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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?” 

    physdotorg
    phys.org

    March 20, 2017
    Lisa Zyga

    1
    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.”

    See the full article here .

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    About Phys.org in 100 Words

    Phys.org™ (formerly Physorg.com) is a leading web-based science, research and technology news service which covers a full range of topics. These include physics, earth science, medicine, nanotechnology, electronics, space, biology, chemistry, computer sciences, engineering, mathematics and other sciences and technologies. Launched in 2004, Phys.org’s readership has grown steadily to include 1.75 million scientists, researchers, and engineers every month. Phys.org publishes approximately 100 quality articles every day, offering some of the most comprehensive coverage of sci-tech developments world-wide. Quancast 2009 includes Phys.org in its list of the Global Top 2,000 Websites. Phys.org community members enjoy access to many personalized features such as social networking, a personal home page set-up, RSS/XML feeds, article comments and ranking, the ability to save favorite articles, a daily newsletter, and other options.

     
  • 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” 

    ScienceAlert

    Science Alert

    7 MAR 2017
    FIONA MACDONALD

    1
    sakkmesterke/Shutterstock.com

    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.

    1
    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.

    2
    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.”

    4
    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.

    See the full article here .

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    About Princeton: Overview

    Princeton University is a vibrant community of scholarship and learning that stands in the nation’s service and in the service of all nations. Chartered in 1746, Princeton is the fourth-oldest college in the United States. Princeton is an independent, coeducational, nondenominational institution that provides undergraduate and graduate instruction in the humanities, social sciences, natural sciences and engineering.

    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.

    Today, more than 1,100 faculty members instruct approximately 5,200 undergraduate students and 2,600 graduate students. The University’s generous financial aid program ensures that talented students from all economic backgrounds can afford a Princeton education.

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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” 

    NewScientist

    New Scientist

    7 December 2016
    Michael Brooks

    1
    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

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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

    COSMOS

    27 September 2016
    Cathal O’Connell

    1
    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

    1
    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.

    2
    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

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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

    COSMOS

    26 September 2016
    Cathal O’Connell

    1
    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.

    2
    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.

    3
    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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  • richardmitnick 7:04 am on September 26, 2016 Permalink | Reply
    Tags: , Australian Centre for Quantum Computation and Communication Technology, , Michelle Simmons, Quantum Computing, ,   

    From COSMOS: Women in STEM – “Michelle Simmons: a quantum queen” 

    Cosmos Magazine bloc

    COSMOS

    26 September 2016
    Elizabeth Finkel

    1
    Michelle Simmons is using a scanning tunnelling microscope (pictured behind her) to build silicon-based quantum devices atom by atom. Nic Walker / Fairfax Syndication

    Building a quantum computer is not for the faint-hearted. These blazingly fast machines could revolutionise computing by ripping through big data, improving everything from tracking financial markets to weather forecasting. But the technology requires shrinking computer bits to the size of an atom.

    And unlike the robust bits of your laptop, quantum bits or qubits are weird and fragile. Trying to corral them is like trying to harness a flock of butterflies.

    And then there is the competition: IBM, Google and Microsoft are all in the race.

    None of this fazes physicist Michelle Simmons. She is confident the team she leads – the Australian Centre for Quantum Computation and Communication Technology – can deliver the most reliable type of quantum computer ever emerge victorious. In their machine, the quixotic qubit is made of stable silicon.

    “I’m not out there to recreate Intel, but I honestly believe our devices will win in the long term,” she says. “They are the most reproducible ones that are out there.”

    Simmons’ audacity is paying off.

    This year the Australian Centre garnered A$45 million from the federal government and businesses. Backing the “space race of the century”, telecommunications company Telstra and the Commonwealth Bank each put in A$10 million; the federal government, A$25 million.

    The boost should allow the Australian team to shrink their timeline for building a 10-qubit processor from 10 to five years.

    They need to work fast: using different types of qubits, MIT and IBM already have a five-qubit processor, Google’s has nine, and Canadian company D-wave controversially boasts 1,000. The Australian team may be behind, but Simmons believes they will win the distance race.

