now it has not been possible to experimentally test the Anderson limit due to the challenges in observing superconducting effects at this scale.

    In the new study published in Nature Communications, Sergio Vlaic and coauthors at the University Paris Sciences et Lettres and French National Centre for Scientific Research (CNRS) designed a nanosystem that allowed them to experimentally investigate the Anderson limit for the first time.

    The Anderson limit arises because, at very small scales, the mechanisms underlying superconductivity essentially stop working. In general, superconductivity occurs when electrons bind together to form Cooper pairs. Cooper pairs have a slightly lower energy than individual electrons, and this difference in energy is the superconducting gap energy. The Cooper pairs’ lower energy inhibits electron collisions that normally create resistance. If the superconducting gap energy gets too small and vanishes—which can occur, for example, when the temperature increases—then the electron collisions resume and the object stops being a superconductor.

    The Anderson limit shows that small size is another way that an object may stop being a superconductor. However, unlike the effects of increasing the temperature, this is not because smaller objects have a smaller superconducting gap energy. Instead, it arises because smaller crystals have fewer electrons, and therefore fewer electron energy levels, than larger crystals do. Since the total possible electron energy of an element stays the same, regardless of size, smaller crystals have larger spacings between their electron energy levels than larger crystals do.

    According to Anderson, this large electronic energy level spacing should pose a problem, and he expected superconductivity to disappear when the spacing becomes larger than the superconducting gap energy. The reason for this, generally speaking, is that one consequence of increased spacing is a decrease in potential energy, which interferes with the competition between kinetic and potential energy that is necessary for superconductivity to occur.

    To investigate what happens to the superconductivity of objects around the Anderson limit, the scientists in the new study prepared large quantities of isolated lead nanocrystals ranging in volume from 20 to 800 nm3.

    Although they could not directly measure the superconductivity of such tiny objects, the researchers could measure something called the parity effect, which results from superconductivity. When an electron is added to a superconductor, the additional energy is partly affected by whether there is an even or odd number of electrons (the parity), which is due to the electrons forming Cooper pairs. If the electrons don’t form Cooper pairs, there is no parity effect, indicating no superconductivity.

    Although the parity effect has previously been observed in large superconductors, this study is the first time that it has been observed in small nanocrystals approaching the Anderson limit. In accordance with Anderson’s predictions from more than 50 years ago, the researchers observed the parity effect for larger nanocrystals, but not for the smallest nanocrystals below approximately 100 nm3.

    The results not only validate the Anderson conjecture, but also extend to a more general area, the Richardson-Gaudin models. These models are equivalent to the conventional theory of superconductivity, the Bardeen Cooper Schrieffer theory, for very small objects.

    “Our experimental demonstration of the Anderson conjecture is also a demonstration of the validity of the Richardson-Gaudin models,” coauthor Hervé Aubin at the University Paris Sciences et Lettres and CNRS told Phys.org. “The Richardson-Gaudin models are an important piece of theoretical works because they can be solved exactly and apply to a wide range of systems; not only to superconducting nanocrystals but also to atomic nuclei and cold fermionic atomic gas, where protons and neutrons, which are fermions like electrons, can also form Cooper pairs.”

    On the more practical side, the researchers expect the results to have applications in future quantum computers.

    “One of the most interesting applications of superconducting islands is their use as Cooper pair boxes employed in quantum bits, the elemental unit of a hypothetical quantum computer,” Aubin said. “So far, Cooper pair boxes used in qubits are much larger than the Anderson limit. Upon reducing the size of the Cooper pair box, quantum computer engineers will eventually have to cope with superconductivity at the Anderson limit.”

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  • richardmitnick 8:25 am on March 8, 2017 Permalink | Reply
    Tags: , Quantum Computing,   

    From Science Alert: “IBM is Rolling out the World’s First Universal ‘Quantum Computing’ Service” 


    Science Alert

    7 MAR 2017


    If you build it, they will come.

    We’re all excited about the potential of quantum computers – devices that will harness strange quantum phenomena to perform calculations far more powerful than anything conventional computers can do today.

