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  • richardmitnick 8:32 pm on September 28, 2015 Permalink | Reply
    Tags: , , Quantum Computing,   

    From WIRED: “The Other Way A Quantum Computer Could Revive Moore’s Law” 

    Wired logo


    Cade Metz

    D-Wave’s quantum chip. Google

    Google is upgrading its quantum computer. Known as the D-Wave, Google’s machine is making the leap from 512 qubits—the fundamental building block of a quantum computer—to more than a 1000 qubits. And according to the company that built the system, this leap doesn’t require a significant increase in power, something that could augur well for the progress of quantum machines.

    Together with NASA and the Universities Space Research Association, or USRA, Google operates its quantum machine at the NASA Ames Research center not far from its Mountain View, California headquarters. Today, D-Wave Systems, the Canadian company that built the machine, said it has agreed to provide regular upgrades to the system—keeping it “state-of-the-art”—for the next seven years. Colin Williams, director of business development and strategic partnerships for D-Wave, calls this “the biggest deal in the company’s history.” The system is also used by defense giant Lockheed Martin, among others.

    Though the D-Wave machine is less powerful than many scientists hope quantum computers will one day be, the leap to 1000 qubits represents an exponential improvement in what the machine is capable of. What is it capable of? Google and its partners are still trying to figure that out. But Google has said it’s confident there are situations where the D-Wave can outperform today’s non-quantum machines, and scientists at the University of Southern California have published research suggesting that the D-Wave exhibits behavior beyond classical physics.

    Over the life of Google’s contract, if all goes according to plan, the performance of the system will continue to improve. But there’s another characteristic to consider. Williams says that as D-Wave expands the number of qubits, the amount of power needed to operate the system stays roughly the same. “We can increase performance with constant power consumption,” he says. At a time when today’s computer chip makers are struggling to get more performance out of the same power envelope, the D-Wave goes against the trend.

    The Qubit

    A quantum computer operates according to the principles of quantum mechanics, the physics of very small things, such as electrons and photons. In a classical computer, a transistor stores a single “bit” of information. If the transistor is “on,” it holds a 1, and if it’s “off,” it holds a 0. But in quantum computer, thanks to what’s called the superposition principle, information is held in a quantum system that can exist in two states at the same time. This “qubit” can store a 0 and 1 simultaneously.

    Two qubits, then, can hold four values at any given time (00, 01, 10, and 11). And as you keep increasing the number of qubits, you exponentially increase the power of the system. The problem is that building a qubit is a extreme difficult thing. If you read information from a quantum system, it “decoheres.” Basically, it turns into a classical bit that houses only a single value.

    D-Wave believes it has found a way around this problem. It released its first machine, spanning 16 qubits, in 2007. Together with NASA, Google started testing the machine when it reached 512 qubits a few years back. Each qubit, D-Wave says, is a superconducting circuit—a tiny loop of flowing current—and these circuits are dropped to extremely low temperatures so that the current flows in both directions at once. The machine then performs calculations using algorithms that, in essence, determine the probability that a collection of circuits will emerge in a particular pattern when the temperature is raised.

    Reversing the Trend

    Some have questioned whether the system truly exhibits quantum properties. But researchers at USC say that the system appears to display a phenomenon called “quantum annealing” that suggests it’s truly operating in the quantum realm. Regardless, the D-Wave is not a general quantum computer—that is, it’s not a computer for just any task. But D-Wave says the machine is well-suited to “optimization” problems, where you’re facing many, many different ways forward and must pick the best option, and to machine learning, where computers teach themselves tasks by analyzing large amount of data.

    D-Wave says that most of the power needed to run the system is related to the extreme cooling. The entire system consumes about 15 kilowatts of power, while the quantum chip itself uses a fraction of a microwatt. “Most of the power,” Williams says, “is being used to run the refrigerator.” This means that the company can continue to improve its performance without significantly expanding the power it has to use. At the moment, that’s not hugely important. But in a world where classical computers are approaching their limits, it at least provides some hope that the trend can be reversed.

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  • richardmitnick 8:11 pm on September 10, 2015 Permalink | Reply
    Tags: , Quantum Computing,   

    From UNSW: “Quantum industry needs more Australian government support” 

    U NSW bloc

    University of New South Wales

    10 Sep 2015
    Myles Gough

    Australia may win the race to build a revolutionary quantum computer, but UNSW global research leader Michelle Simmons warns that without investment we risk losing the industry offshore.

    Scientia Professor Michelle Simmons addresses the Chief Executive Women annual dinner event in Sydney. Photo: supplied

    Australia may be poised to win the international race to build a quantum computer, but without investment to scale-up and industrialise the technology, the long-term benefits could be lost offshore, says UNSW Scientia Professor Michelle Simmons.

    Two weeks after winning the CSIRO Eureka Prize for Leadership in Science, Simmons is again in the spotlight, delivering a guest lecture at the Chief Executive Women’s 2015 annual dinner in Sydney.

    As the Director of the Australian Research Council Centre of Excellence for Quantum Computation and Communication Technology, Simmons has been instrumental in positioning Australia as the front-runner in the global race to build a quantum computer based in silicon.

    Addressing more than 900 of the nation’s top female leaders from the public and private sectors, Simmons spoke about her passion for physics and the importance of science education in high schools.

    She also warned that Australia is at risk of missing out on the long-term benefits of her world-leading research conducted in her Centre.

    “We are at risk of all the technology we have developed, and the trained human capital, being transferred overseas with little long-term benefit to Australia. The significance of this work to Australia should not be underestimated.”

    “Australia has established a unique approach [to developing a quantum computer] with a competitive edge that has been described by our US funding agencies as having a two to three year lead over the rest of the world,” says Simmons.

    Despite leading the world, she says “there is no mechanism in Australia to scale-up what we have achieved and to translate it industrially”.

    “We are at risk of all the technology we have developed, and the trained human capital, being transferred overseas with little long-term benefit to Australia. The significance of this work to Australia should not be underestimated.”

    Michelle Simmons, WINNER 2015 Eureka Prize for Leadership in Science
    Download mp4 here.

    Exponential increase

    Quantum computers are predicted to provide an extraordinary speed-up in computational power. For each quantum [bit] added to a circuit, the processing power doubles.

    Instead of performing calculations one after the other like a conventional computer, these futuristic machines – which exploit the unusual quantum properties of single atoms, the fundamental constituents of all matter – work in parallel, calculating all possible outcomes at the same time.

    They will be ideal for encrypting information and searching huge databases much faster than conventional computers, and for performing tasks beyond the capability of even the most powerful supercomputers, such as modelling complex biological molecules for drug development.

    “It is predicted that 40% of all Australian industry will be impacted if we realise this technology.”

    Simmons says an Australian-made prototype system using technologies patented by her team, where all functional components are manufactured and controlled on the atomic-scale, could be ready within five years.

    The Commonwealth Bank of Australia recently invested $5 million into the project and Simmons says she is “negotiating contracts with several other major computing, communications and aerospace industries both here and abroad”.

    “We are at risk of all the technology we have developed, and the trained human capital, being transferred overseas with little long-term benefit to Australia.” NO image credit.

    But the rest of the world is making giant strides, and putting up big money: the UK government recently put forward £270 million and the Dutch government €300 million to support quantum information research.

