MIT Researchers Discover a Universal Law of Superconductivity

MIT Researchers Discover a Universal Law of Superconductivity

Mathematical description of relationship between thickness, temperature, and resistivity could spur advances.

MIT researchers have discovered a new mathematical relationship — between material thickness, temperature, and electrical resistance — that appears to hold in all superconductors. They describe their findings in the latest issue of Physical Review B.

[caption id="attachment_524" align="aligncenter" width="650"] Atoms of niobium and nitrogen in an ultrathin superconducting film that helped MIT researchers discover a universal law of superconductivity.[/caption]

The result could shed light on the nature of superconductivity and could also lead to better-engineered superconducting circuits for applications like quantum computing and ultralow-power computing.

“We were able to use this knowledge to make larger-area devices, which were not really possible to do previously, and the yield of the devices increased significantly,” says Yachin Ivry, a postdoc in MIT’s Research Laboratory of Electronics, and the first author on the paper.

Ivry works in the Quantum Nanostructures and Nanofabrication Group, which is led by Karl Berggren, a professor of electrical engineering and one of Ivry’s co-authors on the paper. Among other things, the group studies thin films of superconductors.

Superconductors are materials that, at temperatures near absolute zero, exhibit no electrical resistance; this means that it takes very little energy to induce an electrical current in them. A single photon will do the trick, which is why they’re useful as quantum photodetectors. And a computer chip built from superconducting circuits would, in principle, consume about one-hundredth as much energy as a conventional chip.

“Thin films are interesting scientifically because they allow you to get closer to what we call the superconducting-to-insulating transition,” Ivry says. “Superconductivity is a phenomenon that relies on the collective behavior of the electrons. So if you go to smaller and smaller dimensions, you get to the onset of the collective behavior.”

Vexing variation

Specifically, Ivry studied niobium nitride, a material favored by researchers because, in its bulk form, it has a relatively high “critical temperature” — the temperature at which it switches from an ordinary metal to a superconductor. But like most superconductors, it has a lower critical temperature when it’s deposited in the thin films on which nanodevices rely.

Previous theoretical work had characterized niobium nitride’s critical temperature as a function of either the thickness of the film or its measured resistivity at room temperature. But neither theory seemed to explain the results Ivry was getting. “We saw large scatter and no clear trend,” he says. “It made no sense, because we grew them in the lab under the same conditions.”

So the researchers conducted a series of experiments in which they held constant either thickness or “sheet resistance,” the material’s resistance per unit area, while varying the other parameter; they then measured the ensuing changes in critical temperature. A clear pattern emerged: Thickness times critical temperature equaled a constant — call it A — divided by sheet resistance raised to a particular power — call it B.

After deriving that formula, Ivry checked it against other results reported in the superconductor literature. His initial excitement evaporated, however, with the first outside paper he consulted. Though most of the results it reported fit his formula perfectly, two of them were dramatically awry. Then a colleague who was familiar with the paper pointed out that its authors had acknowledged in a footnote that those two measurements might reflect experimental error: When building their test device, the researchers had forgotten to turn on one of the gases they used to deposit their films.

Broadening the scope

The other niobium nitride papers Ivry consulted bore out his predictions, so he began to expand to other superconductors. Each new material he investigated required him to adjust the formula’s constants — A and B. But the general form of the equation held across results reported for roughly three dozen different superconductors.

It wasn’t necessarily surprising that each superconductor should have its own associated constant, but Ivry and Berggren weren’t happy that their equation required two of them. When Ivry graphed A against B for all the materials he’d investigated, however, the results fell on a straight line.

Finding a direct relationship between the constants allowed him to rely on only one of them in the general form of his equation. But perhaps more interestingly, the materials at either end of the line had distinct physical properties. Those at the top had highly disordered — or, technically, “amorphous” — crystalline structures; those at the bottom were more orderly, or “granular.” So Ivry’s initial attempt to banish an inelegance in his equation may already provide some insight into the physics of superconductors at small scales.