    And she is not the only one.

    “I think in the long term, for any number of reasons silicon will be the winner,” says electrical engineer John Randall, president of Texas-based Zyvex labs, an atomic-scale manufacturing company. “Australia can be a big player.”

    The way Simmons sees it, she is tracking a path not unlike the one that conventional computers followed. It took about a decade to advance from the first transistor bit in 1947 to the first silicon chip. The Australian group achieved the first qubit in 2010; if they get to 10 bits in five years, they will be well on track.

    Simmons’ team is used to her blithe confidence. “Michelle is just mapping it out step by step,” says lab head Tony Raeside as he takes me on a tour through two floors of glass-walled, state-of-the-art rooms of the fabrication centre at University of New South Wales (UNSW). Some of the rooms are brand new – the first fruits of the new funds. The big contraptions, like steel monsters in glass enclosures, are scanning tunnelling electron microscopes (STMs). Like a blind person’s fingers scanning braille, their fine tips detect the contours of individual atoms.

    These finely tuned electron microscopes allow skilled operators like Simmons to fulfil a vision imagined 30 years ago by Nobel prize-winning physicist Richard Feynman: to sculpt matter atom by atom. They are also the key to making the silicon qubit.

    Many remain sceptical about the promise of quantum computing. But things are changing. Two years ago, Simmons was invited to give a tutorial at a satellite conference of the International Electronic Devices Meeting, the premier gathering for the electronics field. The organisers vetted every word of her talk to make sure it didn’t contain anything too mind-bending. They needn’t have worried. Her talk was a hit, and last December, they invited her back to give the keynote lecture.

    Simmons has always had an audacious streak. She tells a story about how, as an eight-year-old, she sat silently week after week watching her father, a high-ranking policeman in London, play chess with her elder brother. One day she asked her father if she could play. Her father reluctantly acquiesced and played without paying much attention – until she took his first pawn. By then it was too late. She checkmated him.

    Simmons’ mother was a bank manager, and her grandparents included diplomats and members of the military. She describes her family as “take-responsibility kind of people”. The family DNA also includes a sense of adventure. Her father would always tell her, “don’t take the easy route. Do the most challenging thing”.

    I am sitting opposite Simmons, now 49, in the Quad café on a sunny wintry day at UNSW. She has rushed in for a snatched lunch and is clad in her signature look of casual black, draped across her tall, solid frame. Her hair is short and practical; she wears no jewellery or make-up. There is a soft, gentle femininity about her. As we talk, I wait for a glimpse of the iron fist that must surely reside inside the velvet glove. Leading a team of brilliant physicists bent on world domination must take some doing.

    It was by popular demand that in 2010, 11 years after joining the group, Simmons became their leader, overseeing the entire consortium of 180 researchers from UNSW, University of Queensland, University of Melbourne, Griffith University and Australian National University. The UNSW headquarters hosts four scientific teams, each with its own research leader: Andrew Dzurak, Sven Rogge, Andrea Morello and Simmons. Like mountaineers navigating a maze of crevasses and cliffs, they are trying different paths but are united in their push to scale the peak of silicon-based computing. (See figure.)

    2
    Cosmos Magazine / UNSW

    “The strength of our centre is that we have three parallel pathways marching forwards. Of course, we all love our own children, but we a have respect and regard for the others,” says Dzurak. Leadership here requires the ability to rally and unify the team while belaying your own rope.

    Simmons revels in the different strengths and perspectives of her colleagues. “Physicists have very unique ways of seeing the world,” she says. She also enjoys pushing them out of their comfort zones and seeing them scale new heights. The key to her leadership is her clarity of purpose. “I could always see the obvious way to go forward,” she says. There is the eight-year-old anticipating all the moves ahead. Only today, she is navigating her way through the chessboard of quantum computing.