    “Australia is a fantastic place to innovate,” says Simmons. “We attract the best young people from across the world and we undertake leading international science.

    “Our challenge going forward is how to create the environment, opportunities and industries to keep them here.”

    Choosing Australia

    Simmons can speak from first-hand experience. She came to Australia back in 1999 for two reasons: the first, she says, “was academic freedom to pursue something ambitious and high risk”, and the second “was Australia’s ‘can do’ attitude”.

    In the mid-1990s, Simmons was working as an experimental quantum physicist at the University of Cambridge. She had mastered how to design, fabricate and measure electrical devices, which displayed strong quantum effects, and was looking for a new challenge: “to leapfrog the global IT industry and create devices at the atomic scale.”

    When she was awarded an Australian Fellowship to come to UNSW, she withdrew applications for a fellowship to remain at Cambridge, and another for a faculty position at Stanford University in the US.

    “The UK offered years surrounded by pessimistic academics, who would tell you a thousand reasons why your ideas would not work,” she says. “The US offered a highly competitive environment where you would fight both externally and internally for funds.

    “Australia offered independent fellowships, ability to work on large projects with other academics and the ‘can do’ attitude to give it a go.”

    Once in Australia, she set up a team that is still “unique internationally”.

    “Our goal was to adapt the scanning tunnelling microscope (STM) developed by IBM not just to image atoms, but to manipulate them and to make a functional electronic device where the active component is a single atom.”

    Inside the Australian National Fabrication Facility (ANFF) at UNSW, where much of the work on the quantum computer is carried out. Photo: ANFF-NSW/Paul Henderson-Kelly

    Critics, including senior scientists at IBM, believed there were at least eight insurmountable technical challenges.

    “The consensus view within the scientific community was that the chances … were near impossible,” she says.

    Simmons also had to combine two technologies in a way that had never been done before – the STM, which provides the ability to image and manipulate single atoms, and something known as molecular beam epitaxy, which provides the ability to grow a layer of material atom by atom.

    “When I told the two independent system manufacturers in Germany about the idea, they said they would make a laboratory to my design, but that there would be no guarantee that it would work. It was a $3.5 million risk.

    “To my delight it worked a factor of six better than I had hoped. And over the past decade we have systematically solved all those eight challenges that were predicted to block our way.”

    Her team has since developed the world’s first single atom transistor, as well as the narrowest conducting wires in silicon.

    “Australia is a fantastic place to innovate. We attract the best young people from across the world and we undertake leading international science”. Scientia Professor Michelle Simmons with Research Associate Bent Webber.

    Finding physics

    Simmons’ foray into physics began, in part, thanks to a chess match.

    Simmons used to watch her father and brother playing intense games in her family’s living room in south-east London in the 1970s.

    One day, the eight-year-old observer asked to play, eliciting a “somewhat dismissive and terse” response from her father, she recalls.

    “A girl! Wanting to play chess. Well, he indulged me and did something that I believe changed the course of my life,” she says.

    A surprise victory over her father, and several more over the coming weeks and months, saw Simmons take-up competitive chess at her father’s behest, ultimately becoming the London girls chess champion at 12.

    Ultimately, it wasn’t her calling, but chess, she says, taught her to challenge herself and other people’s expectations, and to pursue something she truly loved.

    That love ended up being physics: “I decided to pick the hardest thing that I could find that I enjoyed. Something that I could imagine I would always look forward to; would have to struggle to understand and would feel euphoric about when I had mastered it.”

    She also credits an excellent physics teacher who challenged and encouraged her – and even lined up a phone conversation with a US astronaut, after he learned this was Simmons’ dream profession.

    “The significance of having a passionate teacher, well versed in the subject they teach, cannot be underestimated,” she says. “Great teachers with high expectations challenge their students to be the best they can be.”

    Simmons has exemplified that belief. She was named NSW Scientist of the Year in 2012, was awarded an ARCl Laureate Fellowship in 2013, and in 2014 joined the likes of Stephen Hawking and Albert Einstein as an elected member of the American Academy of Arts and Science.

    “For me, the next challenge is not one of quantum physics but of finding a way working with Australian government, and industries both here and abroad, to establish a high-tech quantum industry in Australia,” she says.

    “To back its brightest and best and to ensure that Australian innovation stays here in Australia.

    “It’s a challenge that I am up for. I fundamentally believe it is the right thing to do and now is the right time to do it.”

    Based at UNSW, the ARC Centre of Excellence for Quantum Computation and Communication Technology is an interdisciplinary, multi-institute centre with more than 180 researchers. In addition to Simmons, key staff members at UNSW include Scientia Professors Andrew Dzurak and Sven Rogge, and Associate Professor Andrea Morello.

    See the full article for additional links.

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    U NSW Campus

    Welcome to UNSW Australia (The University of New South Wales), one of Australia’s leading research and teaching universities. At UNSW, we take pride in the broad range and high quality of our teaching programs. Our teaching gains strength and currency from our research activities, strong industry links and our international nature; UNSW has a strong regional and global engagement.

    In developing new ideas and promoting lasting knowledge we are creating an academic environment where outstanding students and scholars from around the world can be inspired to excel in their programs of study and research. Partnerships with both local and global communities allow UNSW to share knowledge, debate and research outcomes. UNSW’s public events include concert performances, open days and public forums on issues such as the environment, healthcare and global politics. We encourage you to explore the UNSW website so you can find out more about what we do.

  • richardmitnick 9:56 pm on August 12, 2015 Permalink | Reply
    Tags: , , Quantum Computing   

    From phys.org: “Quantum computing advance locates neutral atoms” 


    August 12, 2015
    A’ndrea Elyse Messer

    “We are studying neutral atom qubits because it is clear that you can have thousands in an apparatus,” said Weiss. “They don’t take up much space and they don’t interact with each other unless we want them to.” Credit: © iStock Photo monsitj

    For any computer, being able to manipulate information is essential, but for quantum computing, singling out one data location without influencing any of the surrounding locations is difficult. Now, a team of Penn State physicists has a method for addressing individual neutral atoms without changing surrounding atoms.

    “There are a set of things that we have to have to do quantum computing,” said David S. Weiss, professor of physics. “We are trying to step down that list and meet the various criteria. Addressability is one step.”

    Quantum computers are constructed and operate in completely different ways from the conventional digital computers used today. While conventional computers store information in bits, 1’s and 0’s, quantum computers store information in qubits. Because of a strange aspect of quantum mechanics called superposition, a qubit can be in both its 0 and 1 state at the same time. The methods of encoding information onto neutral atoms, ions or Josephson junctions—electronic devices used in precise measurement, to create quantum computers—are currently the subject of much research. Along with superposition, quantum computers will also take advantage of the quantum mechanical phenomena of entanglement, which can create a mutually dependent group of qubits that must be considered as a whole rather than individually.

    “Quantum computers can solve some problems that classical computers can’t,” said Weiss. “But they are unlikely to replace your laptop.”

    According to the researchers, one area where quantum computers will be valuable is in factoring very large numbers created by multiplying prime numbers, an approach used in creating difficult-to-break security codes.

    Weiss and his graduate students Yang Wang and Aishwarya Kumar, looked at using neutral atoms for quantum computing and investigated ways to individually locate and address an atom to store and retrieve information. They reported their results in a recent issue of Physical Review Letters.

