Nanotechnology Quantum Computing Global Communications Network


id28229Published on Jul 28, 2013

A fascinating applied Nanotechnology engineering documentary where current research for a quantum computer based global communications network is described. Just another example of how the applications derived from advances in quantum physics and the understanding of quantum mechanics will very soon be changing our everyday lives!

 

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Quantum communication controlled by resonance in ‘artificial atoms’


imagesCAMR5BLR Einstein Judging a FishResearchers at the Niels Bohr Institute, together with colleagues in the US and Australia, have developed a method to control a quantum bit for electronic quantum communication in a series of quantum dots, which behave like artificial atoms in the solid state. The results have been published in the scientific journal Physical Review Letters.

 

The experiments are carried out at ultra low temperatures close to absolute zero, which is minus 273 degrees C.

In a conventional computer, information is made up of bits, comprised of 0’s and 1’s. In a quantum computer the 0 and 1 states can simultaneously exist, allowing a kind of parallel computation in which a large number of computational states are acted upon by the machine at the same time. This can make a quantum computer exponentially faster than a conventional computer. The problem with the quantum world, however, is that you cannot allow these states to be measured, or all of the quantum magic disappears.

“We have developed a new way of controlling the electrons so that the quantum state can be controlled without measurement, using resonances familiar in atomic physics, now applied to these artificial atoms,” explains Professor Charles Marcus, director of the Center for Quantum Devices at the Niels Bohr Institute at the University of Copenhagen.

He explains that they are combining classical solid-state physics on a nanometer scale with resonance techniques of atomic physics. In a semiconducting material (GaAs) there are free electrons that move within the material structure. The information is stored in the spin of the electrons which can turn up or down. But the electrons and their spin must be controlled.

Schematic illustration of the actual ‘box’ with a triple quantum dot, where there is one single electron in each dot.

Captures electrons and controls them

“We capture the electrons in ‘boxes’. Each box consist of a quantum dot, which is an artificial atom. The quantum dots are embedded in the semiconductor and each quantum dot can capture one electron. There needs to be three quantum dots next to each other using nanometer-scale electrostatic metal gates. When we open contact between the ‘boxes’ the electrons can sense each others’ presence. The three spins must coordinate their orientations because it cost extra energy to put electrons with the same spin into the same box. To lower their energy, they not only spread out among the three boxes, but they orient their spins to further lower their energy. The three boxes together form a single entity – a qubit or quantum bit,” explains Charles Marcus.

An electrical signal is now sent from outside and by rapidly opening the boxes the system begins to swing in dynamic vibrations. The researchers can use this to change the quantum state of the electrons.

“By combining three electrons in a triple quantum dot and oscillating an applied electric field at the frequency that separates adjacent energy levels, we can thus control the spins of the electrons without measuring them,” explains Charles Marcus.

Quantum computers for extreme applications

First, the technique itself was discovered. The next step is not just a single sequence with three quantum dots, but several sequences. Each sequence forms one qubit and now a series of qubits need to talk to each other. This could be realised by a quantum computer with more bits.

“The potential of a quantum computer is that it will be able to perform multiple calculations at once. In that way it will be much faster than conventional computers and will be able to solve tasks that cannot currently be solved, because it simply takes too long,” says Charles Marcus.

Quantum computers are not expected to be something everyone will own, but rather an advanced set of tools for researchers who need to make extreme calculations.

The research is described in three articles in Physical Review Letters:

  1. Quantum-Dot-Based Resonant Exchange Qubit >>>
  2. Electrically Protected Resonant Exchange Qubits in Triple Quantum Dots >>>
  3. Two-Qubit Gates for Resonant Exchange Qubits >>>

Nanotechnology – Producing the Quantum computer


Published on May 25, 2013

http://youtu.be/To55y5wPsdU

 

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Nanotechnology. The production of the quantum computer based on the quantum spin of electrons and its application to calculation, computing and technology!

Watch here; Nanotechnology Documentary – Quantum Computing, what it is, how it works!

 

Vaporware: Scientists Use Cloud of Atoms as Optical Memory Device


QDOTS imagesCAKXSY1K 8Talk about storing data in the cloud. Scientists at the Joint Quantum Institute (JQI) of the National Institute of Standards and Technology (NIST) and the University of Maryland have taken this to a whole new level by demonstrating* that they can store visual images within quite an ethereal memory device—a thin vapor of rubidium atoms. The effort may prove helpful in creating memory for quantum computers.

