The microprocessor inside a computer is a single multipurpose chip that has revolutionised people's life, allowing them to use one machine to surf the web, check emails and keep track of finances.

Now, researchers from the University of Bristol in the UK and Nippon Telegraph and Telephone (NTT) in Japan, have pulled off the same feat for light in the quantum world by developing an optical chip that can process photons in an infinite number of ways.

It's a major step forward in creating a quantum supercomputer to solve problems such as designing new drugs, superfast database searches, and performing otherwise intractable mathematics that aren't possible for today's supercomputers.

The fully reprogrammable chip brings together a multitude of existing quantum experiments and can realise a plethora of future protocols that have not even been conceived yet, marking a new era of research for quantum scientists and engineers at the cutting edge of quantum technologies. The work is published in the journal Science on 14 August.

Since before Newton held a prism to a ray of sunlight and saw a spectrum of colour, scientists have understood nature through the behaviour of light. In the modern age of research, scientists are striving to understand nature at the quantum level and to engineer and control quantum states of light and matter.

A major barrier in testing new theories for quantum science and quantum supercomputing is the time and resources needed to build new experiments, which are typically extremely demanding due to the notoriously fragile nature of quantum systems.

This result shows a step change for experiments with photons, and what the future looks like for quantum technologies.

Dr Anthony Laing, who led the project, said: "A whole field of research has essentially been put onto a single optical chip that is easily controlled. The implications of the work go beyond the huge resource savings. Now anybody can run their own experiments with photons, much like they operate any other piece of software on a computer. They no longer need to convince a physicist to devote many months of their life to painstakingly build and conduct a new experiment."

The team demonstrated the chip's unique capabilities by re-programming it to rapidly perform a number of different experiments, each of which would previously have taken many months to build.

Bristol PhD student Jacques Carolan, one of the researchers, added: "Once we wrote the code for each circuit, it took seconds to re-programme the chip, and milliseconds for the chip to switch to the new experiment. We carried out a year's worth of experiments in a matter of hours. What we're really excited about is using these chips to discover new science that we haven't even thought of yet."

The device was made possible because the world's leading quantum photonics group teamed up with Nippon Telegraph and Telephone (NTT), the world's leading telecommunications company.

Professor Jeremy O'Brien, Director of the Centre for Quantum Photonics at Bristol University, explained: "Over the last decade, we have established an ecosystem for photonic quantum technologies, allowing the best minds in quantum information science to hook up with established research and engineering expertise in the telecommunications industry. It's a model that we need to encourage if we are to realise our vision for a quantum computer."

The University of Bristol's pioneering 'Quantum in the Cloud' is the first and only service to make a quantum processor publicly accessible and plans to add more chips like this one to the service so others can discover the quantum world for themselves.

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

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

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

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

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

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

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

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

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

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

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

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

CAPTION A nanoscale view of the new superfast fluorescent system using a transmission electron microscope. The silver cube is just 75-nanometers wide. The quantum dots (red) are sandwiched between the silver cube and a thin gold foil. CREDIT Maiken Mikkelsen, Duke University

Plasmonic device has speed and efficiency to serve optical supercomputers

Researchers have developed an ultrafast light-emitting device that can flip on and off 90 billion times a second and could form the basis of optical supercomputing.

At its most basic level, your smart phone's battery is powering billions of transistors using electrons to flip on and off billions of times per second. But if microchips could use photons instead of electrons to process and transmit data, computers could operate even faster. 

But first engineers must build a light source that can be turned on and off that rapidly. While lasers can fit this requirement, they are too energy-hungry and unwieldy to integrate into computer chips. 

Duke University researchers are now one step closer to such a light source. In a new study, a team from the Pratt School of Engineering pushed semiconductor quantum dots to emit light at more than 90 billion gigahertz. This so-called plasmonic device could one day be used in optical supercomputing chips or for optical communication between traditional electronic microchips.

The study was published online on July 27 in Nature Communications.

"This is something that the scientific community has wanted to do for a long time," said Maiken Mikkelsen, an assistant professor of electrical and computer engineering and physics at Duke. "We can now start to think about making fast-switching devices based on this research, so there's a lot of excitement about this demonstration."

The new speed record was set using plasmonics. When a laser shines on the surface of a silver cube just 75 nanometers wide, the free electrons on its surface begin to oscillate together in a wave. These oscillations create their own light, which reacts again with the free electrons. Energy trapped on the surface of the nanocube in this fashion is called a plasmon.

