Category Archives: Quantum Physics

Amazon Web Services has partnered with the National University of Singapore (NUS) in hopes of improving quantum technology and its applications. The duo announced this week that it has signed a memorandum of understanding.

The collaboration is a quantum engineering program hosted by NUS (QEP), Five-year SG $ 25m ($ 18.5m, 13.3m, 15.6m) initiative launched in 2018 by Singapores National Research Foundation to become a technology that can commercialize the abstract science of quantum physics. Focus on converting.

So far, QEP has eight major research projects that could ultimately outperform todays supercomputers, such as hardware and software, to simulate chemicals and help design drugs. I have supported it. reality.

QEP is currently working with companies to identify the problems they are facing that quantum technology may or may not be able to address soon or soon. QEP director Alexander Lynn said. Register..

For example, we help quantum computing software researchers explore algorithms and simulation techniques that can be applied to real-world data. They aim to address supply chain management, finance, trade, chemistry, and materials challenges. The proposal is currently being considered for financing.

Quantum computing may require a leap of science and engineering to create a working system, but one day it will be able to provide powerful computing tools that go beyond the boundaries of traditional computers. maybe. And if the quantum computer takes off (if it is still in the scientific experiment stage), the communication needs to be quantum protected. These computers may be able to computeally decipher unquantum-protected data.

Some forms of encryption used today can be broken by large quantum computers in the future, which also facilitates the search for alternatives, says Ling.

and Canned statement, NUS said AWS will be able to access the universitys National Quantum-Safe Network. It is a vendor-neutral platform for developing technology and integrating some of it into local fiber networks.

The understanding that we are using quantum communication technology to support experiments with existing fibers is correct, said Tan Lee Chew, managing director of AWS ASEAN. Register.

According to Tan, AWS has the opportunity to support Singapores SmartNation initiatives such as traffic optimization, financial planning, shipping and port operations, and material design applications within commercial organizations.

The goal is to train Singaporean scholars, students, and commercial organizations to develop quantum computing skills.

Quantum technology could help Singapore accelerate the smart nation agenda, Tan added. own products.

Inevitably, there are also some joint public relations activities.

Last August, AWS debuted a cloud-based quantum computing-like service. bracket.. Products that pay only what they need provide access to quantum annealers. A gate-based system built on superconducting cubits and trapped ions. Hybrid quantum and classical algorithm tools. Users work in a Jupyter notebook environment.

The quantum cloud initiative is nothing new. IBM and Microsoft are already doing that.In fact, IBM is already 3 years collaboration Big Blue uses QEP to provide NUS researchers with cloud access to 15 of IBMs current generation quantum computing systems.

How about A huge machine that is AWS, Ling said, an existing relationship already exists. Singapore researchers already had connections with companies working with AWS to provide cloud access to quantum hardware.

AWS leverages Singapore scientists to overcome the hurdles facing quantum computing The Register

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AWS leverages Singapore scientists to overcome the hurdles facing quantum computing The Register - Illinoisnewstoday.com

If you look at the number of stars for long enough, you will probably start to wonder the same thing that scientists and philosophers have for years: if there are so many potential worlds out there, where is everybody, and why is nobody getting in touch?

Solutions to what is known as the Fermi Paradox range from the horrifying to the really horrifying. But a new pre-print paper (not yet peer-reviewed) by aquantum physicist atImperial College London proposes a new possibility: what if alien messages are hidden right there the stars themselves?

Terry Rudolph suggests in his paper that if aliens should wish to communicate vast distances without alerting others to their messages, they might be able to do soin a way that is indistinguishable from thermal radiation for any other species listening to the skies. The basic principle is that species that have spread out among the stars could thensend messages by entangling photons in separate stars, altering the light that is given off by one star byinterfering with another. The receiver could then check in on their message by observing the second star, using linear optics.

A quick refresher from Science ABC.

"Photons can propagate billions of light-years and retain significant quantum coherence," Rudolph writes in the paper."One consequence is therefore that a sufficiently advanced civilization can perform quantum non-demolition measurements of photon number on suitable modes of light being emitted from stars, in such a way that useful large-scale entanglement is distributed by the subsequent free-space propagation of that light through the universe."

The method, he believes, would be particularly appealing to paranoid aliens due to how difficult it would beto distinguish from normal thermal signatures.

"The upshot is that when we look to the stars and see only thermal radiation we typically conclude the universe is empty," he writes. "But perhaps, riding in the correlations of that radiation, the universe is actually bathed in alien chatter and other forms of distributed quantum information processing."

Though the (incredibly complicated) method he suggests is possible according to the laws of physics as we understand them, he is not suggesting that this is a way that aliensarecommunicating, just that it's a method by which theycouldcommunicate. In fact, disappointingly, the only way we may know that technically advanced aliensareusing stars as an intergalactic WhatsAppis... if they tell us that.

"Unfortunately this is all fundamentally hidden from us if quantum theory is correct. And if it isnt correct, then presumably the aliens know that and so are not using this method," Rudolph writes.

"It seems, therefore, that the only way to test this hypothesis is to wait for them to drop by (again?) and let us know which case pertains."

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'Perhaps They Are Everywhere?': Physicist Claims Alien Messages Could Be Hidden In The Stars - IFLScience

Interlayer excitons (glossy ellipsoids), which can form after electrons and holes (red and blue spheres) are separated between optically excited, atomically thin layers (top and bottom sheet). Credit: Markus Plankl (2021)

A team of physicists from Germany, the US and the UK managed to observe the motion of electrons from one atomically thin layer into an adjacent one with nanoscale spatial resolution. The new contact-free nanoscopy concept, which shows great potential for investigations into conducting, nonconducting, and superconducting materials, will be introduced in the new volume of the science journal Nature Photonics.

Nanotechnology sometimes still sounds like science fiction, but is already an integral part of modern electronics in our computers, smart phones, and cars. The size of electronic components, like transistors and diodes has reached the nanoscale, corresponding to only one-millionth of a millimeter. This makes conventional optical microscopes no longer sufficient for inspecting these nanostructures. To develop innovative future nanotechnology, scientists have replaced the optical microscope with much more sophisticated concepts, such as electron or scanning tunneling microscopy. However, these techniques use electrons instead of light, which can influence the properties of nanoscale devices. Furthermore, these important measurement techniques are limited to electrically conducting samples.

A team of physicists around Rupert Huber and Jaroslav Fabian at the Regensburg Center for Ultrafast Nanoscopy (RUN) at Universitt Regensburg together with colleagues Tyler Cocker from Michigan State University, USA, and Jessica Boland from the University of Manchester, UK, have introduced a new technique, which can resolve electron motion on the nanoscale without needing to be electrically contacted. Better still, the new method also reaches unbelievable time resolution as good as one quadrillionth of a second (the femtosecond timescale). Combining these extreme spatial and temporal resolutions makes the recording of slow-motion movies of ultrafast electron dynamics on the nanoscale possible.

The concept behind the technique works similar to contactless payment (Chipcard, Phone, Scanner), which has become an increasingly common component in our lives since the start of the pandemic. These payment methods are based on established frequencies and protocols on the macroscale such as Near Field Communication (NFC). Here, the scientists transferred this idea down to the nanoscale by using a sharp metallic tip as a nano-antenna, which is brought close to the investigated sample.

In contrast to the aforementioned established techniques, where tips are used to drive a current through the sample, the new concept uses a weak alternating electric field to scan the sample contactlessly. The frequency used in the experiments is boosted to the terahertz spectral range, approximately 100,000 times higher than the one used in NFC scanners. Minute changes in these weak electric fields allow for precise conclusions about the local electron motion within the material.

Combining the measurements with a realistic quantum theory shows that the concept even allows for quantitative results. In order to achieve high temporal resolution additionally, the physicists used extremely short light pulses to record crisp snapshots of the movement of electrons over nanometer distances.

The team chose a sample of a new material class called transition metal dichalcogenides, which can be produced in atomically thin layers, as their first test sample. When these sheets are stacked under freely chosen angles, new artificial solids emerge with novel material properties, which are prominently investigated in the Collaborative Research Center 1277 in Regensburg.