This brief animation (click link to launch mp4) by the NIST/JQI team shows the NIST logo they stored within a vapor of rubidium atoms and three different portions of it that they were able to extract at will. Animation combines three actual images from the vapor extracted at different times.

Their work builds on an approach developed at the Australian National University, where scientists showed that a rubidium vapor could be manipulated in interesting ways using magnetic fields and lasers. The vapor is contained in a small tube and magnetized, and a laser pulse made up of multiple light frequencies is fired through the tube. The energy level of each rubidium atom changes depending on which frequency strikes it, and these changes within the vapor become a sort of fingerprint of the pulse’s characteristics. If the field’s orientation is flipped, a second pulse fired through the vapor takes on the exact characteristics of the first pulse—in essence, a readout of the fingerprint.

“With our paper, we’ve taken this same idea and applied it to storing an image—basically moving up from storing a single ‘pixel’ of light information to about a hundred,” says Paul Lett, a physicist with JQI and NIST’s Quantum Measurement Division. “By modifying their technique, we have been able to store a simple image in the vapor and extract pieces of it at different times.”

It’s a dramatic increase in the amount of information that can be stored and manipulated with this approach. But because atoms in a vapor are always in motion, the image can only be stored for about 10 milliseconds, and in any case the modifications the team made to the original technique introduce too much noise into the laser signal to make the improvements practically useful. So, should the term vaporware be applied here after all? Not quite, says Lett—because the whole point of the effort was not to build a device for market, but to learn more about how to create memory for next-generation quantum computers.

“What we’ve done here is store an image using classical physics. However, the ultimate goal is to store quantum information, which a quantum computer will need,” he says. “Measuring what the rubidium atoms do as we manipulate them is teaching us how we might use them as quantum bits and what problems those bits might present. This way, when someone builds a solid-state system for a finished computer, we’ll know how to handle them more effectively.”

*J.B. Clark, Q. Glorieux and P.D. Lett. Spatially addressable readout and erasure of an image in a gradient echo memory. New Journal of Physics, doi: 10.1088/1367-2630/15/3/035005, 06 March 2013.

New qubit control bodes well for future of quantum computing


QDOTS imagesCAKXSY1K 8January  11, 2013

Yale University scientists have found a way to observe quantum information while preserving its integrity, an achievement that offers researchers greater control in the volatile realm of quantum mechanics and greatly improves the prospects of quantum computing.

Quantum computers would be exponentially faster than the most powerful computers of today.

“Our experiment is a dress rehearsal for a type of process essential for quantum computing,” said Michel Devoret, the Frederick William Beinecke Professor of Applied Physics & Physics at Yale and principal investigator of research published Jan. 11 in the journal Science. “What this experiment really allows is an active understanding of quantum mechanics. It’s one thing to stare at a theoretical formula and it’s another thing to be able to control a real quantum object.”

In quantum systems, microscopic units called qubits represent information. Qubits can assume either of two states — “0” or “1” — or both simultaneously. Correctly recognizing, interpreting, and tracking their state is necessary for quantum computing. However, the act of monitoring them usually damages their information content.

The Yale physicists successfully devised a new, non-destructive measurement system for observing, tracking and documenting all changes in a qubit’s state, thus preserving the qubit’s informational value. In principle, the scientists said, this should allow them to monitor the qubit’s state in order to correct for random errors.

“As long as you know what error process has occurred, you can correct,” Devoret said. “And then everything’s fine. You can basically undo the errors.”

qubits_0An innovation by Yale University physicists offers scientists greater control in the volatile realm of quantum mechanics and greatly improves the prospects of quantum computing. Quantum computers would be exponentially faster than the most powerful computers oftoday.

“That’s the key,” said Michael Hatridge, a postdoctoral associate in physics at Yale and lead author of the Science paper, “the ability to talk to the qubit and hear what it’s telling you.”

 

He continued: “A major problem with quantum computing is the finite lifetime of information stored in the qubits, which steadily decays and which must be corrected. We now know that it is possible to do this correction by feedback involving a continuous measurement. Our work advances the prospects of large-scale quantum computers by opening the door to continuous measurement-based quantum feedback.”

The Yale physicists successfully measured one qubit. The challenge ahead is to measure and control many at once, and the team is developing ultra-fast digital electronics for this purpose.

“We are on the threshold between the ability to measure and control one or two qubits, and many,” Hatridge said.