The plasmon creates an intense electromagnetic field between the silver nanocube and a thin sheet of gold placed a mere 20 atoms away. This field interacts with quantum dots -- spheres of semiconducting material just six nanometers wide -- that are sandwiched in between the nanocube and the gold. The quantum dots, in turn, produce a directional, efficient emission of photons that can be turned on and off at more than 90 gigahertz.

"There is great interest in replacing lasers with LEDs for short-distance optical communication, but these ideas have always been limited by the slow emission rate of fluorescent materials, lack of efficiency and inability to direct the photons," said Gleb Akselrod, a postdoctoral research in Mikkelsen's laboratory. "Now we have made an important step towards solving these problems."

"The eventual goal is to integrate our technology into a device that can be excited either optically or electrically," said Thang Hoang, also a postdoctoral researcher in Mikkelsen's laboratory. "That's something that I think everyone, including funding agencies, is pushing pretty hard for."

The group is now working to use the plasmonic structure to create a single photon source -- a necessity for extremely secure quantum communications -- by sandwiching a single quantum dot in the gap between the silver nanocube and gold foil. They are also trying to precisely place and orient the quantum dots to create the fastest fluorescence rates possible.

Aside from its potential technological impacts, the research demonstrates that well-known materials need not be limited by their intrinsic properties.

"By tailoring the environment around a material, like we've done here with semiconductors, we can create new designer materials with almost any optical properties we desire," said Mikkelsen. "And that's an emerging area that's fascinating to think about."

Quantum mechanics does not only allow superposition of quantum states but also superposition of quantum gates. It was shown that superimposing two quantum gates A and B, an unordered quantum computation can run more efficiently than a well-defined order quantum computation (Copyright: Philip Walther Group, University of Vienna).

Since its conception, quantum mechanics has defied our natural way of thinking, and it has forced physicists to come to grips with peculiar ideas. Although they may be difficult to digest, quantum phenomena are real. What's more, in the last decades, scientists have shown that these bizarre quantum effects can be used for many astonishingly powerful applications: from ultra-secure communication to hacking existing secure communications, and from simulating complex quantum systems to efficiently solving large systems of equations.

One of the most exciting and most difficult proposed quantum technologies is the quantum computer. Quantum logic gates are the basic building blocks of a quantum computer, but constructing enough of them to perform a useful computation is difficult. In the usual approach to quantum computing, quantum gates are applied in a specific order, one gate before another. But it was recently realized that quantum mechanics permits one to "superimpose quantum gates." If engineered correctly, this means that a set of quantum gates can act in all possible orders at the same time. Surprisingly, this effect can be used to reduce the total number of gates required for certain quantum computations.

All orders at once

A team led by Philip Walther recently realized that superimposing the order of quantum gates, an idea which was theoretically designed by the group of Caslav Brukner, could be implemented in the laboratory. In a superposition of quantum gate orders, it is impossible -- even in principle -- to know if one operation occurred before another operation, or the other way around. This means that two quantum logic gates A and B can be applied in both orders at the same time. In other words, gate A acts before B and B acts before A. The physicists from Philip Walther's group designed an experiment in which the two quantum logic gates were applied to single photons in both orders.

The results of their experiment confirm that it is impossible to determine which gate acted first -- but the experiment was not simply a curiosity. "In fact, we were able to run a quantum algorithm to characterize the gates more efficiently than any previously known algorithm," says Lorenzo Procopio, lead author of the study. From a single measurement on the photon, they probed a specific property of the two quantum gates thereby confirming that the gates were applied in both orders at once. As more gates are added to the task, the new method becomes even more efficient compared to previous techniques.

The Way Forward

This is the first time that a superposition of quantum gates has been implemented in the lab. At the same time, it was used to successfully demonstrate a new kind of quantum computing. The scientists were able to accomplish a computation with an efficiency that cannot be achieved within the old scheme of quantum computing. This work opens a door for future studies on novel types of quantum computation. Although its full implications are still unknown, this work represents a new, exciting way to connect theoretical research on the foundations of physics to experimental quantum computing.

