Saturday, July 26, 2014

Unleashing the Power of Quantum Dot Triplets

Unleashing the power of quantum dot triplets


Another step towards faster computers relies on three coherently coupled quantum dots used as quantum information units

Quantum computers have yet to materialise. Yet, scientists are making progress in devising suitable means of making such computers faster. One such approach relies on quantum dots—a kind of artificial atom, easily controlled by applying an electric field. A new study demonstrates that changing the coupling of three coherently coupled quantum dots (TQDs) with electrical impulses can help better control them. This has implications, for example, should TQDs be used as quantum information units, which would produce faster quantum computers due to the fact that they would be operated through electrical impulses. These findings have been published in EPJ B by Sahib Babaee Tooski and colleagues affiliated with both the Institute of Molecular Physics at the Polish Academy of Sciences, in Poznan, Poland, the University of Ljubljana and the Jožef Stefan Institute in Slovenia.




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The authors study the interplay between internal electrons—which, due to electron spins, are localised on the different quantum dots. They then compare them with the interactions of the conducting electrons, which, at low temperature, can increase the electrical resistance, due to what is referred to as the Kondo effect. This effect can be induced by coupling one of the quantum dots with the electrodes.


Tooski and colleagues thus demonstrate that by changing the coupling of the quantum dot with the electrodes, they can help induce the quantum phase transition between entangled and disentangled electron states. Such variations are typically detectable through a sudden jump in the entropy and the spin susceptibility. However, theoretical investigations outlined in the paper and based on numerical renormalisation group analysis suggest that the detection of such change is best achieved by measuring the electrical conductance. This is because, as the authors show, the conductance should be different for the entangled and disentangled states.



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Reference: Tooski, S. B. Bułka, B. R. Žitko, R. Ramšak, A. (2014), Entanglement switching via the Kondo effect in triple quantum dots. European Physical Journal B. DOI: 10.1140/epjb/e2014-41025-6


The full-text article is available to journalists on request.


For more information visit: http://www.epj.org


News Release Source :  Unleashing the power of quantum dot triplets

Quantum Leap in Lasers Brightens Future for Quantum Computing

Quantum leap in lasers at Dartmouth brightens future for quantum computing


Dartmouth scientists and their colleagues have devised a breakthrough laser that uses a single artificial atom to generate and emit particles of light. The laser may play a crucial role in the development of quantum computers, which are predicted to eventually outperform today's most powerful supercomputers.




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The study appears in the journal Physical Review B.


The new laser is the first to rely exclusively on superconducting electron pairs. "The fact that we use only superconducting pairs is what makes our work so significant," says Alex Rimberg, a professor of physics and astronomy at Dartmouth. Superconductivity is a condition that occurs when electricity can travel without any resistance or loss of energy.


"The artificial atom is made of nanoscale pieces of superconductor," says Rimberg. "The reason for using the artificial atom is that you can now make it part of an electrical circuit on a chip, something you can't do with a real atom, and it means we have a much clearer path toward interesting applications in quantum computing."


Light from the laser is produced by applying electricity to the artificial atom. This causes electrons to hop across the atom and, in the process, produce photons that are trapped between two superconducting mirrors. The process is "invisible to the human eye; the hopping electrons dance back and forth across the atom in time with the oscillating waves of the light," Rimberg says.


With the new laser, electrical energy is converted to light that has the ability to transmit information to and from a quantum computer. "With a quantum computer, you have to get the information from point A to point B," he says. "A computer that does a calculation but has no way of getting the information anywhere else isn't particularly useful. Our laser might offer an easy way of producing the kinds of weird quantum states of light that could be used to carry quantum information around."


Much the laser development came out of the thesis work of one of Rimberg's former graduate students, Fei Chen, first author on the Physical Review B paper, with help from another graduate student Juliang Li, and postdoctoral researcher Joel Stettenheim.