Other authors of the paper are S. Shankar, M. Mirrahimi, F. Schackert, K. Geerlings, T. Brecht, K.M. Sliwa, B. Abdo, L. Frunzio, S.M. Girvin, and R.J. Schoelkopf.

Support for the research was provided by the National Science Foundation, the United States Army Research Office, the Intelligence Advanced Research Projects Activity, the Agence National de Recherche, and the College de France.

Topological Superconductors: NIST and the U of Maryland


Seeking a robust home for qubits | October 8th, 2012

If quantum computers are ever going to perform all those expected feats of code-breaking and number crunching, then their component qubits—tiny ephemeral quantum cells held in a superposition of internal states—will have to be protected from intervention by the outside world. In other words, decoherence, the loss of the qubits’ quantum integrity, has to be postponed. Now theoretical physicists at the Joint Quantum Institute (JQI) and the University of Maryland have done an important step forward to understand qubits in a real-world setup. In a new study they show, for the first time, that qubits can successfully exist in a so called topological superconductor material even in the presence of impurities in the material and strong interactions among participating electrons.

To see how qubits can enter into their special coherence-protection program, courtesy of “Majorana particles,” an exotic form of excitation, some groundwork has to be laid.

Quantum Materials

topsup-01Figure 1
Credit: Emily Edwards
Click for Hi-Res

Most designs for qubits involve materials where quantum effects are important. In one such material, superconductors (SC), electrons pair up and can then enter into a large ensemble, a supercurrent, which flows through the material without suffering energy loss. Another material is a sandwich of semiconductors which support the quantum Hall effect (QHE). Here, very low temperatures and a powerful external magnetic field force electrons in a thin boundary layer to execute tiny cyclone motions (not exactly, but ok—also isn’t a cyclone a storm?). At the edge of these layers, the electrons, unable to trace out a complete circular path, will creep along the edge, where they constitute a net electrical current.

 

One of the most interesting and useful facts about these electrons at the edge is that they move in one direction. They cannot scatter backwards no matter how many impurities (which in ordinary conductors can lead to energy dissipation) may be in the material. If, furthermore, the electrons can be oriented according to their spin—their intrinsic angular momentum—then we get what is called the quantum spin Hall effect (QSH). In this case all electrons with spin up will circulate around the material (at the edge) in one direction, while electrons with spin down will circulate around in the opposite direction.

The QHE state is depicted in figure 1.

Topological Materials

 

topsup-02Figure 2
Credit: Emily Edwards
Click for Hi-Res

In some materials the underlying magnetism of the nuclei in the atoms making of the material is so strong than no external magnet is needed to create the Hall effects. Mercury-cadmium-telluride compounds are examples of materials called topological insulators. Insulators (not sure how this sentence was supposed to start, but grammatically is currently confusing) because even as electrons move around the edge of the material with very little loss of energy, the interior of these 3-dimensional structures is an insulator; no current flows. The “topological” is a bit harder to explain. Partly the flow of current on the outside bespeaks of geometry: the electrons flow only at the edge and are unable (owing to quantum interactions) from scattering backwards if they meet an impediment.

 

But topology in this case has more to do with the way in which the motion of the electrons in these materials are described in terms of “dispersion relations.” Just as waves of white light will be dispersed into a spectrum of colors when the waves strike the oblique side of a prism, so electron waves (electrons considered as quantum waves) will be “dispersed,” in the sense that electrons with the same energy might have different momenta, depending on how the electrons move through the material in question.

The idea of electron dispersal is often depicted in the form of an energy-level diagram. In insulators (the left panel of Figure 2) electrons remain in a valence band; they don’t have enough energy to visit the conduction band of energies; hence the electrons do not move; the material is an insulator against electricity. In a conductor (middle part) the conduction and valence bands overlap. In the QHE (right panel) electrons in the interior of the material also do not move along; the bulk of the material is an insulator. But for electrons at the edge there is a chance for movement into the conduction band.

Now for the topology: just as a coffee cup is equivalent to a donut topologically—either can be transformed into the other by stretching but not by any tearing—so here the valence band can be transformed into a conduction band (at least for edge states) no matter what impurities might be present in the underlying material. In other words, the “topological” nature of the material offers some protection for the flow of electrons against the otherwise-dissipating effects of impurities.