    Qubits may be weird, but the day-to-day work of building one is very down to earth. The process starts with a commercial computer chip, a pure silicon crystal wafer about 3 millimetres by 10 millimetres, which is placed inside the ultra-high vacuum chamber of the STM. A trickle of hydrogen gas is bled into the chamber, coating the surface of the wafer with a mask of hydrogen one atom thick. Guided by a scan of the atomic landscape, the tip of the microscope probe becomes a nanoscale etching pen. By varying its voltage, it can be used to poke a single hole through the hydrogen mask or scratch out a line of millions of atoms.

    When phosphene gas seeps in, one phosphate atom will parachute into the hole to become the qubit, while others will fill the long scratch to become the wire that measures the qubit’s signal. Next, the crystal wafer is heated to 350 °C for one minute to bond the phosphate atoms. Then the crystal is coaxed to grow over its new components by sprinkling it with a light soot of silicon atoms – a technique known as epitaxy.

    Simmons pioneered this two-day, 25-step manufacturing technique. “Her ability to position atoms with this accuracy is unique,” says Klaus Ensslin, who heads the nanophysics lab at the Swiss research institute ETH in Zurich. Learning to master it is tough; typically it takes a student a half a year.

    Richard Feynman proposed a basic model for a quantum computer in 1982. The Caltech physicist had been part of the tail end of the quantum mechanics revolution that revealed how strange the universe was at the atomic scale. An electron or the nucleus of an atom has a magnetic orientation called “spin” and can exist in one of two spin states: up or down.

    But in the quantum world, the spins can also exist in these states at the same time. This phenomenon is termed “superposition”. Even more mind-bending, two electrons could influence each others’ spin even if they were at opposite ends of the universe. They were said to be “entangled”. Einstein referred to it as “spooky action at a distance”.

    It was these two properties, superposition and entanglement, that led Feynman to speculate that a quantum computer would be able to perform a massive number of calculations in parallel.

    The bits of a classical computer have a value of either 1 or 0 (because they either pass current or not). But a qubit would also have the value of 1 or 0 simultaneously. The long and short of this quantum logic is that hundreds of qubits are predicted to have the same crunching power as billions of classical bits.

    When it comes to problems that stump modern computers, such as finding the prime factors of huge numbers (the basis of encryption) or finding the optimum path between destinations from billions of possible ones, quantum computers would ace it. That’s why banks and companies that deal with vast databases are so keen on the technology.

    But for two decades, quantum computing remained stuck on the drawing board. Computing requires that calculations are done many times to correct errors. But that’s a problem for quantum computers because each time you read the result you influence it.

    In 1995 several people, including Peter Shor at Bell Labs in the US, figured out how to solve the error correction problem. Shor had also written an algorithm for a quantum computer to factorise prime numbers. Galvanised by the possibilities, labs around the world dove in to try to build a quantum computer. For qubits, MIT used ions trapped in a vacuum; IBM used tiny loops of superconducting metal; others tried quantum dots of gallium arsenide.

    The Australians tried something entirely different: silicon.

    3
    Leading different routes to a silicon-based quantum computer (left to right): UNSW’s Sven Rogge, Andrea Morello, Michelle Simmons and Andrew Dzurak. UNSW

    There is an obvious question to be asked. If silicon really is the clear winner for reliable quantum computing, then why did Australia end up with it, and not MIT or IBM?

    Three things seem to have conspired to make Australia the germination ground: good timing, a core group of visionary physicists, and the exceptionally receptive environment of UNSW.

    Australian physicist Bob Clark founded the group in the late 1990s after returning from Oxford where he had helped pioneer low-dimensional physics. Advances in fabricating silicon and gallium arsenide crystals for the semiconductor industry, using extremely low temperatures and strong magnetic fields, were revealing remarkable new states of matter. So-called “electron gases” with novel behaviours lay between the layers of the crystals. Nobel prizes were awarded for those discoveries.

    To continue the work at UNSW, Clark established a silicon nanofabrication facility and the National Magnet Lab. His reputation attracted bright young physicists from around the world, including Andrew Dzurak, an Australian who had completed a PhD at Cambridge.

