Quantum chaos in ultracold gas discovered

Quantum chaos in ultracold gas discovered

The team of Francesca Ferlaino, Institute for Experimental Physics of the University of Innsbruck, Austria, has experimentally shown chaotic behavior of particles in a quantum gas. "For the first time we have been able to observe quantum chaos in the scattering behavior of ultracold atoms," says an excited Ferlaino. The physicists used random matrix theory to confirm their results, thus asserting the universal character of this statistical theory. Nobel laureate Eugene Wigner formulated random matrix theory to describe complex systems in the 1950s. Although interactions between neutrons with atomic nuclei were not well-known then, Wigner was able to reliably predict properties of complex spectra by using random matrices. Today random matrix theory is applied broadly not only in physics but also in number theory, wireless information technology and risk management models in finance to name only a few fields of application. In the Bohigas-Giannoni-Schmit conjecture random matrix theory has been connected to chaotic behavior in quantum mechanical systems. Catalan physicist Oriol Bohigas, who passed away last year, can be considered the father of quantum chaos research.

 Quantum chaos in ultracold gas discovered

Chaos in the quantum world

To observe quantum chaos, the physicists in Innsbruck cool erbium atoms to a few hundred nanokelvin and load them in an optical dipole trap composed of laser beams. They then influence the scattering behavior of the particles by using a magnetic field. After holding the atoms in the trap for 400 milliseconds, the researchers record the atom number remaining in the trap. Thus, the scientists are able to determine at which magnetic field two atoms are coupled to form a weakly-bound molecule. At this magnetic field, so-called Fano-Feshbach resonances emerge. After varying the magnetic field in each experimental cycle and repeating the experiment 14,000 times, the physicists identified 200 resonances. "We were fascinated by how many resonances of this type we found. This is unprecedented in the physics of ultracold quantum gases," says Francesca Ferlaino's team member Albert Frisch. To explain the high density of resonances, the researchers used statistical methods. By using Wigner's random matrix theory the scientists are able to show that different molecular levels are coupled. This has also been confirmed by computer simulations conducted by Svetlana Kotochigova's research group at Temple University in Philadelphia, Pennsylvania, USA. "The particular properties of erbium cause a highly complex coupling behavior between the particles, which can be described as chaotic," explains Ferlaino. Erbium is relatively heavy and highly magnetic, which leads to anisotropic interaction between atoms. "The electron shell of these atoms do not resemble spherical shells but are highly deformed," explains Albert Frisch. "Therefore, the type of interaction between two erbium atoms is significantly different from other quantum gases that have been investigated so far."

Studying chaos experimentally

In contrast to everyday speech, chaos does not mean disorder for the physicists but rather a well-ordered system that, due to its complexity, shows random behavior. Ferlaino is excited about their breakthrough: "We have created an experiment that provides a controlled environment to study chaotic processes. We cannot characterize the behavior of single atoms in our experiment. However, by using statistical methods, we can describe the behavior of all particles." She compares the method with sociology, which studies the behavior of a bigger community of people, whereas psychology describes the relations between individuals. This work also provides new inroads to the investigation of ultracold gases and, thus, ultracold chemistry." Ferlaino is convinced: "Our work represents a turning point in the world of ultracold gases."

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The experiment and statistical analysis were carried out at the Institute for Experimental Physics at the University of Innsbruck. Theoretical support was provided by John L. Bohn from the Joint Institute for Laboratory Astrophysics in Boulder, Colorado, USA and the team of Svetlana Kotochigova at Temple University in Philadelphia, Pennsylvania, USA. The Austrian researchers are supported by the Austrian Science Fund FWF and the European Research Council (ERC).

Publication: Quantum Chaos in Ultracold Collisions of Erbium. Frisch A, Mark M, Aikawa K, and Ferlaino F, Bohn JL, Makrides C, Petrov A, and Kotochigova S. Nature 2014 DOI: 10.1038/nature13137 [arXiv:1312.1972v1, http://arxiv.org/abs/1312.1972v1]

News Release Source :  Quantum chaos in ultracold gas discovered

Physicists discover 'quantum droplet' in semiconductor

JILA physicists discover 'quantum droplet' in semiconductor

BOULDER, Colo -- JILA physicists used an ultrafast laser and help from German theorists to discover a new semiconductor quasiparticle—a handful of smaller particles that briefly condense into a liquid-like droplet.

Physicists discover 'quantum droplet' in semiconductor

Quasiparticles are composites of smaller particles that can be created inside solid materials and act together in a predictable way. A simple example is the exciton, a pairing, due to electrostatic forces, of an electron and a so-called "hole," a place in the material's energy structure where an electron could be, but isn't.

The new quasiparticle, described in the Feb. 27, 2014, issue of Nature* and featured on the journal's cover, is a microscopic complex of electrons and holes in a new, unpaired arrangement. The researchers call this a "quantum droplet" because it has quantum characteristics such as well-ordered energy levels, but also has some of the characteristics of a liquid. It can have ripples, for example. It differs from a familiar liquid like water because the quantum droplet has a finite size, beyond which the association between electrons and holes disappears.

Although its lifetime is only a fleeting 25 picoseconds (trillionths of a second), thequantum droplet is stable enough for research on how light interacts with specialized forms of matter.

"Electron-hole droplets are known in semiconductors, but they usually contain thousands to millions of electrons and holes," says JILA physicist Steven Cundiff, who studies the properties of cutting-edge lasers and what they reveal about matter. "Here we are talking about droplets with around five electrons and five holes.

"Regarding practical benefits, nobody is going to build a quantum droplet widget. But this does have indirect benefits in terms of improving our understanding of how electrons interact in various situations, including in optoelectronic devices."

The JILA team created the new quasiparticle by exciting a gallium-arsenide semiconductor with an ultrafast red laser emitting about 100 million pulses per second. The pulses initially form excitons, which are known to travel around in semiconductors. As laser pulse intensity increases, more electron-hole pairs are created, with quantum droplets developing when the exciton density reaches a certain level. At that point, the pairing disappears and a few electrons take up positions relative to a given hole. The negatively charged electrons and positively charged holes create a neutral droplet. The droplets are like bubbles held together briefly by pressure from the surrounding plasma.