    The researchers first needed to use laser light to create a 3-dimensional lattice of traps for neutral cesium atoms with no more than one atom at each lattice site. Other researchers are investigating ions and superconducting Josephson junctions, but Weiss’s team chose neutral atoms. Research groups at the University of Wisconsin, in France and elsewhere are also investigating neutral atoms for this purpose.

    “We are studying neutral atom qubits because it is clear that you can have thousands in an apparatus,” said Weiss. “They don’t take up much space and they don’t interact with each other unless we want them to.”

    However, Weiss notes that neutral atoms cannot be held in place as well as ions, because background atoms in the near vacuum occasionally knock them out of their traps.

    Once the cesium atoms are in place, the researchers set them to their lowest quantum state by cooling them. They then shift the internal quantum states of the atoms using two perpendicular, circularly polarized addressing beams. Many atoms are shifted, but the targeted atom, which is where the beams cross, is shifted by about twice as much as any other atom. This allows them to then uss microwaves to change the qubit state of the target atom without affecting the states of any other atoms.

    “One atom gate takes about half a millisecond,” said Weiss. “It takes about 5 microseconds to retarget to another atom.”

    Currently, the researchers can only fill about 50 percent of the laser atom traps with atoms, but they can perform quantum gates on those atoms with 93 percent fidelity and cross talk that is too small to measure. The goal is 99.99 percent fidelity. With continued improvements the researchers think that this goal is in reach.

    See the full article here.

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  • richardmitnick 7:59 am on April 27, 2015 Permalink | Reply
    Tags: , , Quantum Computing   

    From COSMOS: “Breakthrough for quantum computers” 

    Cosmos Magazine bloc


    27 Apr 2015
    Cathal O’Connell

    Andrea Morello at work. The computer he and his team are trying to build would use silicon chips not dissimilar to those in a conventional computer.Credit: Marcus Eno

    Electrical engineers at the University of New South Wales trying to develop a silicon quantum computer have cleared one of the last hurdles to building a simple device. The researchers have reported this missing piece in the journal Science Advances.

    “Once you have demonstrated all the parts, then it’s like a Lego box – you can start building up a large architecture by piecing its components together,” project leader Andrea Morello says.

    In their quest to build a silicon quantum computer, Morello and his colleagues have so far been perfecting its basic element, the “quantum bit”. This is a single phosphorus atom entombed in a silicon crystal. Using a carefully tuned magnetic field, the researchers can manipulate the atom’s quantum “spin”, flipping it up or down.

    That phosphorus atom is equivalent to a transistor in an ordinary computer. A transistor is on or off, which is how it represents the 1s and 0s of the binary code the computer uses to process instructions. A quantum bit is more complex. It can be spin-up, spin-down or in a “superposition” of both: 1 and 0 at the same time. Theoretically, this should enable a quantum computer to weigh multiple solutions to a complex problem at once, and solve it at phenomenal speed.

    A quantum computer is “not just a ‘faster’ computer,” Morello says. “They are the equivalent of a jet plane to a bicycle.”

    Last year the UNSW team showed they can write, read and store the spin of a single quantum bit with better than 99.99% accuracy using a magnetic field. But to carry out complex calculations, a quantum computer needs thousands, or even millions of quantum bits, that can all be individually controlled. And for that, the high frequency oscillating magnetic fields Morello has been using to master the control of a single quantum bit are not suitable.

    For a start, the magnetic field generators Morello and his team used are around $100,000 a pop. If they had to use one for each quantum bit in a large array, the cost would be astronomical. There is also a practical problem. Magnetic fields spread, making it impossible to control one quantum bit in an array without inadvertently affecting all its neighbours.

    In their latest work, carried out by experimental physicist Arne Laucht, Morello and his team found a way to control each quantum bit using a simple electrical pulse. Instead of each phosphorus atom having a dedicated magnetic field generator to control it, their new design floods the whole device with a single magnetic field.

    This field is broadcast at a frequency the phosphorus atoms are not tuned in to, and so they don’t feel its magnetic tug. But when a precise electrical pulse is applied to the quantum bit, the electron orbiting the phosphorus atom feels a strong force, stretching its orbit. This distortion to the electron’s orbit works like twisting a tuning knob on a radio – the phosphorus atom is tuned in to the frequency of the magnetic field being broadcast around it, which then causes the quantum bit to flip.

    By timing their electrical pulses, the team can tune the phosphorus atom in and out of the oscillating magnetic field, and so flip the phosphorus atom’s spin into any position they want – up, down or an intermediate superposition – without affecting its neighbours.

    This idea of combining electric and magnetic fields to control individual quantum bits in an array, called “A-gate” control, has been around since 1998. Bruce Kane, an American quantum physicist who was then working at UNSW, proposed it in a paper in Nature that Morello calls “visionary”. Now, 17 years later, technology has caught up with Kane’s ideas as we can now routinely make structures at the scale needed to build his design.

    Kane – now at the University of Maryland and not directly involved in Morello’s research – says he’s been impressed by the “outstanding” work on the design done at UNSW in recent years. The devices work even better than he anticipated. Back in 1998, Kane worried that imperfections in the materials would prevent the device from working as it should. But, he says, the recent work at UNSW, such as the demonstration of an A-gate, proves material imperfections “will not be a show-stopper for silicon quantum computing”.

    Kane cautions that we are still a long way from large-scale quantum computing in silicon, as the challenges that remain, such as moving quantum information around and controlling interactions between large numbers of spins, are daunting. “I continue to believe that large-scale silicon quantum computing will become a reality, but there is still a long, steep road ahead of us,” he says.

    The group is already at work on these challenges. Morello is confident they will have all the elements in place to build a small-scale test-system within 10 years.

    And as for a large-scale quantum computer capable of making useful calculations? Here, Morello is more coy: “To quote Niels Bohr, ‘It’s hard to make predictions, especially about the future’.”

    More on this topic from Cosmos: The quantum spinmeister

    Can physics protect us from Big Brother’s snooping?

    Quantum computing? Yes, no and maybe.

    See the full article here.

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  • richardmitnick 12:24 pm on April 11, 2015 Permalink | Reply
    Tags: , Quantum Computing,   

    From UNSW via phys.org: “Electrical control of quantum bits in silicon paves the way to large quantum computers” 

    U NSW bloc

    University of New South Wales


    April 10, 2015
    Rob Gutro

    Electron wavefunction of a donor under an electrostatic gate. A positive voltage applied to the gate attracts the electron towards the Si-SiO2 interface. This modifies the hyperfine coupling, shifts the resonance frequencies of electron and nucleus, and allows addressing of individual donor qubits in a global microwave field. Credit: A. Laucht, UNSW Australia

    A UNSW-led research team has encoded quantum information in silicon using simple electrical pulses for the first time, bringing the construction of affordable large-scale quantum computers one step closer to reality.

    Lead researcher, UNSW Associate Professor Andrea Morello from the School of Electrical Engineering and Telecommunications, said his team had successfully realised a new control method for future quantum computers.

    The findings were published today in the open-access journal Science Advances.

    Unlike conventional computers that store data on transistors and hard drives, quantum computers encode data in the quantum states of microscopic objects called qubits.

    The UNSW team, which is affiliated with the ARC Centre of Excellence for Quantum Computation & Communication Technology, was first in the world to demonstrate single-atom spin qubits in silicon, reported in Nature in 2012 and 2013.