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Professor Alex Rimberg is available to comment at Alexander.J.Rimberg@dartmouth.edu


Broadcast studios: Dartmouth has TV and radio studios available for interviews. For more information, visit: http://www.dartmouth.edu/~opa/radio-tv-studios/


News Release Source :  Quantum leap in lasers at Dartmouth brightens future for quantum computing

Wednesday, July 16, 2014

Photonic Router - Step Down The Long Road Toward Quantum Computers

The world's first photonic router


Weizmann Institute scientists take another step down the long road toward quantum computers

Weizmann Institute scientists have demonstrated for the first time a photonic router – a quantum device based on a single atom that enables routing of single photons by single photons. This achievement, as reported in Science magazine, is another step toward overcoming the difficulties in building quantum computers.




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At the core of the device is an atom that can switch between two states. The state is set just by sending a single particle of light – or photon – from the right or the left via an optical fiber. The atom, in response, then reflects or transmits the next incoming photon accordingly. For example, in one state, a photon coming from the right continues on its path to the left, whereas a photon coming from the left is reflected backwards, causing the atomic state to flip. In this reversed state, the atom lets photons coming from the left continue in the same direction, while any photon coming from the right is reflected backwards, flipping the atomic state back again. This atom-based switch is solely operated by single photons – no additional external fields are required.


"In a sense, the device acts as the photonic equivalent to electronic transistors, which switch electric currents in response to other electric currents," says Dr. Barak Dayan, head of the Weizmann Institute's Quantum Optics group, including Itay Shomroni, Serge Rosenblum, Yulia Lovsky, Orel Bechler and Gabriel Guendleman of the Chemical Physics Department in the Faculty of Chemistry. The photons are not only the units comprising the flow of information, but also the ones that control the device.


This achievement was made possible by the combination of two state-of-the-art technologies. One is the laser cooling and trapping of atoms. The other is the fabrication of chip-based, ultra-high quality miniature optical resonators that couple directly to the optical fibers. Dayan's lab at the Weizmann Institute is one of a handful worldwide that has mastered both these technologies.


The main motivation behind the effort to develop quantum computers is the quantum phenomenon of superposition, in which particles can exist in many states at once, potentially being able to process huge amounts of data in parallel. Yet superposition can only last as long as nothing observes or measures the system otherwise it collapses to a single state. Therefore, photons are the most promising candidates for communication between quantum systems as they do not interact with each other at all, and interact very weakly with other particles.

Dayan: "The road to building quantum computers is still very long, but the device we constructed demonstrates a simple and robust system, which should be applicable to any future architecture of such computers. In the current demonstration a single atom functions as a transistor – or a two-way switch – for photons, but in our future experiments, we hope to expand the kinds of devices that work solely on photons, for example new kinds of quantum memory or logic gates."
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Dr. Barak Dayan's research is supported by the Benoziyo Endowment Fund for the Advancement of Science. Dr. Dayan is the incumbent of the Joseph and Celia Reskin Career Development Chair.

The Weizmann Institute of Science in Rehovot, Israel, is one of the world's top-ranking multidisciplinary research institutions. Noted for its wide-ranging exploration of the natural and exact sciences, the Institute is home to scientists, students, technicians and supporting staff. Institute research efforts include the search for new ways of fighting disease and hunger, examining leading questions in mathematics and computer science, probing the physics of matter and the universe, creating novel materials and developing new strategies for protecting the environment.

Weizmann Institute news releases are posted on the World Wide Web at http://wis-wander.weizmann.ac.il, and are also available at http://www.eurekalert.org.

Image Credit: Weizmann Institute of Science

News Release Source :  The world's first photonic router

Friday, July 11, 2014

IBM Speed Up Moore's Law for Quantum Computing

IBM Announces $3 Billion Research Initiative To Tackle Chip Grand Challenges For Cloud And Big Data Systems


Scientists and engineers to push limits of silicon technology to 7 nanometers and below and create post-silicon future


ARMONK, N.Y., July 10, 2014 /PRNewswire/ -- IBM (NYSE: IBM) today announced it is investing $3 billion over the next 5 years in two broad research and early stage development programs to push the limits of chip technology needed to meet the emerging demands of cloud computing and Big Data systems. These investments will push IBM's semiconductor innovations from today's breakthroughs into the advanced technology leadership required for the future.