“None of the admitted theory up to now explains with such a broad class of materials the relation of critical temperature with sheet resistance and thickness,” says Claude Chapelier, a superconductivity researcher at France’s Alternative Energies and Atomic Energy Commission. “There are several models that do not predict the same things.”

Chapelier says he would like to see a theoretical explanation for that relationship. But in the meantime, “this is very convenient for technical applications,” he says, “because there is a lot of spreading of the results, and nobody knows whether they will get good films for superconducting devices. By putting a material into this law, you know already whether it’s a good superconducting film or not.”

News Release Source : New law for superconductors

Image Credit : MIT

Scientists Opens The Way to Future Quantum Internet

Scientists Opens The Way to Future Quantum Internet

Researchers TU/e and FOM obtain vital control on the emission of photons

In the same way as we now connect computers in networks through optical signals, it could also be possible to connect future quantum computers in a 'quantum internet'. The optical signals would then consist of individual light particles or photons. One prerequisite for a working quantum internet is control of the shape of these photons. Researchers at Eindhoven University of Technology (TU/e) and the FOM foundation have now succeeded for the first time in getting this control within the required short time. These findings are published today in Nature Communications.

[caption id="attachment_521" align="aligncenter" width="575"] Scientists Opens The Way to Future Quantum Internet[/caption]

Quantum computers are the dream computers of the future. They use the unique physics of the smallest particles- those described by quantum mechanics - to perform calculations. While today's computers use bits that can be either 0 or 1, quantum computers perform calculations with 'qubits', which can be both 0 and 1 at the same time. That creates an unprecedented degree of extra computing power, which gives quantum computers much greater capabilities than today's computers.

Quantum internet

Quantum computers could in principle communicate with each other by exchanging individual photons to create a 'quantum internet'. The shape of the photons, in other words how their energy is distributed over time, is vital for successful transmission of information. This shape must be symmetric in time, while photons that are emitted by atoms normally have an asymmetric shape. Therefore, this process requires external control in order to create a quantum internet.

Optical cavity

Researchers at TU/e and FOM have succeeded in getting the required degree of control by embedding a quantum dot - a piece of semiconductor material that can transmit photons - into a 'photonic crystal', thereby creating an optical cavity. Then the researchers applied a very short electrical pulse to the cavity, which influences how the quantum dot interacts with it, and how the photon is emitted. By varying the strength of this pulse, they were able to control the shape of the transmitted photons.

Within a billionth of a second

The Eindhoven researchers are the first to achieve this, thanks to the use of electrical pulses shorter than nanosecond, a billionth of a second. This is vital for use in quantum communication, as research leader Andrea Fiore of TU/e explains: "The emission of a photon only lasts for one nanosecond, so if you want to change anything you have to do it within that time. It's like the shutter of a high-speed camera, which has to be very short if you want to capture something that changes very fast in an image. By controlling the speed at which you send a photon, you can in principle achieve very efficient exchange of photons, which is important for the future quantum internet."

News Release Source : Control on shape of light particles opens the way to 'quantum internet'

High Value Resistors for Nanoscale Quantum Circuits

High Value Resistors for Nanoscale Quantum Circuits

London Centre for Nanotechnology (LCN) researchers have made new compact, high-value resistors for nanoscale quantum circuits. The resistors could speed the development of quantum devices for computing and fundamental physics research. The researchers describe the thin-film resistors in an article in the Journal of Applied Physics.

[caption id="attachment_513" align="aligncenter" width="650"]             Electron micrograph of two compact resistors in series with a quantum phase-slip nanowire.                               (The nanowire is too small to see at this scale.)[/caption]

One example of an application that requires high-value resistors is the quantum phase-slip (QPS) circuit. A QPS circuit is made from very narrow wires of superconducting material that can exploit a fundamental, counterintuitive quantum mechanical property called quantum tunneling to move magnetic flux to and fro across the wire, over energy barriers that would be insurmountable in the everyday world of classical physics.