A detector image (top) signals the existence of Weyl fermions. The plus and minus signs note whether the particle's spin is in the same direction as its motion — which is known as being right-handed — or in the opposite direction in which it moves, or left-handed. This dual ability allows Weyl fermions to have high mobility. A schematic (bottom) shows how Weyl fermions also can behave like monopole and antimonopole particles when inside a crystal, meaning that they have opposite magnetic-like charges can nonetheless move independently of one another, which also allows for a high degree of mobility. (Image by Su-Yang Xu and M. Zahid Hasan, Princeton Department of Physics)

An international team led by Princeton University scientists has discovered an elusive massless particle theorized 85 years ago. The particle could give rise to faster and more efficient electronics because of its unusual ability to behave as matter and antimatter inside a crystal, according to new research.

The researchers report in the journal Science July 16 the first observation of Weyl fermions, which, if applied to next-generation electronics, could allow for a nearly free and efficient flow of electricity in electronics, and thus greater power, especially for supercomputers, the researchers suggest.

Proposed by the mathematician and physicist Hermann Weyl in 1929, Weyl fermions have been long sought by scientists because they have been regarded as possible building blocks of other subatomic particles, and are even more basic than the ubiquitous, negative-charge carrying electron (when electrons are moving inside a crystal). Their basic nature means that Weyl fermions could provide a much more stable and efficient transport of particles than electrons, which are the principle particle behind modern electronics. Unlike electrons, Weyl fermions are massless and possess a high degree of mobility; the particle's spin is both in the same direction as its motion — which is known as being right-handed — and in the opposite direction in which it moves, or left-handed.

"The physics of the Weyl fermion are so strange, there could be many things that arise from this particle that we're just not capable of imagining now," said corresponding author M. Zahid Hasan, a Princeton professor of physics who led the research team.

Hasan

An international team led by Princeton University scientists has discovered Weyl fermions, elusive massless particles theorized 85 years ago that could give rise to faster and more efficient electronics because of their unusual ability to behave as matter and antimatter inside a crystal. The team included numerous researchers from Princeton's Department of Physics, including (from left to right) graduate students Ilya Belopolski and Daniel Sanchez; Guang Bian, a postdoctoral research associate; corresponding author M. Zahid Hasan, a Princeton professor of physics who led the research team; and associate research scholar Hao Zheng. (Photo by Danielle Alio, Office of Communications)

The researchers' find differs from the other particle discoveries in that the Weyl fermion can be reproduced and potentially applied, Hasan said. Typically, particles such as the famous Higgs boson are detected in the fleeting aftermath of particle collisions, he said. The Weyl fermion, however, was discovered inside a synthetic metallic crystal called tantalum arsenide that the Princeton researchers designed in collaboration with researchers at the Collaborative Innovation Center of Quantum Matter in Beijing and at National Taiwan University.

The Weyl fermion possesses two characteristics that could make its discovery a boon for future electronics, including the development of the highly prized field of efficient quantum computing, Hasan explained.

For a physicist, the Weyl fermions are most notable for behaving like a composite of monopole- and antimonopole-like particles when inside a crystal, Hasan said. This means that Weyl particles that have opposite magnetic-like charges can nonetheless move independently of one another with a high degree of mobility.

The researchers also found that Weyl fermions can be used to create massless electrons that move very quickly with no backscattering, wherein electrons are lost when they collide with an obstruction. In electronics, backscattering hinders efficiency and generates heat. Weyl electrons simply move through and around roadblocks, Hasan said.

"It's like they have their own GPS and steer themselves without scattering," Hasan said. "They will move and move only in one direction since they are either right-handed or left-handed and never come to an end because they just tunnel through. These are very fast electrons that behave like unidirectional light beams and can be used for new types of quantum computing."

Prior to the Science paper, Hasan and his co-authors published a report in the journal Nature Communications in June that theorized that Weyl fermions could exist in a tantalum arsenide crystal. Guided by that paper, the researchers used the Princeton Institute for the Science and Technology of Materials (PRISM) and Laboratory for Topological Quantum Matter and Spectroscopy in Princeton's Jadwin Hall to research and simulate dozens of crystal structures before seizing upon the asymmetrical tantalum arsenide crystal, which has a differently shaped top and bottom.

The crystals were then loaded into a two-story device known as a scanning tunneling spectromicroscope that is cooled to near absolute zero and suspended from the ceiling to prevent even atom-sized vibrations. The spectromicroscope determined if the crystal matched the theoretical specifications for hosting a Weyl fermion. "It told us if the crystal was the house of the particle," Hasan said.