The sample under study was made from two different atomically thin dichalcogenides to test the centerpiece of a futuristic solar cell. When shining green light onto the structure, charge carriers emerge that will move in one or the other direction depending on their polarity the basic principle of a solar cell, which converts light into electricity. The ultrafast charge separation was observed by the scientists in time as well as in space with nanometer precision. To their surprise, the charge separation even works reliably when the dichalcogenide layers lay over tiny impurities like a mini carpet important insights to optimize these new materials for future use in solar cells or computer chips.

The researchers are exhilarated by their insightful results. We cant wait to videotape further fascinating charge transfer processes in insulating, conducting and superconducting materials, explains Markus Plankl, first author of the publication. Postdoctoral colleague and co-author Thomas Siday adds: Insights on the ultrafast transport on the relevant length and timescales will help us to understand how tunneling shapes the functionalities in a wide range of condensed matter systems.

Besides nanostructures in physics, previously elusive quantum processes in biological systems can now be accessed. These results reflect the increasing focus of researchers from biology, chemistry, and physics at the University of Regensburg towards ultrafast nanoscopy, which led to the approval of the new Regensburg Center for Ultrafast Nanoscopy (RUN). The RUN building, which is currently under construction on the University campus, should provide the optimal environment for such an interdisciplinary exploration of the nanocosm.

Reference: Subcycle contact-free nanoscopy of ultrafast interlayer transport in atomically thin heterostructures by M. Plankl, P. E. Faria Junior, F. Mooshammer, T. Siday, M. Zizlsperger, F. Sandner, F. Schiegl, S. Maier, M. A. Huber, M. Gmitra, J. Fabian, J. L. Boland, T. L. Cocker and R. Huber, 13 May 2021, Nature Photonics.DOI: 10.1038/s41566-021-00813-y

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Ultrafast Terahertz Nanoscopy: Social Distancing on the Nanoscale - SciTechDaily

1. The quantum world is lumpy

The quantum world has a lot in common with shoes. You cant just go to a shop and pick out sneakers that are an exact match for your feet. Instead, youre forced to choose between pairs that come in predetermined sizes.

The subatomic world is similar. Albert Einstein won a Nobel Prize for proving that energy is quantized. Just as you can only buy shoes in multiples of half a size, so energy only comes in multiples of the same "quanta" hence the name quantum physics.

The quanta here is the Planck constant, named after Max Planck, the godfather of quantum physics. He was trying to solve a problem with our understanding of hot objects like the sun. Our best theories couldnt match the observations of the energy they kick out. By proposing that energy is quantized, he was able to bring theory neatly into line with experiment.

J. J. Thomson won the Nobel Prize in 1906 for his discovery that electrons are particles. Yet his son George won the Nobel Prize in 1937 for showing that electrons are waves. Who was right? The answer is both of them. This so-called wave-particle duality is a cornerstone of quantum physics. It applies to light as well as electrons. Sometimes it pays to think about light as an electromagnetic wave, but at other times its more useful to picture it in the form of particles called photons.

A telescope can focus light waves from distant stars, and also acts as a giant light bucket for collecting photons. It also means that light can exert pressure as photons slam into an object. This is something we already use to propel spacecraft with solar sails, and it may be possible to exploit it in order to maneuver a dangerous asteroid off a collision course with Earth, according to Rusty Schweickart, chairman of the B612 Foundation.

Wave-particle duality is an example of superposition. That is, a quantum object existing in multiple states at once. An electron, for example, is both here and there simultaneously. Its only once we do an experiment to find out where it is that it settles down into one or the other.

This makes quantum physics all about probabilities. We can only say which state an object is most likely to be in once we look. These odds are encapsulated into a mathematical entity called the wave function. Making an observation is said to collapse the wave function, destroying the superposition and forcing the object into just one of its many possible states.

This idea is behind the famous Schrdingers cat thought experiment. A cat in a sealed box has its fate linked to a quantum device. As the device exists in both states until a measurement is made, the cat is simultaneously alive and dead until we look.

The idea that observation collapses the wave function and forces a quantum choice is known as the Copenhagen interpretation of quantum physics. However, its not the only option on the table. Advocates of the many worlds interpretation argue that there is no choice involved at all. Instead, at the moment the measurement is made, reality fractures into two copies of itself: one in which we experience outcome A, and another where we see outcome B unfold. It gets around the thorny issue of needing an observer to make stuff happen does a dog count as an observer, or a robot?

Instead, as far as a quantum particle is concerned, theres just one very weird reality consisting of many tangled-up layers. As we zoom out towards the larger scales that we experience day to day, those layers untangle into the worlds of the many worlds theory. Physicists call this process decoherence.

Danish physicist Niels Bohr showed us that the orbits of electrons inside atoms are also quantized. They come in predetermined sizes called energy levels. When an electron drops from a higher energy level to a lower energy level, it spits out a photon with an energy equal to the size of the gap. Equally, an electron can absorb a particle of light and use its energy to leap up to a higher energy level.

Astronomers use this effect all the time. We know what stars are made of because when we break up their light into a rainbow-like spectrum, we see colors that are missing. Different chemical elements have different energy level spacings, so we can work out the constituents of the sun and other stars from the precise colors that are absent.

The sun makes its energy through a process called nuclear fusion. It involves two protons the positively charged particles in an atom sticking together. However, their identical charges make them repel each other, just like two north poles of a magnet. Physicists call this the Coulomb barrier, and its like a wall between the two protons.

Think of protons as particles and they just collide with the wall and move apart: No fusion, no sunlight. Yet think of them as waves, and its a different story. When the waves crest reaches the wall, the leading edge has already made it through. The waves height represents where the proton is most likely to be. So although it is unlikely to be where the leading edge is, it is there sometimes. Its as if the proton has burrowed through the barrier, and fusion occurs. Physicists call this effect "quantum tunneling".

Eventually fusion in the sun will stop and our star will die. Gravity will win and the sun will collapse, but not indefinitely. The smaller it gets, the more material is crammed together. Eventually a rule of quantum physics called the Pauli exclusion principle comes into play. This says that it is forbidden for certain kinds of particles such as electrons to exist in the same quantum state. As gravity tries to do just that, it encounters a resistance that astronomers call degeneracy pressure. The collapse stops, and a new Earth-sized object called a white dwarf forms.

Degeneracy pressure can only put up so much resistance, however. If a white dwarf grows and approaches a mass equal to 1.4 suns, it triggers a wave of fusion that blasts it to bits. Astronomers call this explosion a Type Ia supernova, and its bright enough to outshine an entire galaxy.

A quantum rule called the Heisenberg uncertainty principle says that its impossible to perfectly know two properties of a system simultaneously. The more accurately you know one, the less precisely you know the other. This applies to momentum and position, and separately to energy and time.

Its a bit like taking out a loan. You can borrow a lot of money for a short amount of time, or a little cash for longer. This leads us to virtual particles. If enough energy is borrowed from nature then a pair of particles can fleetingly pop into existence, before rapidly disappearing so as not to default on the loan.

Stephen Hawking imagined this process occurring at the boundary of a black hole, where one particle escapes (as Hawking radiation), but the other is swallowed. Over time the black hole slowly evaporates, as its not paying back the full amount it has borrowed.

Our best theory of the universes origin is the Big Bang. Yet it was modified in the 1980s to include another theory called inflation. In the first trillionth of a trillionth of a trillionth of a second, the cosmos ballooned from smaller than an atom to about the size of a grapefruit. Thats a whopping 10^78 times bigger. Inflating a red blood cell by the same amount would make it larger than the entire observable universe today.

As it was initially smaller than an atom, the infant universe would have been dominated by quantum fluctuations linked to the Heisenberg uncertainty principle. Inflation caused the universe to grow rapidly before these fluctuations had a chance to fade away. This concentrated energy into some areas rather than others something astronomers believe acted as seeds around which material could gather to form the clusters of galaxies we observe now.