    Unfortunately, we still don’t have a tangible, large-scale quantum computer to freak out over just yet, but IBM is already preparing for a future when we do, by announcing that they’re rolling out a universal ‘quantum-computing’ service later this year.

    The service will be called IBM Q, and it will give people access to their early-stage quantum computer over the internet to use as they wish – for a fee.

    The big elephant in the room is that, for now, IBM’s quantum computer only runs on five qubits, so it’s not much faster (if any faster) than a conventional computer.

    But their technology is improving all the time. The company has announced it hopes to get to 50 qubits in the next few years, and in the meantime, it’s building the online systems and software so that anyone in the world can access the full power of its quantum computer when it’s ready. IBM Q is a crucial part of that.

    Unlike conventional computers, which use ‘bits’ of either 1 or 0 to code information, quantum computers use a strange phenomenon known as superposition, which allows an atom to be in both the 1 and 0 position at the same time. These quantum bits, or qubits, give quantum computers far more processing power than traditional computers.

    But right now, qubits are hard to make and manipulate, even for more the most high-tech labs. Which is why IBM only has five qubits working together in a computer, despite decades of research. And those qubits have to be cooled to temperatures just above absolute zero in order to function.

    Companies such as Google, and multiple university research labs, have also built primitive quantum computers, and Google has even used theirs to simulate a molecule for the first time, showing the potential of this technology as it scales up.

    But instead of just focussing on the hardware itself, IBM is also interested in the software around quantum computers, and how to give the public access to them.

    “IBM has invested over decades to growing the field of quantum computing and we are committed to expanding access to quantum systems and their powerful capabilities for the science and business communities,” said Arvind Krishna, senior vice president of Hybrid Cloud and director for IBM Research.

    The system builds on the company’s Quantum Experience, which was rolled out last year for free to approved researchers. IBM Q will use similar cloud software, but will also be open to businesses – and, more importantly, any programmers and developers who want to start experimenting with writing code for quantum systems.

    The goal is to have a functional, commercial, cloud-based service ready to go when a fully realised quantum computer does come online.

    “Putting the machine on the cloud is an obvious thing to do,” physicist Christopher Monroe from the University of Maryland, who isn’t involved with IBM, told Davide Castelvecchi over at Scientific American. “But it takes a lot of work in getting a system to that level.”

    The challenge is that while, on paper, a five-qubit machine is pretty easy to simulate and program for, real qubits don’t quite work that way, because you’re working with atoms that can change their behaviour based on environmental conditions

    “The real challenge is whether you can make your algorithm work on real hardware that has imperfections,” Isaac Chuang, a physicist at MIT who doesn’t work with IBM, told Scientific American.

    In their announcement, IBM said that in the past few months, more than 40,000 users have already used Quantum Experience to build and run 275,000 test applications, and 15 research papers have been published based off of it so far.

    And they predict that in future, the quantum service will become even more useful.

    “Quantum computers will deliver solutions to important problems where patterns cannot be seen because the data doesn’t exist and the possibilities that you need to explore to get to the answer are too enormous to ever be processed by classical computers,” said IBM in its announcement.

    There’s no word as yet on how much IBM Q will cost to use, or how users will be approved. But we have to admit it’d be pretty cool to be among the first to play around with quantum computing.

    See the full article here .

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  • richardmitnick 4:23 pm on January 2, 2017 Permalink | Reply
    Tags: , Electron-photon small-talk could have big impact on quantum computing, , Quantum Computing,   

    From Princeton: “Electron-photon small-talk could have big impact on quantum computing” 

    Princeton University
    Princeton University

    December 22, 2016
    Catherine Zandonella

    In a step that brings silicon-based quantum computers closer to reality, researchers at Princeton University have built a device in which a single electron can pass its quantum information to a particle of light. The particle of light, or photon, can then act as a messenger to carry the information to other electrons, creating connections that form the circuits of a quantum computer.

    The research, published today in the journal Science and conducted at Princeton and HRL Laboratories in Malibu, California, represents a more than five-year effort to build a robust capability for an electron to talk to a photon, said Jason Petta, a Princeton professor of physics.