[caption id="attachment_366" align="alignleft" width="350"]IBM Speed Up Moore's Law for Quantum Computing www.quantumcomputingtechnologyaustralia.com-054 IBM Speed Up Moore's Law for Quantum Computing[/caption]

The first research program is aimed at so-called "7 nanometer and beyond" silicon technology that will address serious physical challenges that are threatening current semiconductor scaling techniques and will impede the ability to manufacture such chips. The second is focused on developing alternative technologies for post-silicon era chips using entirely different approaches, which IBM scientists and other experts say are required because of the physical limitations of silicon based semiconductors.

Cloud and big data applications are placing new challenges on systems, just as the underlying chip technology is facing numerous significant physical scaling limits.  Bandwidth to memory, high speed communication and device power consumption are becoming increasingly challenging and critical.

The teams will comprise IBM Research scientists and engineers from Albany and Yorktown, New York; Almaden, California; andEurope. In particular, IBM will be investing significantly in emerging areas of research that are already underway at IBM such as carbon nanoelectronics, silicon photonics, new memory technologies, and architectures that support quantum and cognitive computing.

These teams will focus on providing orders of magnitude improvement in system level performance and energy efficient computing. In addition, IBM will continue to invest in the nanosciences and quantum computing--two areas of fundamental science where IBM has remained a pioneer for over three decades.

7 nanometer technology and beyond
IBM Researchers and other semiconductor experts predict that while challenging, semiconductors show promise to scale from today's 22 nanometers down to 14 and then 10 nanometers in the next several years.  However, scaling to 7 nanometers and perhaps below, by the end of the decade will require significant investment and innovation in semiconductor architectures as well as invention of new tools and techniques for manufacturing.

"The question is not if we will introduce 7 nanometer technology into manufacturing, but rather how, when, and at what cost?" saidJohn Kelly, senior vice president, IBM Research. "IBM engineers and scientists, along with our partners, are well suited for this challenge and are already working on the materials science and device engineering required to meet the demands of the emerging system requirements for cloud, big data, and cognitive systems. This new investment will ensure that we produce the necessary innovations to meet these challenges."

"Scaling to 7nm and below is a terrific challenge, calling for deep physics competencies in processing nano materials affinities and characteristics. IBM is one of a very few companies who has repeatedly demonstrated this level of science and engineering expertise," said Richard Doherty, technology research director, The Envisioneering Group.

Bridge to a "Post-Silicon" Era
Silicon transistors, tiny switches that carry information on a chip, have been made smaller year after year, but they are approaching a point of physical limitation. Their increasingly small dimensions, now reaching the nanoscale, will prohibit any gains in performance due to the nature of silicon and the laws of physics. Within a few more generations, classical scaling and shrinkage will no longer yield the sizable benefits of lower power, lower cost and higher speed processors that the industry has become accustomed to.

With virtually all electronic equipment today built on complementary metal–oxide–semiconductor (CMOS) technology, there is an urgent need for new materials and circuit architecture designs compatible with this engineering process as the technology industry nears physical scalability limits of the silicon transistor.

Beyond 7 nanometers, the challenges dramatically increase, requiring a new kind of material to power systems of the future, and new computing platforms to solve problems that are unsolvable or difficult to solve today. Potential alternatives include new materials such as carbon nanotubes, and non-traditional computational approaches such as neuromorphic computing, cognitive computing, machine learning techniques, and the science behind quantum computing.

As the leader in advanced schemes that point beyond traditional silicon-based computing, IBM holds over 500 patents for technologies that will drive advancements at 7nm and beyond silicon -- more than twice the nearest competitor. These continued investments will accelerate the invention and introduction into product development for IBM's highly differentiated computing systems for cloud, and big data analytics.

Several exploratory research breakthroughs that could lead to major advancements in delivering dramatically smaller, faster and more powerful computer chips, include quantum computing, neurosynaptic computing, silicon photonics, carbon nanotubes, III-V technologies, low power transistors and graphene:

Quantum Computing
The most basic piece of information that a typical computer understands is a bit. Much like a light that can be switched on or off, a bit can have only one of two values: "1" or "0." Described as superposition, this special property of qubits enables quantum computers to weed through millions of solutions all at once, while desktop PCs would have to consider them one at a time.

IBM is a world leader in superconducting qubit-based quantum computing science and is a pioneer in the field of experimental and theoretical quantum information, fields that are still in the category of fundamental science - but one that, in the long term, may allow the solution of problems that are today either impossible or impractical to solve using conventional machines. The team recently demonstrated the first experimental realization of parity check with three superconducting qubits, an essential building block for one type of quantum computer.

