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‘Primitive’ quantum computer outperforms classical computers

A simulation of Brownian motion (random walk) of a dust particle (yellow) that collides with a large set of smaller particles (molecules of a gas) moving with different velocities in different random directions (credit: Lookang et al./CC)

Researchers at the Universities of Bristol and Western Australia have demonstrated a practical use of a “primitive” quantum computer, using an algorithm known as “quantum walk.” They showed that a two-qubit photonics quantum processor can outperform classical computers for this type of algorithm, without requiring more sophisticated quantum computers, such as IBM’s five-qubits cloud-based quantum processor (see IBM makes quantum computing available free on IBM Cloud).

Quantum walk is the quantum-mechanical analog of “random-walk” models such as Brownian motion (for example, the random motion of a dust particle in air). The researchers implemented “continuous-time quantum walk” computations on circulant graphs* in a proof-of-principle experiment.

The probability distribution of quantum walk on an example circulant graph. Sampling this probability distribution is generally hard for a classical computer, but simple on a primitive quantum computer. (credit: University of Bristol)

Jonathan Matthews, PhD., EPSRC Early Career Fellow and Lecturer in the School of Physics and the Centre for Quantum Photonics, explained in an open-access paper in Nature Communications: “An exciting outcome of our work is that we may have found a new example of quantum walk physics that we can observe with a primitive quantum computer, that otherwise a classical computer could not see. These otherwise hidden properties have practical use, perhaps in helping to design more sophisticated quantum computers.”


Microsoft | Quantum Computing 101

* A circulant graph is a graph where every vertex is connected to the same set of relative vertices, as explained in an open-access paper by Salisbury University student Shealyn Tucker, including a practical example of the use of a circulant graph:

Example of a circulent graph depicting how products should be optimally collocated based on which products customers buy at a grocery store (credit: Shealyn Tucker/Salisbury University)

Abstract of Efficient quantum walk on a quantum processor

The random walk formalism is used across a wide range of applications, from modelling share prices to predicting population genetics. Likewise, quantum walks have shown much potential as a framework for developing new quantum algorithms. Here we present explicit efficient quantum circuits for implementing continuous-time quantum walks on the circulant class of graphs. These circuits allow us to sample from the output probability distributions of quantum walks on circulant graphs efficiently. We also show that solving the same sampling problem for arbitrary circulant quantum circuits is intractable for a classical computer, assuming conjectures from computational complexity theory. This is a new link between continuous-time quantum walks and computational complexity theory and it indicates a family of tasks that could ultimately demonstrate quantum supremacy over classical computers. As a proof of principle, we experimentally implement the proposed quantum circuit on an example circulant graph using a two-qubit photonics quantum processor.

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IBM makes quantum computing available free on IBM Cloud

Layout of IBM’s five superconducting quantum bit device. In 2015, IBM scientists demonstrated critical breakthroughs to detect quantum errors by combining superconducting qubits in latticed arrangements, and whose quantum circuit design is the only physical architecture that can scale to larger dimensions. Now, IBM scientists have achieved a further advance by combining five qubits in the lattice architecture, which demonstrates a key operation known as a parity measurement — the basis of many quantum error correction protocols. (credit: IBM Research)

IBM Research has announced that effective Wednesday May 4, it is making quantum computing available free to members of the public, who can access and run experiments on IBM’s quantum processor, via the IBM Cloud, from any desktop or mobile device.

IBM believes quantum computing is the future of computing and has the potential to solve certain problems that are impossible to solve on today’s supercomputers.

The cloud-enabled quantum computing platform, called IBM Quantum Experience, will allow users to run algorithms and experiments on IBM’s quantum processor, work with the individual quantum bits (qubits), and explore tutorials and simulations around what might be possible with quantum computing.

The quantum processor is composed of five superconducting qubits and is housed at the IBM T.J. Watson Research Center in New York. IBM’s quantum architecture can scale to larger quantum systems. It is aimed at building a universal quantum computer that can be programmed to perform any computing task and will be exponentially faster than classical computers for a number of important applications for science and business, IBM says.


IBM | Explore our 360 Video of the IBM Research Quantum Lab

IBM envisions medium-sized quantum processors of 50–100 qubits to be possible in the next decade. With a quantum computer built of just 50 qubits, none of today’s TOP500 supercomputers could successfully emulate it, reflecting the tremendous potential of this technology.