JILA's experimental data on energy levels of individual droplet rings agreed with theoretical calculations by co-authors at the University of Marburg in Germany. JILA researchers found they could tap into each energy level by tailoring the quantum properties of the laser pulses to match the particle correlations within the droplets. The droplets seem stable enough for future systematic studies on interactions between light and highly correlated states of matter. In addition, quasiparticles, in general, can have exotic properties not found in their constituent parts, and thus, can play a role in controlling the behavior of larger systems and devices.

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JILA is a joint institute of the National Institute of Standards and Technology (NIST) and University of Colorado Boulder. Cundiff is a NIST physicist. The JILA research is supported by the National Science Foundation, NIST and the Alexander von Humboldt Foundation.

* A.E. Almand-Hunter, H. Li, S.T. Cundiff, M. Mootz, M. Kira and S.W. Koch. Quantum droplets of electrons and holes. Nature. Feb. 27, 2014.

News Release Source :   JILA physicists discover 'quantum droplet' in semiconductor

Helical electron and nuclear spin order in quantum wires

Helical electron and nuclear spin order in quantum wires

Physicists at the University of Basel have observed a spontaneous magnetic order of electron and nuclear spins in a quantum wire at temperatures of 0.1 kelvin. In the past, this was possible only at much lower temperatures, typically in the microkelvin range. The coupling of nuclei and electrons creates a new state of matter whereby a nuclear spin order arises at a much higher temperature. The results are consistent with a theoretical model developed in Basel a few years ago, as reported by the researchers in the scientific journal Physical Review Letters.

Helical electron and nuclear spin order in quantum wires

The researchers, led by Professor Dominik Zumbühl from the University of Basel's Department of Physics, used quantum wires made from the semiconductor gallium arsenide. These are one-dimensional structures in which the electrons can move in only one spatial direction.

At temperatures above 10 kelvin, the quantum wires exhibited universal, quantized conductance, suggesting that the electron spins were not ordered. However, when the researchers used liquid helium to cool the wires to a temperature below 100 millikelvin (0.1 kelvin), the electronic measurements showed a drop in conductance by a factor of two, which would suggest a collective orientation of the electron spin. This state also remained constant when the researchers cooled the sample to even lower temperatures, down to 10 millikelvin.

Electron-nuclear spin coupling

The results are exceptional because this is the first time that nuclear spin order has been measured at temperatures as high as 0.1 kelvin. Previously, spontaneous nuclear spin order was observed only at much lower temperatures, typically below 1 microkelvin; i.e. five orders of magnitude lower in temperature.

The reason why nuclear spin order is possible already at 0.1 kelvin is that the nuclei of the gallium and arsenic atoms in these quantum wires couple to the electrons, which themselves act back on the nuclear spins, which again interact with the electrons, and so on. This feedback mechanism strongly amplifies the interaction between the magnetic moments, thus creating the combined nuclear and electron spin magnetism. This order is further stabilized by the fact that the electrons in such quantum wires have strong mutual interactions, bumping into each other like railcars on a single track.

Helical electron and nuclear spin order

Interestingly, in the ordered state, the spins of the electrons and nuclei do not all point in the same direction. Instead, they take the form of a helix rotating along the quantum wire. This helical arrangement is predicted by a theoretical model described by Professor Daniel Loss and collaborators at the University of Basel in 2009. According to this model, the conductance drops by a factor of two in the presence of a nuclear spin helix. All other existing theories are incompatible with the data from this experiment.

A step closer to the development of quantum computers

The results of the experiment are important for fundamental research, but are also interesting for the development of quantum computers based on electron spin as a unit of information (proposed by Daniel Loss and David P. DiVincenzo in 1997). In order for electron spins to be used for computation, they must be kept stable for a long period. However, the difficulty of controlling nuclear spins presents a major source of error for the stability of electron spins.

The work of the Basel physicists opens up new avenues for mitigating these disruptive nuclear spin fluctuations: with the nuclear spin order achieved in the experiment, it may be possible to generate much more stable units of information in the quantum wires.

In addition, the nuclear spins can be controlled with electronic fields, which was not previously possible. By applying a voltage, the electrons are expelled from the semiconductor, which dissolves the electron-nucleus coupling and the helical order.

International research partnership

The work was conducted by an international team led by Professor Dominik Zumbühl from the University of Basel's Department of Physics; the team received support in the measurements from Harvard University (Professor Amir Yacoby). The nanowires originated from Princeton University (Loren N. Pfeiffer and Ken West).

The research was co-funded by the European Research Council, the Swiss National Science Foundation, the Basel Center for Quantum Computing and Quantum Coherence (Basel QC2 Center), the Swiss Nanoscience Institute and the NCCR Quantum Science & Technology (QSIT).

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News Release Source :  Helical electron and nuclear spin order in quantum wires

Scientists use 'voting' and 'penalties' to overcome errors in quantumoptimization

Scientists use 'voting' and 'penalties' to overcome errors in quantum optimization

Study demonstrates that when the D-Wave quantum processor is led astray by noise, error correction can ensure that it functions as intended

Seeking a solution to decoherence—the "noise" that prevents quantum processors from functioning properly—scientists at USC have developed a strategy of linking quantum bits together into voting blocks, a strategy that significantly boosts their accuracy.

Scientists use 'voting' and 'penalties' overcome errors in quantum optimization

In a paper published today in Nature Communications, the team found that their method results in at least a five-fold increase in the probability of reaching the correct answer when the processor solves the largest problems tested by the researcher, involving hundreds of qubits.

The team, led by Daniel Lidar—director of the USC-Lockheed Martin Quantum Computing Center at the USC Viterbi School of Engineering—ran their tests on the 512-quantum-bit D-Wave Two processor. The D-Wave Two is among the first commercially available quantum processors, a device so advanced that there are only two in use outside the Canadian company where they were built: The first one went to USC and Lockheed Martin, and the second to NASA and Google.

"We have demonstrated that our quantum annealing correction strategy significantly improves the success probability of the D-Wave Two processor on the benchmark problem of antiferromagnetic chains, and are planning to next use it on computationally hard problems," Lidar said. His team includes graduate student Kristen Pudenz and postdoctoral fellow Tameem Albash.