    Around 1996, Bruce Kane, a junior scientist from Bell Labs in the US arrived to work on the low-dimensional physics of gallium arsenide crystals. Clark also suggested that he try working with silicon.

    Months later, Kane appeared in Clark’s office bearing a hard-back notebook filled with calculations. In his spare time at UNSW, Kane had worked out a design for the basic elements of a quantum computer.

    Kane’s idea was entirely different from the other approaches in play. He conceived a way to make a qubit using the computer industry’s standard materials. Kane’s qubit would be a single phosphorous atom embedded in a silicon crystal. Because the phosphorous atom is very close in size to the silicon atom, it should cause minimal disturbance to the silicon crystal. Pure silicon, whose atomic nuclei have zero spin, would provide a noiseless background against which to read the spin of the phosphorous nucleus.

    By the time Kane was scribbling in his notebook, it was clear that noise was a limitation of other types of qubits; it was interfering with the ability of the qubit to hold its signal long enough to do some processing – its so-called “coherence” time. While other systems had coherence times of a 1,000th of a second or less, in theory silicon would provide the qubits with entire seconds to carry out its processing.

    Clark stayed up all night reading Kane’s paper. By morning it was clear to him this was a work of genius. It was also clear to him that he would move heaven and earth to bring the idea to fruition. The team filed a patent and published a paper in Nature in 1998. Physicists read it and were enthralled. But there weren’t many who were eager to give it a try.

    Manipulating single atoms to build the qubit was only the start. No-one knew how to read the spin signal of a single electron or nucleus; the available technologies read signals from a million of them.

    “It was a theory on paper but I never thought it would work,” recalls Ensslin.

    But there was one person who did. She was just a junior scientist at Cambridge, but she had a reputation as a world leader in fabricating quantum electronic devices. Dzurak, her former Cambridge colleague, had already regaled her with tales of sunny Sydney skies and the blues of Bondi Beach. In 1999, Simmons joined the budding group of UNSW visionaries.

    4
    A chilled chamber cools the silicon qubits to 0.02 degrees above absolute zero, which allows information to be read and written onto them. Marcus Enno

    When Simmons arrived at UNSW, she was no stranger to daunting problems.

    She encountered one of the first as a 16-year‑old at a rough inner London comprehensive school. In her final two years, the school decided to road-test “independent learning”; it was so independent her class had no chemistry teacher. Most of her classmates failed the year. But Simmons unpacked boxes with the textbooks and chemistry experiments and figured out a DIY chemistry course. It was an experience that forged her signature trait: self-reliance. “ I still believe the best way you learn is by yourself,” she says.

    Years later, that self-reliance got her a dream collaboration with NASA while still working on her PhD. “I love everything space,” she says. Simmons was to test the potential of perfectly symmetrical 3-D crystals grown on the space shuttle to advance solar cell technology.

    But tragedy struck. In 1988, her supervisor, who was fond of taking a weekend swim between two bays in Northeast England, didn’t make it to the second bay. Simmons thought of moving on to another university but her supervisor’s wife entreated her to continue her husband’s work. She stayed. But then the space crystals didn’t cooperate – what came back on the shuttle was a sludge. Seeking help from other Durham professors, she took a different tack. It required combining cadmium sulphide and cadmium telluride – an unusual combination of a cubic and hexagonal crystal. The resultant material was superior to silicon, achieving a record efficiency for solar cells at that time.

    Her success took her to Cambridge, where her modest project was to build the world’s fastest transistor using crystals of gallium arsenide. Known as “the material of the future”, unlike silicon, it could be fabricated as a single crystal, not just in a horizontal layer but also the vertical layer, offering the possibility of 3-D computer chips.

    Simmons became a master fabricator. Even so, she struggled. Out of 10 chips, they all behaved differently. She could not see how the gallium arsenide crystal could ever be useful on an industrial scale or how it could deliver reproducible results as a qubit.