    The team has already improved the control of these qubits to an accuracy of above 99% and established the world record for how long quantum information can be stored in the solid state, as published in Nature Nanotechnology in 2014.

    It has now demonstrated a key step that had remained elusive since 1998.

    “We demonstrated that a highly coherent qubit, like the spin of a single phosphorus atom in isotopically enriched silicon, can be controlled using electric fields, instead of using pulses of oscillating magnetic fields,” explained UNSW’s Dr Arne Laucht, post-doctoral researcher and lead author of the study.

    Associate Professor Morello said the method works by distorting the shape of the electron cloud attached to the atom, using a very localized electric field.

    “This distortion at the atomic level has the effect of modifying the frequency at which the electron responds.

    “Therefore, we can selectively choose which qubit to operate. It’s a bit like selecting which radio station we tune to, by turning a simple knob. Here, the ‘knob’ is the voltage applied to a small electrode placed above the atom.”

    Electron wave in a phosphorus atom, distorted by a local electric field. Credit: Dr. Arne Laucht

    The findings suggest that it would be possible to locally control individual qubits with electric fields in a large-scale quantum computer using only inexpensive voltage generators, rather than the expensive high-frequency microwave sources.

    Moreover, this specific type of quantum bit can be manufactured using a similar technology to that employed for the production of everyday computers, drastically reducing the time and cost of development.

    The device used in this experiment was fabricated at the NSW node of the Australian National Fabrication Facility, in collaboration with the group led by UNSW Scientia Professor Andrew Dzurak.

    Dr. Arne Laucht (left) and assistant professor Andrea Morello (right). Credit: Paul Henderson-Kelly/UNSW

    Key to the success of this electrical control method is the placement of the qubits inside a thin layer of specially purified silicon, containing only the silicon-28 isotope.

    “This isotope is perfectly non-magnetic and, unlike those in naturally occurring silicon, does not disturb the quantum bit,” Associate Professor Morello said.

    The purified silicon was provided through collaboration with Professor Kohei Itoh from Keio University in Japan.

    See the full article here.

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    U NSW Campus

    Welcome to UNSW Australia (The University of New South Wales), one of Australia’s leading research and teaching universities. At UNSW, we take pride in the broad range and high quality of our teaching programs. Our teaching gains strength and currency from our research activities, strong industry links and our international nature; UNSW has a strong regional and global engagement.

    In developing new ideas and promoting lasting knowledge we are creating an academic environment where outstanding students and scholars from around the world can be inspired to excel in their programs of study and research. Partnerships with both local and global communities allow UNSW to share knowledge, debate and research outcomes. UNSW’s public events include concert performances, open days and public forums on issues such as the environment, healthcare and global politics. We encourage you to explore the UNSW website so you can find out more about what we do.

  • richardmitnick 5:36 am on March 5, 2015 Permalink | Reply
    Tags: , , Quantum Computing   

    From NYT: “Researchers Report Milestone in Developing Quantum Computer” 

    New York Times

    The New York Times

    MARCH 4, 2015

    This device contains nine qubits, the very unstable basic elements of quantum computing equivalent to bits in a regular computer. In the array, each qubit interacts with its neighbors to protect them from error. Credit Julian Kelly/University of California, Santa Barbara, via Google

    Scientists at the University of California, Santa Barbara, and at Google reported on Wednesday in the journal Nature that they had made a significant advance that brings them a step closer to developing a quantum computer.

    Researchers have been pursuing the development of computers that exploit quantum mechanical effects since the 1990s, because of their potential to vastly expand the performance of conventional computers. The goal has long remained out of reach, however, because the computers are composed of basic elements known as qubits that have remained, despite decades of engineering research, highly unstable.

    In contrast to a bit, which is the basic element of a conventional computer and can represent either a zero or a one, a qubit can exist in a state known as superposition, in which it can represent both a zero and a one simultaneously.

    If the qubits are then placed in an entangled state — physically separate but acting with many other qubits as if connected — they can represent a vast number of values simultaneously.

    To date, matrices of qubits that are simultaneously in superposition and entangled have eluded scientists because they are ephemeral, with the encoded information dissipating within microseconds.

    The university and Google researchers reported, however, that they had succeeded in creating an error-correction system that stabilized a fragile array of nine qubits. The researchers said they had accomplished this by creating circuits in which additional qubits were used to observe the state of the computing qubits without altering their state.

    But an important asterisk remains, according to scientists who read an early version of the paper. The Nature paper stated the researchers had succeeded in preserving only the limited “classical” states, rather than the more complex quantum information that would be needed to create a system that outperforms today’s computers.

    The importance of the advance is that the scientists have developed evidence that the system becomes more stable as they interconnect more qubits in the error-checking array. This suggests that far larger arrays of qubits, composed of thousands or tens of thousands of qubits, might be able to control the errors that have until now bedeviled scientists.

    “We have for the first time in the long history of quantum computing an actual device, where we can test all of our ideas about error detection,” said Rami Barends, a quantum electronics engineer at Google and one of the authors of the paper.

    Julian Kelly, another Google quantum electronics engineer, said there remained significant challenges in manufacturing materials for quantum computing.

    In some cases, the scientists are able to rely on existing semiconductor technology, but there are many steps for which they will have to invent approaches.

    The research was reported by scientists working in the laboratory of John M. Martinis, a physicist at the university. In September, Google announced it would join efforts to build a quantum computer as part of the recently established Quantum Artificial Intelligence Laboratory. Under that agreement, Dr. Martinis joined Google while keeping his teaching role, and members of his laboratory became Google employees.

    While the researchers described their new circuit as a significant advance, they acknowledged that they had not yet solved all of the problems that prevented the building of a working quantum computer.

    “While the basic physical processes behind quantum error correction are feasible, many challenges remain, such as improving the logic operations behind error correction and testing protection from phase-flip errors,” the scientists noted in a statement posted on the company’s website.

    In a discussion of the Nature paper on his website, the M.I.T. physicist Scott Aaronson suggested that the achievement represented about half the progress required to build a fully functional quantum computer.

    Google is not the only computing company collaborating with academic researchers in advancing quantum computing. IBM is working with scientists at Yale, and Microsoft is working separately with researchers at the University of California, Santa Barbara, supporting the Station Q research laboratory it created there in 2006.

    See the full article here.

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  • richardmitnick 5:46 pm on January 28, 2015 Permalink | Reply
    Tags: , Isotropy, , Quantum Computing,   

    From UC Berkeley: “Quantum computer as detector shows space is not squeezed” 

    UC Berkeley

    UC Berkeley

    January 28, 2015
    Robert Sanders

    As the Earth rotates every 24 hours, the orientation of the ions in the quantum computer/detector changes with respect to the Sun’s rest frame. If space were squeezed in one direction and not another, the energies of the electrons in the ions would have shifted with a 12-hour period. Hartmut Haeffner image.

    A new experiment by University of California, Berkeley, physicists used partially entangled atoms – identical to the qubits in a quantum computer – to demonstrate more precisely than ever before that this is true, to one part in a billion billion.

    The classic experiment that inspired Albert Einstein was performed in Cleveland by Albert Michelson and Edward Morley in 1887 and disproved the existence of an “ether” permeating space through which light was thought to move like a wave through water. What it also proved, said Hartmut Häffner, a UC Berkeley assistant professor of physics, is that space is isotropic and that light travels at the same speed up, down and sideways.