The marvelous properties of superconductors and topological materials can be combined. If a one-dimensional topological specimen—a nanowire made from indium and arsenic—is draped across a superconductor (niobium, say) then the superconductivity can extend into the wire (proximity effect). And in this conjunction of materials, still another hotly-pursued effect can come into play.

Majorana Particles

 

topsup-03Figure 3
The orange balls (the letter gamma) mark the two places, at either end of the nanowire, where the Majorana excitations appear. The wire sits atop the superconductor. (Credit: Alejandro Lobos)
Click for Hi-Res

One last concept is needed here—Majorana particles—named for the Italian physicist Ettore Majorana, who predicted in 1937 the existence of a class of particle that would serve as its own antiparticle. Probably this object would not exist usefully in the form of a single real particle but would, rather, appear in a material as a quasiparticle, an ensemble excitation of many electrons.

 

Some scientists believe that qubits made from Majorana pulses excited in topological materials (and benefitting from the same sort of topological protection that benefits, say, electrons in QHE materials) would be much more immune from decoherence than other qubits based on conventional particles.

Specifically Sankar Das Sarma and his colleagues at the University of Maryland (JQI and the Condensed Matter Theory Center) predicted that Majorana particles would appear in topological quantum nanowires. In fact part of the Majorana excitation would appear at both ends of the wire. These predictions were borne out. It is precisely the separation of these two parts (each of which constitutes a sort of “half electron”) that confers some of the anticipated coherence-protection: a qubit made of that Majorana excitation would not be disrupted by merely a local irregularity in the wire.

A recent experiment in Holland provides preliminary evidence for exactly this occurrence (***).

Robust Qubits Amid Disorder

 

topsup-04Figure 4
Here is a picture of the prospective experimental setup in which Majorana particles could be made in a hybrid superconductor-nanowire material. In (a) the topological semiconductor bridges a gap between two parts of a superconductor. The letter phi represents an external magnetic field which can tailor conditions in the semiconductor. In (b) the immediate nanowire (gray rod) environment is shown. L is the size of the gap between the superconductor halves while L1 is the distance over which the underlying superconductivity will persist within the overlying semiconductor. (c) shows how the superconductor (SC)-semiconductor (SM) sandwich can be further tuned by a nearby electrical circuit. (Credit: Lutchyn, Sau and Das Sarma)
Click for Hi-Res

One of the authors of the new study, Alejandro Lobos, said that the earlier Maryland prediction, useful as it was, was still somewhat idealistic in that it didn’t fully grapple with the presence of impurities, a fact of life which all engineers of actual computers must confront. This is what the new paper, which appears in the journal Physical Review Letters, addresses.

 

The problem of impurities or defects (which flowing electrons encounter as a form of disorder) is especially important for components which are two or even one dimensional in nature. The same is true for the repulsive force among electrons. “In 3-dimensional materials,” said Lobos, “electrons (and their screening clouds of surrounding holes) can avoid each other thanks to the availability of space. They can just go around each other. In 1-D materials, this is not possible, since electrons cannot pass each other. In 1D, if one electron wants to move, it has to move all the other electrons! This ensures that excitations in a 1D metal are necessarily collective, as opposed to the single-particle excitations existing in a 3D metal.

So, in summary, the new Maryland work shows that disorder and electron interactions, two things that normally work to disrupt superconductivity, can be overcome with careful engineering of the material. “A number of important theoretical studies before ours have focused on the destabilizing effects of either disorder or interaction on topological superconductors,” said Lobos. “These studies showed the extent to which a topological superconductor could survive under these effects separately. But to make contact with real materials, disorder and interactions have to be considered on equal footing and simultaneously, a particular requirement imposed by the one-dimensional geometry of the system. It was then an important question to determine if it was possible to stabilize a topological superconductor under their simultaneous presence. The good news is that the answer is yes: despite their detrimental effect, there is still a sizable range of parameters where topological superconductors hosting Majorana excitations can exist. That’s the main result of our study, which will be useful to understand and characterize topological superconductors in more realistic situations.”

 

(*) The Joint Quantum Institute is operated jointly by the National Institute of Standards and Technology in Gaithersburg, MD and the University of Maryland in College Park.

(**) “Interplay of disorder and interaction in Majorana quantum wires,” Alejandro M. Lobos, Roman M. Lutchyn, and S. Das Sarma, Physical Review Letters, 5 October 2012

(***) See earlier Majorana JQI press release and several pertinent research papers

Alejandro M. Lobos, (301)405-0603, alobos@umd.edu

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