Lidar added that all quantum information processors are expected to be highly susceptible to decoherence, so that error correction is viewed as an essential and inescapable part of quantum computing.

Quantum processors encode data in qubits, which have the capability of representing the two digits of one and zero at the same time – as opposed to traditional bits, which can encode distinctly either a one or a zero. This property, called superposition, along with the ability of quantum states to "interfere" (cancel or reinforce each other like waves in a pond) and "tunnel" through energy barriers, is what may one day allow quantum processors ultimately perform optimization calculations much faster than traditional processors.

Decoherence knocks qubits out of superposition, forcing them to behave as traditional bits, and robbing them of their edge over traditional processors.

Pudenz, Albash and Lidar developed and tested a strategy of grouping three qubits together into larger blocks of encoded qubits that can be decoded by a "majority vote." This way, if decoherence affects one of the qubits and causes it to "flip" to the incorrect value, the other two qubits in the block ensure that the data is still correctly encoded and can be correctly decoded by out-voting the errant qubit.

These voting blocks of qubits are then magnetically tied to a fourth qubit in such a way that if any one "flips" then all four must flip. In effect, it makes the whole block of four so massive that it's difficult for one lonely qubit acting under the influence of decoherence to throw a wrench in the works.

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This research was funded by the Army Research Office, the Lockheed Martin Corporation, and the National Science Foundation.

Read the study at: http://www.nature.com/ncomms/2014/140206/ncomms4243/full/ncomms4243.html

News Release Source :  Scientists use 'voting' and 'penalties' to overcome errors in quantum optimization

Integration brings quantum computer a step closer

Integration brings quantum computer a step closer

An international research group led by the University of Bristol has made an important advance towards a quantum computer by shrinking down key components and integrating them onto a silicon microchip.

Integration brings quantum computer a step closer

Scientists and engineers from an international collaboration led by Dr Mark Thompson from the University of Bristol have, for the first time, generated and manipulated single particles of light (photons) on a silicon chip – a major step forward in the race to build a quantum computer.

Quantum computers and quantum technologies in general are widely anticipated as the next major technology advancement, and are poised to replace conventional information and computing devices in applications ranging from ultra-secure communications and high-precision sensing to immensely powerful computers. While many of the components for a quantum computer already exist, for a quantum computer to be realised, these components need to be integrated onto a single chip.

Featuring today on the front cover of Nature Photonics, this latest advancement is one of the important pieces in the jigsaw needed in order to realise a quantum computer. While previous attempts have required external light sources to generate the photons, this new chip integrates components that can generate photons inside the chip. "We were surprised by how well the integrated sources performed together," admits Joshua Silverstone, lead author of the paper. "They produced high-quality identical photons in a reproducible way, confirming that we could one day manufacture a silicon chip with hundreds of similar sources on it, all working together. This could eventually lead to an optical quantum computer capable of perform enormously complex calculations."

"Single-photon detectors, sources and circuits have all been developed separately in silicon but putting them all together and integrating them on a chip is a huge challenge," explains group leader Mark Thompson. "Our device is the most functionally complex photonic quantum circuit to date, and was fabricated by Toshiba using exactly the same manufacturing techniques used to make conventional electronic devices."

The group, which, includes researchers from Toshiba Corporation (Japan), Stanford University (US), University of Glasgow (UK) and TU Delft (The Netherlands), now plans to integrate the remaining necessary components onto a chip, and show that large-scale quantum devices using photons are possible.

"Our group has been making steady progress towards a functioning quantum computer over the last five years," said Thompson. "We hope to have a photon-based device which can rival modern computing hardware for highly-specialised tasks within the next couple of years."

Much of the work towards this goal will be carried out at Bristol's new Centre for Doctoral Training in Quantum Engineering, which will train a new generation of engineers, scientists and entrepreneurs to harness the power of quantum mechanics using state-of-the-art engineering technique to make real world and useful quantum enhanced devices. This innovative centre bridges the gaps between physics, engineering, mathematics and computer science, working closely with chemists and biologists while interacting strongly with industry.

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News Release Source : Integration brings quantum computer a step closer

Quantum Physics Could Make Secure, Single-use Computer Memories Possible

Quantum physics could make secure, single-use computer memories possible

Computer security systems may one day get a boost from quantum physics, as a result of recent research from the National Institute of Standards and Technology (NIST). Computer scientist Yi-Kai Liu has devised away to make a security device that has proved notoriously difficult to build—a "one-shot" memory unit, whose contents can be read only a single time.

Quantum physics could make secure, single-use computer memories possible

The research, which Liu is presenting at this week's Innovations in Theoretical Computer Science conference,* shows in theory how the laws of quantum physics could allow for the construction of such memory devices. One-shot memories would have a wide range of possible applications such as protecting the transfer of large sums of money electronically. A one-shot memory might contain two authorization codes: one that credits the recipient's bank account and one that credits the sender's bank account, in case the transfer is canceled. Crucially, the memory could only be read once, so only one of the codes can be retrieved, and hence, only one of the two actions can be performed—not both.

"When an adversary has physical control of a device—such as a stolen cell phone—software defenses alone aren't enough; we need to use tamper-resistant hardware to provide security," Liu says. "Moreover, to protect critical systems, we don't want to rely too much on complex defenses that might still get hacked. It's better if we can rely on fundamental laws of nature, which are unassailable."

Unfortunately, there is no fundamental solution to the problem of building tamper-resistant chips, at least not using classical physics alone. So scientists have tried involving quantum mechanics as well, because information that is encoded into a quantum system behaves differently from a classical system.

Liu is exploring one approach, which stores data using quantum bits, or "qubits," which use quantum properties such as magnetic spin to represent digital information. Using a technique called "conjugate coding, "two secret messages—such as separate authorization codes—can be encoded into the same string of qubits, so that a user can retrieve either one of the two messages. But as the qubits can only be read once, the user cannot retrieve both.

The risk in this approach stems from a more subtle quantum phenomenon: "entanglement," where two particles can affect each other even when separated by great distances. If an adversary is able to use entanglement, he can retrieve both messages at once, breaking the security of the scheme.