    Princeton Professor of Physics Jason Petta, from left, and physics graduate students David Zajac and Xiao Mi, have built a device that is a step forward for silicon-based quantum computers, which when built will be able to solve problems beyond the capabilities of everyday computers. The device isolates an electron so that can pass its quantum information to a photon, which can then act as a messenger to carry the information to other electrons to form the circuits of the computer. (Photo by Denise Applewhite, Office of Communications)

    “Just like in human interactions, to have good communication a number of things need to work out — it helps to speak the same language and so forth,” Petta said. “We are able to bring the energy of the electronic state into resonance with the light particle, so that the two can talk to each other.”

    The discovery will help the researchers use light to link individual electrons, which act as the bits, or smallest units of data, in a quantum computer. Quantum computers are advanced devices that, when realized, will be able to perform advanced calculations using tiny particles such as electrons, which follow quantum rules rather than the physical laws of the everyday world.

    Each bit in an everyday computer can have a value of a 0 or a 1. Quantum bits — known as qubits — can be in a state of 0, 1, or both a 0 and a 1 simultaneously. This superposition, as it is known, enables quantum computers to tackle complex questions that today’s computers cannot solve.

    Simple quantum computers have already been made using trapped ions and superconductors, but technical challenges have slowed the development of silicon-based quantum devices. Silicon is a highly attractive material because it is inexpensive and is already widely used in today’s smartphones and computers.

    The qubit consists of a single electron that is trapped below the surface of a silicon chip (gray). The green, pink and purple wires on top of the silicon structure deliver precise voltages to the qubit. The purple plate reduces electronic interference that can destroy the qubit’s quantum information. By adjusting the voltages in the wires, the researchers can trap a single electron in a double quantum dot and adjust its energy so that it can communicate its quantum information to a nearby photon. (Photo courtesy of the Jason Petta research group, Department of Physics)

    The researchers trapped both an electron and a photon in the device, then adjusted the energy of the electron in such a way that the quantum information could transfer to the photon. This coupling enables the photon to carry the information from one qubit to another located up to a centimeter away.

    Quantum information is extremely fragile — it can be lost entirely due to the slightest disturbance from the environment. Photons are more robust against disruption and can potentially carry quantum information not just from qubit to qubit in a quantum computer circuit but also between quantum chips via cables.

    For these two very different types of particles to talk to each other, however, researchers had to build a device that provided the right environment. First, Peter Deelman at HRL Laboratories, a corporate research-and-development laboratory owned by the Boeing Company and General Motors, fabricated the semiconductor chip from layers of silicon and silicon-germanium. This structure trapped a single layer of electrons below the surface of the chip. Next, researchers at Princeton laid tiny wires, each just a fraction of the width of a human hair, across the top of the device. These nanometer-sized wires allowed the researchers to deliver voltages that created an energy landscape capable of trapping a single electron, confining it in a region of the silicon called a double quantum dot.

    The researchers used those same wires to adjust the energy level of the trapped electron to match that of the photon, which is trapped in a superconducting cavity that is fabricated on top of the silicon wafer.

    Prior to this discovery, semiconductor qubits could only be coupled to neighboring qubits. By using light to couple qubits, it may be feasible to pass information between qubits at opposite ends of a chip.

    The electron’s quantum information consists of nothing more than the location of the electron in one of two energy pockets in the double quantum dot. The electron can occupy one or the other pocket, or both simultaneously. By controlling the voltages applied to the device, the researchers can control which pocket the electron occupies.

    “We now have the ability to actually transmit the quantum state to a photon confined in the cavity,” said Xiao Mi, a graduate student in Princeton’s Department of Physics and first author on the paper. “This has never been done before in a semiconductor device because the quantum state was lost before it could transfer its information.”

    A fully packaged device for trapping and manipulating single electrons and photons. A series of on-chip electrodes (lower left and upper right) lead to the formation of a double quantum dot that confines a single electron below the surface of the chip. The photon, which is free to move within the full 7-millimeter span of the cavity, exchanges quantum information with the electron inside the double quantum dot. (Photo courtesy of the Jason Petta research group, Department of Physics)

    The success of the device is due to a new circuit design that brings the wires closer to the qubit and reduces interference from other sources of electromagnetic radiation. To reduce this noise, the researchers put in filters that remove extraneous signals from the wires that lead to the device. The metal wires also shield the qubit. As a result, the qubits are 100 to 1,000 times less noisy than the ones used in previous experiments.