In 2006, scientists from the Kavli Institute of Nanoscience in the Netherlands proposed that a QPS circuit could be used to redefine the amp – a standard unit of measure for electrical current – by linking it to fundamental properties of the universe  (as opposed to a physical system kept in a standards lab). Other scientific groups have also proposed using QPS devices as qubits in quantum computers – the fundamental unit of quantum information at the heart of such computers.

As the LCN's Paul Warburton explained, resistors are needed to isolate the fragile quantum states in QPS devices from the noisy classical world. “In the application as a current standard, the resistors also enable the device to operate stably,” he added.

Yet standard materials used to make resistors for integrated circuits do not typically provide enough resistance in a small enough form to meet the requirements for QPS circuits.

Warburton and his colleagues turned to the compound chromium oxide to create new high-value, compact nanoscale resistors. The researchers created thin films of chromium oxide using a technique called sputter deposition. They were able to tune the resistance of the chromium oxide films by controlling the oxygen content of the films: the higher the oxygen content, the higher the resistance.

“Replacing chromium with oxygen affects both the numbers of electrons available to carry current and also the availability of paths for electrons to hop through the material,” explained Warburton.

The researchers cooled their resistors to 4.2 degrees Kelvin and measured the resistivity for a range of oxygen-to-chromium mass ratios. Poorly conducting materials, such as the chromium oxide thin films the researchers tested, generally have higher resistance at low temperatures, and any resistor used in a QPS device would have to operate at a cold enough temperature that quantum effects would dominate over classical effects. For the resistors with the highest oxygen content, the researchers measured a resistance high enough to be compatible with most QPS circuit requirements.

The teams also characterized the contact resistance of one chromium oxide thin film at an interface with niobium-silicon. Creating a QPS circuit with niobium silicon nanowires would be one way to measure a new quantum standard for the amp. The team found that creating a gold intermediate layer between the chromium oxide and the niobium-silicon lowers the contact resistance – a favorable outcome. The team next plans to incorporate their new resistors into QPS devices.

Journal link: Compact chromium oxide thin film resistors for use in nanoscale quantum circuits J. Appl. Phys. 116, 224501 (2014); http://dx.doi.org/10.1063/1.4901933

Picture credit: James Sagar, Nick Constantino, Chris Nash, Jon Fenton and Paul Warburton

News Source (for more information) :  London Centre for Nanotechnology

Quantum Transistors for Post-CMOS Era

Quantum Transistors for Post-CMOS Era

An odd, iridescent material that's puzzled physicists for decades turns out to be an exotic state of matter that could open a new path to quantum computers and other next-generation electronics.

[caption id="attachment_510" align="aligncenter" width="604"] Samarium hexaboride, abbreviated SmB6, is a compound made of the metal samarium and the rare metalloid boron. University of Michigan researchers have confirmed its unusual electrical properties and shown how it could advance the development of next-generation transistors for quantum computers.[/caption]

Physicists at the University of Michigan have discovered or confirmed several properties of the compound samarium hexaboride that raise hopes for finding the silicon of the quantum era. They say their results also close the case of how to classify the material—a mystery that has been investigated since the late 1960s.

The researchers provide the first direct evidence that samarium hexaboride, abbreviated SmB₆, is a topological insulator. Topological insulators are, to physicists, an exciting class of solids that conduct electricity like a metal across their surface, but block the flow of current like rubber through their interior. They behave in this two-faced way despite that their chemical composition is the same throughout.

The U-M scientists used a technique called torque magnetometry to observe tell-tale oscillations in the material's response to a magnetic field that reveal how electric current moves through it. Their technique also showed that the surface of samarium hexaboride holds rare Dirac electrons, particles with the potential to help researchers overcome one of the biggest hurdles in quantum computing.

These properties are particularly enticing to scientists because SmB₆ is considered a strongly correlated material. Its electrons interact more closely with one another than most solids. This helps its interior maintain electricity-blocking behavior.

