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)

 

New UK's Network of Quantum Technology Hubs

Quantum Leap as Clark unveils UK's network of Quantum Technology Hubs

A new £120 million national network of Quantum Technology Hubs, that will explore the properties of quantum mechanics and how they can be harnessed for use in technology, has been unveiled today at the University of Birmingham.

[caption id="attachment_494" align="aligncenter" width="650"] New UK's Network of Quantum Technology Hubs[/caption]

The new network will involve 17 universities and 132 companies and will be funded by the Engineering and Physical Sciences Research Council (EPSRC) from the £270 million investment in the UK National Quantum Technologies Programme announced by the Chancellor, George Osborne in his Autumn Statement of 2013.

The network will consist of four hubs which were selected after a competitive peer reviewed process. They will be led by the universities of Birmingham, Glasgow, Oxford and York.

This programme will deliver a suite of research and innovation investments from a number of partners including EPSRC, Innovate UK, BIS, National Physical Laboratory (NPL), Government Communications Headquarters (GCHQ), Defence Science and Technology Laboratory (Dstl) and the Knowledge Transfer Network (KTN).

Greg Clark, Minister of State for Universities, Science and Cities said: This exciting new Quantum Hubs network will push the boundaries of knowledge and exploit new technologies, to the benefit of healthcare, communications and security.

This investment in Quantum technologies has the potential to bring game-changing advantages to future timing, sensing and navigation capabilities that could support multi-billion pound markets in the UK and globally.

Today’s announcement is another example of the Government’s recognition of the UK’s science base and its critical contribution to our sustained economic growth.

Professor Philip Nelson, EPSRC’s Chief Executive said: These new Hubs will build on our previous investments in quantum science. They will draw together scientists, engineers and technologists from across the UK who will explore how we can exploit the intriguing properties of the quantum realm. The area offers great promise, and the Hubs will keep the UK at the leading edge of this exciting field.

The capabilities in Quantum Technologies offer potentially transformative impacts in key areas such as quantum metrology and sensors; quantum simulators; quantum computers and quantum secure communications.

News Release Source :  Quantum Leap as Clark unveils UK's network of Quantum Technology Hubs

Creation of Global Quantum Communications

Global Quantum Communications – No Longer the Stuff of Fiction?

Neither quantum computers nor quantum cryptography will become prevalent technologies without memory systems able to manipulate quantum information easily and effectively. The Faculty of Physics at the University of Warsaw has recently made inroads into popularizing quantum information technologies by creating an atomic memory with outstanding parameters and an extremely simple construction.

[caption id="attachment_488" align="aligncenter" width="530"] An atomic memory (glowing green). From left to right: Michał Dąbrowski, Radek Chrapkiewicz and Wojciech Wasilewski. (Source: FUW, R. Chrapkiewicz)[/caption]

Following years of tests in physics laboratories, the first quantum technologies are slowly emerging into wider applications. One example is quantum cryptography – an encryption method providing an almost full guarantee of secure data transmission, currently being introduced by military forces and banking institutions. Processing quantum information and sending it over long distances has so far been severely limited due to a lack of adequate memories. A solution is now within reach: the Faculty of Physics at the University of Warsaw (FUW), Poland, has created a fully-functioning atomic memory with a simple, reliable construction and numerous potential applications, including in telecommunications.

“The greatest challenge in the construction of our quantum memory was the precise selection of system parameters that would allow it to save, store and read quantum information effectively. We have also found a novel way of reducing noise during detection,” says Dr. Wojciech Wasilewski (FUW).

Contemporary fiber-optic communications involve the transmission of classical information using laser light propagated inside optical fiber cables. Attenuation causes the light signal in the optical fiber cable to weaken as the distance it travels increases. When long optical fiber cables are used, laser amplifiers multiplicating photons are placed along them at intervals of approximately 100 km. These turn a weak signal comprising a low number of photons into a strong signal with high numbers of photons.

However, in quantum communications it is the individual photons and their quantum states that are important. Here signal amplification of the signal does not simply mean increasing the number of photons, but rather preserving their original, undisturbed quantum states. Unfortunately, quantum information cannot be duplicated with impunity: performing any measurement of the quantum state of the photon will inevitably affect its original state. The impossibility of quantum cloning, co-discovered by the Polish physicist Prof. Wojciech Żurek, places fundamental limitations on the operations that can be conducted on quantum information.

In 2001, a team of physicists from the University of Innsbruck and Harvard University proposed the DLCZ quantum transmission protocol, making it possible to send quantum information over long distances. Under this protocol, quantum information reaching each relay point along the channel must be stored there for a sufficiently long time to ensure that attempts at transmitting it to the next node are successful, as confirmed via a normal signal. In such a protocol, therefore, a key role is played by quantum memory in which quantum information needs to be stored for a sufficiently long time.

“Until now, quantum memory required highly sophisticated laboratory equipment and complex techniques chilling the systems to extremely low temperatures approaching absolute zero. The atomic memory device we have been able to create operates at far higher temperatures, in the region of tens of degrees Celsius, which are significantly easier to maintain,” notes Radek Chrapkiewicz, doctoral student at the Faculty and co-author of the paper in the renowned journal Optics Express.

The main element of the memory device constructed by the University of Warsaw physicists is a glass chamber 2.5 cm in diameter and 10 cm long, with rubidium-coated sides, filled with a noble gas. When the tube is heated gently, rubidium pairs fill the inside, with the noble gas restricting their movement and thereby reducing noise. When quantum information is stored in such a memory, photons from the laser beam “imprint” quantum states on many rubidium atoms. Other photons are emitted at the same time; their detection confirms that the information has been saved. Information stored in the memory can then be retrieved using another specially selected laser pulse.

To record and retrieve quantum information, the researchers used advanced methods of light filtering (patent pending) and an innovative camera of their own design. This camera, able to detect individual photons, is characterized by extremely low noise levels and a speed tens of times higher than existing cameras.

“The stability of the quantum information stored in our memory lasts from a few microseconds up to tens of microseconds. You’d be forgiven for asking how such short-lived memory could be useful at all, but bear in mind that it depends on the application. In telecommunications, microsecond timescales are sufficient to conduct several attempts at transmitting a quantum signal to the next relay station,” stresses Michał Dąbrowski, a doctoral student from the Faculty.