“Quantum computing is becoming a reality and it will extend computation far beyond what is imaginable with today’s computers,” said Arvind Krishna, senior vice president and director, IBM Research. “This moment represents the birth of quantum cloud computing. By giving hands-on access to IBM’s experimental quantum systems, the IBM Quantum Experience will make it easier for researchers and the scientific community to accelerate innovations in the quantum field, and help discover new applications for this technology.”

This leap forward in computing could lead to the discovery of new pharmaceutical drugs and completely safeguard cloud computing systems, IBM believes. It could also unlock new facets of artificial intelligence (which could lead to future, more powerful Watson technologies), develop new materials science to transform industries, and search large volumes of big data.

The IBM Quantum Experience


IBM | Running an experiment in the IBM Quantum Experience

Coupled with software expertise from the IBM Research ecosystem, the team has built a dynamic user interface on the IBM Cloud platform that allows users to easily connect to the quantum hardware via the cloud.

In the future, users will have the opportunity to contribute and review their results in the community hosted on the IBM Quantum Experience and IBM scientists will be directly engaged to offer more research and insights on new advances. IBM plans to add more qubits and different processor arrangements to the IBM Quantum Experience over time, so users can expand their experiments and help uncover new applications for the technology.

IBM employs superconducting qubits that are made with superconducting metals on a silicon chip and can be designed and manufactured using standard silicon fabrication techniques. Last year, IBM scientists demonstrated critical breakthroughs to detect quantum errors by combining superconducting qubits in latticed arrangements, and whose quantum circuit design is the only physical architecture that can scale to larger dimensions.


IBM | IBM Brings Quantum Computing to the Cloud

Now, IBM scientists have achieved a further advance by combining five qubits in the lattice architecture, which demonstrates a key operation known as a parity measurement — the basis of many quantum error correction protocols.

By giving users access to IBM’s experimental quantum systems, IBM believes it will help businesses and organizations begin to understand the technology’s potential, for universities to grow their teaching programs in quantum computing and related subjects, and for students (IBM’s potential future customers) to become aware of promising new career paths. And of course, to raise IBM’s marketing profile in this emerging field.

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Capturing a single photon

Capturing a single photon from a pulse of light (credit: Weizmann Institute of Science)

Weizmann Institute of Science researchers have managed to isolate a single photon out of a pulse of light. Single photons may be the backbone of future quantum communication systems, the researchers say.

The mechanism relies on a physical effect that they call “single-photon Raman interaction” (SPRINT). “The advantage of SPRINT is that it is completely passive; it does not require any control fields — just the interaction between the atom and the optical pulse,” said Barak Dayan, PhD, head of the Weizmann Institute Quantum Optics group.

The experimental setup involves laser cooling and trapping of atoms (in this case, rubidium), optical nanofibers, and fabrication of chip-based, ultrahigh-quality glass microspheres.

Previously, a low-reflectivity beam splitter directing a small fraction of the incoming light toward a detector was used, with low success rates.

“The ability to divert a single photon from a flow could be harnessed for various tasks, from creating nonclassical states of light that are useful for basic scientific research, through eavesdropping on imperfect quantum-cryptography systems that rely on single photons, to increasing the security of your own quantum-communication systems,” Dayan said.

The findings of this research appeared Nov. 23, 2015 in Nature Photonics.

Abstract of Extraction of a single photon from an optical pulse

Removing a single photon from a pulse is one of the most elementary operations that can be performed on light, having both fundamental significance and practical applications in quantum communication and computation. So far, photon subtraction, in which the removed photon is detected and therefore irreversibly lost, has been implemented in a probabilistic manner with inherently low success rates using low-reflectivity beam splitters. Here we demonstrate a scheme for the deterministic extraction of a single photon from an incoming pulse. The removed photon is diverted to a different mode, enabling its use for other purposes, such as a photon number-splitting attack on quantum key distribution protocols. Our implementation makes use of single-photon Raman interaction (SPRINT) with a single atom near a nanofibre-coupled microresonator. The single-photon extraction probability in our current realization is limited mostly by linear loss, yet probabilities close to unity should be attainable with realistic experimental parameters.

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Quantum entanglement achieved at room temperature in macroscopic semiconductor wafers

Paul Klimov, a graduate student in the University of Chicago’s Institute for Molecular Engineering, adjusts the intensity of a laser beam during an experiment. (credit: Awschalom Group/University of Chicago)

Researchers in Prof. David Awschalom’s group at the Institute for Molecular Engineering have demonstrated macroscopic entanglement at room temperature and in a small (33 millitesla) magnetic field.