    But when Simmons read Kane’s paper, she sniffed a game-changer. And so when the call from UNSW came, she enthusiastically accepted. It was a place, she recalls, where “peoples’ eyes didn’t glaze over” at crazy ideas.

    Physicists say there is no way to underestimate the difficulty of what Simmons and the group have achieved so far. “All of our Australian friends are pioneers. They pushed this [silicon-based] technology when we gave up,” says Ensslin. But, he adds, “Michelle is truly courageous. She pushed through with amazing tenacity for 10 years”.

    Somehow, Simmons manages to have a life beyond work. She is the mother of three children, aged eight, 11 and 12. Her husband also has a high-powered career as an academic consultant. Her husband’s family makes it all possible, she says. Each time she gave birth, she moved into her husband’s family home for a couple of weeks. Now her kids are older, with different needs. “They are all terrific, but it doesn’t get any easier,” she says. The relentless travel is difficult, but “they believe what I am doing is important”, she adds.

    Her unwavering dedication has not gone unnoticed by the scientific community. Simmons has won a string of Australian and international awards. Last May, she received the prestigious Feynman prize. The judges credited her with creating “the new field of atomic electronics”.

    But Simmons spends little of her own time fabricating atomic electronics now. Her energies are directed towards leading the team’s ascent.

    There’s still a long distance to travel from two qubits to 10. And the pressure to deliver over the next five years is huge.

    Simmons is undaunted. Ever the policeman’s daughter, she remains focused, driven by a sense of responsibility and unafraid to face a challenge. “All my life, I’ve always thought, ‘well this is another little problem, this is what we’ve got to do’ and I’ve always wanted to get on with it,” she says. “It’s all working the way Bruce Kane imagined. That’s what gives me that audacity.”

    See the full article here .

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  • richardmitnick 10:57 am on September 19, 2016 Permalink | Reply
    Tags: , , , Quantum Computing   

    From New Scientist- “Revealed: Google’s plan for quantum computer supremacy” 

    NewScientist

    New Scientist

    31 August 2016 [This just now appeared in social media.]
    Jacob Aron

    1
    Superconducting qubits are tops. UCSB

    The field of quantum computing is undergoing a rapid shake-up, and engineers at Google have quietly set out a plan to dominate.

    SOMEWHERE in California, Google is building a device that will usher in a new era for computing. It’s a quantum computer, the largest ever made, designed to prove once and for all that machines exploiting exotic physics can outperform the world’s top supercomputers.

    And New Scientist has learned it could be ready sooner than anyone expected – perhaps even by the end of next year.

    The quantum computing revolution has been a long time coming. In the 1980s, theorists realised that a computer based on quantum mechanics had the potential to vastly outperform ordinary, or classical, computers at certain tasks. But building one was another matter. Only recently has a quantum computer that can beat a classical one gone from a lab curiosity to something that could actually happen. Google wants to create the first.

    The firm’s plans are secretive, and Google declined to comment for this article. But researchers contacted by New Scientist all believe it is on the cusp of a breakthrough, following presentations at conferences and private meetings.
    .

    “They are definitely the world leaders now, there is no doubt about it,” says Simon Devitt at the RIKEN Center for Emergent Matter Science in Japan. “It’s Google’s to lose. If Google’s not the group that does it, then something has gone wrong.”

    We have had a glimpse of Google’s intentions. Last month, its engineers quietly published a paper detailing their plans (arxiv.org/abs/1608.00263). Their goal, audaciously named quantum supremacy, is to build the first quantum computer capable of performing a task no classical computer can.

    “It’s a blueprint for what they’re planning to do in the next couple of years,” says Scott Aaronson at the University of Texas at Austin, who has discussed the plans with the team.

    So how will they do it? Quantum computers process data as quantum bits, or qubits. Unlike classical bits, these can store a mixture of both 0 and 1 at the same time, thanks to the principle of quantum superposition. It’s this potential that gives quantum computers the edge at certain problems, like factoring large numbers. But ordinary computers are also pretty good at such tasks. Showing quantum computers are better would require thousands of qubits, which is far beyond our current technical ability.