    Jeffrey Cady, a 2015 graduate, helped develop the filters to reduce the noise as part of his undergraduate senior thesis, and graduate student David Zajac led the effort to use overlapping electrodes to confine single electrons in silicon quantum dots.

    Eventually the researchers plan to extend the device to work with an intrinsic property of the electron known as its spin. “In the long run we want systems where spin and charge are coupled together to make a spin qubit that can be electrically controlled,” Petta said. “We’ve shown we can coherently couple an electron to light, and that is an important step toward coupling spin to light.”

    David DiVincenzo, a physicist at the Institute for Quantum Information in RWTH Aachen University in Germany, who was not involved in the research, is the author of an influential 1996 paper outlining five minimal requirements necessary for creating a quantum computer. Of the Princeton-HRL work, in which he was not involved, DiVincenzo said: “It has been a long struggle to find the right combination of conditions that would achieve the strong coupling condition for a single-electron qubit. I am happy to see that a region of parameter space has been found where the system can go for the first time into strong-coupling territory.”

    Funding for this research was provided by Army Research Office grant No. W911NF-15-1-0149, the Gordon and Betty Moore Foundation’s EPiQS Initiative through grant GBMF4535, and the National Science Foundation (DMR-1409556 and DMR-1420541). The material is based upon work supported by the U.S. Department of Defense under contract H98230-15-C0453.

    The paper, Strong Coupling of a Single Electron in Silicon to a Microwave Photon by X. Mi, J. V. Cady, D. M. Zajac, P. W. Deelman, J. R. Petta, was published in the journal Science online on Thursday, Dec. 22.

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    About Princeton: Overview

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    As a world-renowned research university, Princeton seeks to achieve the highest levels of distinction in the discovery and transmission of knowledge and understanding. At the same time, Princeton is distinctive among research universities in its commitment to undergraduate teaching.

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  • richardmitnick 1:48 pm on December 13, 2016 Permalink | Reply
    Tags: , , Quantum Computing   

    From New Scientist: “Quantum computers ditch all the lasers for easier engineering” 


    New Scientist

    7 December 2016
    Michael Brooks

    Lasers are not the only option. Richard Kail/Science Photo Library

    They will be the ultimate multitaskers – but quantum computers might take a bit of juggling to operate. Now, a team has simplified their inner workings.

    Computers that take advantage of quantum laws allowing particles to exist in multiple states at the same time promise to run millions of calculations at once. One of the candidate technologies involves ion traps, which hold and manipulate charged particles, called ions, to encode information.

    But to make a processor that works faster than a classical computer would require millions of such traps, each controlled with its own precisely aligned laser – making it extremely complicated.

    Now, Winfried Hensinger at the University of Sussex in the UK and his colleagues have replaced the millions of lasers with some static magnets and a handful of electromagnetic fields. “Our invention has led to a radical simplification of the engineering required, which means we are now able to construct a large-scale device,” he says.

    In their scheme, each ion is trapped by four permanent magnets, with a controllable voltage across the trap. The entire device is bathed in a set of tuned microwave and radio-frequency electromagnetic fields. Tweaking the voltage shifts the ions to a different position in the magnetic field, changing their state.

    Promising technology

    The researchers have already used this idea to build and operate a quantum logic gate, a building block of a processor. This particular gate involves entangling two ions – in other words, linking their quantum states such that they are fully dependent on each other. Hensinger says this is the most difficult kind of logic gate to build.

    Manas Mukherjee at the National University of Singapore is impressed with the new technology. “It’s a promising development, with good potential for scaling up,” he says.

    That’s exactly what the team is planning: they hope to have a trial device containing tens of ions ready within four years.

    The fact that the device uses current technologies such as techniques for silicon-chip manufacturing means there are no known roadblocks to scaling up to create a useful quantum computer.