As well as helping to prove that light is quantum, Einstein argued in favor of another effect that he dubbed spooky action at distance. Today we know that this quantum entanglement is real, but we still dont fully understand whats going on. Lets say that we bring two particles together in such a way that their quantum states are inexorably bound, or entangled. One is in state A, and the other in state B.

The Pauli exclusion principle says that they cant both be in the same state. If we change one, the other instantly changes to compensate. This happens even if we separate the two particles from each other on opposite sides of the universe. Its as if information about the change weve made has traveled between them faster than the speed of light, something Einstein said was impossible.

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10 mind-boggling things you should know about quantum physics

July 30, 2021

Todd Krauss, chair of the Department of Chemistry at the University of Rochester, and his fellow researchers are joining a $73 million initiative, funded by the US Department of Energy, to advance quantum science and technology. Krausss project, Understanding coherence in lightmatter interfaces for quantum science, is one of 29 projects intended to help scientists better understand and to harness the quantum world in order to eventually benefit people and society.

Its exciting to see the University recognized for its work in the emerging field of quantum information science, says Krauss.

The University has a long history in quantum science, dating back to physicist Leonard Mandelconsidered a pioneer in quantum opticsin the 1960s. And Krauss says he and his colleagues are now building on the work of Mandel and other giants at Rochester, as well as leveraging the talents of the Universitys current crop of quantum researchers.

Quantum science represents the next technological revolution and frontier in the Information Age, and America stands at the forefront, said Secretary of Energy Jennifer M. Granholm as part of the DOEs announcement of the funding. At DOE, were investing in the fundamental research, led by universities and our National Labs, that will enhance our resiliency in the face of growing cyber threats and climate disasters, paving the path to a cleaner, more secure future.

One of the principle challenges in this line of research, explains Krauss, is that quantum states of matter are typically stable only at temperatures below 10 degrees Kelvin; thats roughly 441 degrees Fahrenheit. By comparison, the coldest recorded temperature on Earth was 128.6 at Russias Vostok station in Antarctica in 1983. If stability can be achieved at room temperature, then the benefits of quantum applications can be realized on a broader scale.

More robust quantum states could yield exponentially faster computers, extremely responsive chemical or biological sensors, as well as more secure communication systems, an area that Krausss project is focused on. In quantum state communications, it will be possible to know when someone else is monitoring your messaging, says Krauss.

Krauss is being awarded $1.95 million over three years for his project on light-matter interfaces. Basically, says Krauss, were sticking colloidal nanoparticles into optical cavities in order to interact the nanoparticles with the quantum-light of the cavity. The work will be divided among four researchers:

We are excited to be taking the field of quantum optics in completely new and uncharted directions with our studies of the quantum optics of nanoparticles, says Krauss.

Tags: Arts and Sciences, Department of Chemistry, Hajim School of Engineering and Applied Sciences, Institute of Optics, Nick Vamivakas, Pengfei Huo, quantum optics, quantum physics, research funding, Todd Krauss

Category: Science & Technology

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Rochester researchers join national initiative to advance quantum science - University of Rochester

The development of an ultrathin magnet that operates at room temperature could lead to new applications in computing and electronics such as high-density, compact spintronic memory devices and new tools for the study of quantum physics.

The ultrathinmagnet,which was recentlyreported in the journal Nature Communications, could make big advances in next-gen memory devices, computing, spintronicsand quantum physics. It was discovered by scientists at the Department of Energys Lawrence Berkeley National Laboratory (Berkeley Lab) and UC Berkeley.

Were the first to make a room-temperature 2D magnet that is chemically stable under ambient conditions, said senior authorJie Yao, afaculty scientist in Berkeley Labs Materials Sciences Division and associate professor ofmaterials science and engineering at UC Berkeley.

This discovery is exciting because it not only makes 2D magnetism possible at room temperature, but it also uncovers a new mechanism to realize 2Dmagneticmaterials, added Rui Chen, a UC Berkeley graduate student in theYao Research Groupand lead author on the study.

The magnetic component of todays memory devices is typically made of magnetic thin films. But at the atomic level, these materials are still three-dimensional hundreds or thousands of atoms thick. For decades, researchers have searched for ways to make thinner and smaller 2D magnets and thus enable data to be stored at a much higher density.

Previous achievements in the field of 2D magnetic materials have brought promising results. But these early 2D magnets lose their magnetism and become chemically unstable at room temperature.

State-of-the-art 2D magnets need very low temperatures to function. But for practical reasons, a data center needs to run at room temperature, Yao said. Our 2D magnet is not only the first that operates at room temperature or higher, but it is also the first magnet to reach the true 2D limit: Its as thin as a single atom!

The researchers say that their discovery will also enable new opportunities to study quantum physics. It opens up every single atom for examination, which may reveal how quantum physics governs each single magnetic atom and the interactions between them, Yao said.

The researchers synthesized the new 2D magnet called a cobalt-doped van der Waals zinc-oxide magnet from a solution of graphene oxide, zinc, and cobalt.

Just a few hours of baking in a conventional lab oven transformed the mixture into a single atomic layer of zinc-oxide with a smattering of cobalt atoms sandwiched between layers of graphene.

In a final step, the graphene is burned away, leaving behind just a single atomic layer of cobalt-doped zinc-oxide.

With our material, there are no major obstacles for industry to adopt our solution-based method, said Yao. Its potentially scalable for mass production at lower costs.

To confirm that the resulting 2D film is just one atom thick, Yao and his team conducted scanning electron microscopy experiments at Berkeley LabsMolecular Foundryto identify the materials morphology, and transmission electron microscopy (TEM) imaging to probe the material atom by atom.

X-ray experiments at Berkeley LabsAdvanced Light Sourcecharacterized the 2D materials magnetic parameters under high temperature.

Additional X-ray experiments at SLAC National Accelerator Laboratorys Stanford Synchrotron Radiation Lightsource verified the electronic and crystal structures of the synthesized 2D magnets. And at Argonne National Laboratorys Center for Nanoscale Materials, the researchers employed TEM to image the 2D materials crystal structure and chemical composition.

The researchers found that the graphene-zinc-oxide system becomes weakly magnetic with a 5-6 percentconcentration of cobalt atoms. Increasing the concentration of cobalt atoms to about 12 percentresults in a very strong magnet.

To their surprise, a concentration of cobalt atoms exceeding 15 percentshifts the 2D magnet into an exotic quantum state of frustration, whereby different magnetic states within the 2D system are in competition with each other.

And unlike previous 2D magnets, which lose their magnetism at room temperature or above, the researchers found that the new 2D magnet not only works at room temperature but also at 100 degrees Celsius (212 degrees Fahrenheit).

Our 2D magnetic system shows a distinct mechanism compared to previous 2D magnets, said Chen. And we think this unique mechanism is due to the free electrons in zinc oxide.

When you command your computer to save a file, that information is stored as a series of ones and zeroes in the computers magnetic memory, such as the magnetic hard drive or a flash memory.

And like all magnets, magnetic memory devices contain microscopic magnets with two poles north and south, the orientations of which follow the direction of an external magnetic field. Data is written or encoded when these tiny magnets are flipped to the desired directions.

According to Chen, zinc oxides free electrons could act as an intermediary that ensures the magnetic cobalt atoms in the new 2D device continue pointing in the same direction and thus stay magnetic even when the host, in this case the semiconductor zinc oxide, is a nonmagnetic material.

Free electrons are constituents of electric currents. They move in the same direction to conduct electricity, Yao added, comparing the movement of free electrons in metals and semiconductors to the flow of water molecules in a stream of water.

The new material which can be bent into almost any shape without breaking, and is a million times thinner than a sheet of paper could help advance the application of spin electronics or spintronics, a new technology that uses the orientation of an electrons spin rather than its charge to encode data. Our 2D magnet may enable the formation of ultra-compact spintronic devices to engineer the spins of the electrons, Chen said.

I believe that the discovery of this new, robust, truly two-dimensional magnet at room temperature is a genuine breakthrough, said co-author Robert Birgeneau, a faculty senior scientist in Berkeley Labs Materials Sciences Division and professor of physics at UC Berkeley who co-led the study.