    “Michelson and Morley proved that space is not squeezed,” Häffner said. “This isotropy is fundamental to all physics, including the Standard Model of physics. If you take away isotropy, the whole Standard Model will collapse. That is why people are interested in testing this.”

    The Standard Model of elementary particles, with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    The Standard Model of particle physics describes how all fundamental particles interact, and requires that all particles and fields be invariant under Lorentz transformations, and in particular that they behave the same no matter what direction they move.

    Häffner and his team conducted an experiment analogous to the Michelson-Morley experiment, but with electrons instead of photons of light. In a vacuum chamber he and his colleagues isolated two calcium ions, partially entangled them as in a quantum computer, and then monitored the electron energies in the ions as Earth rotated over 24 hours.

    If space were squeezed in one or more directions, the energy of the electrons would change with a 12-hour period. It didn’t, showing that space is in fact isotropic to one part in a billion billion (10^18), 100 times better than previous experiments involving electrons, and five times better than experiments like Michelson and Morley’s that used light.

    The results disprove at least one theory that extends the Standard Model by assuming some anisotropy of space, he said.

    Häffner and his colleagues, including former graduate student Thaned Pruttivarasin, now at the Quantum Metrology Laboratory in Saitama, Japan, will report their findings in the Jan. 29 issue of the journal Nature.

    Entangled qubits

    Häffner came up with the idea of using entangled ions to test the isotropy of space while building quantum computers, which involve using ionized atoms as quantum bits, or qubits, entangling their electron wave functions, and forcing them to evolve to do calculations not possible with today’s digital computers. It occurred to him that two entangled qubits could serve as sensitive detectors of slight disturbances in space.

    “I wanted to do the experiment because I thought it was elegant and that it would be a cool thing to apply our quantum computers to a completely different field of physics,” he said. “But I didn’t think we would be competitive with experiments being performed by people working in this field. That was completely out of the blue.”

    He hopes to make more sensitive quantum computer detectors using other ions, such as ytterbium, to gain another 10,000-fold increase in the precision measurement of Lorentz symmetry. He is also exploring with colleagues future experiments to detect the spatial distortions caused by the effects of dark matter particles, which are a complete mystery despite comprising 27 percent of the mass of the universe.

    “For the first time we have used tools from quantum information to perform a test of fundamental symmetries, that is, we engineered a quantum state which is immune to the prevalent noise but sensitive to the Lorentz-violating effects,” Häffner said. “We were surprised the experiment just worked, and now we have a fantastic new method at hand which can be used to make very precise measurements of perturbations of space.”

    Other co-authors are UC Berkeley graduate student Michael Ramm, former UC Berkeley postdoc Michael Hohensee of Lawrence Livermore National Laboratory, and colleagues from the University of Delaware and University of Maryland and institutions in Russia. The work was supported by the National Science Foundation.

    See the full article here.

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  • richardmitnick 2:54 pm on January 22, 2015 Permalink | Reply
    Tags: , , Quantum Computing   

    From Quanta: “Quantum Computing Without Qubits” 

    Quanta Magazine
    Quanta Magazine

    January 22, 2015
    Peter Byrne

    A quantum computing pioneer explains why analog simulators may beat out general-purpose digital quantum machines — for now.

    Ivan Deutsch is building quantum computers out of base-16 “qudits,” quantum information units that can assume any number of “d” states.

    For more than 20 years, Ivan H. Deutsch has struggled to design the guts of a working quantum computer. He has not been alone. The quest to harness the computational might of quantum weirdness continues to occupy hundreds of researchers around the world. Why hasn’t there been more to show for their work? As physicists have known since quantum computing’s beginnings, the same characteristics that make quantum computing exponentially powerful also make it devilishly difficult to control. The quantum computing “nightmare” has always been that a quantum computer’s advantages in speed would be wiped out by the machine’s complexity.

    Yet progress is arriving on two main fronts. First, researchers are developing unique quantum error-correction techniques that will help keep quantum processors up and running for the time needed to complete a calculation. Second, physicists are working with so-called analog quantum simulators — machines that can’t act like a general-purpose computer, but rather are designed to explore specific problems in quantum physics. A classical computer would have to run for thousands of years to compute the quantum equations of motion for just 100 atoms. A quantum simulator could do it in less than a second.

    Quanta Magazine spoke with Deutsch about recent progress in the field, his hopes for the near future, and his own work at the University of New Mexico’s Center for Quantum Information and Control on scaling up binary quantum bits into base-16 digits.

    QUANTA MAGAZINE: Why would a universal quantum machine be so uniquely powerful?

    IVAN DEUTSCH: In a classical computer, information is stored in retrievable bits binary coded as 0 or 1. But in a quantum computer, elementary particles inhabit a probabilistic limbo called superposition where a “qubit” can be coded as 0 and 1.

    Here is the magic: Each qubit can be entangled with the other qubits in the machine. The intertwining of quantum “states” exponentially increases the number of 0s and 1s that can be simultaneously processed by an array of qubits. Machines that can harness the power of quantum logic can deal with exponentially greater levels of complexity than the most powerful classical computer. Problems that would take a state-of-the-art classical computer the age of our universe to solve, can, in theory, be solved by a universal quantum computer in hours.

    What is the quantum computing “nightmare”?

    The same quantum effects that make a quantum computer so blazingly fast also make it incredibly difficult to operate. From the beginning, it has not been clear whether the exponential speed up provided by a quantum computer would be cancelled out by the exponential complexity needed to protect the system from crashing.

    Is the situation hopeless?

    Not at all. We now know that a universal quantum computer will not require exponential complexity in design. But it is still very hard.

    So what’s the problem, and how do we get around it?

    The hardware problem is that the superposition is so fragile that the random interaction of a single qubit with the molecules composing its immediate surroundings can cause the entire network of entangled qubits to delink or collapse. The ongoing calculation is destroyed as each qubit transforms into a digitized classical bit holding a single value: 0 or 1.

    A test-bed quantum computer that Deutsch is working on with his colleague Poul Jessen at the University of Arizona. Courtesy of Poul Jessen

    In classical computers, we reduce the inevitable loss of information by designing a lot of redundancy into the system. Error-correcting algorithms compare multiple copies of the output. They select the most frequent answer and discard the rest of the data as noise. We cannot do that with a quantum computer, because trying to directly compare qubits will crash the program. But we are gradually learning how to keep systems of entangled qubits from collapsing.

    The major obstacle, to my mind, is creating error-correcting software that can keep data from being corrupted as the calculation proceeds toward the final readout. The great trick is to design and implement an algorithm that only measures the errors and not the data, thus preserving the superposition that contains the correct answer.

    Will that end the nightmare?

    It turns out that the error correction technique itself introduces errors. One of the most wonderful advances in quantum computing was recognizing that, in theory, we can correct the new errors without requiring 100 percent precision, allowing minor background noise to pollute the calculation as it rolls along. We cannot actually do this — yet. The main reason that we do not have a working universal quantum computer is that we are still experimenting with how to implant such a “fault-tolerant” algorithm into a quantum circuit. Right now we can control 10 qubits reasonably well. But there is no error-correcting technique, to my knowledge, capable of controlling the thousands of qubits needed to construct a universal machine.