Skillfully harnessing subtle quantum optics phenomena has enabled the University of Warsaw researchers to significantly reduce noise levels in the quantum signals. When the information is retrieved, most of the noise is carried away by photons which are emitted by the memory cells in a different direction than the photons carrying the relevant quantum information.

A single atomic quantum memory cell, as developed at the Faculty of Physics at the University of Warsaw, can also store light with several different spatial modes (types of vibration). This means this is currently the solution with the highest capacity. In real quantum telecommunications applications, a single cell of this new type could serve as a buffer memory for several fiber-optic cables concurrently.

This work on atomic quantum memory cells received funding from the Polish National Research Centre.

Physics and Astronomy first appeared at the University of Warsaw in 1816, under the then Faculty of Philosophy. In 1825 the Astronomical Observatory was established. Currently, the Faculty of Physics' Institutes include Experimental Physics, Theoretical Physics, Geophysics, Department of Mathematical Methods in Physics and an Astronomical Observatory. Research covers almost all areas of modern physics, on scales from the quantum to the cosmological. The Faculty's research and teaching staff includes ca. 200 university teachers, of which 88 are employees with the title of professor. The Faculty of Physics, University of Warsaw, is attended by ca. 1000 students and more than 170 doctoral students..

SCIENTIFIC PAPERS:

“Hamiltonian design in readout from room-temperature Raman atomic memory”; M. Dąbrowski, R. Chrapkiewicz, W. Wasilewski; Optics Express, Vol. 22, Issue 21, pp. 26076-26091 (2014); http://dx.doi.org/10.1364/OE.22.026076

News Release Source : Global Quantum Communications – No Longer the Stuff of Fiction?

Image Credit :  Faculty of Physics University of Warsaw

MIT Researchers Provide New Two-Dimensional Quantum Materials

New 2-D quantum materials for nanoelectronics

MIT team provides theoretical roadmap to making 2-D electronics with novel properties.

MIT News Office
November 20, 2014

Researchers at MIT say they have carried out a theoretical analysis showing that a family of two-dimensional materials exhibits exotic quantum properties that may enable a new type of nanoscale electronics.

[caption id="attachment_482" align="aligncenter" width="650"] MIT Researchers Provide New Two-Dimensional Quantum Materials[/caption]

These materials are predicted to show a phenomenon called the quantum spin Hall (QSH) effect, and belong to a class of materials known as transition metal dichalcogenides, with layers a few atoms thick. The findings are detailed in a paper appearing this week in the journal Science, co-authored by MIT postdocs Xiaofeng Qian and Junwei Liu; assistant professor of physics Liang Fu; and Ju Li, a professor of nuclear science and engineering and materials science and engineering.

QSH materials have the unusual property of being electrical insulators in the bulk of the material, yet highly conductive on their edges. This could potentially make them a suitable material for new kinds of quantum electronic devices, many researchers believe.

But only two materials with QSH properties have been synthesized, and potential applications of these materials have been hampered by two serious drawbacks: Their bandgap, a property essential for making transistors and other electronic devices, is too small, giving a low signal-to-noise ratio; and they lack the ability to switch rapidly on and off. Now the MIT researchers say they have found ways to potentially circumvent both obstacles using 2-D materials that have been explored for other purposes.

Existing QSH materials only work at very low temperatures and under difficult conditions, Fu says, adding that “the materials we predicted to exhibit this effect are widely accessible. … The effects could be observed at relatively high temperatures.”

“What is discovered here is a true 2-D material that has this [QSH] characteristic,” Li says. “The edges are like perfect quantum wires.”

The MIT researchers say this could lead to new kinds of low-power quantum electronics, as well as spintronics devices — a kind of electronics in which the spin of electrons, rather than their electrical charge, is used to carry information.

Graphene, a two-dimensional, one-atom-thick form of carbon with unusual electrical and mechanical properties, has been the subject of much research, which has led to further research on similar 2-D materials. But until now, few researchers have examined these materials for possible QSH effects, the MIT team says. “Two-dimensional materials are a very active field for a lot of potential applications,” Qian says — and this team’s theoretical work now shows that at least six such materials do share these QSH properties.

The MIT researchers studied materials known as transition metal dichalcogenides, a family of compounds made from the transition metals molybdenum or tungsten and the nonmetals tellurium, selenium, or sulfur. These compounds naturally form thin sheets, just atoms thick, that can spontaneously develop a dimerization pattern in their crystal structure. It is this lattice dimerization that produces the effects studied by the MIT team.

While the new work is theoretical, the team produced a design for a new kind of transistor based on the calculated effects. Called a topological field-effect transistor, or TFET, the design is based on a single layer of the 2-D material sandwiched by two layers of 2-D boron nitride. The researchers say such devices could be produced at very high density on a chip and have very low losses, allowing high-efficiency operation.

By applying an electric field to the material, the QSH state can be switched on and off, making possible a host of electronic and spintronic devices, they say.

In addition, this is one of the most promising known materials for possible use in quantum computers, the researchers say. Quantum computing is usually susceptible to disruption — technically, a loss of coherence — from even very small perturbations. But, Li says, topological quantum computers “cannot lose coherence from small perturbations. It’s a big advantage for quantum information processing.”

Because so much research is already under way on these 2-D materials for other purposes, methods of making them efficiently may be developed by other groups and could then be applied to the creation of new QSH electronic devices, Qian says.

Nai Phuan Ong, a professor of physics at Princeton University who was not connected to this work, says, "Although some of the ideas have been mentioned before, the present system seems especially promising. This exciting result will bridge two very active subfields of condensed matter physics, topological insulators and dichalcogenides.”

The research was supported by the National Science Foundation, the U.S. Department of Energy, and the STC Center for Integrated Quantum Materials. Qian and Liu contributed equally to the work.

News Release Source :  New 2-D quantum materials for nanoelectronics

Image Credit : MIT News

Piece of The Quantum Puzzle

A piece of the quantum puzzle

UCSB physicists demonstrate the high level of controllability needed to explore ideas in quantum simulations

While the Martinis Lab at UC Santa Barbara has been focusing on quantum computation, former postdoctoral fellow Pedram Roushan and several colleagues have been exploring qubits (quantum bits) for quantum simulation on a smaller scale. Their research appears in the current edition of the journal Nature.