Neurosynaptic Computing
Bringing together nanoscienceneuroscience, and supercomputing, IBM and university partners have developed an end-to-end ecosystem including a novel non-von Neumann architecture, a new programming language, as well as applications. This novel technology allows for computing systems that emulate the brain's computing efficiency, size and power usage. IBM's long-term goal is to build a neurosynaptic system with ten billion neurons and a hundred trillion synapses, all while consuming only one kilowatt of power and occupying less than two liters of volume.

Silicon Photonics
IBM has been a pioneer in the area of CMOS integrated silicon photonics for over 12 years, a technology that integrates functions for optical communications on a silicon chip, and the IBM team has recently designed and fabricated the world's first monolithic silicon photonics based transceiver with wavelength division multiplexing.  Such transceivers will use light to transmit data between different components in a computing system at high data rates, low cost, and in an energetically efficient manner.

Silicon nanophotonics takes advantage of pulses of light for communication rather than traditional copper wiring and provides a super highway for large volumes of data to move at rapid speeds between computer chips in servers, large datacenters, and supercomputers, thus alleviating the limitations of congested data traffic and high-cost traditional interconnects.

Businesses are entering a new era of computing that requires systems to process and analyze, in real-time, huge volumes of information known as Big Data. Silicon nanophotonics technology provides answers to Big Data challenges by seamlessly connecting various parts of large systems, whether few centimeters or few kilometers apart from each other, and move terabytes of data via pulses of light through optical fibers.

III-V technologies
IBM researchers have demonstrated the world's highest transconductance on a self-aligned III-V channel metal-oxide semiconductor (MOS) field-effect transistors (FETs) device structure that is compatible with CMOS scaling. These materials and structural innovation are expected to pave path for technology scaling at 7nm and beyond.  With more than an order of magnitude higher electron mobility than silicon, integrating III-V materials into CMOS enables higher performance at lower power density, allowing for an extension to power/performance scaling to meet the demands of cloud computing and big data systems.

Carbon Nanotubes
IBM Researchers are working in the area of carbon nanotube (CNT) electronics and exploring whether CNTs can replace silicon beyond the 7 nm node.  As part of its activities for developing carbon nanotube based CMOS VLSI circuits, IBM recently demonstrated -- for the first time in the world -- 2-way CMOS NAND gates using 50 nm gate length carbon nanotube transistors.

IBM also has demonstrated the capability for purifying carbon nanotubes to 99.99 percent, the highest (verified) purities demonstrated to date, and transistors at 10 nm channel length that show no degradation due to scaling--this is unmatched by any other material system to date.

Carbon nanotubes are single atomic sheets of carbon rolled up into a tube. The carbon nanotubes form the core of a transistor device that will work in a fashion similar to the current silicon transistor, but will be better performing. They could be used to replace the transistors in chips that power data-crunching servers, high performing computers and ultra fast smart phones.

Carbon nanotube transistors can operate as excellent switches at molecular dimensions of less than ten nanometers – the equivalent to 10,000 times thinner than a strand of human hair and less than half the size of the leading silicon technology. Comprehensive modeling of the electronic circuits suggests that about a five to ten times improvement in performance compared to silicon circuits is possible.

To see how carbon nanotubes are made, click here: https://www.flickr.com/photos/ibm_research_zurich/8124102857/in/set-72157631852280328

Graphene
Graphene is pure carbon in the form of a one atomic layer thick sheet.  It is an excellent conductor of heat and electricity, and it is also remarkably strong and flexible.  Electrons can move in graphene about ten times faster than in commonly used semiconductor materials such as silicon and silicon germanium. Its characteristics offer the possibility to build faster switching transistors than are possible with conventional semiconductors, particularly for applications in the handheld wireless communications business where it will be a more efficient switch than those currently used.

Recently in 2013, IBM demonstrated the world's first graphene based integrated circuit receiver front end for wireless communications. The circuit consisted of a 2-stage amplifier and a down converter operating at 4.3 GHz.