However, Liu has observed that in certain kinds of physical systems, it is very difficult to create and use entanglement, and shows in his paper that this obstacle turns out to be an advantage: Liu presents a mathematical proof that if an adversary is unable to use entanglement in his attack, that adversary will never be able to retrieve both messages from the qubits. Hence, if the right physical systems are used, the conjugate coding method is secure after all.

"It's fascinating how entanglement—and the lack thereof—is the key to making this work," Liu says. "From a practical point of view, these quantum devices would be more expensive to fabricate, but they would provide a higher level of security. Right now, this is still basic research. But there's been a lot of progress in this area, so I'm optimistic that this will lead to useful technologies in the real world."

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*Y-K Liu. "Building one-time memories from isolated qubits." Paper presented at the ITCS 20-14 Innovations in Theoretical Computer Science meeting, Princeton University, Jan. 11-14, 2014. More info at http://itcs2014.wordpress.com/program/.


Presented paper is here

Building one-time memories from isolated qubits

News Release Source : Quantum physics could make secure, single-use computer memories possible

Possibility of Cloning Quantum Data from The Past

LSU researcher shows possibility of cloning quantum information from the past

Popular television shows such as "Doctor Who" have brought the idea of time travel into the vernacular of popular culture. But problem of time travel is even more complicated than one might think. LSU's Mark Wilde has shown that it would theoretically be possible for time travelers to copy quantum data from the past.

Possibility of Cloning Quantum Data from The Past

It all started when David Deutsch, a pioneer of quantum computing and a physicist at Oxford, came up with a simplified model of time travel to deal with the paradoxes that would occur if one could travel back in time. For example, would it be possible to travel back in time to kill one's grandfather? In the Grandfather paradox, a time traveler faces the problem that if he kills his grandfather back in time, then he himself is never born, and consequently is unable to travel through time to kill his grandfather, and so on. Some theorists have used this paradox to argue that it is actually impossible to change the past.

"The question is, how would you have existed in the first place to go back in time and kill your grandfather?" said Mark Wilde, an LSU assistant professor with a joint appointment in the Department of Physics and Astronomy and with the Center for Computation and Technology, or CCT.

Deutsch solved the Grandfather paradox originally using a slight change to quantum theory, proposing that you could change the past as long as you did so in a self-consistent manner.

"Meaning that, if you kill your grandfather, you do it with only probability one-half," Wilde said. "Then, he's dead with probability one-half, and you are not born with probability one-half, but the opposite is a fair chance. You could have existed with probability one-half to go back and kill your grandfather."

But the Grandfather paradox is not the only complication with time travel. Another problem is the no-cloning theorem, or the no "subatomic Xerox-machine" theorem, known since 1982. This theorem, which is related to the fact that one cannot copy quantum data at will, is a consequence of Heisenberg's famous Uncertainty Principle, by which one can measure either the position of a particle or its momentum, but not both with unlimited accuracy. According to the Uncertainty Principle, it is thus impossible to have a subatomic Xerox-machine that would take one particle and spit out two particles with the same position and momentum – because then you would know too much about both particles at once.

"We can always look at a paper, and then copy the words on it. That's what we call copying classical data," Wilde said. "But you can't arbitrarily copy quantum data, unless it takes the special form of classical data. This no-cloning theorem is a fundamental part of quantum mechanics – it helps us reason how to process quantum data. If you can't copy data, then you have to think of everything in a very different way."

But what if a Deutschian closed timelike curve did allow for copying of quantum data to many different points in space? According to Wilde, Deutsch suggested in his late 20th century paper that it should be possible to violate the fundamental no-cloning theorem of quantum mechanics. Now, Wilde and collaborators at the University of Southern California and the Autonomous University of Barcelona have advanced Deutsch's 1991 work with a recent paper inPhysical Review Letters (DOI: 10.1103/PhysRevLett.111.190401). The new approach allows for a particle, or a time traveler, to make multiple loops back in time – something like Bruce Willis' travels in the Hollywood film "Looper."

"That is, at certain locations in spacetime, there are wormholes such that, if you jump in, you'll emerge at some point in the past," Wilde said. "To the best of our knowledge, these time loops are not ruled out by the laws of physics. But there are strange consequences for quantum information processing if their behavior is dictated by Deutsch's model."

A single looping path back in time, a time spiral of sorts, behaving according to Deutsch's model, for example, would have to allow for a particle entering the loop to remain the same each time it passed through a particular point in time. In other words, the particle would need to maintain self-consistency as it looped back in time.

"In some sense, this already allows for copying of the particle's data at many different points in space," Wilde said, "because you are sending the particle back many times. It's like you have multiple versions of the particle available at the same time. You can then attempt to read out more copies of the particle, but the thing is, if you try to do so as the particle loops back in time, then you change the past."

To be consistent with Deutsch's model, which holds that you can only change the past as long as you can do it in a self-consistent manner, Wilde and colleagues had to come up with a solution that would allow for a looping curve back in time, and copying of quantum data based on a time traveling particle, without disturbing the past.

"That was the major breakthrough, to figure out what could happen at the beginning of this time loop to enable us to effectively read out many copies of the data without disturbing the past," Wilde said. "It just worked."

However, there is still some controversy over interpretations of the new approach, Wilde said. In one instance, the new approach may actually point to problems in Deutsch's original closed timelike curve model.

"If quantum mechanics gets modified in such a way that we've never observed should happen, it may be evidence that we should question Deutsch's model," Wilde said. "We really believe that quantum mechanics is true, at this point. And most people believe in a principle called Unitarity in quantum mechanics. But with our new model, we've shown that you can essentially violate something that is a direct consequence of Unitarity. To me, this is an indication that something weird is going on with Deutsch's model. However, there might be some way of modifying the model in such a way that we don't violate the no-cloning theorem."

Other researchers argue that Wilde's approach wouldn't actually allow for copying quantum data from an unknown particle state entering the time loop because nature would already "know" what the particle looked like, as it had traveled back in time many times before.

But whether or not the no-cloning theorem can truly be violated as Wilde's new approach suggests, the consequences of being able to copy quantum data from the past are significant. Systems for secure Internet communications, for example, will likely soon rely on quantum security protocols that could be broken or "hacked" if Wilde's looping time travel methods were correct.