    Instead, Google wants to claim the prize with just 50 qubits. That’s still an ambitious goal – publicly, they have only announced a 9-qubit computer – but one within reach.

    To help it succeed, Google has brought the fight to quantum’s home turf. It is focusing on a problem that is fiendishly difficult for ordinary computers but that a quantum computer will do naturally: simulating the behaviour of a random arrangement of quantum circuits.

    Any small variation in the input into those quantum circuits can produce a massively different output, so it’s difficult for the classical computer to cheat with approximations to simplify the problem. “They’re doing a quantum version of chaos,” says Devitt. “The output is essentially random, so you have to compute everything.”

    To push classical computing to the limit, Google turned to Edison, one of the most advanced supercomputers in the world, housed at the US National Energy Research Scientific Computing Center. Google had it simulate the behaviour of quantum circuits on increasingly larger grids of qubits, up to a 6 × 7 grid of 42 qubits.

    This computation is difficult because as the grid size increases, the amount of memory needed to store everything balloons rapidly. A 6 × 4 grid needed just 268 megabytes, less than found in your average smartphone. The 6 × 7 grid demanded 70 terabytes, roughly 10,000 times that of a high-end PC.

    Google stopped there because going to the next size up is currently impossible: a 48-qubit grid would require 2.252 petabytes of memory, almost double that of the top supercomputer in the world. If Google can solve the problem with a 50-qubit quantum computer, it will have beaten every other computer in existence.

    Eyes on the prize

    By setting out this clear test, Google hopes to avoid the problems that have plagued previous claims of quantum computers outperforming ordinary ones – including some made by Google.

    Last year, the firm announced it had solved certain problems 100 million times faster than a classical computer by using a D-Wave quantum computer, a commercially available device with a controversial history. Experts immediately dismissed the results, saying they weren’t a fair comparison.

    Google purchased its D-Wave computer in 2013 to figure out whether it could be used to improve search results and artificial intelligence. The following year, the firm hired John Martinis at the University of California, Santa Barbara, to design its own superconducting qubits. “His qubits are way higher quality,” says Aaronson.

    It’s Martinis and colleagues who are now attempting to achieve quantum supremacy with 50 qubits, and many believe they will get there soon. “I think this is achievable within two or three years,” says Matthias Troyer at the Swiss Federal Institute of Technology in Zurich. “They’ve showed concrete steps on how they will do it.”

    Martinis and colleagues have discussed a number of timelines for reaching this milestone, says Devitt. The earliest is by the end of this year, but that is unlikely. “I’m going to be optimistic and say maybe at the end of next year,” he says. “If they get it done even within the next five years, that will be a tremendous leap forward.”

    The first successful quantum supremacy experiment won’t give us computers capable of solving any problem imaginable – based on current theory, those will need to be much larger machines. But having a working, small computer could drive innovation, or augment existing computers, making it the start of a new era.

    Aaronson compares it to the first self-sustaining nuclear reaction, achieved by the Manhattan project in Chicago in 1942. “It might be a thing that causes people to say, if we want a full-scalable quantum computer, let’s talk numbers: how many billions of dollars?” he says.

    Solving the challenges of building a 50-qubit device will prepare Google to construct something bigger. “It’s absolutely progress to building a fully scalable machine,” says Ian Walmsley at the University of Oxford.

    For quantum computers to be truly useful in the long run, we will also need robust quantum error correction, a technique to mitigate the fragility of quantum states. Martinis and others are already working on this, but it will take longer than achieving quantum supremacy.

    Still, achieving supremacy won’t be dismissed.

    “Once a system hits quantum supremacy and is showing clear scale-up behaviour, it will be a flare in the sky to the private sector,” says Devitt. “It’s ready to move out of the labs.”

    “The field is moving much faster than expected,” says Troyer. “It’s time to move quantum computing from science to engineering and really build devices.”