[caption id="attachment_478" align="aligncenter" width="650"] This image shows a top down view of the gmon qubit chip (0.6 cm x 0.6 cm) connected to microwave frequency control lines (copper) with thin wire bonds.[/caption]

"While we're waiting on quantum computers, there are specific problems from various fields ranging from chemistry to condensed matter that we can address systematically with superconducting qubits," said Roushan, who is now a quantum electronics engineer at Google. "These quantum simulation problems usually demand more control over the qubit system." Earlier this year, John M. Martinis and several members of his UCSB lab joined Google, which established a satellite office at UCSB.

In conjunction with developing a general-purpose quantum computer, Martinis' team worked on a new qubit architecture, which is an essential ingredient for quantum simulation, and allowed them to master the seven parameters necessary for complete control of a two-qubit system. Unlike a classical computer bit with only two possible states -- 0 and 1 -- a qubit can be in either state or a superposition of both at the same time, creating many possibilities of interaction.

One of the crucial specifications -- Roushan refers to them as control knobs or switches -- is the connectivity, which determines whether or not, and how, the two qubits interact. Think of the two qubits as people involved in a conversation. The researchers have been able to control every aspect -- location, content, volume, tone, accent, etc. -- of the communication. In quantum simulation, full control of the system is a holy grail and becomes more difficult to achieve as the size of the system grows.

"There are lots of technological challenges, and hence learnings involved in this project," Roushan said. "The icing on the cake is a demonstration that we chose from topology." Topology, the mathematical study of shapes and spaces, served as a good demonstration of the power of full control of a two-qubit system.

In this work, the team demonstrates a quantum version of Gauss's law. First came the 19th-century Gauss-Bonnet theorem, which relates the total local curvature of the surface of a geometrical object, such as a sphere or a doughnut, to the number of holes in the object (zero for the sphere and one for the doughnut). "Gauss's law in electromagnetism essentially provides the same relation: Measuring curvature on the surface -- in this case, an electric field -- tells you something about what is inside the surface: the charge," Roushan explained.

The novelty of the experiment is how the curvature was measured. Project collaborators at Boston University suggested an ingenious method: sensing the curvature through movement. How local curvature affects the motion can be understood from another analogy with electromagnetism: the Lorentz force law, which says that a charged particle in a magnetic field, which curves the space, is deflected from the straight pass. In their quantum system, the researchers measured the amount of deflection along one meridian of a sphere's curve and deduced the local curvature from that.

"When you think about it, it is pretty amazing," Roushan said. "You do not need to go inside to see what is in there. Moving on the surface tells you all you need to know about what is inside a surface."

This kind of simulation -- arbitrary control over all parameters in a closed system -- contributes to a body of knowledge that is growing, and the paper describing that demonstration is a key step in that direction. "The technology for quantum computing is in its infancy in a sense that it's not fully clear what platform and what architecture we need to develop," Roushan said. "It's like a computer 50 years ago. We need to figure out what material to use for RAM and for the CPU. It's not obvious so we try different architectures and layouts. One could argue that what we've shown is very crucial for coupling qubits when you're asking for a full-fledged quantum computer."

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Lead co-authors are UCSB's Charles Neill and Yu Chen, of Google Inc., Santa Barbara. Other UCSB co-authors include Rami Barends, Brooks Campbell, Zijun Chen, Ben Chiaro, Andrew N. Cleland, Andrew Dunsworth, Michael Fang, Julian Kelly, Nelson Leung, Anthony Megrant, Josh Mutus, Peter O'Malley, Chris Quintana, Amit Vainsencher, Jim Wenner and Ted White, as well as Evan Jeffrey, Martinis and Daniel Sank of Google Inc., Santa Barbara, and Michael Kolodrubetz and Anatoli Polkovnikov of Boston University.

This work was supported by the National Science Foundation (NSF), the Office of the Director of National Intelligence and the Intelligence Advanced Research Projects Activity. Devices were made at the UCSB Nanofabrication Facility, part of the NSF-funded National Nanotechnology Infrastructure Network and the NanoStructures Cleanroom Facility.

News Release Source :  A piece of the quantum puzzle

Image Credit: Michael Fang, Martinis Lab

Australian Quantum Physicist Michelle Simmons to Head New Quantum Journal

Education Minister launches first Nature Partner Journal in Australia

University of New South Wales (UNSW)

04 November 2014

A new scientific journal focusing on the rapidly developing areas of quantum research that promise to revolutionise the processing and transmission of information has been launched today at UNSW by the Federal Minister for Education Christopher Pyne.

[caption id="attachment_471" align="aligncenter" width="650"] Australian Quantum Physicist Michelle Simmons to Head New Quantum Journal[/caption]

npj Quantum Information is an international open-access journal and the first Nature Partner Journal based in Australia.

Professor Michelle Simmons, Director of the Australian Research Council Centre of Excellence for Quantum Computation and Communication Technology at UNSW, has been appointed to the prestigious role of Editor-in-Chief of the journal.

npj Quantum Information will combine research at the forefront of quantum computing, quantum communication and quantum information theory, covering topics including optics, atomic physics, semiconductor physics, superconducting physics and computer science.

Recent advances in instrumentation mean matter can be manipulated at the smallest scales – at the level of single atoms of matter or single photons of light. Scientists predict these areas of research will bring dramatic increases in computational power, and the ability to transmit information absolutely securely.

Professor Simmons said: “The 21st century will be the quantum information century, as the properties of quantum physics are exploited to develop powerful new, secure technologies for transmitting and processing information. New commercial and intellectual opportunities are emerging for nations that are able to discover, patent and exploit technologies in these areas.”

She highlighted the importance of the open access model for scientific journals: “While discovery is converging across fields, advances are still reported in disparate journals.

npj Quantum Information aims to change that, providing an open-access home for all aspects of this rapidly developing discipline.”

“The ARC plays an important role in the global research effort – the race to develop the quantum computer could be the space race of the 21st century,” said Federal Education Minister, the Hon Christopher Pyne MP, who toured the ARC Centre of Excellence laboratories at UNSW.

“Australia has a reputation for excellent research of international standing. The ARC Centre of Excellence for Quantum Computation and Communication Technology is strengthening this reputation and the new Nature Partner Journal will provide an important focus on this rapidly changing and exciting area of research,” he said.

David Swinbanks, Managing Director of Macmillan Science and Education, Australia and New Zealand said: “This launch is particularly exciting because it is our first partner journal partnered with an Australian institution.