"If an adversary, if a malicious person, were to have access to these time loops, then they could break the security of quantum key distribution," Wilde said. "That's one way of interpreting it. But it's a very strong practical implication because the big push of quantum communication is this secure way of communicating. We believe that this is the strongest form of encryption that is out there because it's based on physical principles."

Today, when you log into your Gmail or Facebook, your password and information encryption is not based on physical principles of quantum mechanical security, but rather on the computational assumption that it is very difficult for "hackers" to factor mathematical products of prime numbers, for example. But physicists and computer scientists are working on securing critical and sensitive communications using the principles of quantum mechanics. Such encryption is believed to be unbreakable – that is, as long as hackers don't have access to Wilde's looping closed timelike curves.

"This ability to copy quantum information freely would turn quantum theory into an effectively classical theory in which, for example, classical data thought to be secured by quantum cryptography would no longer be safe," Wilde said. "It seems like there should be a revision to Deutsch's model which would simultaneously resolve the various time travel paradoxes but not lead to such striking consequences for quantum information processing. However, no one yet has offered a model that meets these two requirements. This is the subject of open research."

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News Release Source : LSU researcher shows possibility of cloning quantum information from the past

Scientists Achieves Secure Cloud Computing with Quantum Computers

International team of scientists achieves secure “cloud” computing with quantum computers

A team of scientists, including CIFAR Junior Fellow Anne Broadbent, have demonstrated that “blind quantum computing” is now possible. An experiment published in the latest issue of Science, showed that quantum computation could occur and keep the input, data processing and output unknown to the quantum computer.

Scientists Achieves Secure Cloud Computing with Quantum Computers

With the use of light particles, the scientists were able to send data to a quantum computer, have the computer solve the computation and then send the data back, all without revealing the information being transmitted. This proved that quantum computers could perform calculations required of a user remotely, but have no means to find out what it is actually doing. Everything would remain perfectly encrypted - a level of security not achievable in the current world of classical computing.
Quantum computers are considered the next generation in high performance computing, able to solve complicated problems deemed impossible for classical computers. In present day, large scale data storage and data processing is handled through remote centres of supercomputers. Everything is performed in a “cloud”, but the challenge to date has been ensuring that a users’ data stays private.

These findings are an exciting breakthrough for scientists hoping to achieve secure and global quantum computing.

Read more : International team achieves breakthrough in secure quantum computing

News Source : International team of scientists achieves secure “cloud” computing with quantum computers

Quantum physics secures new cryptography scheme

Quantum physics secures new cryptography scheme

The way we secure digital transactions could soon change. An international team has demonstrated a form of quantum cryptography that can protect people doing business with others they may not know or trust – a situation encountered often on the internet and in everyday life, for example at a bank's ATM.

Quantum physics secures new cryptography scheme

"Having quantum cryptography to hand is a realistic prospect, I think. I expect that quantum technologies will gradually become integrated with existing devices such as smartphones, allowing us to do things like identify ourselves securely or generate encryption keys," says Stephanie Wehner, a Principal Investigator at the Centre for Quantum Technologies (CQT) at the National University of Singapore, and co-author on the paper.

In cryptography, the problem of providing a secure way for two mutually distrustful parties to interact is known as 'two-party secure computation'. The new work, published in Nature Communications, describes the implementation using quantum technology of an important building block for such schemes.

CQT theorists Wehner and Nelly Ng teamed up with researchers at the Institute for Quantum Computing (IQC) at the University of Waterloo, Canada, for the demonstration.

"Research partnerships such as this one between IQC and CQT are critical in moving the field forward," says Raymond Laflamme, Executive Director at the Institute for Quantum Computing. "The infrastructure that we've built here at IQC is enabling exciting progress on quantum technologies."

"CQT and IQC are two of the world's largest, leading research centres in quantum technologies. Great things can happen when we combine our powers," says Artur Ekert, Director of CQT.

The experiments performed at IQC deployed quantum-entangled photons in such a way that one party, dubbed Alice, could share information with a second party, dubbed Bob, while meeting stringent restrictions. Specifically, Alice has two sets of information. Bob requests access to one or the other, and Alice must be able to send it to him without knowing which set he's asked for. Bob must also learn nothing about the unrequested set. This is a protocol known as 1-2 random oblivious transfer (ROT).

ROT is a starting point for more complicated schemes that have applications, for example, in secure identification. "Oblivious transfer is a basic building block that you can stack together, like lego, to make something more fantastic," says Wehner.

Today, taking money out of an ATM requires that you put in a card and type in your PIN. You trust the bank's machine with your personal data. But what if you don't trust the machine? You might instead type your PIN into your trusted phone, then let your phone do secure quantum identification with the ATM (see artist's impression). Ultimately, the aim is to implement a scheme that can check if your account number and PIN matches the bank's records without either you or the bank having to disclose the login details to each other.

Unlike protocols for ROT that use only classical physics, the security of the quantum protocol cannot be broken by computational power. Even if the attacker had a quantum computer, the protocol would remain secure.

Its security depends only on Alice and Bob not being able to store much quantum information for long. This is a reasonable physical assumption, given today's best quantum memories are able to store information for minutes at most. Moreover, any improvements in memory can be matched by changes in the protocol: a bigger storage device simply means more signals have to be sent in order to achieve security. (The idea of 'noisy storage' securing quantum cryptography was developed by Wehner in earlier papers.)

To start the ROT protocol, Alice creates pairs of entangled photons. She measures one of each pair and sends the other to Bob to measure. Bob chooses which photons he wants to learn about, dividing his data accordingly without revealing his picks to Alice. Both then wait for a length of time chosen such that any attempt to store quantum information about the photons is likely to fail. To complete the oblivious transfer, Alice then tells Bob which measurements she made, and they both process their data in set ways that ensure the result is correct and secure within a pre-agreed margin of error.

In the demonstration performed at IQC, Alice and Bob achieved a random oblivious transfer of 1,366 bits. The whole process took about three minutes.

The experiment adapted devices built to do a more standard form of quantum cryptography known as quantum key distribution (QKD), a scheme that generates random numbers for scrambling communication. Devices for QKD are already commercially available, and miniaturised versions of this experiment are in principle possible using integrated optics. In the future, people might carry hand-held quantum devices that can perform this kind of feat.