    Is that what you’re working on?

    I study the information processing capabilities of trapped atoms. My colleague Poul Jessen at the University of Arizona and I are pushing the logical power beyond binary-based qubits. For example, what if we can control the superposition of an atom with, say, 16 different energy levels? Using base 16, we can then store what we call a “qudit” in a single atom. That would move us beyond the information processing speed obtainable by a base 2 system, the qubit.

    What other options do we have?

    There may be significant applications available for making non-universal machines: Special purpose, analog quantum simulators designed to solve specific problems, such as how room-temperature superconductors work or how a particular protein folds.

    Are these actually computers?

    They are not universal machines capable of solving any type of question. But say that I want to model global climate change. One way to do this is to write down a mathematical model and then solve the equations on a digital computer. That is typically what climate scientists do. Another way is to try to simulate some aspect of the earth’s climate in a controllable experiment. I can create a simple physical system that obeys the same laws of motion as the system I’m trying to model — mixing nitrogen, oxygen, and hydrogen in a tank, for example. What goes on inside the tank is a real-world computation that tells me something about atmospheric turbulence under certain conditions.

    It is the same with an analog quantum simulator — I use one controllable physical system to simulate another. For example, successfully simulating a superconductor with such a device would reveal the quantum mechanics of high-temperature superconductivity. That could lead to the manufacture of non-brittle superconducting materials for many uses, including building less-fragile quantum circuits. Hopefully, we can learn how to build a robust universal digital computer by experimenting with analog simulators.

    Has anyone built a working analog quantum simulator?

    In 2002, a group at the Max Planck Institute in Germany built an optical lattice — a super-chilled egg carton made of light — and controlled it by pulsing different strengths of laser beams at it. This was a fundamentally analog device designed to obey quantum mechanical equations of motion. The short story is that it successfully simulated how atoms transition between acting as superfluids or insulators. That experiment has sparked a lot of research in analog quantum computing with optical lattices and cold atom traps.

    What are the main challenges for these quantum simulators?

    Because the evolution of the analog simulation is not digitized, the software cannot correct the tiny errors that accumulate during the calculation as we could error-correct noise on a universal machine. The analog device must keep a quantum superposition intact long enough for the simulation to run its course without resorting to digital error correction. This is a particular challenge for the analog approach to quantum simulation.

    Is the D-Wave machine a quantum simulator?

    The D-Wave prototype is not a universal quantum computer. It is not digital, nor error-correcting, nor fault tolerant. It is a purely analog machine designed to solve a particular optimization problem. It is unclear if it qualifies as a quantum device.

    Will a scalable quantum computer be deployed during your lifetime?

    We are pushing past the nightmare. Around the world, many university-based labs are working hard to remove or bypass the road block of fault tolerance. Academic researchers are leading the way, intellectually. For example, the groups of Rob Schoelkopf and Michel H. Devoret at Yale are taking superconducting technologies close to fault-tolerance.

    But constructing a working universal digital quantum computer will likely require mobilizing industrial-scale resources. To that end, IBM is exploring quantum computing with superconducting circuits with personnel largely from the Yale groups. Google is working with John Martinis’s lab at the University of California, Santa Barbara. HRL Laboratories is working on silicon-based quantum computing. Lockheed Martin is exploring ion traps. And who knows what the National Security Agency is up to.

    But generally in academic labs, without these industrial-scale resources, scientists are focusing more and more on learning how to control analog quantum simulators. There is short-term fruit to be picked in that arena — both intellectually and in the currency of academics: publishable papers.

    Are you willing to settle for analog?

    I favor pursuing the digital approach full force. Before I die, I would love to see just one universal logical qubit that can be indefinitely error corrected. It would instantly be classified by the government, of course. But I dream on, regardless.

    See the full article here.

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    Formerly known as Simons Science News, Quanta Magazine is an editorially independent online publication launched by the Simons Foundation to enhance public understanding of science. Why Quanta? Albert Einstein called photons “quanta of light.” Our goal is to “illuminate science.” At Quanta Magazine, scientific accuracy is every bit as important as telling a good story. All of our articles are meticulously researched, reported, edited, copy-edited and fact-checked.

  • richardmitnick 7:23 am on January 9, 2015 Permalink | Reply
    Tags: , , Quantum Computing   

    From MIT: “Toward quantum chips” 

    MIT News

    January 9, 2015
    Larry Hardesty | MIT News Office

    A team of researchers has built an array of light detectors sensitive enough to register the arrival of individual light particles, or photons, and mounted them on a silicon optical chip. Such arrays are crucial components of devices that use photons to perform quantum computations.

    One of the researchers’ new photon detectors, deposited athwart a light channel — or “waveguide” (horizontal black band) — on a silicon optical chip.
    Image courtesy of Nature Communications

    Single-photon detectors are notoriously temperamental: Of 100 deposited on a chip using standard manufacturing techniques, only a handful will generally work. In a paper appearing today in Nature Communications, the researchers at MIT and elsewhere describe a procedure for fabricating and testing the detectors separately and then transferring those that work to an optical chip built using standard manufacturing processes.

    In addition to yielding much denser and larger arrays, the approach also increases the detectors’ sensitivity. In experiments, the researchers found that their detectors were up to 100 times more likely to accurately register the arrival of a single photon than those found in earlier arrays.

    “You make both parts — the detectors and the photonic chip — through their best fabrication process, which is dedicated, and then bring them together,” explains Faraz Najafi, a graduate student in electrical engineering and computer science at MIT and first author on the new paper.

    Thinking small

    According to quantum mechanics, tiny physical particles are, counterintuitively, able to inhabit mutually exclusive states at the same time. A computational element made from such a particle — known as a quantum bit, or qubit — could thus represent zero and one simultaneously. If multiple qubits are “entangled,” meaning that their quantum states depend on each other, then a single quantum computation is, in some sense, like performing many computations in parallel.

    With most particles, entanglement is difficult to maintain, but it’s relatively easy with photons. For that reason, optical systems are a promising approach to quantum computation. But any quantum computer — say, one whose qubits are laser-trapped ions or nitrogen atoms embedded in diamond — would still benefit from using entangled photons to move quantum information around.

    “Because ultimately one will want to make such optical processors with maybe tens or hundreds of photonic qubits, it becomes unwieldy to do this using traditional optical components,” says Dirk Englund, the Jamieson Career Development Assistant Professor in Electrical Engineering and Computer Science at MIT and corresponding author on the new paper. “It’s not only unwieldy but probably impossible, because if you tried to build it on a large optical table, simply the random motion of the table would cause noise on these optical states. So there’s been an effort to miniaturize these optical circuits onto photonic integrated circuits.”

    The project was a collaboration between Englund’s group and the Quantum Nanostructures and Nanofabrication Group, which is led by Karl Berggren, an associate professor of electrical engineering and computer science, and of which Najafi is a member. The MIT researchers were also joined by colleagues at IBM and NASA’s Jet Propulsion Laboratory.


    The researchers’ process begins with a silicon optical chip made using conventional manufacturing techniques. On a separate silicon chip, they grow a thin, flexible film of silicon nitride, upon which they deposit the superconductor niobium nitride in a pattern useful for photon detection. At both ends of the resulting detector, they deposit gold electrodes.