Previously, scientists have overcome the thermodynamic barrier and achieved macroscopic entanglement in solids and liquids by going to ultra-low temperatures (-270 degrees Celsius) and applying huge magnetic fields (1,000 times larger than that of a typical refrigerator magnet) or using chemical reactions.

In an open-access paper published in the Nov. 20 issue of Science Advances, the researchers explain that they used infrared laser light to order (preferentially align) the magnetic states of thousands of electrons and nuclei and then used electromagnetic pulses to entangle them. This procedure caused pairs of electrons and nuclei in a macroscopic 40 micrometer-cubed volume (the volume of a red blood cell) in a silicon carbide (SiC, also known as carborundum) semiconductor wafer to become entangled.

“The ability to produce robust entangled states in an electronic-grade semiconductor at ambient conditions has important implications on future quantum devices,” Awschalom said. In the short term, the research could enable quantum sensors that use entanglement as a resource for beating the sensitivity limit of traditional (non-quantum) sensors and for biological sensing inside a living organism, using entanglement-enhanced magnetic resonance imaging probes, according to the researchers.

They said that it might even be possible in the long term to go from entangled states on the same SiC chip to entangled states across distant SiC chips via macroscopic quantum states, as opposed to single quantum states (in single atoms). Such long-distance entangled states have been proposed for synchronizing global positioning satellites and for communicating information secured from eavesdroppers.

The institute is a partnership of the University of Chicago and and Argonne National Laboratory.

Abstract of Quantum entanglement at ambient conditions in a macroscopic solid-state spin ensemble

Entanglement is a key resource for quantum computers, quantum-communication networks, and high-precision sensors. Macroscopic spin ensembles have been historically important in the development of quantum algorithms for these prospective technologies and remain strong candidates for implementing them today. This strength derives from their long-lived quantum coherence, strong signal, and ability to couple collectively to external degrees of freedom. Nonetheless, preparing ensembles of genuinely entangled spin states has required high magnetic fields and cryogenic temperatures or photochemical reactions. We demonstrate that entanglement can be realized in solid-state spin ensembles at ambient conditions. We use hybrid registers comprising of electron-nuclear spin pairs that are localized at color-center defects in a commercial SiC wafer. We optically initialize 103 identical registers in a 40-μm3 volume (with Embedded Image fidelity) and deterministically prepare them into the maximally entangled Bell states (with 0.88 ± 0.07 fidelity). To verify entanglement, we develop a register-specific quantum-state tomography protocol. The entanglement of a macroscopic solid-state spin ensemble at ambient conditions represents an important step toward practical quantum technology.

 

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Semantic Scholar uses AI to transform scientific search

Example of the top return in a Semantic Scholar search for “quantum computer silicon” constrained to overviews (52 out of 1,397 selected papers since 1989) (credit: AI2)

The Allen Institute for Artificial Intelligence (AI2) launched Monday (Nov. 2) its free Semantic Scholar service, intended to allow scientific researchers to quickly cull through the millions of scientific papers published each year to find those most relevant to their work.

Semantic Scholar leverages AI2’s expertise in data mining, natural-language processing, and computer vision, according to according to Oren Etzioni, PhD, CEO at AI2. At launch, the system searches more than three million computer science papers, and will add scientific categories on an ongoing basis.

With Semantic Scholar, computer scientists can:

  • Home in quickly on what they are looking for, with advanced selection filtering tools. Researchers can filter search results by author, publication, topic, and date published. This gets the researcher to the most relevant result in the fastest way possible, and reduces information overload.
  • Instantly access a paper’s figures and findings. Unique among scholarly search engines, this feature pulls out the graphic results, which are often what a researcher is really looking for.
  • Jump to cited papers and references and see how many researchers have cited each paper, a good way to determine citation influence and usefulness.
  • Be prompted with key phrases within each paper to winnow the search further.

Example of figures and tables extracted from the first document discovered (“Quantum computation and quantum information”) in the search above (credit: AI2)

How Semantic Scholar works

Using machine reading and vision methods, Semantic Scholar crawls the web, finding all PDFs of publicly available scientific papers on computer science topics, extracting both text and diagrams/captions, and indexing it all for future contextual retrieval.