“npj Quantum Information will be open access, free immediately upon publication for anyone who wants to read it. The open access model is especially important in the field of quantum information where the research is growing rapidly but has historically been fragmented. Our hope is that open access will stimulate sharing of ideas across these communities. Additionally, it will facilitate knowledge transfer to up and coming entrepreneurial businesses that are springing up in this area,” Mr Swinbanks said.

npj Quantum Information is now accepting submissions. Professor Simmons anticipates that the journal willpublish papers from both fundamental and applied areas, which could include reports about the fundamental relationship between quantum mechanics and information, the practical steps that are being taken to realise a quantum computer, algorithms opening new pathways for quantum information processing, exquisitely sensitive quantum sensors, the development of secure quantum communications across a global scale and emerging applications of quantum entanglement such as teleportation.

Professor Simmons is a world leader in the field of quantum computing and holds an ARC Laureate Fellowship at UNSW. She has published more than 350 papers in refereed journals including Nature, Nature Nanotechnology, Nature Physics, Nature Materials and Nature Communications. In 2012, her research group developed the world’s smallest transistor, marking a technological achievement 10 years ahead of industry predictions. Her laboratory is the only one in the world able to make atomically precise devices in silicon, including the thinnest conducting wires yet produced, which are 1000 times narrower than a human hair. A member of the Australian Academy of Science since 2006, she was named NSW Scientist of the Year in 2012. In 2014, she became an elected member of the American Academy of Arts and Sciences.

For more on the journal check the website: www.nature.com/npjqi/

News Release Source : Education Minister launches first Nature Partner Journal in Australia

Researchers Linked between String Field Theory and Quantum Mechanics

String field theory could be the foundation of quantum mechanics

USC scientists uncover a connection that could be a huge boost to string theory

Two USC researchers have proposed a link between string field theory and quantum mechanics that could open the door to using string field theory — or a broader version of it, called M-theory — as the basis of all physics.

[caption id="attachment_467" align="aligncenter" width="660"] Two USC researchers used string field theory to try to validate quantum mechanics.[/caption]

“This could solve the mystery of where quantum mechanics comes from,” said Itzhak Bars, USC Dornsife College of Letters, Arts and Sciences professor and lead author of the paper.

Bars collaborated with Dmitry Rychkov, his Ph.D. student at USC. The paper was published online on Oct. 27 by the journal Physics Letters.

Rather than use quantum mechanics to validate string field theory, the researchers worked backwards and used string field theory to try to validate quantum mechanics.

In their paper, which reformulated string field theory in a clearer language, Bars and Rychov showed that a set of fundamental quantum mechanical principles known as “commutation rules’’ that may be derived from the geometry of strings joining and splitting.

“Our argument can be presented in bare bones in a hugely simplified mathematical structure,” Bars said. “The essential ingredient is the assumption that all matter is made up of strings and that the only possible interaction is joining/splitting as specified in their version of string field theory.”

The history of string theory

Physicists have long sought to unite quantum mechanics and general relativity, and to explain why both work in their respective domains. First proposed in the 1970s, string theory resolved inconsistencies of quantum gravity and suggested that the fundamental unit of matter was a tiny string, not a point, and that the only possible interactions of matter are strings either joining or splitting.

Four decades later, physicists are still trying to hash out the rules of string theory, which seem to demand some interesting starting conditions to work (like extra dimensions, which may explain why quarks and leptons have electric charge, color and “flavor” that distinguish them from one another).

At present, no single set of rules can be used to explain all of the physical interactions that occur in the observable universe.

On large scales, scientists use classical, Newtonian mechanics to describe how gravity holds the moon in its orbit or why the force of a jet engine propels a jet forward. Newtonian mechanics is intuitive and can often be observed with the naked eye.

On incredibly tiny scales, such as 100 million times smaller than an atom, scientists use relativistic quantum field theory to describe the interactions of subatomic particles and the forces that hold quarks and leptons together inside protons, neutrons, nuclei and atoms.

An invaluable framework

Quantum mechanics is often counterintuitive, allowing for particles to be in two places at once, but has been repeatedly validated from the atom to the quarks. It has become an invaluable and accurate framework for understanding the interactions of matter and energy at small distances.

Quantum mechanics is extremely successful as a model for how things work on small scales, but it contains a big mystery: the unexplained foundational quantum commutation rules that predict uncertainty in the position and momentum of every point in the universe.

“The commutation rules don’t have an explanation from a more fundamental perspective, but have been experimentally verified down to the smallest distances probed by the most powerful accelerators. Clearly the rules are correct, but they beg for an explanation of their origins in some physical phenomena that are even deeper,” Bars said.

The difficulty lies in the fact that there’s no experimental data on the topic — testing things on such a small scale is currently beyond a scientist’s technological ability.

The research was funded by the Department of Energy.

News Release Source :  String field theory could be the foundation of quantum mechanics

Image Credit : Photo/astrophysics.pro

Academics are Challenging The Foundations of Quantum Science

New quantum theory is out of this parallel world

Griffith University academics are challenging the foundations of quantum science with a radical new theory based on interactions between parallel universes.

[caption id="attachment_462" align="aligncenter" width="468"] The Director of Griffith’s Centre for Quantum Dynamics,                              Professor Howard Wiseman[/caption]

In a paper published in the prestigious journal Physical Review X,Professor Howard Wiseman and Dr Michael Hall from Griffith’sCentre for Quantum Dynamics, and Dr Dirk-Andre Deckert from the University of California, take interacting parallel worlds out of the realm of science fiction and into that of hard science.

The team proposes that parallel universes really exist, and that they interact. That is, rather than evolving independently, nearby worlds influence one another by a subtle force of repulsion. They show that such an interaction could explain everything that is bizarre about quantum mechanics.

Quantum theory is needed to explain how the universe works at the microscopic scale, and is believed to apply to all matter. But it is notoriously difficult to fathom, exhibiting weird phenomena which seem to violate the laws of cause and effect.

As the eminent American theoretical physicist Richard Feynman once noted: “I think I can safely say that nobody understands quantum mechanics.”

However, the “Many-Interacting Worlds” approach developed at Griffith University provides a new and daring perspective on this baffling field.

“The idea of parallel universes in quantum mechanics has been around since 1957,” says Professor Wiseman.

“In the well-known “Many-Worlds Interpretation”, each universe branches into a bunch of new universes every time a quantum measurement is made. All possibilities are therefore realised – in some universes the dinosaur-killing asteroid missed Earth. In others, Australia was colonised by the Portuguese.