"We did the experiment with big and bulky optics taking metres of space, but you can well imagine this technology being shrunk down to sit happily next to classical processing circuits on a small little microchip. The field of integrated quantum optics has been progressing in leaps and bounds, and most of the key pieces required to implement ROT have already been successfully demonstrated in integrated setups a few millimetres in size," says Chris Erven, who performed the experiments at IQC as a PhD student under the supervision of Raymond Laflamme and Gregor Weihs. Weihs is now at the University of Innsbruck, Austria. Erven is now a postdoctoral fellow at the University of Bristol, UK.


Reference:

"An Experimental Implementation of Oblivious Transfer in the Noisy Storage Model", Nature Communications DOI:10.1038/ncomms4418 (2014)

http://www.nature.com/naturecommunications

A preprint is available at http://arxiv.org/abs/1308.5098

News Release Source :  Quantum physics secures new cryptography scheme

Opens The Door to Multi-Party Quantum Communication

Experiment opens the door to multi-party quantum communication

In the world of quantum science, Alice and Bob have been talking to one another for years. Charlie joined the conversation a few years ago, but now with spacelike separation, scientists have measured that their communication occurs faster than the speed of light.

Opens The Door to Multi-Party Quantum Communication

For the first time, physicists at the Institute for Quantum Computing (IQC) at the University of Waterloo have demonstrated the distribution of three entangled photons at three different locations (Alice, Bob and Charlie) several hundreds of metres apart, proving quantum nonlocality for more than two entangled photons.

The findings of the experiment, Experimental Three-Particle Quantum Nonlocality under Strict Locality Conditions, are published in Nature Photonics today.

Once described by Einstein as "spooky action at a distance", this three-photon entanglement leads to interesting possibilities for multi-party quantum communication.

Nonlocality describes the ability of particles to instantaneously know about each other's state, even when separated by large distances. In the quantum world, this means it might be possible to transfer information instantaneously – faster than the speed of light. This contravenes what Einstein called the "principle of local action," the rule that distant objects cannot have direct influence on one another, and that an object is directly influenced only by its immediate surroundings.

To truly test that the hidden local variables are not responsible for the correlation between the three photons, IQC scientists needed the experiment to close what is known as the locality loophole. They achieved this separation of the entangled photons in a way that did not allow for a signal to coordinate the behaviour of the photons, but beaming the entangled photons to trailers parked in fields several hundred meters from their lab.

"Correlations measured from quantum systems can tell us a lot about nature at the most fundamental level," said co-author Professor Kevin Resch, Canada Research Chair in Optical Quantum Technologies and recent winner of the E.W.R. Steacie Fellowship from the Natural Sciences and Engineering Research Council of Canada (NSERC). "Three-particle entanglement is more complex than that of pairs. We can exploit the complex behaviour to rule out certain descriptions of nature or as a resource for new quantum technologies.

The project team studied the correlations of three photons in a Greenberger-Horne-Zeilinger (GHZ) state – a type of entangled quantum state involving at least three particles.

First, photon triplets were generated in Resch's lab – the Alice in the experiment. Then, the first photon was delayed in a 580m optical fibre in the lab while the two other photons travelled up 85m of optical fibre to the rooftop where they were sent through two telescopes. Both photons were then sent to two trailers, Bob and Charlie, about 700m away from the source and from each other.

To maintain the spacelike separate in the experiment, a fourth party, Randy, located in a third trailer randomly selected each of the measurements that Alice was to perform on her photons in the lab.

Each trailer contained detectors, time-tagging devices developed by IQC spin off company Universal Quantum Devices (UQD), and quantum random number generators. To ensure the locality loophole was closed, the random number generators determined how the photon at each trailer would be measured independently. The UQD time tagging devices also ensured the measurements happened in a very small time window (three nanoseconds), meaning that no information could possibly be transmitted from one location to the other during the measurement period ¬– a critical condition to prove the non-locality of entanglement.

"The idea of entangling three photons has been around for a long time," said Professor Thomas Jennewein, a co-author of the paper. "It took the right people with the right knowledge to come together to make the experiment happen in the short time it did. IQC had the right mix at the right time."

The experiment demonstrated the distribution of three entangled particles, which can eventually be used to do more than pairwise communication where only one party can communicate with another. It opens the possibility for multipartite quantum communication protocols, including Quantum Key Distribution (QKD), third man cryptography and quantum secret sharing.

"The interesting result is that we now have the ability to do more than paired quantum communication," said the paper's lead author Chris Erven, a former IQC PhD student who is now a research assistant at the University of Bristol. "QKD, so far, has been a pairwise system – meaning that it works best and with less assumptions when you're only talking with one other person. This is the first experiment where you can now imagine a network of people connected in different ways using the correlations between three or more photons."

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The team from the Institute for Quantum Computing and the Department of Physics and Astronomy in the Faculty of Science at the University of Waterloo included students Chris Erven, Evan Meyer-Scott, Kent Fisher, Jonathan Lavoie, Christopher Pugh, Jean-Paul Bourgoin, Laura Richards, Nickolay Gigov, postdoctoral fellow Brendon Higgins, Professor Jennewein, Professor Resch and Raymond Laflamme, executive director of IQC.

The project team also included former IQC postdoctoral fellows Robert Prevedel, now at the Max F. Perutz Laboratories (MFPL) and the Institute for Molecular Pathology (IMP); Zhizhong Yan, now at Macquarie University; Krister Shalm now at the National Institute of Standards and Technology (NIST); and former faculty member Gregor Weihs, now at the Institut fur Experimentalphysik at the University of Innsbruck.

News Release Source :  Experiment opens the door to multi-party quantum communication

Ultracold molecules promising for quantum computing

'Ultracold' molecules promising for quantum computing, simulation

WEST LAFAYETTE, Ind. – Researchers have created a new type of "ultracold" molecule, using lasers to cool atoms nearly to absolute zero and then gluing them together, a technology that might be applied to quantum computing, precise sensors and advanced simulations.

Ultracold molecules promising for quantum computing

"It sounds counterintuitive, but you can use lasers to take away the kinetic energy, resulting in radical cooling," said Yong P. Chen, an associate professor of physics and electrical and computer engineering at Purdue University.