Using natural language processing, the system identifies the top papers, extracts filtering information and topics, and sorts by what type of paper and how influential its citations are. It provides the scientist with a simple user interface (optimized for mobile) that maps to academic researchers’ expectations.

Filters such as topic, date of publication, author and where published are built in. It includes smart, contextual recommendations for further keyword filtering as well. Together, these search and discovery tools provide researchers with a quick way to separate wheat from chaff, and to find relevant papers in areas and topics that previously might not have occurred to them.

Semantic Scholar builds from the foundation of other research-paper search applications such as Google Scholar, adding AI methods to overcome information overload.

“Semantic Scholar is a first step toward AI-based discovery engines that will be able to connect the dots between disparate studies to identify novel hypotheses and suggest experiments that would otherwise be missed,” said Etzione. “Our goal is to enable researchers to find answers to some of science’s thorniest problems.”

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How to build a full-scale quantum computer in silicon

Physical layout of the surface code* quantum computer. The system comprises three layers. The 2D donor qubit array resides in the middle layer. A mutually perpendicular (crisscross) pattern of control gates in the upper and lower planes form a regular 3D grid of cells. (credit: Charles D. Hill et al./Science Advances)

A new 3D silicon-chip architecture based on single-atom quantum bits has been designed by researchers at UNSW Australia (The University of New South Wales) and the University of Melbourne.

The use of silicon makes it compatible with existing atomic-scale fabrication techniques, providing a way to build a large-scale quantum computer.**

The scientists and engineers from the Australian Research Council Centre of Excellence for Quantum Computation and Communication Technology (CQC2T), headquartered at UNSW, previously demonstrated a fabrication strategy. But the hard part in scaling up to an operational quantum computer was the architecture: How to precisely control multiple qubits in parallel across an array of many thousands of qubits and constantly correct for quantum errors in calculations.

The CQC2T collaboration says they have now designed such a device. In a study published Friday (Oct. 30) in an open-access paper in Science Advances, the CQC2T team describes a new silicon architecture that uses atomic-scale qubits aligned to control lines (essentially very narrow wires) inside a 3D design.


UNSW | How to build a quantum computer in silicon

Error correction

Errors (caused by decoherence and other quantum noise) are endemic to quantum computing, so error correction protocols are essential in creating a practical system that can be scaled up to larger numbers of qubits.

“The great thing about this work, and architecture, is that it gives us an endpoint,” says UNSW Scientia Professor Michelle Simmons, study co-author and Director of the CQC2T. “We now know exactly what we need to do in the international race to get there.”

In the team’s conceptual design, they have moved from the conventional one-dimensional array (in a line) of qubits to a two-dimensional array (in a surface), which is far more tolerant of errors. This qubit layer is “sandwiched” between two layers of control wires arranged in a 3D grid.

By applying voltages to a subset of these wires, multiple qubits can be controlled in parallel, performing a series of operations using far fewer controls. They can also perform the 2D surface-code* error correction protocols, so any computational errors that creep into the calculation can be corrected faster than they occur.

The researchers believe their structure is scalable to millions of qubits, and that means they may be on the fast track to a full-scale quantum processor.

* “Surface code is a powerful quantum error correcting code that can be defined on a 2D square lattice of qubits with only nearest neighbor interactions.” — Austin G. Fowler et al. Surface code quantum error correction incorporating accurate error propagation. arXiv, 4/2010

** In classical computers, data is rendered as binary bits, which are always in one of two states: 0 or 1. However, a qubit can exist in both of these states at once, a condition known as a superposition. A qubit operation exploits this quantum weirdness by allowing many computations to be performed in parallel (a two-qubit system performs the operation on 4 values, a three-qubit system on 8, and so on). As a result, quantum computers will far exceed today’s most powerful supercomputers, and offer enormous advantages for a range of complex problems, such as rapidly scouring vast databases, modeling financial markets, optimizing huge metropolitan transport networks, and modeling complex biological molecules.