“But critics question the reality of these other universes, since they do not influence our universe at all. On this score, our “Many Interacting Worlds” approach is completely different, as its name implies.”

Professor Wiseman and his colleagues propose that:

• The universe we experience is just one of a gigantic number of worlds. Some are almost identical to ours while most are very different;


• All of these worlds are equally real, exist continuously through time, and possess precisely defined properties;


• All quantum phenomena arise from a universal force of repulsion between ‘nearby’ (i.e. similar) worlds, which tends to make them more dissimilar.

Dr Hall says the “Many-Interacting Worlds” theory may even create the extraordinary possibility of testing for the existence of other worlds.

“The beauty of our approach is that if there is just one world our theory reduces to Newtonian mechanics, while if there is a gigantic number of worlds it reproduces quantum mechanics,” he says.

“In between it predicts something new that is neither Newton’s theory nor quantum theory.

“We also believe that, in providing a new mental picture of quantum effects, it will be useful in planning experiments to test and exploit quantum phenomena.”

The ability to approximate quantum evolution using a finite number of worlds could have significant ramifications in molecular dynamics, which is important for understanding chemical reactions and the action of drugs.

Professor Bill Poirier, Distinguished Professor of Chemistry at Texas Tech University, has observed: “These are great ideas, not only conceptually, but also with regard to the new numerical breakthroughs they are almost certain to engender.”

From chemistry to quantum physics to science fiction, this theory opens new and exciting territory.

Go to:https://journals.aps.org/prx/abstract/10.1103/PhysRevX.4.041013

News Release Source :  New quantum theory is out of this parallel world

Image Credit : Centre for Quantum Dynamics, Griffith University

New Research Suggests The Electron's Quantum State Separated into Parts

Can the wave function of an electron be divided and trapped?

Electrons are elementary particles — indivisible, unbreakable. But new research suggests the electron's quantum state — the electron wave function — can be separated into many parts. That has some strange implications for the theory of quantum mechanics.

PROVIDENCE, R.I. [Brown University] 28/10/2014

New research by physicists from Brown University puts the profound strangeness of quantum mechanics in a nutshell — or, more accurately, in a helium bubble.

[caption id="attachment_456" align="aligncenter" width="580" class=" "] The electron wave function
A canister of liquid helium inside the blue cylinder allowed researchers to experiment with tiny electron bubbles only 3.6 nanometers in diameter. The work suggests that the wave function of an electron can be split and parts of it trapped in smaller bubbles.[/caption]

Experiments led by Humphrey Maris, professor of physics at Brown, suggest that the quantum state of an electron — the electron’s wave function — can be shattered into pieces and those pieces can be trapped in tiny bubbles of liquid helium. To be clear, the researchers are not saying that the electron can be broken apart. Electrons are elementary particles, indivisible and unbreakable. But what the researchers are saying is in some ways more bizarre.

In quantum mechanics, particles do not have a distinct position in space. Instead, they exist as a wave function, a probability distribution that includes all the possible locations where a particle might be found. Maris and his colleagues are suggesting that parts of that distribution can be separated and cordoned off from each other.

“We are trapping the chance of finding the electron, not pieces of the electron,” Maris said. “It’s a little like a lottery. When lottery tickets are sold, everyone who buys a ticket gets a piece of paper. So all these people are holding a chance and you can consider that the chances are spread all over the place. But there is only one prize — one electron — and where that prize will go is determined later.”

If Maris’s interpretation of his experimental findings is correct, it raises profound questions about the measurement process in quantum mechanics. In the traditional formulation of quantum mechanics, when a particle is measured — meaning it is found to be in one particular location — the wave function is said to collapse.

“The experiments we have performed indicate that the mere interaction of an electron with some larger physical system, such as a bath of liquid helium, does not constitute a measurement,” Maris said. “The question then is: What does?”

And the fact that the wave function can be split into two or more bubbles is strange as well. If a detector finds the electron in one bubble, what happens to the other bubble?

"It really raises all kinds of interesting questions," Maris said.

The new research is published in the Journal of Low Temperature Physics.

Electron bubbles

Scientists have wondered for years about the strange behavior of electrons in liquid helium cooled to near absolute zero. When an electron enters the liquid, it repels surrounding helium atoms, forming a bubble in the liquid about 3.6 nanometers across. The size of the bubble is determined by the pressure of the electron pushing against the surface tension of the helium. The strangeness, however, arises in experiments dating back to the 1960s looking at how the bubbles move.

In the experiments, a pulse of electrons enters the top of a helium-filled tube, and a detector registers the electric charge delivered when electron bubbles reach the bottom of the tube. Because the bubbles have a well-defined size, they should all experience the same amount of drag as they move, and should therefore arrive at the detector at the same time. But that’s not what happens. Experiments have detected unidentified objects that reach the detector before the normal electron bubbles. Over the years, scientists have cataloged 14 distinct objects of different sizes, all of which seem to move faster than an electron bubble would be expected to move.

“They’ve been a mystery ever since they were first detected,” Maris said. “Nobody has a good explanation.”

Several possibilities have been proposed. The unknown objects could be impurities in the helium—charged particles knocked free from the walls of the container. Another possibility is that the objects could be helium ions — helium atoms that have picked up one or more extra electrons, which produce a negative charge at the detector.

But Maris and his colleagues, including Nobel laureate and Brown physicist Leon Cooper, believe a new set of experiments puts those explanations to rest.

New experiments

The researchers performed a series of electron bubble mobility experiments with much greater sensitivity than previous efforts. They were able to detect all 14 of the objects from previous work, plus four additional objects that appeared frequently over the course of the experiments. But in addition to those 18 objects that showed up frequently, the study revealed countless additional objects that appeared more rarely.

In effect, Maris says, it appears there aren’t just 18 objects, but an effectively infinite number of them, with a “continuous distribution of sizes” up to the size of the normal electron bubble.

“That puts a dagger in the idea that these are impurities or helium ions,” Maris said. “It would be hard to imagine that there would be that many impurities, or that many previously unknown helium ions.”

The only way the researchers can think of to explain the results is through “fission” of the wave function. In certain situations, the researchers surmise, electron wave functions break apart upon entering the liquid, and pieces of the wave function are caught in separate bubbles. Because the bubbles contain less than the full wave function, they’re smaller than normal electron bubbles and therefore move faster.