Physicists are using lasers to achieve such extreme cooling, reducing the temperature to nearly absolute zero, or minus 273 degrees Celsius (minus 459 degrees Fahrenheit) - the lowest temperature possible in the universe.

At these temperatures atoms are brought to a near standstill, making possible new kinds of chemical interactions that are predominantly quantum mechanical in nature. The process is performed inside of an apparatus called a magneto-optical trap, a system that uses a vacuum chamber, magnetic coils and a series of lasers to cool and trap the atoms.

"This is our test tube," said Daniel S. Elliott, a professor of electrical and computer engineering and physics. "In ultracold chemistry, molecules are really moving slowly so they have a long time to interact with each other."

Other researchers have used the method to create cold molecules out of atoms of other alkali metals, which are relatively easy to turn into ultracold molecules. The Purdue researchers are the first to achieve the milestone with the alkali metals lithium and rubidium, in work led by Chen and Elliott.

Findings are detailed in a research paper that appeared as a "Rapid Communication" in the February issue of the journal Physical Review A, a publication of the American Physical Society. The paper was authored by former Purdue physics doctoral student Sourav Dutta, who has graduated; graduate students John Lorenz and Adeel Altaf; Elliott and Chen. The paper is available online at http://pra.aps.org/abstract/PRA/v89/i2/e020702

The method is called photoassociation: two atoms are merged using lasers to induce a chemical bond between them, forming a molecule. These molecules may contain two of the same types of atoms - making them homonuclear - or they can contain two different types of atoms, heteronuclear, such as the case with the lithium-rubidium molecules created by the team.

If the molecules are heteronuclear there is a difference in electric charge between these two atoms and the molecule is said to be polar. This difference in charge is called a dipole moment, which enables interaction between molecules. The greater the dipole moment, the stronger the interaction.

The lithium-rubidium molecule is potentially ideal for various applications, including quantum computing, because it has a significant dipole moment, which can enable these molecules to be used as "quantum bits."

Quantum computers would take advantage of a phenomenon described by quantum theory called "entanglement." Instead of only the states of one and zero used in conventional computer processing, there are many possible "entangled quantum states" in between one and zero, dramatically increasing the capacity to process information.

"In quantum computing the larger the dipole moment the stronger the interaction would be between molecules, and you need that interaction," Elliott said. "They need to interact with each other in order to affect each other, the key to entanglement."

Another potential advantage for the lithium-rubidium molecule is that it can be produced in large quantities.

"The rate of production is much greater for lithium-rubidium than for other bi-alkali-metal molecules," Chen said. "That was a pleasant surprise. It was already known that it has the third- largest dipole moment among bi-alkali-metal molecules, but nobody expected it would be made so efficiently."

Ultracold means temperatures less than about one thousandth of degree above absolute zero. Achieving such frigid extremes requires reducing the kinetic energy of molecules as well as their "internal excitation energies," which are stored in three ways: the rotation of the molecule itself, the vibrations of the atomic nuclei, and the movement of electrons in "shells" surrounding the nuclei. The combined energy of the trio is called rovibronic, a shortened version of rotational, vibrational and electronic.

"We are reporting a highly efficient production of ultracold lithium-rubidium molecules by photoassociation," Dutta said. "This provides the first step towards the production of such ultracold lithium-rubidium molecules in their ground, polar state."

Molecules in their "ground state" have the lowest possible rovibronic energy, which would make them more stable and easier to control.

A related research paper was also published by the team in January in the journal Europhysics Letters, a publication of the European Physical Society. That paper is available online at http://iopscience.iop.org/0295-5075/104/6/63001/article

"Lithium rubidium is one of the last bi-alkali molecules to be made cold, and we are the first to do this," Chen said. "People knew virtually nothing about these molecules."

Ultimately, researchers are seeking more efficient methods for the production of ultracold molecules.

The research has been funded by Purdue's Bilsland Dissertation Fellowship, the National Science Foundation, Army Research Office, and more recently by a research incentive grant from Purdue's Office of Vice President for Research.

The research falls within a field called AMO, for atomic, molecular, and optical physics, an area under expansion at Purdue.

"AMO physics is an exciting area in the landscape of experimental and theoretical physics," Elliott said. "Seven years ago we had one person working in this area."

Since then, the department has added three faculty members working in AMO and is in the process of adding more.

"Purdue is positioned to become a leader in AMO physics," Chen said.

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Writer: Emil Venere, 765-494-4709, venere@purdue.edu

Sources: Yong Chen, 765-494-0947, yongchen@purdue.edu
Daniel S. Elliott, 765-494-3442, elliottd@ecn.purdue.edu

Related websites:
Yong Chen: http://www.physics.purdue.edu/people/faculty/yongchen.shtml
Daniel S. Elliott: https://engineering.purdue.edu/ECE/People/profile?resource_id=2925

ABSTRACT

Photoassociation of ultracold LiRb∗ molecules: Observation of high efficiency and unitarity-limited rate saturation Sourav Dutta,1 , * John Lorenz,1 Adeel Altaf,1 , D. S. Elliott,1, 2 and Yong P. Chen 1, 2
1 Department of Physics, Purdue University
2 School of Electrical and Computer Engineering, Purdue University

We report the production of ultracold heteronuclear 7 Li85 Rb molecules in excited electronic states by photoassociation (PA) of ultracold 7 Li and 85 Rb atoms. PA is performed in a dual-species 7 Li-85 Rb magneto-optical trap (MOT) and the PA resonances are detected using trap loss spectroscopy. We identify several strong PA resonances below the Li (2s 2 S1/2 ) + Rb (5p 2 P3/2 ) asymptote and xperimentally determine the long range C6 dispersion coefficients. We find a molecule formation rate (PLiRb ) of 3.5 × 10 7 s−1 and a PA rate coefficient (KPA ) of 1.3 × 10−10 cm3 /s, the highest among heteronuclear bi-alkali-metal molecules. At large PA laser intensity we observe the saturation of the PA rate coefficient (KPA ) close to the theoretical value at the unitarity limit.