Abstract of A surface code quantum computer in silicon

The exceptionally long quantum coherence times of phosphorus donor nuclear spin qubits in silicon, coupled with the proven scalability of silicon-based nano-electronics, make them attractive candidates for large-scale quantum computing. However, the high threshold of topological quantum error correction can only be captured in a two-dimensional array of qubits operating synchronously and in parallel—posing formidable fabrication and control challenges. We present an architecture that addresses these problems through a novel shared-control paradigm that is particularly suited to the natural uniformity of the phosphorus donor nuclear spin qubit states and electronic confinement. The architecture comprises a two-dimensional lattice of donor qubits sandwiched between two vertically separated control layers forming a mutually perpendicular crisscross gate array. Shared-control lines facilitate loading/unloading of single electrons to specific donors, thereby activating multiple qubits in parallel across the array on which the required operations for surface code quantum error correction are carried out by global spin control. The complexities of independent qubit control, wave function engineering, and ad hoc quantum interconnects are explicitly avoided. With many of the basic elements of fabrication and control based on demonstrated techniques and with simulated quantum operation below the surface code error threshold, the architecture represents a new pathway for large-scale quantum information processing in silicon and potentially in other qubit systems where uniformity can be exploited.

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Engineered viruses provide quantum-based enhancement of energy transport

Rendering of a virus used in the MIT experiments. The light-collecting centers, called chromophores, are in red, and chromophores that just absorbed a photon of light are glowing white. After the virus is modified to adjust the spacing between the chromophores, energy can jump from one set of chromophores to the next faster and more efficiently. (credit: the researchers and Lauren Alexa Kaye)

MIT engineers have achieved a significant efficiency boost in a light-harvesting system, using genetically engineered viruses to achieve higher efficiency in transporting energy from receptors to reaction centers where it can be harnessed, making use of the exotic effects of quantum mechanics. Emulating photosynthesis in nature, it could lead to inexpensive and efficient solar cells or light-driven catalysis,

This achievement in coupling quantum research and genetic manipulation, described this week in the journal Nature Materials, was the work of MIT professors Angela Belcher, an expert on engineering viruses to carry out energy-related tasks, and Seth Lloyd, an expert on quantum theory and its potential applications, and 15 collaborators at MIT and in Italy.

The “Quantum Goldilocks Effect”

In photosynthesis, a photon hits a receptor called a chromophore, which in turn produces an exciton — a quantum particle of energy. This exciton jumps from one chromophore to another until it reaches a reaction center, where that energy is harnessed to build the molecules that support life, or photosynthesis.

But the hopping pathway of excitons is random and inefficient unless it takes advantage of quantum effects that allow it, in effect, to take multiple pathways at once and select the best ones, behaving more like a wave than a particle.

To do that, the chromophores have to be arranged just right, with exactly the right amount of space between them. This, Lloyd explains, is known as the “Quantum Goldilocks Effect.”

Molecular models of the genetically engineered viruses. Left virus has long inter-binding site distances of 16Å and 33Å within two proteins. Right virus has closer inter-binding site distances of approximately 10Å and 13Å, achieving faster excitation-energy transport speed. (credit: Heechul Park et al./Nature Materials)

That’s where the virus comes in. By engineering a virus that Belcher has worked with for years, the team was able to get it to bond with multiple synthetic chromophores — or, in this case, organic dyes. The researchers were then able to produce many varieties of the virus, with slightly different spacings between those synthetic chromophores, and select the ones that performed best.

In the end, they were able to more than double excitons’ speed, increasing the distance they traveled before dissipating — a significant improvement in the efficiency of the process.

The project started from a chance meeting at a conference in Italy. Lloyd and Belcher, a professor of biological engineering, were reporting on different projects they had worked on, and began discussing the possibility of a project encompassing their very different expertise. Lloyd, whose work is mostly theoretical, pointed out that the viruses Belcher works with have the right length scales to potentially support quantum effects.

In 2008, Lloyd had published a paper demonstrating that photosynthetic organisms transmit light energy efficiently because of these quantum effects. When he saw Belcher’s report on her work with engineered viruses, he wondered if that might provide a way to artificially induce a similar effect, in an effort to approach nature’s efficiency.

“I had been talking about potential systems you could use to demonstrate this effect, and Angela said, ‘We’re already making those,’” Lloyd recalls. Eventually, after much analysis, “We came up with design principles to redesign how the virus is capturing light, and get it to this quantum regime.”

Within two weeks, Belcher’s team had created their first test version of the engineered virus. Many months of work then went into perfecting the receptors and the spacings.

Once the team engineered the viruses, they were able to use laser spectroscopy and dynamical modeling to watch the light-harvesting process in action, and to demonstrate that the new viruses were indeed making use of quantum coherence to enhance the transport of excitons.

“It was really fun,” Belcher says. “A group of us who spoke different [scientific] languages worked closely together, to both make this class of organisms, and analyze the data. That’s why I’m so excited by this.”