Our results are even better than what we expected, which is really exciting. Most of the time in science, experiments can be very challenging, Yao said. But when you finally realize something new, its always very fulfilling.

Co-authors on the paper include researchers from Berkeley Lab, including Alpha NDiaye and Padraic Shafer of the Advanced Light Source; UC Berkeley; UC Riverside; Argonne National Laboratory; and Nanjing University and the University of Electronic Science and Technology of China.

The Advanced Light Source and Molecular Foundry are DOE national user facilities at Berkeley Lab.

The Stanford Synchrotron Radiation Lightsource is a DOE national user facility at SLAC National Accelerator Laboratory.

The Center for Nanoscale Materials is a DOE national user facility at Argonne National Laboratory.

This work was funded by the DOE Office of Science, the Intel Corporation, and the Bakar Fellows Program at UC Berkeley.

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News Scientists create the world's thinnest magnet - University of California

Professor Michael Biercuk is CEO of quantum tech startup Q-CTRL.

Researchers at the University of Sydney and quantum control startup Q-CTRL have announced a way to identify sources of error in quantum computers through machine learning, providing hardware developers the ability to pinpoint performance degradation with unprecedented accuracy and accelerate paths to useful quantum computers.

A joint scientific paper detailing the research, titled Quantum Oscillator Noise Spectroscopy via Displaced Cat States, has been published inPhysical Review Letters, the worlds premier physical science research journal and flagship publication of the American Physical Society (APS Physics).

Focused on reducing errors caused by environmental noise - the Achilles heel of quantum computing - the University of Sydney team developed a technique to detect the tiniest deviations from the precise conditions needed to execute quantum algorithms using trapped ion and superconducting quantum computing hardware. These are the core technologies used by world-leading industrial quantum computing efforts at IBM, Google, Honeywell, IonQ, and others.

The University team is based at the Quantum Control Laboratory led by Professor Michael Biercukin the Sydney Nanoscience Hub.

Topinpoint the source of the measured deviations, Q-CTRL scientists developed a new way to process the measurement results using custom machine-learning algorithms. In combination with Q-CTRLs existing quantum control techniques, the researchers were also able to minimise the impact of background interference in the process. This allowed easy discrimination between real noise sources that could be fixed and phantom artefacts of the measurements themselves.

Combining cutting-edge experimental techniques with machine learning has demonstrated huge advantages in the development of quantum computers, said Dr Cornelius Hempel of ETH Zurich who conducted the research while at the University of Sydney. The Q-CTRL team was able to rapidly develop a professionally engineered machine learning solution that allowed us to make sense of our data and provide a new way to see the problems in the hardware and address them.

Q-CTRL CEO Professor Biercuk said: The ability to identify and suppress sources of performance degradation in quantum hardware is critical to both basic research and industrial efforts building quantum sensors and quantum computers.

Quantum control, augmented by machine learning, has shown a pathway to make these systems practically useful and dramatically accelerate R&D timelines, he said.

The published results in a prestigious, peer-reviewed journal validate the benefit of ongoing cooperation between foundational scientific research in a university laboratory and deep-tech startups. Were thrilled to be pushing the field forward through our collaboration.

Q-CTRL was spun-out of the University of Sydney by Professor Michael Biercuk from the School of Physics. The startup builds quantum control infrastructure software for quantum technology end-users and R&D professionals across all applications.

Q-CTRL has assembled the worlds foremost team of expert quantum-control engineers, providing solutions to many of the most advanced quantum computing and sensing teams globally. Q-CTRL is funded by SquarePeg Capital, Sierra Ventures, Sequoia Capital China, Data Collective, Horizons Ventures, Main Sequence Ventures and In-Q-Tel. Q-CTRL has international headquarters in Sydney, Los Angeles, and Berlin.

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Q-CTRL: machine learning technique to pinpoint quantum errors - News - The University of Sydney

Jul 28th 2021

AS HE LIKED to tell it, there were three epiphanies in Steven Weinbergs life. The first came in a wooden box. It was a chemistry set, passed on by a cousin who was tired of it. As he played with the chemicals in it, and found that each reacted differently because of atoms, a vast thought struck him: if he learned about atoms, he would know how the whole world worked.

The second epiphany came when, as a teenager, he paid a routine visit to his local library in New York. On the table was a book called Heat, open to a page of equations. Among them was the elegant, unknown swirl of an integral sign. It showed that with a mathematical formula, and a magic symbol, science could express something as rudimentary as the glow of a candle flame. His third awakening, when he was in his 20s and already a professor of physics, was the discovery that a mathematical theory could be applied to the whole dazzling array of stars and planets, dark space beyond them and, he concluded, everything.

All regularities in nature followed from a few simple laws. Not all were known yet; but they would be. In the end he was sure they would combine into a set of equations simple enough to put on a T-shirt, like Einsteins E=mc2. It was just a matter of continually querying and searching. In the strange circumstance of finding himself conscious and intelligent on a rare patch of ordinary matter that was able to sustain life, doggedly asking questions was the least he could do.

His signal achievement was to discover, in the 1960s, a new level of simplicity in the universe. There were then four known universal forcesgravity and electromagnetism, both of which operate at large scales, and the strong and weak nuclear forces, both of which are appreciable only at small scales. Electromagnetism was explained by a quantum field theory; similar theories for the nuclear forces were eagerly being sought.

In quantum field theories, forces are mediated by particles called bosons; the boson involved in electromagnetism is the photon, the basic particle of light. He and others showed that a theory of the weak force required three bosons: the W+ and the W-, which carried electric charges, and the Z0, which did not. The W particles were at play in the observable universe; they were responsible for some sorts of radioactive decay. The Z was notional until, in 1973, researchers at CERN, Europes great particle-physics lab, observed neutral currents between the particles they were knocking together. These had never been seen before, and could be explained only by the Z. In 1979 the Nobel prize duly followed.

In his understated way, he called his contribution very satisfactory. It was not just that the weak force and the electromagnetic force could be explained by similar tools. At high energies they were basically the same thing.

That triumph of unification increased his curiosity about the only point where such high energies were known to have existed: the Big Bang. In his book The First Three Minutes, in 1977, he described the immediate aftermath, to the point where the hyper-hot cosmic soup had cooled enough for atomic nuclei to form. He saw early on how deeply particle physics and cosmology were intertwined, and became fascinated by the idea of a universe dominated by unobservable dark energy and dark matter in which ordinary matter (the stars and the planets and us) was merely a small contamination. He longed for CERN s Large Hadron Collider to find evidence of dark matter. It caused him lasting frustration that Congress in 1993 had cancelled the Superconducting Super Collider, which was to have been even bigger.

Whatever was found, he was sure it would fit into the simple scheme of natures laws. Quantum mechanics, however, troubled him. He worried that its determinism implied that the world was endlessly splitting, generating myriad parallel histories and universes in which the constants in nature would have different values. Goodbye to a unified theory of everything, if that were so.

Such a unified law would have given him satisfaction but, he knew, no comfort. Natures laws were impersonal, cold and devoid of purpose. Certainly there was no God-directed plan. As he wrote at the end of The First Three Minutes, the more the universe seemed comprehensible, the more it seemed pointless. No saying of his became more famous, but the next paragraph softened it: humans gave the universe their own point and purpose by the way they lived, by loving each other and by creating art.

He set the example by marrying Louise, his college sweetheart, devouring opera and theatre, revelling in the quirky liberalism of Austin, where he taught at the University of Texas for almost four decades, and looking for theories in physics that would carry the same sense of inevitability he found so beautiful in chamber music, or in poetry. He still thought of human existence as accidental and tragic, fundamentally. But from his own little island of warmth and love, art and science, he managed a wry smile.

What angered him most was the persistence of religion. It had not only obstructed and undermined science in the age of Galileo and Copernicus; it had also survived Darwin, whose theory of evolution had shocked it more sharply than anything physics did. And it was still there, an alternative theory of the world that corroded free inquiry. For even if the laws of nature could be reduced to one, scientists would still ask: Why? Why this theory, not another? Why in this universe, and not another?