Next Generation Low Power Transistors
In addition to new materials like CNTs, new architectures and innovative device concepts are required to boost future system performance. Power dissipation is a fundamental challenge for nanoelectronic circuits. To explain the challenge, consider a leaky water faucet -- even after closing the valve as far as possible water continues to drip -- this is similar to today's transistor, in that energy is constantly "leaking" or being lost or wasted in the off-state.

A potential alternative to today's power hungry silicon field effect transistors are so-called steep slope devices. They could operate at much lower voltage and thus dissipate significantly less power. IBM scientists are researching tunnel field effect transistors (TFETs). In this special type of transistors the quantum-mechanical effect of band-to-band tunneling is used to drive the current flow through the transistor. TFETs could achieve a 100-fold power reduction over complementary CMOS transistors, so integrating TFETs with CMOS technology could improve low-power integrated circuits.

Recently, IBM has developed a novel method to integrate III-V nanowires and heterostructures directly on standard silicon substrates and built the first ever InAs/Si tunnel diodes and TFETs using InAs as source and Si as channel with wrap-around gate as steep slope device for low power consumption applications.

"In the next ten years computing hardware systems will be fundamentally different as our scientists and engineers push the limits of semiconductor innovations to explore the post-silicon future," said Tom Rosamilia, senior vice president, IBM Systems and Technology Group. "IBM Research and Development teams are creating breakthrough innovations that will fuel the next era of computing systems."

IBM's historic contributions to silicon and semiconductor innovation include the invention and/or first implementation of: the single cell DRAM, the "Dennard scaling laws" underpinning "Moore's Law", chemically amplified photoresists, copper interconnect wiring, Silicon on Insulator, strained engineering, multi core microprocessors, immersion lithography, high speed silicon germanium (SiGe), High-k gate dielectrics, embedded DRAM, 3D chip stacking, and Air gap insulators.

IBM researchers also are credited with initiating the era of nano devices following the Nobel prize winning invention of the scanning tunneling microscope which enabled nano and atomic scale invention and innovation.

IBM will also continue to fund and collaborate with university researchers to explore and develop the future technologies for the semiconductor industry. In particular, IBM will continue to support and fund university research through private-public partnerships such as the NanoElectornics Research Initiative (NRI), and the Semiconductor Advanced Research Network (STARnet), and the Global Research Consortium (GRC) of the Semiconductor Research Corporation.

Media Contacts
Steve Tomasco
IBM Media Relations
stomasc@us.ibm.com
917-687-4588

Ari Entin
IBM Media Relations
aentin@us.ibm.com
408-927-2272

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News Release Source : IBM Announces $3 Billion Research Initiative To Tackle Chip Grand Challenges For Cloud And Big Data Systems

Tuesday, July 8, 2014

Superconducting Silicon Quantum Devices

Superconducting-silicon qubits


Using a bottom-up approach to make hybrid quantum devices

Theorists propose a way to make superconducting quantum devices such as Josephson junctions and qubits, atom-by-atom, inside a silicon crystal. Such systems could combine the most promising aspects of silicon spin qubits with the flexibility of superconducting circuits. The researcher's results have now been published in Nature Communications (1).





[caption id="attachment_358" align="aligncenter" width="542"]Silicon Quantum Devices www.quantumcomputingtechnologyaustralia.com-053 Examples of superconducting-silicon quantum devices. (left) A superconducting loop interrupted at two points by junctions can form a superconducting flux qubit or a superconducting quantum interference device, or SQUID. Currents flowing in the loop can be used to measure the strength of a magnetic field threading the loop. The currents (flowing in either direction) can also be used to constitute a qubit. (middle) Separating the superconducting wires by an insulator, in this case pure, crystalline silicon, forms a Josephson junction. (right) Precisely placed, highly doped regions within the semiconductor form the superconducting wires.[/caption]


High quality silicon is one of the historical foundations of modern computing. But it is also promising for quantum information technology. In particular, electron and nuclear spins in pure silicon crystals have been measured to have excellent properties as long-lived qubits, the equivalent of bits in conventional computers.


In a paper appearing this week in Nature Communications, Yun-Pil Shim and Charles Tahan from the University of Maryland and the Laboratory for Physical Sciences (on the College Park, MD campus) have shown how superconducting qubits and devices can be constructed out of silicon (2). Doing so can potentially combine the good quantum properties of silicon and the ubiquity of semiconductor technology with the flexibility of superconducting devices. They propose using "bottom-up" nano-fabrication techniques to construct precisely placed superconducting regions within silicon or germanium and show that such "wires" can be used to make superconducting tunnel junctions and other useful superconducting devices.