    See the full article here .

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  • richardmitnick 10:09 am on August 29, 2016 Permalink | Reply
    Tags: , , Quantum Computing   

    From physicsworld.com: “Nonlinear optical quantum-computing scheme makes a comeback” 

    physicsworld
    physicsworld.com

    Aug 29, 2016
    Hamish Johnston

    A debate that has been raging for 20 years about whether a certain interaction between photons can be used in quantum computing has taken a new twist, thanks to two physicists in Canada. The researchers have shown that it should be possible to use “cross-Kerr nonlinearities” to create a cross-phase (CPHASE) quantum gate. Such a gate has two photons as its input and outputs them in an entangled state. CPHASE gates could play an important role in optical quantum computers of the future.

    Photons are very good carriers of quantum bits (qubits) of information because the particles can travel long distances without the information being disrupted by interactions with the environment. But photons are far from ideal qubits when it comes to creating quantum-logic gates because photons so rarely interact with each other.

    One way around this problem is to design quantum computers in which the photons do not interact with each other. Known as “linear optical quantum computing” (LOQC), it usually involves preparing photons in a specific quantum state and then sending them through a series of optical components, such as beam splitters. The result of the quantum computation is derived by measuring certain properties of the photons.

    Simpler quantum computers

    One big downside of LOQC is that you need lots of optical components to perform basic quantum-logic operations – and the number quickly becomes very large to make an integrated quantum computer that can perform useful calculations. In contrast, quantum computers made from logic gates in which photons interact with each other would be much simpler – at least in principle – which is why some physicists are keen on developing them.

    This recent work on cross-Kerr nonlinearities has been carried out by Daniel Brod and Joshua Combes at the Perimeter Institute for Theoretical Physics and Institute for Quantum Computing in Waterloo, Ontario. Brod explains that a cross-Kerr nonlinearity is a “superidealized” interaction between two photons that can be used to create a CPHASE quantum-logic gate.

    This gate takes zero, one or two photons as input. When the input is zero or one photon, the gate does nothing. But when two photons are present, the gate outputs both with a phase shift between them. One important use of such a gate is to entangle photons, which is vital for quantum computing.

    The problem is that there is no known physical system – trapped atoms, for example – that behaves exactly like a cross-Kerr nonlinearity. Physicists have therefore instead looked for systems that are close enough to create a practical CPHASE. Until recently, it looked like no appropriate system would be found. But now Brod and Combes argue that physicists have been too pessimistic about cross-Kerr nonlinearities and have shown that it could be possible to create a CPHASE gate – at least in principle.

    From A to B via an atom

    Their model is a chain of interaction sites through which the two photons propagate in opposite directions. These sites could be pairs of atoms, in which the atoms themselves interact with each other. The idea is that one photon “A” will interact with one of the atoms in a pair, while the other photon “B” interacts with the other atom. Because the two atoms interact with each other, they will mediate an interaction between photons A and B.

    Unlike some previous designs that implemented quantum error correction to protect the integrity of the quantum information, this latest design is “passive” and therefore simpler.

    Brod and Combes reckon that a high-quality CPHASE gate could be made using five such atomic pairs. Brod told physicsworld.com that creating such a gate in the lab would be difficult, but if successful it could replace hundreds of components in a LOQC system.

    As well as pairs of atoms, Brod says that the gate could be built from other interaction sites such as individual three-level atoms or optical cavities. He and Combes are now hoping that experimentalists will be inspired to test their ideas in the lab. Brod points out that measurements on a system with two interaction sites would be enough to show that their design is valid.

    The work is described in Physical Review Letters. Brod and Combes have also teamed-up with Julio Gea-Banacloche of the University of Arkansas to write a related paper that appears in Physical Review A. This second work looks at their design in more detail.

    See the full article here .

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    PhysicsWorld is a publication of the Institute of Physics. The Institute of Physics is a leading scientific society. We are a charitable organisation with a worldwide membership of more than 50,000, working together to advance physics education, research and application.

    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
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