Inexpensive and efficient solar cells or light-driven catalysis

While this initial result is essentially a proof of concept rather than a practical system, it points the way toward an approach that could lead to inexpensive and efficient solar cells or light-driven catalysis, the team says. So far, the engineered viruses collect and transport energy from incoming light, but do not yet harness it to produce power (as in solar cells) or molecules (as in photosynthesis). But this could be done by adding a reaction center, where such processing takes place, to the end of the virus where the excitons end up.

“This is exciting and high-quality research,” says Alán Aspuru-Guzik, a professor of chemistry and chemical biology at Harvard University who was not involved in this work. The research, he says, “combines the work of a leader in theory (Lloyd) and a leader in experiment (Belcher) in a truly multidisciplinary and exciting combination that spans biology to physics to potentially, future technology.”

“Access to controllable excitonic systems is a goal shared by many researchers in the field,” Aspuru-Guzik adds. “This work provides fundamental understanding that can allow for the development of devices with an increased control of exciton flow.”

The research was supported by the Italian energy company Eni through the MIT Energy Initiative. The team included researchers at the University of Florence, the University of Perugia, and Eni.


MIT | See how researchers genetically engineer viruses to more efficiently transport energy.

Abstract of Enhanced energy transport in genetically engineered excitonic networks

One of the challenges for achieving efficient exciton transport in solar energy conversion systems is precise structural control of the light-harvesting building blocks. Here, we create a tunable material consisting of a connected chromophore network on an ordered biological virus template. Using genetic engineering, we establish a link between the inter-chromophoric distances and emerging transport properties. The combination of spectroscopy measurements and dynamic modelling enables us to elucidate quantum coherent and classical incoherent energy transport at room temperature. Through genetic modifications, we obtain a significant enhancement of exciton diffusion length of about 68% in an intermediate quantum-classical regime.

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Light-controlled ‘quantum Etch-a-Sketch’ could lead to advanced computers and quantum microchips

Artist’s rendition of optically defined quantum circuits in a topological insulator (credit: Peter Allen)

Penn State University and University of Chicago researchers say an accidental discovery of a “quantum Etch-a-Sketch” may lead to a new way to use beams of light to draw and erase quantum circuits, and that could lead to the next generation of advanced computers and quantum microchips.

The new technique is based on “topological insulators” (a material that behaves as an insulator in its interior but whose surface contains conducting states, meaning that electrons can only move along the surface of the material). The electrons in topological insulators have unique quantum properties that many scientists believe will be useful for developing spin-based electronics (such as disk drives) and quantum computers.

However, making even the simplest experimental circuits with topological insulators has proved difficult because traditional semiconductor engineering techniques tend to destroy their fragile quantum properties. Even a brief exposure to air can reduce their quality.

The researchers have now discovered a rewriteable “optical fabrication” process that allows them to “tune” the energy of electrons in these materials using light instead of chemicals — without ever having to touch the material itself. They used this effect to draw and erase one of the central components of a transistor — the p-n junction — in a topological insulator for the first time.

An accidental discovery

Optical fabrication (draw/erase) of a topological insulator (credit: Andrew L. Yeats et al./Science Advances)

Curiously, the scientists made the discovery when they noticed that a particular type of fluorescent light in the lab caused the surface of strontium titanate (the substrate material on which they had grown their samples) to become electrically polarized by ultraviolet light. The room lights happened to emit it at just the right wavelength. It turned out that the electric field from the polarized strontium titanate was leaking into the topological insulator layer, changing its electronic properties.

They found by intentionally focusing beams of light on their samples, they could draw electronic structures that persisted long after the light was removed. “It’s like having a sort of quantum Etch-a-Sketch in our lab,” said said David D. Awschalom, Liew Family Professor and deputy director in the Institute of Molecular Engineering at the University of Chicago. They also found that bright red light counteracted the effect of the ultraviolet light, allowing the researchers to both write (with UV) and erase (with red light).

“Instead of spending weeks in the clean room and potentially contaminating our materials, now we can sketch and measure devices for our experiments in real time,” said Awschalom. “When we’re done, we just erase it and make something else. We can do this in less than a second.”

To test whether the new technique might interfere with the unique properties of topological insulators, the team measured their samples in high magnetic fields. They found promising signatures of an effect called “weak anti-localization,” which arises from quantum interference between the different simultaneous paths that electrons can take through a material when they behave as waves.