There was, he reflected, no end to the chain of whys. So he did not stop asking or wondering. He liked to review and grade his predecessors, from the ancient Greeks onwards, chastising them for failing to use the data they had, but also sympathising with their lack of machines advanced enough to prove their ideas. The human tragedy was never to understand why things were as they were. Yet, for all that, he could echo Ptolemy: I know that I am mortal and the creature of a day, but when I search out the massed wheeling circles of the stars, my feet no longer touch the EarthI take my fill of ambrosia, the food of the gods.

This article appeared in the Obituary section of the print edition under the headline "Natures laws"

See the rest here:

Steven Weinberg died on July 23rd - The Economist

Eureka! A research team featuring dozens of scientists working in partnership with Googles quantum computing labs may have created the worlds first time crystal inside a quantum computer.

This is the kind of news that makes me want to jump up and do a happy dance.

These scientists may have produced an entirely new phase of matter. Im going to do my best to explain what that means and why I personally believe this is the most important scientificbreakthrough in our lifetimes.

However, for the sake of clarity, theres two points I need to make first:

In colloquial terms, its a big screw you to Sir Isaac Newton.

Time crystals are a new phase of matter. For the sake of simplicity, lets imagine a cube of ice.

When you put a cube of ice in glass of water, youre introducing two separate entities (the ice cube and the liquid water) to each other at two different temperatures.

Everyone knows that the water will get colder (thats why we put the ice in there) and, over time, the ice will get warmer and turn into water. Eventually youll just have a glass of room-temperature water.

We call this process thermal equilibrium.

Most people are familiar with Newtons first law of motion, its the one that says an object at rest tends to stay at rest and an object inmotion tends to stay in motion.

An important side-effect of this law of physics is that it means a perpetual motion machine is classically impossible.

According to classical physics, the universe is always moving towards entropy. In other words: if we isolate an ice cube and a room-temperature glass of water from all other external forces, the water will always melt the ice cube.

The entropy (the movement towards change) of any system will always remain the same if there are no processes, and it will always increase if there are processes.

Since our universe has stars exploding, black holes sucking, and people lighting things on fire chemical processes entropy is always increasing.

Except when it comes to time crystals. Time crystals dont give a damn what Newton or anyone else thinks. Theyre lawbreakers and heart takers. They can, theoretically, maintain entropy even when theyre used in a process.

Think about a crystal youre familiar with, such as a snowflake. Snowflakes arent just beautiful because each one is unique, theyre also fascinating formations that nearly break the laws of physics themselves.

Crystalline structures form in the physical world because, for whatever fundamental scientific reason, the atoms within them want to exist in certain exact points.

Want is a really weird word to use when were talking about atoms Im certainly not implying theyre sentient but its hard to describe the tendency toward crystalline structures in abstracts such as why.

A time crystal is a new phase of matter that, simplified, would be like having a snowflake that constantly cycled back and forth between two different configurations. Its a seven-pointed lattice one moment and a ten-pointed lattice the next, or whatever.

Whats amazing about time crystals is that when they cycle back and forth between two different configurations, they dont lose or use any energy.

Time crystals can survive energy processes without falling victim to entropy. The reason theyre called time crystals is because they can have their cake and eat it too.

They can be in a state of having eaten the whole cake, and then cycle right back to a state of still having the cake and they can, theoretically, do this forever and ever.

Most importantly, they can do this inside of an isolated system. That means they can consume the cake and then magically make it reappear over and over again forever, without using any fuel or energy.

Literally everyone should care. As I wrote back in 2018, time crystals could be the miracle quantum computing needs.

Nearly every far-future tech humans can imagine, from teleportation to warp drives and from artificial food synthesizers to perpetual motion reactors capable of powering the world without burning fuels or harnessing energy, will require quantum computing systems.

Quantum computers can solve really hard problems. Unfortunately, theyre brittle. Its hard to build them, hard to maintain them, hard to get them to do anything, and even harder to interpret the results they give. This is because of something called decoherence, which works a lot like entropy.

Computer bits in the quantum world, qubits, share a funky feature of quantum mechanics that makes them act differently when observed than when theyre left alone. That sort of makes any direct measurements of qubit states (reading the computers output) difficult.

But time crystals want to be coherent. So putting them inside a quantum computer, and using them to conduct computer processes could potentially serve an incredibly important function: ensuringquantum coherence.

[Greetings Humanoids! Did you know we have a newsletter all about AI and quantum computing? You can subscribe to itright here]

No. No, no, no, no no. Dont get me wrong. This is baby steps. This is infancy research. This is Antony van Leeuwenhoek becoming the first person to use a microscope to look at a drop of water under magnification.

What Googles done, potentially, is prove that humans can manufacture time crystals. In the words of the researchers themselves:

These results establish a scalable approach to study non-equilibrium phases of matter on current quantum processors.

Basically they believe theyve proven the concept, so now its time to see what can be done with it.

Time crystals have always been theoretical. And by always, I mean: since 2012 when they were first hypothesized.

If Googles actually created time-crystals, it could accelerate the timeline for quantum computing breakthroughs from maybe never to maybe within a few decades.

At the far-fetched, super-optimistic end of things we could see the creation of a working warp drive in our lifetimes. Imagine taking a trip to Mars or the edge of our solar system, and being back home on Earth in time to catch the evening news.

And, even on the conservative end with more realistic expectations, its not hard to imagine quantum computing-based chemical and drug discovery leading to universally-effective cancer treatments.

This could be the big eureka weve all been waiting for. I cant wait to see what happens in peer-review.

If you want to know more, you can read Googles paper here. And if youre looking for a technical deep-dive into the scientific specifics of what the researchers accomplished in the lab, this piece on Quanta Magazine byNatalie Wolchover is the bees knees.

Continue reading here:

Google's 'time crystals' could be the greatest scientific achievement of our lifetimes - The Next Web

Strange ideas can come from ordinary places. This one came from Texas. In 1981, John A. Wheeler, the father of the black hole and a theoretical physicist at the University of Texas in Austin, threw a party. The guests were all young physicists with a common interest in the foundations of computing, a topic that Wheeler believed--correctly--would become increasingly important in the years to come.

It was at this party that a conversation with Charles Bennett, an IBM physicist, sparked an idea in the mind of Oxford University researcher David Deutsch. It struck him that computer theory was based on Newton's laws, not the more fundamental description of the universe provided by quantum theory.

At the time, the computer industry was beginning to fret over the future of microchips. How many calculations per second would be ultimately possible, how much heat would this produce, and could silicon survive the constant baking? To help them, computer scientists turned to the theory developed in the 1930s by the pioneer of their field, Alan Turing. But at Wheeler's party, said Deutsch, "I could see immediately that using the laws [of quantum mechanics] would give a different answer."

Deutsch began work on a paper that is now generally regarded as a classic in the field. Published in 1985, it describes how a computer might run using the strange rules of quantum mechanics and why such a computer differs fundamentally from ordinary computers.

Fifteen years later, the revolution that Deutsch started has reached global proportions. Quantum computers are no longer seen as weird curiosities but as the powerful future of the computer industry, and the debate is shifting from whether they will ever become a reality to when they will do so. The excitement is not due to their power, although they undoubtedly will be more powerful than today's models. Their big selling point, the killer app if you like, is that they can solve problems and carry out simulations that are basically impossible on conventional computers.

Such is the potential of these devices that the list of companies funding research programs sounds like a roll call of the world's biggest telecommunications and computer businesses. They include IBM, Hewlett-Packard, Lucent Technologies, AT&T, and Microsoft. There is even a New York Citybased start-up called MagiQ Technologies that hopes to make money by developing intellectual property in this field.

One of the strongest forces driving the development of quantum computers is the fear they will crack with ease secret codes that are impervious to other computers. The alarm bells started ringing in 1994, when Peter Shor of AT&T's Bell Laboratories in New Jersey showed that quantum computers were far faster than their ordinary brethren at factoring numbers.