This deeper understanding of samarium hexaboride raises the possibility that engineers might one day route the flow of electric current in quantum computers like they do on silicon in conventional electronics, said Lu Li, assistant professor of physics in the College of Literature, Science, and the Arts and a co-author of a paper on the findings published in Science.

"Before this, no one had found Dirac electrons in a strongly correlated material," Li said. "We thought strong correlation would hurt them, but now we know it doesn't. While I don't think this material is the answer, now we know that this combination of properties is possible and we can look for other candidates."

The drawback of samarium hexaboride is that the researchers only observed these behaviors at ultracold temperatures.

Quantum computers use particles like atoms or electrons to perform processing and memory tasks. They could offer dramatic increases in computing power due to their ability to carry out scores of calculations at once. Because they could factor numbers much faster than conventional computers, they would greatly improve computer security.

In quantum computers, "qubits" stand in for the 0s and 1s of conventional computers' binary code. While a conventional bit can be either a 0 or a 1, a qubit could be both at the same time—only until you measure it, that is. Measuring a quantum system forces it to pick one state, which eliminates its main advantage.

Dirac electrons, named after the English physicist whose equations describe their behavior, straddle the realms of classical and quantum physics, Li said. Working together with other materials, they could be capable of clumping together into a new kind of qubit that would change the properties of a material in a way that could be measured indirectly, without the qubit sensing it. The qubit could remain in both states.

While these applications are intriguing, the researchers are most enthusiastic about the fundamental science they've uncovered.

"In the science business you have concepts that tell you it should be this or that and when it's two things at once, that's a sign you have something interesting to find," said Jim Allen, an emeritus professor of physics who studied samarium hexaboride for 30 years. "Mysteries are always intriguing to people who do curiosity-driven research."

Allen thought for years that samarium hexaboride must be a flawed insulator that behaved like a metal at low temperatures because of defects and impurities, but he couldn't align that with all of its other properties.

"The prediction several years ago about it being a topological insulator makes a lightbulb go off if you're an old guy like me and you've been living with this stuff your whole life," Allen said.

In 2010, Kai Sun, assistant professor of physics at U-M, led a group that first posited that SmB₆ might be a topological insulator. He and Allen were also involved in seminal U-M experiments led by physics professor Cagliyan Kurdak in 2012 that showed indirectly that the hypothesis was correct.

"But the scientific community is always critical," Sun said. "They want very strong evidence. We think this experiment finally provides direct proof of our theory."

The paper is titled "Two-dimensional Fermi surfaces in Kondo Insulator SmB₆." It was funded by the U.S. Department of Energy and the National Science Foundation. The U-M Mcubed program also provided seed funds for this research.

News Release Source :  45-year physics mystery shows a path to quantum transistors

For more Information Research :  Two-dimensional Fermi surfaces in Kondo insulator SmB₆ (Subscription Link)

Related Information on Research  :  Topological Insulator, Nano Transistors for Post-CMOS Era

Controlled Emission and Spatial Splitting of Electron Pairs Demonstrated

Controlled Emission and Spatial Splitting of Electron Pairs Demonstrated

In quantum optics, generating entangled and spatially separated photon pairs (e.g. for quantum cryptography) is already a reality. So far, it has, however, not been possible to demonstrate an analogous generation and spatial separation of entangled electron pairs in solids. Physicists from Leibniz University Hannover and from the Physikalisch-Technische Bundesanstalt (PTB) have now taken a decisive step in this direction. They have demonstrated for the first time the on-demand emission of electron pairs from a semiconductor quantum dot and verified their subsequent splitting into two separate conductors. Their results have been published in the current online issue of the renowned journal "Nature Nanotechnology".