To better understand the physics behind the effect, the researchers conducted a number of control measurements, which showed that the optical effect is not unique to topological insulators; it can also act on other materials grown on strontium titanate.

“In a way, the most exciting aspect of this work is that it should be applicable to a wide range of nanoscale materials such as complex oxides, graphene, and transition metal dichalcogenides,” said Awschalom. “It’s not just that it’s faster and easier. This effect could allow electrical tuning of materials in a wide range of optical, magnetic, and spectroscopic experiments where electrical contacts are extremely difficult or simply impossible.”

The research was published October 9, 2015 in an open-access paper in a new AAAS journal, Science Advances.

Abstract of Persistent Optical Gating of a Topological Insulator

The spin-polarized surface states of topological insulators (TIs) are attractive for applications in spintronics and quantum computing. A central challenge with these materials is to reliably tune the chemical potential of their electrons with respect to the Dirac point and the bulk bands. We demonstrate persistent, bidirectional optical control of the chemical potential of (Bi,Sb)2Te3 thin films grown on SrTiO3. By optically modulating a space-charge layer in the SrTiO3 substrates, we induce a persistent field effect in the TI films comparable to electrostatic gating techniques but without additional materials or processing. This enables us to optically pattern arbitrarily shaped p- and n-type regions in a TI, which we subsequently image with scanning photocurrent microscopy. The ability to optically write and erase mesoscopic electronic structures in a TI may aid in the investigation of the unique properties of the topological insulating phase. The gating effect also generalizes to other thin-film materials, suggesting that these phenomena could provide optical control of chemical potential in a wide range of ultrathin electronic systems.

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First two-qubit logic gate built in silicon

Artist’s impression of the two-qubit logic gate device developed at UNSW. Each of the two electron qubits (red and blue) has a spin, or magnetic field, indicated by the arrow directions. Metal electrodes on the surface are used to manipulate the qubits, which interact to create an entangled quantum state. (credit: Tony Melov/UNSW)

University of New South Wales (UNSW) and Keio University engineers have built the first quantum logic gate in silicon, making calculations between two qubits* of information possible and clearing the final hurdle to making silicon quantum computers a reality.

The significant advance appears today (Oct. 5, 2015) in the journal Nature.

“What we have is a game changer,” said team leader Andrew Dzurak, Scientia Professor and Director of the Australian National Fabrication Facility at UNSW. “Because we use essentially the same device technology as existing computer chips, we believe it will be much easier to manufacture a full-scale processor chip than for any of the leading designs, which rely on more exotic technologies.”


University of New South Wales

“If quantum computers are to become a reality, the ability to conduct one- and two-qubits calculations are essential,” said Dzurak, who jointly led the team in 2012 that demonstrated the first ever silicon qubit, also reported in Nature.

Until now, using silicon, it had not been possible to make two quantum bits “talk” to each other and thereby create a logic gate. The new result means that all of the physical building blocks for a silicon-based quantum computer have now been successfully constructed, allowing engineers to finally begin the task of designing and building a functioning quantum computer, the researchers say.

Dzurak noted that the team had recently “patented a design for a full-scale quantum computer chip that would allow for millions of our qubits … using standard industrial manufacturing techniques to build the world’s first quantum processor chip. … That has major implications for the finance, security, and healthcare sectors.”

He said that a key next step for the project is to identify the right industry partners to work with to manufacture the full-scale quantum processor chip.

Dzurak’s research is supported by the Australian Research Council via the Centre of Excellence for Quantum Computation and Communication Technology, the U.S. Army Research Office, the State Government of New South Wales in Australia, the Commonwealth Bank of Australia, and the University of New South Wales. Veldhorst acknowledges support from the Netherlands Organisation for Scientific Research. The quantum logic devices were constructed at the Australian National Fabrication Facility, which is supported by the federal government’s National Collaborative Research Infrastructure Strategy (NCRIS).

* In classical computers, data is rendered as binary bits, which are always in one of two states: 0 or 1. A quantum bit (or ‘qubit’) can exist in both of these states at once, a condition known as a superposition. A qubit operation exploits this quantum weirdness by allowing many computations to be performed in parallel (a two-qubit system performs the operation on 4 values, a three-qubit system on 8, and so on).