In their new paper, Maris and his team lay out a mechanism by which fission could happen that is supported by quantum theory and is in good agreement with the experimental results. The mechanism involves a concept in quantum mechanics known as reflection above the barrier.

In the case of electrons and helium, it works like this: When an electron hits the surface of the liquid helium, there’s some chance that it will cross into the liquid, and some chance that it will bounce off and carom away. In quantum mechanics, those possibilities are expressed as part of the wave function crossing the barrier, and part of it being reflected. Perhaps the small electron bubbles are formed by the portion of the wave function that goes through the surface. The size of the bubble depends on how much wave function goes through, which would explain the continuous distribution of small electron bubble sizes detected in the experiments.

The idea that part of the wave function is reflected at a barrier is standard quantum mechanics, Cooper said. “I don’t think anyone would argue with that,” he said. “The non-standard part is that the piece of the wave function that goes through can have a physical effect by influencing the size of the bubble. That is what is radically new here.”

Further, the researchers propose what happens after the wave function enters the liquid. It’s a bit like putting a droplet of oil in a puddle of water. “Sometime your drop of oil forms one bubble,” Maris said, “Sometimes it forms two, sometimes 100.”

There are elements within quantum theory that suggest a tendency for the wave function to break up into specific sizes. By Maris’s calculations, the specific sizes one might expect to see correspond roughly to the 18 frequently occurring electron bubble sizes.

“We think this offers the best explanation for what we see in the experiments,” Maris said. We’ve got this body of data that goes back 40 years. The experiments are not wrong; they’ve been done by multiple people. We have a tradition called Occam’s razor, where we try to come up with the simplest explanation. This, so far as we can tell, is it.”

But it does raise some interesting questions that sit on the border of science and philosophy. For example, it’s necessary to assume that the helium does not make a measurement of the actual position of the electron. If it did, any bubble found not to contain the electron would, in theory, simply disappear. And that, Maris says, points to one of the deepest mysteries of quantum theory.

“No one is sure what actually constitutes a measurement. Perhaps physicists can agree that someone with a Ph.D. wearing a white coat sitting in the lab of a famous university can make measurements. But what about somebody who really isn’t sure what they are doing? Is consciousness required? We don’t really know.”

Authors on the paper in addition to Maris were former Brown postdoctoral researcher Wanchun Wei, graduate student Zhuolin Xie, and George Seidel, professor emeritus of physics.

News Release Source : Can the wave function of an electron be divided and trapped?

Image Credit : Mike Cohea/Brown University

American Aircraft Helps Quantum Technology Take Flight

1980s American aircraft helps quantum technology take flight

What does a 1980s experimental aircraft have to do with state-of-the art quantum technology? Lots, as shown by new research from the Quantum Control Laboratory at the University of Sydney, and published in Nature Physics today.

[caption id="attachment_452" align="aligncenter" width="695"] American Aircraft Helps Quantum Technology Take Flight[/caption]

Over several years a team of scientists has taken inspiration from aerospace research and development programs to make unusually shaped experimental aircraft fly.

"It always amazed me that the X-29, an American airplane that was designed like a dart being thrown backwards, was able to fly. Achieving this, in 1984, came through major advances in a discipline called control engineering that were able to stabilise the airplane," said Associate Professor Michael Biercuk, from the School of Physics and director of the Quantum Control Laboratory.

"We became interested in how similar concepts could play a role in bringing quantum technologies to reality. If control engineering can turn an unstable dart into a high-performance fighter jet, it's pretty amazing to think what it can do for next-generation quantum technologies."

The result is that the researchers have been able to turn fragile quantum systems into useful pieces of advanced tech useful for everything from computation and communications to building specialised sensors for industry. The trick was figuring out how to protect them from their environments using control theory.

The big challenge facing quantum technologies is they are very sensitive to random 'noise' in surrounding environments, said Professor Biercuk. "Noise, in this case, is a bit like local electromagnetic weather experienced by a piece of hardware. Imagine your television only worked when the weather was perfectly sunny. Something needs to be done to make that technology more functional, even on the grey days."

The new field of quantum control engineering provided a path forward. The first step was trying to pinpoint how noise would affect a quantum system while it performed some task, which is fiendishly difficult.

"We were able to calculate how much damage is done to a quantum state using so-called transfer functions tailored to specific operations – for instance, manipulating a quantum system as a part of a computation," according to co-lead author, PhD student Harrison Ball.

The next issue was to show that the theoretical techniques actually worked.

"One of our main achievements has been to show – using experiments on real quantum systems in the form of atoms in a special trap – that the transfer functions were excellent at predicting how quantum systems changed in response to environmental noise."

With new capabilities to predict the effect of the environment on quantum systems, it became possible to protect them by applying the right control techniques.

"Similar to the control system that kept an aerodynamically unstable plane aloft, experiments revealed that our new techniques were able to keep the atoms performing useful computations," said Biercuk. "Turn off the new control techniques and they would crash and burn."

"Achieving this is a grand challenge for the entire community," according to Ball, and it is especially important as researchers move from proof-of-principle demonstrations to trying to develop real quantum technologies.

Working to make those technologies a reality is the aim of Prof. Biercuk and his colleagues in the ARC Centre for Engineered Quantum Systems.

"This may sound like futuristic fantasy, but the navigation system in your car works because of an early quantum technology – atomic clocks," according to Biercuk.

"We know that exotic phenomena like quantum systems being in two places at once, and even the ability to teleport quantum states, are real and accessible in the laboratory. Now we are trying to actually put them to work, and that means figuring out how to coax quantum systems into doing new and useful things."

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Image Credit: NASA photo by Larry Sammons

News Release Source : 1980s American aircraft helps quantum technology take flight

Australian Teams Set New Records for Silicon Quantum Computing

Australian teams set new records for silicon quantum computing

UNSW Newsroom
13 October 2014

Two research teams working in the same laboratories at UNSW Australia have found distinct solutions to a critical challenge that has held back the realisation of super powerful quantum computers.

[caption id="attachment_445" align="aligncenter" width="500"] Australian Teams Set New Records for Silicon Quantum Computing[/caption]

The teams created two types of quantum bits, or "qubits" – the building blocks for quantum computers – that each process quantum data with an accuracy above 99%. The two findings have been published simultaneously today in the journal Nature Nanotechnology.