[caption id="attachment_505" align="aligncenter" width="588"] This image shows an etched semiconducting channel with electron source (A) and barrier (B). The electron pairs are emitted by the source and split at the barrier into two separate electric conductors (arrow).[/caption]

A precise control and manipulation of quantum-mechanical states could pave the way for promising applications such as quantum computers and quantum cryptography. In quantum optics, such experiments have already been performed for some time. This, for example, allows the controlled generation of pairs of entangled, but spatially separated photons, which are of essential importance for quantum cryptography. An analogous generation and spatial separation of entangled electrons in solids would be of fundamental importance for future applications, but could not be demonstrated yet. The results from Hannover and Braunschweig are a decisive step in this direction.

As an electron source, the physicists from Leibniz University Hannover and from PTB used so-called semiconductor single-electron pumps. Controlled by voltage pulses, these devices emit a defined number of electrons. The single-electron pump was operated in such a way that it released exactly one electron pair per pulse into a semiconducting channel. A semitransparent electronic barrier divides the channel into two electrically distinct areas. A correlation measurement then recorded whether the electron pairs traversed the barrier, or whether they were reflected or split by the barrier. It could be shown that for suitable parameters, more than 90 % of the electron pairs were split and spatially separated by the barrier. This is an important step towards the envisioned generation and separation of entangled electron pairs in semiconductor components.

For More Information : http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.275.html

News Release Source :  Electron pairs on demand

Graphene Read Head for Quantum Computers

Graphene layer reads optical information from nanodiamonds electronically

Nitrogen-vacancy centers in diamonds could be used to construct vital components for quantum computers. But hitherto it has been impossible to read optically written information from such systems electronically. Using a graphene layer, a team of scientists headed by Professor Alexander Holleitner of the Technische Universität München (TUM) has now implemented just such a read unit.

[caption id="attachment_498" align="aligncenter" width="650"] This image shows a laboratory set-up measuring the interaction between graphene and nano-diamonds with implanted nitrogen-vacancy centers.[/caption]

Ideally, diamonds consist of pure carbon. But natural diamonds always contain defects. The most researched defects are nitrogen-vacancy centers comprising a nitrogen atom and a vacancy. These might serve as highly sensitive sensors or as register components for quantum computers. However, until now it has not been possible to extract the optically stored information electronically.

A team headed by Professor Alexander Holleitner, physicist at the TU München and Frank Koppens, physics professor at the Institut de Ciencies Fotoniques near Barcelona, have now devised just such a methodology for reading the stored information. The technique builds on a direct transfer of energy from nitrogen-vacancy centers in nanodiamonds to a directly neighboring graphene layer.

Non-radiative energy transfer

When laser light shines on a nanodiamond, a light photon raises an electron from its ground state to an excited state in the nitrogen-vacancy center. "The system of the excited electron and the vacated ground state can be viewed as a dipole," says Professor Alexander Holleitner. "This dipole, in turn, induces another dipole comprising an electron and a vacancy in the neighboring graphene layer."

In contrast to the approximately 100 nanometer large diamonds, in which individual nitrogen-vacancy centers are insulated from each other, the graphene layer is electrically conducting. Two gold electrodes detect the induced charge, making it electronically measureable.

Picosecond electronic detection

Essential for this experimental setup is that the measurement is made extremely quickly, because the generated electron-vacancy pairs disappear after only a few billionths of a second. However, the technology developed in Holleitners laboratory allows measurements in the picosecond domain (trillionths of a second). The scientists can thus observe these processes very closely.

"In principle our technology should also work with dye molecules," says doctoral candidate Andreas Brenneis, who carried out the measurements in collaboration with Louis Gaudreau. "A diamond has some 500 point defects, but the methodology is so sensitive that we should be able to even measure individual dye molecules."

As a result of the extremely fast switching speeds of the nanocircuits developed by the researchers, sensors built using this technology could be used not only to measure extremely fast processes. Integrated into future quantum computers they would allow clock speeds ranging into the terahertz domain.

News Release Source :  Possible read head for quantum computers

Images Credit : Technische Universität München (TUM)