Qubits in superconductors and semiconductors

Superconducting circuits, made from superconducting metals and Josephson tunnel junctions (which allow superconducting electron pairs to tunnel between two superconductors), are exceptionally customizable and can produce devices ranging from magnetic field sensors to classical logic circuits. They are also likely to play a big role in processing quantum information, where they can be used as a platform for qubits, tiny quantum systems which reside in a superposition of quantum states.


Several types of superconducting circuits have been used to implement qubits and quantum logic gates with different properties and potential uses. For example, in one kind of circuit current can flow in either of two directions. These alternatives constitute the two superposed states needed for establishing a qubit. The two states can be labeled "0" and "1" in analogy with classical bits. Microwave pulses can drive transitions between the two levels allowing for quantum logic gates.


In general, quantum systems are delicate objects and are susceptible to noise and other environmental factors which diminish performance. Prospective quantum circuits must preserve qubits from outside interference for as long as the quantum calculation proceeds. Despite rapid progress in the quality of superconducting qubits (qubit lifetimes can now surpass 100 microseconds), qubit gate error rates are still limited by loss in the metals, insulators, substrates, and interfaces that make up the heterogeneous superconducting devices.


Spin qubits, are an example of qubits realized in a solid-state, silicon context. Spin is a quantum property of particles like an electron; physicists often think of an electron's spin as being like a small magnet, which will naturally point along the direction of an applied magnetic field. Here the 0 and the 1 states correspond to the two possible orientations of the electron spin, either up or down. Because spin is naturally decoupled from charge in some systems (meaning the information stored in the direction of the spin will not be ruined by moving the electron or by it being shaken by electric noise), spin qubits are thought to be promising candidates for a robust qubit design. Further, the use of epitaxial semiconductor devices, and the ability to bury spin qubits deep inside a semiconductor medium, far away from noise at interfaces and surfaces, has resulted in qubits that live for seconds or even hours in some situations, much longer than superconducting qubits to date.


Practical devices

Shim and Tahan propose to use the best features of superconductor and semiconductor qubits. They aim to make superconducting wires and junctions, from which qubits and sensors can be made, by placing (or "doping") acceptor atoms (such as boron or aluminum, elements which readily accept extra electrons) in silicon in precise regions within the crystal. They suggest that a recently developed technique from the silicon qubit community, "STM hydrogen lithography", can be used to do just that. Pioneered by Michelle Simmons at the University of New South Wales, a scanning tunneling microscope (STM) tip is used to selectively remove hydrogen atoms on the surface of silicon (or germanium). Doping gas such as phosphine can then be introduced, allowing the selective placement of impurities down to a single atomic site. "If acceptor atoms can be placed at sufficient density over enough layers, then superconducting regions can be fabricated within the silicon and then encapsulated with crystalline silicon," says Dr. Shim.


In some STM efforts as many as one-in-four silicon atoms have been replaced in this manner. And generally the higher the dopant density, the higher the critical superconducting temperature will be.


Scientists first learned about 10 years ago that silicon can be made superconducting if doped to sufficient density with acceptor atoms such as boron. In recent years, the quality of such superconducting silicon systems has improved greatly, yielding material with superconducting critical temperatures approaching 1 Kelvin and still leaving the crystal in good condition (in other words, it's still silicon).


By calculating the properties of these superconducting-semiconductor regions, Shim and Tahan show that wires with sufficient critical temperature can be constructed practically with the bottom-up hydrogen lithography approach. They also show that Josephson tunnel junctions and weak links, the fundamental non-linearity from which superconducting circuits can be constructed, can also be made. Finally they show that the previously-demonstrated superconducting qubit types (seen in metal samples) can be constructed in this silicon system as well and provide the geometric requirements needed for fabrication.


"There is ongoing effort to make the tunneling barrier epitaxial to improve its quality," said Charles Tahan, "but no previous work to make the whole device out of a single semiconductor crystal. As far as we know, this is the first proposal on the feasibility of SC silicon for Josephson junctions and qubits. I'm also excited about these systems' potential for other devices such as sensors and particle detectors."