Abstract of A two-qubit logic gate in silicon

Quantum computation requires qubits that can be coupled in a scalable manner, together with universal and high-fidelity one- and two-qubit logic gates. Many physical realizations of qubits exist, including single photons, trapped ions, superconducting circuits, single defects or atoms in diamond and silicon, and semiconductor quantum dots, with single-qubit fidelities that exceed the stringent thresholds required for fault-tolerant quantum computing. Despite this, high-fidelity two-qubit gates in the solid state that can be manufactured using standard lithographic techniques have so far been limited to superconducting qubits, owing to the difficulties of coupling qubits and dephasing in semiconductor systems. Here we present a two-qubit logic gate, which uses single spins in isotopically enriched silicon and is realized by performing single- and two-qubit operations in a quantum dot system using the exchange interaction, as envisaged in the Loss–DiVincenzo proposal. We realize CNOT gates via controlled-phase operations combined with single-qubit operations. Direct gate-voltage control provides single-qubit addressability, together with a switchable exchange interaction that is used in the two-qubit controlled-phase gate. By independently reading out both qubits, we measure clear anticorrelations in the two-spin probabilities of the CNOT gate.

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A new distance record for quantum teleportation via photons

This graphic describes how researchers at the National Institute of Standards and Technology (NIST) have “teleported” or transferred quantum information carried in light particles over 100 kilometers (km) of optical fiber, four times farther than the previous record. (credit: K. Irvine/NIST)

Researchers at the National Institute of Standards and Technology (NIST) have “teleported” (transferred) quantum information carried in photons over 100 kilometers (km) of optical fiber — four times farther than the previous record.

The experiment confirmed that quantum communication is feasible over long distances in fiber, according to the researchers. Other research groups have teleported quantum information over longer distances in free space (wirelessly), but fiber-optic cables offer more options for network design, the NIST researchers note.

Teleportation is useful in both quantum communications and quantum computing, which allow advancements in unbreakable encryption and code-breaking, respectively.

The new record, described in an open-access paper in Optica, involved the transfer of quantum information from one photon (its specific time slot in a sequence) to another photon* over 102 km of spooled fiber in a NIST laboratory in Colorado.

The achievement was made possible by NIST’s advanced single-photon detectors.

“Only about 1 percent of photons make it all the way through 100 km of fiber,” NIST’s Marty Stevens says. “We never could have done this experiment without these new detectors, which can measure this incredibly weak signal.”

Quantum internet

The new NTT/NIST teleportation technique could be used to make devices called quantum repeaters that could resend data periodically, extending network reach, perhaps enough to eventually build a “quantum internet.”

Previously, researchers thought quantum repeaters might need to rely on atoms or other matter, instead of light, a difficult engineering challenge that would also slow down transmission.*

* Various quantum states can be used to carry information; the NTT/NIST experiment used quantum states that indicate when in a sequence of time slots a single photon arrives. That teleportation method is novel in that four of NIST’s photon detectors were positioned to filter out specific quantum states. (See graphic for an overview of how the teleportation process works.) The detectors rely on superconducting nanowires made of molybdenum silicide. They can record more than 80 percent of arriving photons, revealing whether they are in the same or different time slots each just 1 nanosecond long. The experiments were performed at wavelengths commonly used in telecommunications.

Because the experiment filtered out and focused on a limited combination of quantum states, teleportation could be successful in only 25 percent of the transmissions at best. Thanks to the efficient detectors, researchers successfully teleported the desired quantum state in 83 percent of the maximum possible successful transmissions, on average. All experimental runs with different starting properties exceeded the mathematically significant 66.7 percent threshold for proving the quantum nature of the teleportation process.

Abstract of Quantum teleportation over 100  km of fiber using highly efficient superconducting nanowire single-photon detectors

Quantum teleportation is an essential quantum operation by which we can transfer an unknown quantum state to a remote location with the help of quantum entanglement and classical communication. Since the first experimental demonstrations using photonic qubits and continuous variables, the distance of photonic quantum teleportation over free-space channels has continued to increase and has reached >100  km. On the other hand, quantum teleportation over optical fiber has been challenging, mainly because the multifold photon detection that inevitably accompanies quantum teleportation experiments has been very inefficient due to the relatively low detection efficiencies of typical telecom-band single-photon detectors. Here, we report on quantum teleportation over optical fiber using four high-detection-efficiency superconducting nanowire single-photon detectors (SNSPDs). These SNSPDs make it possible to perform highly efficient multifold photon measurements, allowing us to confirm that the quantum states of input photons were successfully teleported over 100 km of fiber with an average fidelity of 83.7±2.0%.