Finding the factors of large numbers is so difficult for conventional computers that code-makers rely on this weakness of theirs to protect sensitive data. With the development of quantum computers, these codes will be obsolete. As soon as the first modest-sized quantum computer is switched on, governments and their militaries will be forced to concede that many of their codes are unsafe. Understandably, they are keen to find out just what quantum computers can do, and various national laboratories have begun substantial programs, in particular the U.S. National Institute of Standards and Technology in Boulder, Colo.; Los Alamos National Laboratory in New Mexico; and the United Kingdom equivalent, the Defence Evaluation and Research Agency in Malvern.

Aside from its promise for espionage there is the new physics unveiled almost daily by scientists trying to understand quantum information and how to control it. Quantum computers are becoming tiny laboratories in which scientists can test the theories of quantum mechanics with greater precision than ever before. Arguably the strongest team in the world making such discoveries is at the University of Oxford. Smaller groups exist at places such as MIT, Caltech, and a group of Australian universities, with influential individuals scattered throughout the United States, Europe, and Israel. After a late start, Japan has begun a concerted effort to catch up.

ILLUSTRATION: STEVE STANKIEWICZ

How Spin States Can Make Qubits: The spin of a particle in a dc magnetic field is analogous to a spinning top that is precessing around the axis of the field. In such a field, the particle assumes one of two states, spin up or spin down, which can represent 0 and 1 in digital logic. A particle in one spin state can be pushed toward another by a radio frequency pulse perpendicular to the magnetic field. A pulse of the right frequency and duration will flip the spin completely [top]. A shorter RF pulse will tip the spin into a superposition of the up and down state [bottom], allowing simultaneous calculations on both states.

Quantum information

Digital information appears mundane stuff. The 0s and 1s of binary code can be easily measured, copied, and moved around. But assign a piece of information to a quantum particle, and it takes on the bizarre characteristics of the quantum world. This fundamental unit of quantum information is called a quantum bit, or qubit (pronounced cue bit), and it is quite different from its classical counterpart.

For a start, a qubit can be both a 0 and 1 at the same time. Take the spin of an electron--a property that can be imagined as the spin of a top with its axis pointing either up or down [see figure, above}. The up or down spin can correspond to a 0 or 1. But the electron can also be placed in a ghostly dual existence, known as a superposition of states, in which it is both up and down, a 0 and a 1, at the same time. Carry out a calculation using the electron, and you perform it simultaneously on both the 0 and the 1, two calculations for the price of one.

At first glance, this may not seem impressive, but add more qubits and the numbers become much more persuasive. While 1 qubit can be in a superposition of two states, 0 and 1, two qubits can be in a superposition of four states--00, 01, 10, and 11--representing four numbers at once. The increase is exponential: with m qubits, it is possible to carry out a single calculation on 2m numbers in parallel. With only a few hundred qubits, it is possible to represent simultaneously more numbers than there are atoms in the universe.

Algorithms, entanglement, and error correction

Of course, once the calculation has finished, the answer must be obtained. A simple measurement destroys the superposition, leaving the system in one state or another. Unfortunately, it is rarely possible to determine in advance which state this will be, and that is a problem. The goal is to ensure that the measurement produces the answer of interest, and it can be reached by exploiting the phenomenon of quantum interference. Each of the superposed states has a probability associated with it that has a wavelike behavior--it can interfere with the probabilities of other states destructively or constructively. Getting the desired answer to a calculation means processing the information in such a way that undesired solutions interfere destructively, leaving only the wanted state, or a few more or less wanted states, at the end. The process is known as a quantum algorithm, and its design challenges physicists, mathematicians, and computer scientists. A final measurement then gives the desired answer, or in the case of a few final states, a series of measurements gives their probability distribution from which the desired answer can be calculated.

Quantum algorithms have the potential to be dramatically faster than their conventional counterparts. A good example is an algorithm for searching through lists that was developed by Lov Grover at Lucent Technologies' Bell Laboratories, in Murray Hill, N.J. The problem is to find a person's name in a telephone directory, given his or her phone number. If the directory contains N entries, then on average, you would have to search through N/2 entries before you find it. Grover's quantum algorithm does better. It finds the name after searching through only (check)N entries, on average. So for a directory of 10 000 names, the task would require (check)(10 000) = 100 steps, rather than 5000. The algorithm works by first creating a superposition of all 10 000 entries in which each entry has the same likelihood of appearing in response to a measurement made on the system. Then, to increase the probability of a measurement producing the required entry, the superposition is subjected to a series of quantum operations that recognize the required entry and increase its chances of appearing. (Remember that the recognition is possible because you have the phone number but not the name.)

ILLUSTRATION: STEVE STANKIEWICZ

Entangled Particles: If two particles, both in states of superposition, are entangled, measuring one forces both to assume complementary states.

Coherence/Decoherence: the ability of a quantum system to maintain a superposition of states. Decoherence is the process by which interactions with the environment destroy superposition, forcing a system into one state or another.

Entanglement: the state in which two quantum systems in indeterminate states are linked so that measuring or manipulating one system instantaneously manipulates the second.

Qubit: a unit of information used in quantum computing. It is distinct from an ordinary bit in that it can encode a superposition of values.

Spin: a quantum mechanical property of particles that in certain cases can take only two mutually exclusive values. It is used widely in nuclear magnetic resonance.

Superposition: if a physical system such as a particle can be found in more than one state and its state is unknown, it exists in a superposition of those states. That is, if there are two possible states, the system can be said to exist in both at once until its state is actually measured. Such a measurement collapses the system onto one state or another.

Teleportation: communication between two parties using entangled particles. Through the entanglement the state of one particle can be transferred to another distant particle with which it is entangled.

Vibrational State: the quantized state of the collective motion of ions in a linear ion trap. The vibrational state can encode a qubit and is used to link the ions during calculations.

As if superposed values and probability waves were not counterintuitive enough, another strange phenomenon is prominent in the new science of quantum information. In the '30s, scientists fiercely debated whether what quantum mechanics predicted had a real existence or whether its strangeness was due to some deficiency in the theory. In particular, Albert Einstein could not believe that the universe was built as quantum mechanics claimed. So, together with his colleagues Boris Podolsky and Nathan Rosen, he devised a thought experiment to find holes in the new theory.

The thought experiment centers on the behavior of pairs of particles that, according to quantum theory, are joined together--entangled--in a profound way that has no analog in the classical world. Prod one, and it seems the other instantly feels the influence, no matter how far away it might be [see figure, above]. The three scientists pointed out that this process would have to involve a faster-than-light signal passing between the particles--an impossibility. Their conclusion became known as the EPR (Einstein-Podolsky-Rosen) paradox and the entangled particles as EPR pairs.

The debate was resolved by John Bell, a theorist at CERN, the European laboratory for particle physics near Geneva, and the French physicist Alain Aspect. They proved that the Siamese twins of the quantum world, EPR pairs, indeed behave in the way predicted by quantum mechanics. However, the experiment also showed that there is no faster-than-light signal and that entanglement cannot be used for superluminal communication. Rather than communicating, EPR pairs share the same existence, the same destiny, if you like. Entanglement is now one of the key phenomena exploited in quantum information processing. Today the EPR experiment is performed almost daily around the world.

If creating entanglement and superposition has become a commonplace event compared with 10 years ago, quantum information remains fragile stuff. Ordinary interactions with the environment destroy qubits and the information they contain, a process known as decoherence. (Its opposite, coherence, is the ability of a qubit to maintain such quantum characteristics as superposition.) If quantum information is to pass into the world of computer science, a process of error correction is needed to protect against decoherence [see Defining Terms, left].

Initially, physicists believed that such a technique was impossible, because detecting and correcting errors would mean measuring the state of a quantum system and so destroying the information it contained. Still, by the early '90s Deutsch had shown this need not be the case. And in 1994 Andrew Steane at the University of Oxford and Peter Shor at AT&T's Bell Laboratories in New Jersey independently discovered practical quantum error-correction algorithms.