Beyond the possibility of superconducting circuits built inside a homogeneous silicon crystal, engineered superconducting-semiconductor devices like these could be used to build other types of exotic quantum many-body systems, at the atomic scale, and even act as testbeds for our understanding of superconductivity itself.



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(1) "Bottom-up superconducting and Josephson junction devices inside a Group-IV semiconductor," Yun-Pil Shim and Charles Tahan. Nature Communications (2014), published online ZZZ July 2014. DOI: 10.1038/ncomms5225. On arXiv:1309.0015


(2) The Laboratory for Physical Sciences is affiliated with the Joint Quantum Institute (JQI).


Charles Tahan, ctahan@lps.umd.edu; Yun Pil Shim, ypshim@umd.edu


Press contact at JQI: Phillip F. Schewe, pschewe@umd.eduhttp://jqi.umd.edu/


Image Credit : LPS


News Release Source :  Superconducting-silicon qubits

Sunday, July 6, 2014

Graphene Qubits? : From Pencil Marks to Quantum Computers

From pencil marks to quantum computers


Introducing graphene

One of the hottest materials in condensed matter research today is graphene.


Graphene had an unlikely start: it began with researchers messing around with pencil marks on paper. Pencil "lead" is actually made of graphite, which is a soft crystal lattice made of nothing but carbon atoms. When pencils deposit that graphite on paper, the lattice is laid down in thin sheets. By pulling that lattice apart into thinner sheets – originally using Scotch tape – researchers discovered that they could make flakes of crystal just one atom thick.


The name for this atom-scale chicken wire is graphene. Those folks with the Scotch tape, Andre Geim and Konstantin Novoselov, won the 2010 Nobel Prize for discovering it. "As a material, it is completely new – not only the thinnest ever but also the strongest," wrote the Nobel committee. "As a conductor of electricity, it performs as well as copper. As a conductor of heat, it outperforms all other known materials. It is almost completely transparent, yet so dense that not even helium, the smallest gas atom, can pass through it."




[caption id="attachment_352" align="alignleft" width="400"]From Pencil Marks to Quantum Computers From Pencil Marks to Quantum Computers[/caption]
Developing a theoretical model of graphene

Graphene is not just a practical wonder – it's also a wonderland for theorists. Confined to the two-dimensional surface of the graphene, the electrons behave strangely. All kinds of new phenomena can be seen, and new ideas can be tested. Testing new ideas in graphene is exactly what Perimeter researchers Zlatko Papić and Dmitry (Dima) Abanin set out to do.


"Dima and I started working on graphene a very long time ago," says Papić. "We first met in 2009 at a conference in Sweden. I was a grad student and Dima was in the first year of his postdoc, I think."


The two young scientists got to talking about what new physics they might be able to observe in the strange new material when it is exposed to a strong magnetic field.


"We decided we wanted to model the material," says Papić. They've been working on their theoretical model of graphene, on and off, ever since. The two are now both at Perimeter Institute, where Papić is a postdoctoral researcher and Abanin is a faculty member. They are both cross-appointed with the Institute for Quantum Computing (IQC) at the University of Waterloo.


In January 2014, they published a paper in Physical Review Letters (PRL) presenting new ideas about how to induce a strange but interesting state in graphene – one where it appears as if particles inside it have a fraction of an electron's charge.


It's called the fractional quantum Hall effect (FQHE), and it's head turning. Like the speed of light or Planck's constant, the charge of the electron is a fixed point in the disorienting quantum universe.


Every system in the universe carries whole multiples of a single electron's charge. When the FQHE was first discovered in the 1980s, condensed matter physicists quickly worked out that the fractionally charged "particles" inside their semiconductors were actually quasiparticles – that is, emergent collective behaviours of the system that imitate particles.


Graphene is an ideal material in which to study the FQHE. "Because it's just one atom thick, you have direct access to the surface," says Papić. "In semiconductors, where FQHE was first observed, the gas of electrons that create this effect are buried deep inside the material. They're hard to access and manipulate. But with graphene you can imagine manipulating these states much more easily."