The Princeton team took the crystals passing the spectromicroscope test to the Lawrence Berkeley National Laboratory in California to be tested with high-energy accelerator-based photon beams. Once fired through the crystal, the beams' shape, size and direction indicated the presence of the long-elusive Weyl fermion.

First author Su-Yang Xu, a postdoctoral research associate in Princeton's Department of Physics, said that the work was unique for encompassing theory and experimentalism.

"The nature of this research and how it emerged is really different and more exciting than most of other work we have done before," Xu said. "Usually, theorists tell us that some compound might show some new or interesting properties, then we as experimentalists grow that sample and perform experiments to test the prediction. In this case, we came up with the theoretical prediction ourselves and then performed the experiments. This makes the final success even more exciting and satisfying than before."

In pursuing the elusive particle, the researchers had to pull from a number of disciplines, as well as just have faith in their quest and scientific instincts, Xu said.

"Solving this problem involved physics theory, chemistry, material science and, most importantly, intuition," he said. "This work really shows why research is so fascinating, because it involved both rational, logical thinking, and also sparks and inspiration."

Weyl, who worked at the Institute for Advanced Study, suggested his fermion as an alternative to the theory of relativity proposed by his colleague Albert Einstein. Although that application never panned out, the characteristics of his theoretical particle intrigued physicists for nearly a century, Hasan said. Actually observing the particle was a trying process — one ambitious experiment proposed colliding high-energy neutrinos to test if the Weyl fermion was produced in the aftermath, he said.

Hasan2

Hasan (pictured) and his research group researched and simulated dozens of crystal structures before finding the one suitable for holding Weyl fermions. Once fashioned, the crystals were loaded into this two-story device known as a scanning tunneling spectromicroscope to ensure that they matched theoretical specifications. Located in the Laboratory for Topological Quantum Matter and Spectroscopy in Princeton's Jadwin Hall, the spectromicroscope is cooled to near absolute zero and suspended from the ceiling to prevent even atom-sized vibrations. (Photo by Danielle Alio, Office of Communications)

The hunt for the Weyl fermion began in the earliest days of quantum theory when physicists first realized that their equations implied the existence of antimatter counterparts to commonly known particles such as electrons, Hasan said.

"People figured that although Weyl's theory was not applicable to relativity or neutrinos, it is the most basic form of fermion and had all other kinds of weird and beautiful properties that could be useful," he said.

"After more than 80 years, we found that this fermion was already there, waiting. It is the most basic building block of all electrons," he said. "It is exciting that we could finally make it come out following Weyl's 1929 theoretical recipe."

Ashvin Vishwanath, a professor of physics at the University of California-Berkeley who was not involved in the study, commented: "Professor Hasan's experiments report the observation of both the unusual properties in the bulk of the crystal as well as the exotic surface states that were theoretically predicted. While it is early to say what practical implications this discovery might have, it is worth noting that Weyl materials are direct 3-D electronic analogs of graphene, which is being seriously studied for potential applications."

The team included numerous researchers from Princeton's Department of Physics, including graduate students Ilya Belopolski, Nasser Alidoust and Daniel Sanchez; Guang Bian, a postdoctoral research associate; associate research scholar Hao Zheng; and Madhab Neupane, a Princeton postdoctoral research associate now at the Los Alamos National Laboratory; and Class of 2015 undergraduate Pavel Shibayev.

Other co-authors were Chenglong Zhang, Zhujun Yuan and Shuang Jia from Peking University; Raman Sankar and Fangcheng Chou from National Taiwan University; Guoqing Chang, Chi-Cheng Lee, Shin-Ming Huang, BaoKai Wang and Hsin Lin from the National University of Singapore; Jie Ma from Oak Ridge National Laboratory; and Arun Bansil from Northeastern University. Wang is also affiliated with Northeastern University, and Jia is affiliated with the Collaborative Innovation Center of Quantum Matter in Beijing.

The paper, "Discovery of Weyl fermions and topological Fermi arcs," was published online by Science on July 16. The work was supported by the Gordon and Betty Moore Foundations Emergent Phenomena in Quantum Systems (EPiQS) Initiative (grant no. GBMF4547); the Singapore National Research Foundation (grant no. NRF-NRFF2013-03); the National Basic Research Program of China(grant nos. 2013CB921901 and 2014CB239302); the U.S. Department of Energy (grant no. DE-FG-02-05ER462000); and the Taiwan Ministry of Science and Technology (project no. 102-2119-M- 002-004).

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