    Then, to one end of the silicon nitride film, they attach a small droplet of polydimethylsiloxane, a type of silicone. They then press a tungsten probe, typically used to measure voltages in experimental chips, against the silicone.

    “It’s almost like Silly Putty,” Englund says. “You put it down, it spreads out and makes high surface-contact area, and when you pick it up quickly, it will maintain that large surface area. And then it relaxes back so that it comes back to one point. It’s like if you try to pick up a coin with your finger. You press on it and pick it up quickly, and shortly after, it will fall off.”

    With the tungsten probe, the researchers peel the film off its substrate and attach it to the optical chip.

    In previous arrays, the detectors registered only 0.2 percent of the single photons directed at them. Even on-chip detectors deposited individually have historically topped out at about 2 percent. But the detectors on the researchers’ new chip got as high as 20 percent. That’s still a long way from the 90 percent or more required for a practical quantum circuit, but it’s a big step in the right direction.

    “This work is a technical tour de force,” says Robert Hadfield, a professor of photonics at the University of Glasgow who was not involved in the research. “There is potential for scale-up to large circuits requiring hundreds of detectors using commercial pick-and-place technology.”

    See the full article here.

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  • richardmitnick 3:56 pm on December 9, 2014 Permalink | Reply
    Tags: , , Quantum Computing   

    From NOVA: “Is There Anything Beyond Quantum Computing?” 



    Thu, 10 Apr 2014
    Scott Aaronson

    A quantum computer is a device that could exploit the weirdness of the quantum world to solve certain specific problems much faster than we know how to solve them using a conventional computer. Alas, although scientists have been working toward the goal for 20 years, we don’t yet have useful quantum computers. While the theory is now well-developed, and there’s also been spectacular progress on the experimental side, we don’t have any computers that uncontroversially use quantum mechanics to solve a problem faster than we know how to solve the same problem using a conventional computer.

    Credit: Marcin Wichary/Flickr, under a Creative Commons license.

    Yet some physicists are already beginning to theorize about what might lie beyond quantum computers. You might think that this is a little premature, but I disagree. Think of it this way: From the 1950s through the 1970s, the intellectual ingredients for quantum computing were already in place, yet no one broached the idea. It was as if people were afraid to take the known laws of quantum physics and see what they implied about computation. So, now that we know about quantum computing, it’s natural not to want to repeat that mistake! And in any case, I’ll let you in on a secret: Many of us care about quantum computing less for its (real but modest) applications than because it defies our preconceptions about the ultimate limits of computation. And from that standpoint, it’s hard to avoid asking whether quantum computers are “the end of the line.”

    Now, I’m emphatically not asking a philosophical question about whether a computer could be conscious, or “truly know why” it gave the answer it gave, or anything like that. I’m restricting my attention to math problems with definite right answers: e.g., what are the prime factors of a given number? And the question I care about is this: Is there any such problem that couldn’t be solved efficiently by a quantum computer, but could be solved efficiently by some other computer allowed by the laws of physics?

    Here I’d better explain that, when computer scientists say “efficiently,” they mean something very specific: that is, that the amount of time and memory required for the computation grows like the size of the task raised to some fixed power, rather than exponentially. For example, if you want to use a classical computer to find out whether an n-digit number is prime or composite—though not what its prime factors are!—the difficulty of the task grows only like n cubed; this is a problem classical computers can handle efficiently. If that’s too technical, feel free to substitute the everyday meaning of the word “efficiently”! Basically, we want to know which problems computers can solve not only in principle, but in practice, in an amount of time that won’t quickly blow up in our faces and become longer than the age of the universe. We don’t care about the exact speed, e.g., whether a computer can do a trillion steps or “merely” a billion steps per second. What we care about is the scaling behavior: How does the number of steps grow as the number to be factored, the molecule to be simulated, or whatever gets bigger and bigger? Scaling behavior is where we see profound differences between today’s computers and quantum computers; it’s the whole reason why anyone wants to build quantum computers in the first place. So, could there be a physical device whose scaling behavior is better than quantum computers’?

    The Simulation Machine

    A quantum computer, as normally envisioned, would be a very specific kind of quantum system: one built up out of “qubits,” or quantum bits, which exist in “superpositions” of the “0” and “1” states. It’s not immediately obvious that a machine based on qubits could simulate other kinds of quantum-mechanical systems, for example, systems involving particles (like electrons and photons) that can move around in real space. And if there are systems that are hard to simulate on standard, qubit-based quantum computers, then those systems themselves could be thought of as more powerful kinds of quantum computers, which solve at least one problem—the problem of simulating themselves—faster than is otherwise possible.
    “It looks likely that a single device, a quantum computer, would in the future be able to simulate all of quantum chemistry and atomic physics efficiently.”

    So maybe Nature could allow more powerful kinds of quantum computers than the “usual” qubit-based kind? Strong evidence that the answer is “no” comes from work by Richard Feynman in the 1980s, and by Seth Lloyd and many others starting in the 1990s. They showed how to take a wide range of realistic quantum systems and simulate them using nothing but qubits. Thus, just as today’s scientists no longer need wind tunnels, astrolabes, and other analog computers to simulate classical physics, but instead represent airflow, planetary motions, or whatever else they want as zeroes and ones in their digital computers, so too it looks likely that a single device, a quantum computer, would in the future be able to simulate all of quantum chemistry and atomic physics efficiently.

    So far, we’ve been talking about computers that can simulate “standard,” non-relativistic quantum mechanics. If we want to bring special relativity into the picture, we need quantum field theory—the framework for modern particle physics, as studied at colliders like the LHC—which presents a slew of new difficulties. First, many quantum field theories aren’t even rigorously defined: It’s not clear what we should program our quantum computer to simulate. Also, in most quantum field theories, even a vacuum is a complicated object, like an ocean surface filled with currents and waves. In some sense, this complexity is a remnant of processes that took place in the moments after the Big Bang, and it’s not obvious that a quantum computer could efficiently simulate the dynamics of the early universe in order to reproduce that complexity. So, is it possible that a “quantum field theory computer” could solve certain problems more efficiently than a garden-variety quantum computer? If nothing else, then at least the problem of simulating quantum field theory?

    While we don’t yet have full answers to these questions, over the past 15 years we’ve accumulated strong evidence that qubit quantum computers are up to the task of simulating quantum field theory. First, Michael Freedman, Alexei Kitaev, and Zhenghan Wang showed how to simulate a “toy” class of quantum field theories, called topological quantum field theories (TQFTs), efficiently using a standard quantum computer. These theories, which involve only two spatial dimensions instead of the usual three, are called “topological” because in some sense, the only thing that matters in them is the global topology of space. (Interestingly, along with Michael Larsen, these authors also proved the converse: TQFTs can efficiently simulate everything that a standard quantum computer can do.)

    Then, a few years ago, Stephen Jordan, Keith Lee, and John Preskill gave the first detailed, efficient simulation of a “realistic” quantum field theory using a standard quantum computer. (Here, “realistic” means they can simulate a universe containing a specific kind of particle called scalar particles. Hey, it’s a start.) Notably, Jordan and his colleagues solve the problem of creating the complicated vacuum state using an algorithm called “adiabatic state preparation” that, in some sense, mimics the cooling the universe itself underwent shortly after the Big Bang. They haven’t yet extended their work to the full Standard Model of particle physics, but the difficulties in doing so are probably surmountable.