"For quantum computing to become a reality we need to operate the bits with very low error rates," says Scientia Professor Andrew Dzurak, who is Director of the Australian National Fabrication Facility at UNSW, where the devices were made.

"We've now come up with two parallel pathways for building a quantum computer in silicon, each of which shows this super accuracy," adds Associate Professor Andrea Morello from UNSW's School of Electrical Engineering and Telecommunications.



The UNSW teams, which are also affiliated with the ARC Centre of Excellence for Quantum Computation & Communication Technology, were first in the world to demonstrate single-atom spin qubits in silicon, reported in Nature in 2012 and 2013.

Now the team led by Dzurak has discovered a way to create an "artificial atom" qubit with a device remarkably similar to the silicon transistors used in consumer electronics, known as MOSFETs. Post-doctoral researcher Menno Veldhorst, lead author on the paper reporting the artificial atom qubit, says, "It is really amazing that we can make such an accurate qubit using pretty much the same devices as we have in our laptops and phones".

Meanwhile, Morello's team has been pushing the "natural" phosphorus atom qubit to the extremes of performance. Dr Juha Muhonen, a post-doctoral researcher and lead author on the natural atom qubit paper, notes: "The phosphorus atom contains in fact two qubits: the electron, and the nucleus. With the nucleus in particular, we have achieved accuracy close to 99.99%. That means only one error for every 10,000 quantum operations."

Dzurak explains that, "even though methods to correct errors do exist, their effectiveness is only guaranteed if the errors occur less than 1% of the time. Our experiments are among the first in solid-state, and the first-ever in silicon, to fulfill this requirement."

The high-accuracy operations for both natural and artificial atom qubits is achieved by placing each inside a thin layer of specially purified silicon, containing only the silicon-28 isotope. This isotope is perfectly non-magnetic and, unlike those in naturally occurring silicon, does not disturb the quantum bit. The purified silicon was provided through collaboration with Professor Kohei Itoh from Keio University in Japan.

The next step for the researchers is to build pairs of highly accurate quantum bits. Large quantum computers are expected to consist of many thousands or millions of qubits and may integrate both natural and artificial atoms.

Morello's research team also established a world-record "coherence time" for a single quantum bit held in solid state. "Coherence time is a measure of how long you can preserve quantum information before it's lost," Morello says. The longer the coherence time, the easier it becomes to perform long sequences of operations, and therefore more complex calculations.

The team was able to store quantum information in a phosphorus nucleus for more than 30 seconds. "Half a minute is an eternity in the quantum world. Preserving a 'quantum superposition' for such a long time, and inside what is basically a modified version of a normal transistor, is something that almost nobody believed possible until today," Morello says.

"For our two groups to simultaneously obtain these dramatic results with two quite different systems is very special, in particular because we are really great mates," adds Dzurak.

The quantum bit devices were constructed at UNSW at the Australian National Fabrication Facility, with support from researchers at the University of Melbourne and the Australian National University. The research was funded by: the Australian Research Council, the US Army Research Office, the NSW Government, UNSW Australia and the University of Melbourne.

 

News Release Source : Australian teams set new records for silicon quantum computing

Image Credit : UNSW

Quantum computing will Create Smarter and Creative Robots

Pressing The Accelerator on Quantum Robotics

Quantum computing will allow for the creation of powerful computers, but also much smarter and more creative robots than conventional ones. This was the conclusion arrived at by researchers from the Complutense University of Madrid (UCM) and Austria, who have confirmed that quantum tools help robots learn and respond much faster to the stimuli around them.

[caption id="attachment_440" align="aligncenter" width="500"] Quantum computing will Create Smarter and Creative Robots[/caption]

SINC | October 02 2014 09:00

Quantum mechanics has revolutionised the world of communications and computers by introducing algorithms which are much quicker and more secure in transferring information. Now researchers from the Complutense University of Madrid (UCM) and the University of Innsbruck (Austria) have published a study in the journal ‘Physical Review X’ which states that these tools can be used to apply to robots, automatons and the other agents that use artificial intelligence (AI).

They demonstrate for the first time that quantum machines can respond the best and act the fastest against the environment surrounding them. More specifically, they adapt to situations where the conventional ones, which are much slower, cannot finish the learning and response processes.

“In the case of very demanding and ‘impatient’ environments, the outcome is that the quantum robot can adapt itself and survive, while the classic robot is destined to collapse,” explains G. Davide Paparo and Miguel A. Martín-Delgado, the two researchers from UCM who have participated in the study.

Their theoretical work has focused on using quantum computing to accelerate ahead with one of the most difficult points to resolve in information technology: machine learning, which is used to create highly accurate models and predictions. It is applied, for example, to know how the climate or an illness will evolve or in the development of Internet search engines.

More creative quantum robots

“Building a model is actually a creative act, but conventional computers are no good at it,” says Martín-Delgado. “That is where quantum computing comes into play. The advances it brings are not only quantitative in terms of greater speed, but also qualitative: adapting better to environments where the classic agent does not survive. This means that quantum robots are more creative”.

The authors assess the scope of their study as such: “It means a step forward towards the most ambitious objective of artificial intelligence: the creation of a robot that is intelligent and creative, and that is not designed for specific tasks”.

This work comes under a new discipline, the so-called ‘quantum AI’, an area in which the company Google has started to invest millions of dollars via the creation of a specialised laboratory in collaboration with the NASA.

Referencia bibliográfica:

Giuseppe Davide Paparo, Vedran Dunjko, Adi Makmal, Miguel Angel Martin-Delgado, and Hans J. Briegel. “Quantum Speedup for Active Learning Agents”. Phys. Rev. X 4, 031002, 8 de julio de 2014.

News Release Source : Pressing the accelerator on quantum robotics

Image Credit : SINC

Quantum environmentalism

Quantum environmentalism

Putting a qubit's surroundings to good use

Where are the quantum computers? Aren't they supposed to be speeding up decryption and internet searches? After two decades of research, you still can't find them in stores. Well, it took two decades or more of research dedicated to semiconductors and circuit integration before we had digital computers. For quantum computers too it will take technology more time to catch up to the science

[caption id="attachment_436" align="aligncenter" width="512" class=" "] The electron spin (black arrow) inside a quantum dot interacts magnetically with the effective magnetic field (orange arrow) of the nuclei of the atoms forming the atomic environment of the electron.[/caption]

Meanwhile, research devoted to exploring bizarre quantum phenomena must continue to overcome or reduce a litany of practical obstacles before quantum computing can be realized. Not the least of these obstacles remains the isolation of qubits---the repository of quantum information---from their surroundings. A qubit, or better yet an ensemble of qubits, exists in a superposition of two or more possible states. The trouble is that superposition is a fragile condition, and the manipulation and final readout of those states are in danger of being undone if a qubit interacts with its environment. A new paper, published in Nature Physics (1) addresses this problem by demonstrating a new type of qubit control, one that actually makes productive use of a qubit's proximity to its surroundings.