    It won’t be plain sailing, though. Scaling up will mean creating magnetic fields that vary in strength over relatively short distances. This a significant engineering challenge, says Mukherjee. Then there’s the challenge of handling waste heat, which becomes more problematic as the processor gets bigger. “As with any architecture, you need low heating rates,” he says.

    Journal reference: Physical Review Letters, DOI: 10.1103/PhysRevLett.117.220501

    See the full article here .

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  • richardmitnick 8:16 am on September 27, 2016 Permalink | Reply
    Tags: , , Quantum Computing, Stopped light   

    From COSMOS: “Stopped light means go for quantum computers (eventually)” 

    Cosmos Magazine bloc


    27 September 2016
    Cathal O’Connell

    A cold cloud of atoms (red smear in the centre) holds light in place. Ben Buchler / ANU

    Australian physicists have brought quantum computing a step closer by bringing light to a standstill. This kind of system, reported in Nature Physics, could be used to store light in a quantum memory or build optical gates – two vital components in the futuristic goal of assembling a light-based quantum computer.

    The researchers from the Australian National University in Canberra liken the experiment to a scene from the 2015 film Star Wars: The Force Awakens when the character Kylo Ren used the Force to stop a laser blast mid-air.

    “Of course, we’re not using the Force, we’re using a light-matter interaction,” says study co-author Geoff Campbell, adding that the movie scene does give an intuitive idea about what the experiment was about.

    The work follows 20 years of research into slowing or stopping the fastest phenomenon in the universe. Light barrels along through a vacuum at three hundred billion metres per second.

    In 1999 physicists managed to slow it to 17 metres per second in a cloud of cold gas.

    And by 2013, scientists at the University of Darmstadt in Germany stopped it entirely, for a full minute, inside an opaque crystal.

    But what physicists call ‘stopped light’ is not quite what you might imagine from that Star Wars scene.

    When physicists stop light, it’s actually only the light’s information that’s held in place – imprinted on surrounding atoms as light is absorbed. They can then retrieve this information by setting it in motion again as another light wave, for instance.

    This storage and retrieval of light information could be vital for building light-based quantum computers.

    The new experiment is a new variation on the stopped light technique, called ‘stationary light’. To pull it off, the Australian team shone infrared lasers into an ultra-cold cloud of rubidium atoms which excited atoms in two locations.

    The two excited groups of atoms then exchanged photons in a self-sustaining interaction – a bit like two groups of excited supporters exchanging chants at a football game.

    This optical chanting is called stationary light, because it preserves the information of the original light sent into the cloud – although only for a fraction of a second.

    While light has ground to a halt before, the Australian team managed to create a self-correcting arrangement – something which has never been done but makes preparation a lot easier. They were also able to image the cloud of atoms side-on and show the light exchange in action.

    The physicists see the experiment as an important step towards building a quantum logic gate, a critical element of optical quantum computers.

    Although some quantum logic gates have been built, they have been probabilistic, meaning they only work some of the time and can’t be scaled up. Building more reliable quantum gates hinges on finding a way to get two particles of light to interact.

    “The problem is photons tend not to talk to one another,” says Ben Buchler, who led the research.

    Using this technique, holding the light in place could give it more of a chance to interact, he adds: “That’s the building block for a quantum gate which is essential to a quantum computer.”

    See the full article here .

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  • richardmitnick 8:16 am on September 26, 2016 Permalink | Reply
    Tags: , , , Quantum Computing   

    From Niels Bohr Institute: “Effective reflection of light for quantum technology” 

    Niels Bohr Institute bloc

    Niels Bohr Institute

    23 September 2016
    No writer credit found

    Quantum Technology: Light usually spreads out in all directions and when the light hits an object, it is reflected and is scattered even more. So light is normally quite uncontrollable. But researchers want to be able to control light all the way down to the atomic level in order to develop future quantum technologies. Researchers at the Niels Bohr Institute have therefore developed a new method where they create a very strong interaction between light and atoms, which means that the light can be controlled and reflected on a glass fiber. The results are published in the scientific journal, Physical Review Letters [link is below].