    So, if we’re looking for areas of physics that a quantum computer would have trouble simulating, we’re left with just one: quantum gravity. As you might have heard, quantum gravity has been the white whale of theoretical physicists for almost a century. While there are deep ideas about it (most famously, string theory), no one really knows yet how to combine quantum mechanics with [Albert] Einstein’s general theory of relativity, leaving us free to project our hopes onto quantum gravity—including, if we like, the hope of computational powers beyond those of quantum computers!

    Boot Up Your Time Machine

    But is there anything that could support such a hope? Well, quantum gravity might force us to reckon with breakdowns of causality itself, if closed timelike curves (i.e., time machines to the past) are possible. A time machine is definitely the sort of thing that might let us tackle problems too hard even for a quantum computer, as David Deutsch, John Watrous and I have pointed out. To see why, consider the “Shakespeare paradox,” in which you go back in time and dictate Shakespeare’s plays to him, to save Shakespeare the trouble of writing them. Unlike with the better-known “grandfather paradox,” in which you go back in time and kill your grandfather, here there’s no logical contradiction. The only “paradox,” if you like, is one of “computational effort”: somehow Shakespeare’s plays pop into existence without anyone going to the trouble to write them!
    “A time machine is definitely the sort of thing that might let us tackle problems too hard even for a quantum computer.”

    Using similar arguments, it’s possible to show that, if closed timelike curves exist, then under fairly mild assumptions, one could “force” Nature to solve hard combinatorial problems, just to keep the universe’s history consistent (i.e., to prevent things like the grandfather paradox from arising). Notably, the problems you could solve that way include the NP-complete problems: a class that includes hundreds of problems of practical importance (airline scheduling, chip design, etc.), and that’s believed to scale exponentially in time even for quantum computers.

    Of course, it’s also possible that quantum gravity will simply tell us that closed timelike curves can’t exist—and maybe the computational superpowers they would give us if they did exist is evidence that they must be forbidden!

    Simulating Quantum Gravity

    Going even further out on a limb, the famous mathematical physicist Roger Penrose has speculated that quantum gravity is literally impossible to simulate using either an ordinary computer or a quantum computer, even with unlimited time and memory at your disposal. That would put simulating quantum gravity into a class of problems studied by the logicians Alan Turing and Kurt Gödel in the 1930s, which includes problems way harder than even the NP-complete problems—like determining whether a given computer program will ever stop running (the “halting problem”). Penrose further speculates that the human brain is sensitive to quantum gravity effects, and that this gives humans the ability to solve problems that are fundamentally unsolvable by computers. However, virtually no other expert in the relevant fields agrees with the arguments that lead Penrose to this provocative position.

    What’s more, there are recent developments in quantum gravity that seem to support the opposite conclusion: that is, they hint that a standard quantum computer could efficiently simulate even quantum-gravitational processes, like the formation and evaporation of black holes. Most notably, the AdS/CFT correspondence, which emerged from string theory, posits a “duality” between two extremely different-looking kinds of theories. On one side of the duality is AdS (Anti de Sitter): a theory of quantum gravity for a hypothetical universe that has a negative cosmological constant, effectively causing the whole universe to be surrounded by a reflecting boundary. On the other side is a CFT (Conformal Field Theory): an “ordinary” quantum field theory, without gravity, that lives only on the boundary of the AdS space. The AdS/CFT correspondence, for which there’s now overwhelming evidence (though not yet a proof), says that any question about what happens in the AdS space can be translated into an “equivalent” question about the CFT, and vice versa.
    “Even if a quantum gravity theory seems ‘wild’—even if it involves nonlocality, wormholes, and other exotica—there might be a dual description of the theory that’s more ‘tame,’ and that’s more amenable to simulation by a quantum computer.”

    This suggests that, if we wanted to simulate quantum gravity phenomena in AdS space, we might be able to do so by first translating to the CFT side, then simulating the CFT on our quantum computer, and finally translating the results back to AdS. The key point here is that, since the CFT doesn’t involve gravity, the difficulties of simulating it on a quantum computer are “merely” the relatively prosaic difficulties of simulating quantum field theory on a quantum computer. More broadly, the lesson of AdS/CFT is that, even if a quantum gravity theory seems “wild”—even if it involves nonlocality, wormholes, and other exotica—there might be a dual description of the theory that’s more “tame,” and that’s more amenable to simulation by a quantum computer. (For this to work, the translation between the AdS and CFT descriptions also needs to be computationally efficient—and it’s possible that there are situations where it isn’t.)

    The Black Hole Problem

    So, is there any other hope for doing something in Nature that a quantum computer couldn’t efficiently simulate? Let’s circle back from the abstruse reaches of string theory to some much older ideas about how to speed up computation. For example, wouldn’t it be great if you could program your computer to do the first step of a computation in one second, the second step in half a second, the third step in a quarter second, the fourth step in an eighth second, and so on—halving the amount of time with each additional step? If so, then much like in Zeno’s paradox, your computer would have completed infinitely many steps in a mere two seconds!

    Or, what if you could leave your computer on Earth, working on some incredibly hard calculation, then board a spaceship, accelerate to close to the speed of light, then decelerate and return to Earth? If you did this, then Einstein’s special theory of relativity firmly predicts that, depending on just how close you got to the speed of light, millions or even trillions of years would have elapsed in Earth’s frame of reference. Presumably, civilization would have collapsed and all your friends would be long dead. But if, hypothetically, you could find your computer in the ruins and it was still running, then you could learn the answer to your hard problem!

    We’re now faced with a puzzle: What goes wrong if you try to accelerate computation using these sorts of tricks? The key factor is energy. Even in real life, there are hobbyists who “overclock” their computers, or run them faster than the recommended speed; for example, they might run a 1000 MHz chip at 2000 MHz. But the well-known danger in doing this is that your microchip might overheat and melt! Indeed, it’s precisely because of the danger of overheating that your computer has a fan. Now, the faster you run your computer, the more cooling you need—that’s why many supercomputers are cooled using liquid nitrogen. But cooling takes energy. So, is there some fundamental limit here? It turns out that there is. Suppose you wanted to cool your computer so completely that it could perform about 1043 operations per second—that is, one about operation per Planck time (where a Planck time, ~10-43 seconds, is the smallest measurable unit of time in quantum gravity). To run your computer that fast, you’d need so much energy concentrated in so small a space that, according to general relativity, your computer would collapse into a black hole!

    And the story is similar for the “relativity computer.” There, the more you want to speed up your computer, the closer you have to accelerate your spaceship to the speed of light. But the more you accelerate the spaceship, the more energy you need, with the energy diverging to infinity as your speed approaches that of light. At some point, your spaceship will become so energetic that it, too, will collapse into to a black hole.

    Now, how do we know that collapse into a black hole is inevitable—that there’s no clever way to avoid it? The calculation combines Newton’s gravitational constant G with Planck’s constant h, the central constant of quantum mechanics. That means one is doing a quantum gravity calculation! I’ll end by letting you savor the irony: Even as some people hope that a quantum theory of gravity might let us surpass the known limits of quantum computers, quantum gravity might play just the opposite role, enforcing those limits.

    See the full article here.

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    Stem Education Coalition

    NOVA is the highest rated science series on television and the most watched documentary series on public television. It is also one of television’s most acclaimed series, having won every major television award, most of them many times over.

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