The experimental work was performed at the Cavendish Laboratory at Cambridge University in the UK. JQI (2) scientist Jacob Taylor provided the theoretical input to the work over the course of five years of cross-Atlantic collaboration.

ENVIRONMENTAL-ASSISTED CONTROL Qubits come in many forms but have one essential thing in common: they all embody a physical system---whether in the form of a photon or atom or electron or electrical current---which exists in two quantum states simultaneously. In the Cambridge experiment the qubit is a single electron trapped in a semiconductor. The two quantum states, in this case, consist of the two orientations for the electron's tiny spin, either up or down.

The electron's trap is a tiny region of the semiconductor called a quantum dot, essentially a zero-dimensional volume housing a single active electron. When studied at a temperature of about 4 K, the electron spreads as a wave across the 10 nanometer length of the quantum dot. The dot can be considered to be an artificial atom in which the electron (instead of orbiting a single nucleus) is confined in the crystalline lattice of millions of atoms. And like electrons in regular atoms, the select electron in a quantum dot possesses an energy spectrum of discrete energies.

The dot is fabricated using existing nanotechology. In substance it is a tiny lump of indium arsenide (InAs) grown amidst surrounding thicker layers of gallium arsenide (GaAs). Recall the story of the princess and the pea; a girl confirms her status as a princess by detecting a very faint lumpiness created by the presence of a single pea sandwiched between much larger mattresses.

Here the pea is the InAs quantum dot and the mattresses are the GaAs layers. The two species of semiconductor blend, InAs and GaAs, are incommensurate, meaning that their natural atomic spacings are slightly different. In practice this ensures that the atoms in the InAs dot are under some stress amid the surrounding GaAs atom layers. This stress, in turn, leads to the growth of such small 'pea'-like dots, confining the electron inside. Fortunately, one can probe the properties of these buried quantum dots via light, as the materials are transparent, allowing experimentalists to control and manipulate the electron using lasers.

The electron's energy spectrum is made still more complicated by the faint magnetic interaction between it and the nuclei of all those indium and arsenic atoms. Each of those nuclei has a net magnetic field and behaves as if it were a tiny magnet. Those nuclei, in the absence of an external magnetic field, are pointing in all different directions. But at any one moment, the totality of the nuclei exert a single, effective field, which the electron senses. The collective nuclei form a magnetic "environment" for the electron. In just a moment we'll see how the electron is employed as a qubit and how its environment---usually viewed as a threat to maintaining the quantum integrity of the qubit---is actually put to use in the process of manipulating and reading out the qubit.

QUANTUM DARK STATES If a laser strikes the quantum dot at certain wavelengths corresponding to allowed electron energies, the dot will absorb and then reemit light. It will fluoresce. Things get more interesting, however, if the electron can be coaxed into not absorbing light. To do this a dark state must be created. Generally, if a laser beam is tuned to the energy difference between two quantum levels, the atom can absorb laser light and be promoted to the more excited of the two states. The continued presence of the laser light will later stimulate the atom to re-emit the light and return to its lower (ground) state. In some circumstances this absorption/emission process can be stymied if there are closely spaced ground states.

Two laser beams, one tuned to promote the atom from state 1 to state 3 and one tuned to promote the atom from state 2 to state 3, can interfere with each other, leaving state 3 unreachable. This destructive interference can be turned on and off by changing the relative phase of the two laser beams.

This method is employed in the Cambridge experiment. The quantum dot qubit consists of the lone electron being maintained in a state of superposition (spin up and spin down) by laser light. Moreover, the presence of the underlying nuclear magnetic field forming the electron's "environment" helps establish a two-part ground state. Thereafter, the combination of the environment, plus the presence of two carefully-tuned laser beams allows the state of the qubit to be controlled (swiveled around in space) and even to be read out when detectors glimpse the electron as a bright (fluorescent) or a dark object.

In quantum computing, the explicit state of the qubit is unknown (spin up or spin down); this is positively a necessary condition---a required indeterminacy---for the quantum computation to continue. The only thing required is that one knows the relative change in the qubit, such as whether it was swiveled through an angle of 90 or 180 degrees. In conventional computing an example would be a NOT gate, which changes a bit from a 1 state into a 0 state or vice versa. It's only important to know the relative change in the bit, not its actual value. "What is profound is that the electron spin is always in the same quantum superposition, but the physical state evolves with the nuclear field," notes Mete Atature, the Cambridge researcher leading the experimental study.

"Usually you need an external magnetic field to manipulate qubits," says Jacob Taylor, "but this single field can't be used also to read out the final state of a qubit. In our all-optical approach we don't use a field so we can perform manipulation and readout with a single setup."

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1. "Environment-assisted quantum control of a solid-state spin via coherent dark states," Jack Hansom, Carsten H. H. Schulte, Claire Le Gall, Clemens Matthiesen, Edmund Clarke, Maxime Hugues, Jacob M. Taylor, Mete Atatüre, Nature Physics, Nature Physics 10, 725–730 (2014), doi:10.1038/nphys3077; published online 7 September 2014

2. The Joint Quantum Institute (JQI) is operated by the National Institute for Standards and Technology and the University of Maryland (http://jqi.umd.edu/)

3. Previous press release about 3-electron quantum dot qubits: http://jqi.umd.edu/news/resonant-exchange-qubits.

4. Previous press release about reducing noise in quantum-dot qubits: http://jqi.umd.edu/news/reducing-noise-qubit-arrays

5. Previous press release about Bloch spheres and how to manipulate qubits: http://jqi.umd.edu/news/quantum-longitude

Jacob Taylor, jmtaylor@umd.edu

Press contact at JQI: Phillip F. Schewe, pschewe@umd.edu, 301-405-0989. http://jqi.umd.edu/

 

News Release Source :  Quantum environmentalism