    The experiments are carried out in a glass chamber with an ultra thin optical glass fiber stretched across it. The optical glass fiber has a diameter of 500 nanometers – that is 1000 times smaller than the diameter of a strand of hair. In the glass chamber there is also a gas of caesium atoms, which is cooled down to 50 micro-degrees Kelvin, which is almost absolute zero at minus 273 degrees Celsius. Due to the ultracold temperature, the caesium atoms are almost motionless and they are held close to the surface of the glass fiber. Using laser light, the researchers can push the individual atoms a bit so that they are evenly spaced along the surface of the glass fiber.

    Mirror effect. “We now send laser light through the glass fiber. The light has a particular wavelength and when the fiber is thinner than the wavelength of the light, the light moves along the surface of the fiber, where the atoms sit in a row. When the light hits the first atom, a strong interaction is created between the light and the atom and the atom moves with the light wave. With the atom’s precise distance, which matches the wavelength of the light, you get a backwards reflection of the light,” explains Jürgen Appel, associate professor in the research group Quantop at the Niels Bohr Institute at the University of Copenhagen.

    He explains that it is this backward reflection that is so important. When the light hits the next atom in the row the same thing happens – and the next, and the next. For every time the light hits an atom, a small part of the light is reflected and sent backwards.
    “We have managed to divert more than 10 percent of the light. With only 1000 atoms, an interaction is created that is just as strong as a glass plate with billions of atoms. We have created a mirror that effectively reflects light and we can even turn it on and off,” says Jürgen Appel.

    Such an on/off mirror based on just a few atoms can be used to improve the interaction between individual atoms and the fiber-guided light and it can be developed to improve entangled quantum states in connection with captured atomic systems for future quantum technology.

    Link to the article in Physical Review Letters: http://physics.aps.org/articles/v9/109

    See the full article here .

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    Niels Bohr Institute Campus

    The Niels Bohr Institute (Danish: Niels Bohr Institutet) is a research institute of the University of Copenhagen. The research of the institute spans astronomy, geophysics, nanotechnology, particle physics, quantum mechanics and biophysics.

    The Institute was founded in 1921, as the Institute for Theoretical Physics of the University of Copenhagen, by the Danish theoretical physicist Niels Bohr, who had been on the staff of the University of Copenhagen since 1914, and who had been lobbying for its creation since his appointment as professor in 1916. On the 80th anniversary of Niels Bohr’s birth – October 7, 1965 – the Institute officially became The Niels Bohr Institute.[1] Much of its original funding came from the charitable foundation of the Carlsberg brewery, and later from the Rockefeller Foundation.[2]

    During the 1920s, and 1930s, the Institute was the center of the developing disciplines of atomic physics and quantum physics. Physicists from across Europe (and sometimes further abroad) often visited the Institute to confer with Bohr on new theories and discoveries. The Copenhagen interpretation of quantum mechanics is named after work done at the Institute during this time.

    On January 1, 1993 the institute was fused with the Astronomic Observatory, the Ørsted Laboratory and the Geophysical Institute. The new resulting institute retained the name Niels Bohr Institute.

  • richardmitnick 7:29 am on September 26, 2016 Permalink | Reply
    Tags: , Bragg reflection, , , Quantum Computing, world’s most ethereal mirrors – made of just 1000 or 2000 atoms   

    From COSMOS: “Physicists build mirrors from just 1,000 atoms” 

    Cosmos Magazine bloc


    26 September 2016
    Cathal O’Connell

    A conceptual drawing of how optical circuits may look in future computers. RICHARD KAIL/Getty Images

    Mirror, mirror in mid-air. Two independent teams of physicists have created the world’s most ethereal mirrors – made of just 1,000 or 2,000 atoms suspended in a vacuum.

    The mirrors are held in space like beads on a string. By controlling the spacing between the atoms, the physicists could make the strings reflect up to 75% of the light shone on them.

    The reflectivity can be switched rapidly on or off, just by applying a few bursts of light – so the new mirrors could be useful for controllably bouncing light around optical circuits. And because the atoms interact with one another as well as the light, the set-up might be useful for linking quantum bits (or “qubits”) together in a quantum computer.

    The mirrors were created by two independent groups, one in France and the other in Denmark. Both are described in the current issue of Physical Review Letters.