The problem is similar to reproducing in one place a message that has been constructed in another. If the message is sent over a channel or stored in a place noisy enough to distort some of the bits in the sequence, how can the receiver recognize the message? By adding redundancy to the message so that the sender can correct bits that have been distorted.

Shor and Steane came up with the quantum equivalent of sending the same bit three times. The extra qubits are known as ancillas. Measuring these qubits tells the receiver what errors have occurred and how to correct the qubits that are part of the message.

NMR leads the charge

The first big breakthrough for scientists building actual quantum computers came in the mid-'90s, when they discovered how to carry out calculations using the techniques of nuclear magnetic resonance (NMR). The key idea was that a single molecule can act like a tiny computer. Information is stored in the orientation of nuclear spins in the molecule, each nucleus holding one qubit. And the interaction between the nuclear spins, known as spin-spin coupling, serves to mediate logic operations. In a strong magnetic field, these nuclei precess around the direction of the magnetic field at frequencies that depend on their chemical environment.

For instance, in a 9.3-tesla field, a carbon-13 nucleus in a chloroform molecule precesses at about 100 MHz. By zapping the molecule with radio waves tuned to these resonant frequencies, it is possible to manipulate each nucleus individually to carry out logic operations. The manipulation might involve flipping a nucleus from a 1 to a 0, a so-called one-qubit operation or single-bit rotation; or it might involve two linked nuclei in a two-qubit operation, in which the value of one nucleus is flipped in a way that depends on the value of the other.

Chloroform made with the carbon-13 isotope is a good example of a molecule that can act as a two-qubit quantum computer, because its hydrogen and carbon-13 nuclei can be addressed individually by the radio waves. A quantum calculation is then carried out by encoding a program--a sequence of one- and two-qubit operations--as a series of RF pulses. The results are then read out by listening for the magnetic induction signal generated by the precessing nuclei at the end of the calculation. That signal indicates the orientation of the nuclear spin.

Nuclear magnetic resonance sounds like the dream solution to a thorny problem. Nuclei are naturally isolated from the noise of the outside world and so can maintain coherence for many seconds, enough time to perform hundreds of logic operations. In addition, NMR is a mature technology, having been used over many years for imaging and chemical analysis.

But the technique has some severe limitations. Single molecules do not produce a signal strong enough to be observed. Instead, NMR experiments must involve huge numbers of molecules (of the order of 1023) so that their combined magnetic induction signal is large enough to be picked up. (These molecules are usually distributed in a solvent, so the first quantum computers actually have liquid hearts.)

To begin a calculation, the initial state of the computer must be known. But in a material at room temperature, the spin up and spin down states are distributed almost equally and at random. In other words, the state of each of the many computers in solution cannot be known, rendering any subsequent calculation meaningless.

ILLUSTRATION: STEVE STANKIEWICZ

Quantum Logic: One of the most important logic elements in quantum computing is the controlled-NOT gate, similar to a controllable inverter circuit. In such an element, the state of one qubit, the control qubit, determines whether the final state of a second qubit, the input qubit, will be inverted by a series of RF pulses.

But never say die. In 1997, two groups independently came to quantum computing's rescue. Isaac Chuang, now at IBM's Almaden Laboratory near San Jose, Calif., and Neil Gershenfeld at the Massachusetts Institute of Technology (MIT), in Cambridge, found that they could turn a small natural bias--say, toward spinning up rather than down with respect to the magnetic field--in the nuclei of some molecules to advantage. They could use it to establish a kind of artificial ground state (00 for a two-qubit stystem) from which to start a calculation. At the same time, David Cory, also at MIT, and Amr Fahmy and Timothy Havel, both from Harvard University, in Cambridge, Mass., discovered that by bombarding the sample with radio pulses they could effectively "jam" the signal from all but the ground state.

To carry out useful calculations, the computer must be able to perform any logical operation. For quantum computers, there are two logic operations from which all other operations can be derived, rather like the AND and NOT gates in classical computing. One involves rotating a single qubit. The other, carried out on two qubits and called a controlled-NOT gate, flips or fails to flip one qubit depending on the state of another to which it is coupled [see figure, above]. Both these operations are straightforward: simply bombard the liquid sample with the appropriate sequence of radio pulses. Since 1997, these two groups and others, notably at Los Alamos and Oxford University, have built liquid NMR quantum computers with up to seven qubits to perform simple algorithms, one of which even belongs to the mathematical family of Shor's code-cracking formula [see "Quantum Code Cracking Creeps Closer," Spectrum, October 2000].

Unfortunately, quantum computers based on liquid NMR will never be much more powerful than this. The readout signals they produce plummet exponentially with the number of qubits involved in the calculation, because the proportion of molecules found in the appropriate starting state decreases. So scientists do not expect to be able to handle any more than a dozen qubits or so before the signal becomes indistinguishable from the background. Attempts to build machines that can handle more than 10 qubits continue, but if nontrivial quantum computing is ever to become possible, some other approach is needed.

Refrigerated ions

A technology that is less in the public eye than NMR has attracted others. In 1995 Ignacio Cirac and Peter Zoller of the University of Innsbruck, in Austria, suggested using ion traps to build quantum logic gates. The technology behind ion traps is already used for spectroscopy and to improve time and frequency standards, but huge advances are needed for quantum computation. The idea is that a number of ultra-cold ions can be trapped using a device known as a linear radio-frequency Paul trap. This device sets up a high-frequency RF field that holds the ions tightly in two dimensions but only weakly in the third dimension. Because the ions have the same charge, they repel each other and tend to arrange themselves in a straight line, equally spaced, like beads on an elastic string. The arrangement allows them to vibrate as a group in ways important for quantum computing.

The qubits are initially stored in the internal spin states of the ions relative to a background magnetic field. They are written to the ions using a pulsed, oscillating magnetic field, which flips the bits or places them in a superposition of up and down states, depending on its duration. An advantage of ion traps is that this superposition is extremely robust, lasting for at least as long as the qubits in NMR, ample time to carry out the desired logic operations.

ILLUSTRATION: STEVE STANKIEWICZ

Computing in an Ion Trap: Ions are lined up in a trap by RF energy from four electrodes, then chilled using lasers [top]. The electrostatic repulsion between the ions couples their individual motion as if they were connected by springs [middle]. The coupled motion, or vibrational state, can be used to transfer quantum information from one qubit to another. Basically, a pulse of energy equal to the difference between the quantum state of the ion and the vibrational state of the two ions (0 or 1) leads the ion to swap its internal state for the vibrational state. A similar pulse to the other ion performs another swap, transferring the original state of the first qubit to the second.

To share the qubits between the ions, scientists turn to the ion vibrations. The aim is to chill the ions until as a group they are absolutely still. This is the ground state of the system. Inject a little energy, and the ions begin to vibrate. But being quantum particles, the ions can exist in a superposition of the ground state and the vibratory state, so the vibration can be used to store a qubit. Because the ions all take part in the vibration, this qubit is shared among them. It's as if this collective motion is a kind of databus, allowing all the ions to temporarily share the information and become entangled. This sharing allows the IF and THEN type operations that are the building blocks of computer logic gates. For example, an instruction might be: IF the vibrational state is 1, THEN flip the qubit in the first ion's internal spin state. Researchers at the National Institute of Science and Technology (NIST) have already demonstrated that a string of four ions can be entangled and have said that more should be possible.

At least five groups around the world are working on ion trap quantum computers, but David Wineland's team at NIST is widely regarded as the leader. His group has built a 2-qubit logic gate using a single beryllium ion cooled to its vibrating ground state. Using a laser focused on the ion, the group superimposes on the background magnetic field a second magnetic field with a magnitude that varies with the position of the ions. The ion's vibration causes it to experience an oscillating magnetic field, and when the frequency of the oscillation matches the energy difference between the ion's two spin states, energy is transferred from the spin to the vibrational state, mapping the quantum information to the vibrational from the spin state [see figure, above]. This is the basis of a controlled-NOT gate and was realized in 1995 only a few months after Cirac and Zoller's announcement. Reading the data involves scattering light off the ion, since a spin up ion can be made to scatter strongly, while a spin down ion will scatter hardly at all.