In the January paper, Abanin and Papić reported novel types of FQHE states that could arise in bilayer graphene – that is, in two sheets of graphene laid one on top of another – when it is placed in a strong perpendicular magnetic field. In an earlier work from 2012, they argued that applying an electric field across the surface of bilayer graphene could offer a unique experimental knob to induce transitions between FQHE states. Combining the two effects, they argued, would be an ideal way to look at special FQHE states and the transitions between them.


Experimental tests

Two experimental groups – one in Geneva, involving Abanin, and one at Columbia, involving both Abanin and Papić – have since put the electric field + magnetic field method to good use. The paper by the Columbia group appears in the July 4 issue of Science . A third group, led by Amir Yacoby of Harvard, is doing closely related work.


"We often work hand in hand with experimentalists," says Papić. "One of the reasons I like condensed matter is that often even the most sophisticated, cutting-edge theory stands a good chance of being quickly checked with experiment."


Inside both the magnetic and electric field, the electrical resistance of the graphene demonstrates the strange behaviour characteristic of the FQHE. Instead of resistance that varies in a smooth curve with voltage, resistance jumps suddenly from one level to another, and then plateaus – a kind of staircase of resistance. Each stair step is a different state of matter, defined by the complex quantum tangle of charges, spins, and other properties inside the graphene.


"The number of states is quite rich," says Papić. "We're very interested in bilayer graphene because of the number of states we are detecting and because we have these mechanisms – like tuning the electric field – to study how these states are interrelated, and what happens when the material changes from one state to another."


For the moment, researchers are particularly interested in the stair steps whose "height" is described by a fraction with an even denominator. That's because the quasiparticles in that state are expected to have an unusual property.


There are two kinds of particles in our three-dimensional world: fermions (such as electrons), where two identical particles can't occupy one state, and bosons (such as photons), where two identical particles actually want to occupy one state. In three dimensions, fermions are fermions and bosons are bosons, and never the twain shall meet.


But a sheet of graphene doesn't have three dimensions – it has two. It's effectively a tiny two-dimensional universe, and in that universe, new phenomena can occur. For one thing, fermions and bosons can meet halfway – becoming anyons, which can be anywhere in between fermions and bosons. The quasiparticles in these special stair-step states are expected to be anyons.


In particular, the researchers are hoping these quasiparticles will be non-Abelian anyons, as their theory indicates they should be. That would be exciting because non-Abelian anyons can be used in the making of qubits.



Graphene qubits?

Qubits are to quantum computers what bits are to ordinary computers: both a basic unit of information and the basic piece of equipment that stores that information. Because of their quantum complexity, qubits are more powerful than ordinary bits and their power grows exponentially as more of them are added. A quantum computer of only a hundred qubits can tackle certain problems beyond the reach of even the best non-quantum supercomputers. Or, it could, if someone could find a way to build stable qubits.


The drive to make qubits is part of the reason why graphene is a hot research area in general, and why even-denominator FQHE states – with their special anyons– are sought after in particular. "A state with some number of these anyons can be used to represent a qubit," says Papić. "Our theory says they should be there and the experiments seem to bear that out – certainly the even-denominator FQHE states seem to be there, at least according to the Geneva experiments."


That's still a step away from experimental proof that those even-denominator stair-step states actually contain non-Abelian anyons. More work remains, but Papić is optimistic: "It might be easier to prove in graphene than it would be in semiconductors. Everything is happening right at the surface."


It's still early, but it looks as if bilayer graphene may be the magic material that allows this kind of qubit to be built. That would be a major mark on the unlikely line between pencil lead and quantum computers.



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Further exploration

January PRL paper mentioned above: http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.046602


Experimental paper from the Geneva graphene group, including Abanin: http://pubs.acs.org/doi/abs/10.1021/nl5003922


Experimental paper from the Columbia graphene group, including both Abanin and Papić: http://arxiv.org/abs/1403.2112. This paper is featured in the journal Science.


Related experiment on bilayer graphene by Amir Yacoby's group at Harvard: http://www.sciencemag.org/content/early/2014/05/28/science.1250270


The Nobel Prize press release on graphene, mentioned above: http://www.nobelprize.org/nobel_prizes/physics/laureates/2010/press.html


News Release Source : From pencil marks to quantum computers