    In a regular mirror, such as a polished metal surface, light is reflected because it interacts with the cloud of unattached electrons floating free in the metal, causing them to wobble. These wobbling electrons then re-emit the light. (These electrons are also the reason metals conduct electricity, so that’s the connection between shininess and conductivity.)

    But the new mirrors use something called Bragg reflection, which is a bit different.

    As Australia-born British physicist William Lawrence Bragg discovered in 1912, light waves scattering off layers in a crystal are reinforced at certain angles – those where neighbouring light waves return in lock step.

    Building on this work, just three years later, Bragg and his dad, William Henry Bragg, won the Nobel Prize for physics for using X-rays to figure out the structure of crystals. This kicked off the whole field of X-ray crystallography – instrumental a few decades later in unravelling the double helix structure of DNA.

    But when the spacing between the crystal layers is just right, the scattering angle is 90 degrees and so the crystal strongly reflects the light back where it came from – a special case known as Bragg reflection.

    A schematic showing a Bragg diffraction – the usual scattering of light that occurs when two beams with identical wavelength and phase approach a crystalline solid and are scattered off two different atoms within it. The lower beam traverses an extra length of 2dsinθ. But when the angle θ of the light hitting the surface is 90°, the light is reflected straight back the way it came – a Bragg reflection. WIKIMEDIA COMMONS

    Whereas regular mirrors can reflect any visible wavelength (that’s why mirrors appear to have no “colour” of their own), a Bragg mirror only reflects one wavelength. So don’t expect to see yourself in one.

    But that’s no limitation for communications technology, or optical circuits, which involve shuttling around light of a single wavelength.

    In 2011, a German team managed to turn a cloud of cold atoms into a Bragg mirror. They crisscrossed beams of lasers to arrange the atoms of the cloud into a lattice with just the right spacing. Although they achieved 80% reflection, they needed 10 million atoms to do it.

    Now two teams have dramatically reduced the number of atoms needed to make a useful mirror. Instead of simply shining a beam of light into a cloud of atoms (as the German group did), the teams transmit light along microscopically thin optical fibres. Atoms precisely positioned next to the fibre do the reflecting.

    When light travels along very thin optical fibres, some of the light spills out forming a so-called evanescent field – you can picture the field as a glowing halo around the fibre.

    Because the light is intensely confined in this halo, the interaction with any nearby atoms is very strong. This means only 1,000 or so atoms are needed to achieve a reflection, versus tens of millions for the cloud situation.

    The groups created their strings of single atoms by holding them in place using a laser beam running parallel to the fibre, and just a few hundred nanometres away, via the optical tweezers effect.

    Each string was evenly spaced with atoms every few hundred nanometres and was about one millimetre long.

    The physicists then sent another beam of light along the fibre – and this one interacted with the string of atoms through the halo of its evanescent field. When the spacing between the atoms was tuned just right, the Bragg condition applied – and much of the light was reflected back along the fibre in the opposite direction.

    The Danish team could reflect about 10% of the light using a string of 1,300 caesium atoms. While the French team reflected 75% using 2,000 atoms, also of caesium. The increased reflectivity achieved by the French group was not just a factor of more atoms in a row, they also had better control over the positions of their atoms.

    The mirror could be rapidly disassembled and reassembled simply by knocking the atoms out of their ordered state, and then replacing them. This mirror switching mechanism could be very useful for making optical switches in light-based circuitry.

    Red light sent through an optical fibre is visible in the fibre segment that is just a few hundred nanometres in diameter in this Danish experiment. J. Appel / University of Copenhagen

    The atoms also interact with one another via the light field, and over quite a long range. This kind of interaction could be used to simulate less tangible quantum interactions, or even for linking quantum bits (or “qubits”) together in quantum computers.

    All these applications will need stronger interactions between the light and the atoms – in effect a reflectivity much closer to 100%.

    Both teams have some tricks up their sleeves to achieve this, such as using longer sections of very thin fibre, or by reshaping the surface of the fibre to increase the interaction.

    As Wolfgang Ketterle, a physicist in quantum optics at the Massachusetts Institute of Technology told the American Physical Society, these works represent “a major advance in engineering and controlling how atoms scatter light”.

    See the full article here .

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