Ion traps, too, have their limitations. One is the short decoherence time of the qubits after transfer to the vibrational "databus." Because the ions are charged, the vibrations are strongly influenced by stray electric fields, causing decoherence. Nonetheless, the group is confident that this tendency can be overcome by isolating the trap better from the environment. Ion traps also suffer from problems of scalability. The more ions there are in the trap, the greater the risk of tapping into uncontrollable vibrational states and so destroying the calculation. The next step will be to build adjacent traps, each holding only a few ions, and sending quantum information from one trap to another, either by physically moving the ions or by a phenomenon peculiar to quantum information called teleportation.

The alternatives

While liquid NMR is doomed because of the problems of working at room temperature, several groups are looking into carrying out NMR-type manipulations on single atoms in the solid state. A proposal from Bruce Kane at the University of Maryland in particular has attracted attention. His idea is to bury an array of phosphorus atoms in silicon and overlay it with an insulating layer, on top of which sits a like array of electrodes, each of which can apply a voltage to the atom beneath it. The ingenious aspect of this setup is how Kane proposes to control the spin of each nucleus.

Just as in NMR, the spin of the nuclei can be flipped by being zapped with radio waves of just the right energy--but, of course, these radio waves would flip every nucleus. Now phosphorus atoms have a single electron in their outer shell that interacts with the nuclear spin in a complex way. Applying a voltage to the atom changes the energy required to address both the nuclear and the electronic spin, and therefore it changes the frequency of the radio waves needed to flip the nucleus. So by applying a voltage to a specific electrode and zapping the array with the new frequency, it is possible to address a single nucleus.

But to perform a controlled-NOT logic operation, two qubits have to become entangled. Kane also has a way of doing this. Voltages applied between adjacent phosphorus atoms in the array can turn on and off the interactions between the outer electrons in each atom, allowing two-qubit operations.

Of course, the theory is all very well. The difficulty is actually building such a device, and Kane's collaborators are already working on it. At the Centre for Quantum Computer Technology at the University of New South Wales, in Australia, Robert Clark heads a team that is hoping to overcome many of the obstacles Kane's device faces. First up is the difficulty of creating the atomic array and preventing the phosphorus atoms from migrating within the silicon.

Kane is setting up a lab to study another challenging aspect of his device: the readout. Once the one- or two-qubit operation has been completed, the result has to be read out from the nuclear spins. Once again, Kane relies on the link between nuclear and electronic spins to get an answer. By very carefully measuring the spin of the electron, he said, it is possible to infer the spin of the nucleus. Measuring the spin of a single electron has never been done, but Kane said this should be possible shortly.

Kane's idea has attracted so much attention because many of these logic gates can be linked together to form a large quantum computer, though doing so may take some time. New South Wales's Clark believes that a handful of qubits might be possible in the medium term.

The quantum phenomena of superconductivity may also prove useful for building quantum computers. In 1999, at the Delft University of Technology in the Netherlands, a team designed a superconducting circuit in which superposed counter-rotating currents could prove useful for storing and manipulating qubits. The circuit consists of a loop with three or four Josephson junctions for measuring the circuit's state. The fact that it is made by conventional electron-beam lithographic techniques makes it particularly conducive to large-scale integration. However, superconducting circuits have short decoherence times, and today's techniques for measuring the states of the circuits are too invasive for useful manipulation of qubits.

A more advanced solid-state technology is the quantum dot, essentially a semiconductor trap holding a discrete number of electrons. These have been studied since the early 1990s because the trapped electrons act like artificial atoms, with their own periodic table and chemistry. Then in 1998, David DiVincenzo of IBM and Daniel Loss of the University of Basel, in Switzerland, proposed using quantum dots as the building block of a quantum computer, and a variety of ideas have since been put forward for exploiting the dots' quantum properties for computation. One idea is a two-qubit system consisting of two electrons shared by four quantum dots in a square. The electrons, seeking to minimize their energy, occupy opposite corners of the square, and since this arrangement has two configurations, they exist as a superposition that is manipulable through electrodes at the corners of the square. A number of other techniques involve reading and writing data to the dots with laser pulses and placing a single nucleus at the center of each dot that can be addressed with NMR techniques, rather as in Kane's proposal.

A quantum Internet

The problems in scaling up many of these ideas have persuaded many scientists that if quantum computing is to become useful any time soon, it will have to involve networking small quantum computers together. But sending quantum information from one place to another is tricky. One option is to physically move the qubits, but then they would be liable to decoherence. In 1993, however, Charles Bennett, from IBM's Thomas J. Watson Laboratory in Yorktown Heights, N.Y., and a few colleagues came up with a different option: teleportation.

Teleportation utilizes the deep link that entanglement sets up between one point in the universe and another. Bennett theorized that entanglement could act as a kind of phone line down which to send quantum information--in other words, create an entangled pair of particles and send one of them to the receiver while keeping the other [see "Quantum Teleportation"]. This process links these two points in a way that allows the exchange of quantum information from one qubit to another.

Bennett and his colleagues had to wait four years to see their predictions verified. In 1997, in a small room at the University of Innsbruck, in Austria, a group of physicists led by Anton Zeilinger performed the first teleportation experiment. Zeilinger's travelers were photons and he was sending them only a meter or so, from one side of the lab to the other. Today, more than three years later, Zeilinger is working on the next step, which is to teleport photons over distances of a kilometer.

Soon after Zeilinger's breakthrough, Cirac and Zoller proposed that teleportation could become the basis of a kind of quantum Internet. And in March of 2000, Seth Lloyd and Selim Shahriar at MIT and Philip Hemmer at the U.S. Air Force Research Laboratory, in Lincoln, Mass., suggested sending entangled photons over optical fibers to nodes containing cold atoms that would absorb the photons and so store the entanglement. This entanglement could then be used for error correction, teleportation, and various other valuable applications. A number of groups are working on this idea, including Jeff Kimble at the California Institute of Technology and Eli Yablonovitch at the University of California at Los Angeles. They hope to have a three-node network running within 10 years.

Some scientists hope for even greater things from entanglement, believing it will be so useful that it will one day be traded as a currency over the quantum Internet. Considerable progress will be required before anything remotely like that becomes possible. Even so, the pace of innovation in quantum computing has already exceeded most scientists' wildest dreams. Only five years ago, many were confident that quantum computers would not be built for 20 years, yet NMR proved them wrong within a year. Only the bravest forecaster would dare to predict how the field will stand five years from now.

With only a few hundred qubits it is possible to represent simultaneously more numbers than there are atoms in the universe

Spectrum Editor: Samuel K. Moore

JUSTIN MULLINS [p. 42], a contributing editor, is a freelance science writer based in Oxford, England. He is a consulting technology editor for New Scientist.

For an overview of quantum computing techniques and the peculiarities of quantum information, try Introduction to Quantum Computing and Information, edited by Hoi-Kwong Lo, Sandu Popescu, and Tim Spiller, and published in 1998 by World Scientific (Singapore).

Some important papers in quantum computing include: "Bulk Spin-Resonance Quantum Computation," by N. Gershenfeld and I. L. Chuang, Science, Vol. 275, p. 350 (1997); "Quantum Logic Gates in Optical Lattices," by G. Brennen, C. Caves, I. Deutsch, and P. Jessen, Physics Review Letters, Vol. 82, p. 1060 (1999); "A Silicon-Based Nuclear Spin Quantum Computer," by B. E. Kane, Nature, Vol. 393, p. 133 (1998); and Teleportation and the Quantum Internet, by S. Lloyd, M. Shahriar, and P. Hemmer, available from the Los Alamos Archive (http://xxx.lanl.gov).

An informative Web site explaining quantum teleportation is http://info.uibk.ac.at/c/c7/c704/qo/ photon/_teleport/index.html.

Nuclear magnetic resonance (NMR) is explained in an on-line book at http://www.cis.rit.edu/htbooks/nmr/. The author is Joseph P. Hornak.

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The Topsy Turvy World of Quantum Computing - IEEE Spectrum