News Release Source :    'Ultracold' molecules promising for quantum computing, simulation

Diamond Defect Boosts Quantum Technology

Diamond defect boosts quantum technology

Washington, D.C.—New research shows that a remarkable defect in synthetic diamond produced by chemical vapor deposition allows researchers to measure, witness, and potentially manipulate electrons in a manner that could lead to new "quantum technology" for information processing. The study is published in the January 31, 2014, issue of Physical Review Letters.

Diamond Boosts Quantum Technology

Normal computers process bits, the fundamental ones and zeros, one at a time. But in quantum computing, a "qubit" can be a one or a zero at the same time. This duplicitous state can allow multitasking at an astounding rate, which could exponentially increase the computing capacity of a tiny, tiny machine.

An "NV-" center can be created within a diamond's scaffold-like structure by replacing a missing carbon atom with a nitrogen atom (N)that has trapped an electron making the center negatively charged. Scientists can monitor the center's behavior and thereby provide a window for understanding how electrons respond to different conditions. The center has the potential to serve as a qubit in future quantum computers.

Electrons occupy different orbits around their atom and, by analogy, spin like the Earth. For the first time, Struzhkin and his team, led by Marcus Doherty of the Australian National University, observed what happens to electrons in these NV- centers under high-pressure and normal temperatures. Coauthor of the study, Viktor Struzhkin at the Carnegie Institution for Science, explained: "Our technique offers a powerful new tool for analyzing and manipulating electrons to advance our understanding of high-pressure superconductivity, as well as magnetic and electrical properties."

Struzhkin and team subjected single-crystal diamonds to pressures up to 600,000 times atmospheric pressure at sea level (60 gigapascals, GPa) in a diamond anvil cell and observed how electron spin and motion were affected. They optically excited the NV- centers with light and scanned microwave frequencies in a process called optically detected magnetic resonance to determine any changes. The NV- center is very sensitive to magnetic fields, electrical fields, and stress.

Until now, researchers thought that the orbits of the electrons that contribute to the defect's electronic structure and spin dynamics were localized to the area immediately surrounding the vacancy. Doherty explained: "Our team found instead that the electrons also orbit more distant atoms and that the span of their orbits contract with increasing pressure."

In addition to overturning previous beliefs about the electron orbits, the researchers found a sensitive means to measure pressure. This method can detect changes in pressure of about 10 atmospheres in one second, even up to pressures of 500,000 atmospheres (50 GPa).

"This work demonstrates that defects in diamond have great potential as quantum sensors of high pressure phenomena and, conversely, that high pressure can be employed to study the quantum phenomena of the defects," remarked Doherty.

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This work was supported by BES/DOE, DOE-NNSA, the Australian Research Council Discovery Project, Centre of Excellence for Quantum Computation and Communications Technology, and the Alexander von Humboldt Foundation.

The Carnegie Institution for Science is a private, nonprofit organization headquartered in Washington, D.C., with six research departments throughout the U.S. Since its founding in 1902, the Carnegie Institution has been a pioneering force in basic scientific research. Carnegie scientists are leaders in plant biology, developmental biology, astronomy, materials science, global ecology, and Earth and planetary science.

News Release Source :  Diamond defect boosts quantum technology

Seeking quantum-ness: D-Wave chip passes rigorous tests

Seeking quantum-ness: D-Wave chip passes rigorous tests

With cutting-edge technology, sometimes the first step scientists face is just making sure it actually works as intended.

The USC Viterbi School of Engineering is home to the USC-Lockheed Martin Quantum Computing Center (QCC), a super-cooled, magnetically shielded facility specially built to house the first commercially available quantum computing processors – devices so advanced that there are only two in use outside the Canadian lab where they were built: The first one went to USC and Lockheed Martin, and the second to NASA and Google.

Seeking quantum-ness: D-Wave chip passes rigorous tests

Since USC's facility opened in October 2011, a key task for researchers has been to determine whether D-Wave processors operate as hoped – using the special laws of quantum mechanics to offer potentially higher-speed processing, instead of operating in a classical, traditional way.

An international collaboration of scientists has now published several papers rejecting classical models of the first-generation D-Wave One processor housed at USC, including one on an elaborate test of all 108 of the chip's functional quantum bits ("qubits"). The test demonstrates that the D-Wave One behaved in a way that agrees with a model called "quantum Monte Carlo," yet disagreed with two candidate classical models that could have described the processor in the absence of quantum effects.

The research was published on Feb. 28 by Nature Physics.

"The challenge is that the tests we can perform on the USC-based D-Wave processor can't directly 'prove' that the D-Wave processor is quantum – we can only disprove candidate classical models one at a time," said QCC Director Prof. Daniel Lidar. "But so far we find that the D-Wave processor is always consistent with our quantum models. Our tests continually get more rigorous and complex."

Add this to recent work involving USC Information Sciences Institute researcher Federico Spedalieri demonstrating entanglement in a chip at the company's headquarters in Burnaby BC as well as previous testing of a smaller group of qubits by Spedalieri, Lidar and their collaborators, and the evidence is mounting that quantum effects are at play in the D-Wave processors.

Quantum processors encode data in qubits, which have the capability of representing the two digits of one and zero at the same time – as opposed to traditional bits, which can encode distinctly either a one or a zero. This property, called superposition, along with the ability of quantum states to "interfere" (cancel or reinforce each other like waves in a pond) and "tunnel" through energy barriers, is what may one day allow quantum processors ultimately perform optimization calculations much faster than traditional processors.

Optimization problems can take many forms, and quantum processors have been theorized to be useful for a variety of big data problems like stock portfolio optimization, image recognition and classification, and detecting anomalies, such as rooting out bugs in complex software.

The first quantum chip housed at the QCC was a 128-qubit D-Wave One, which was replaced about a year ago with the 512-qubit D-Wave Two. Though every chip is unique, the repeated validation of the older chip bodes well for its successor, which shares the same architecture.

"Our work is part of a large scale effort by the research community aimed at validating the potential of quantum information processing, which we all hope might one day surpass its classical counterparts," Lidar said.

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This research was funded by the Swiss National Science Foundation, the Army Research Office, the Lockheed Martin Corporation, DARPA, and the National Science Foundation.

News Release Source :  Seeking quantum-ness: D-Wave chip passes rigorous tests