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 >>>

A recipe for making perfectly ordered quantum dots


QDOTS imagesCAKXSY1K 8Researchers from the Johannes Kepler University in Linz, Austria, have shown that they can grow uniformly sized quantum dots on pit-patterned substrates with inter-dot distances varying from a few hundred nanometres to several microns on one and the same sample. Being able to accurately control inter-dot distances in this way will be useful for making a variety of electronic and optoelectronic devices with novel functionalities, such as single photon emitters.

MGrydlik1

Quantum dots in pits

The researchers began by defining the most important parameters for molecular beam epitaxy growth of strictly ordered germanium dots on pit-patterned silicon substrates. They then showed that these growth parameters are closely linked and need to be adjusted with respect to each other for optimal growth. Indeed, the initial pit shape and size, as well the growth conditions of the Si buffer layer, have to be adjusted to provide suitable preconditions – or a solid foundation – for the growth of Ge quantum dots with the desired size, composition and nucleation position.

Ordered QDs

The team also showed that the two-dimensional Ge wetting layer between pits can act as a stabilizer that prevents the dots from changing shape and inhibits the formation of dislocations in ordered dots. These findings allow perfectly ordered and homogeneous Ge dots to be fabricated on one and the same sample, even if the pit-period is varied from a few hundred nanometres to several microns.

Finally, by showing that the growth of InAs dots on GaAs substrates can also be controlled in this way, the researchers say that many aspects described in their work might be of great use when growing ordered epitaxial quantum dots made from other materials, such as different group III-V semiconductors.

More details of the work can be found in the journal Nanotechnology.

About the author

The studies were carried out at the Institute of Semiconductor and Solid State Physics at the Johannes Kepler Universtiy, Linz (JKU). Martyna Grydlik is currently a post-doc at the Leibniz Institute for Solid State and Materials Research, Dresden (IFW). Gregor Langer is working as a research scientist at Recendt, the Research Center for Nondestructive Testing GmbH. Thomas Fromherz is group leader and Friedrich Schäffler is a full professor at the Institute of Semiconductor and Solid State Physics at the JKU, Linz. Moritz Brehm is currently Erwin Schrödinger fellow of the Austrian science funds FWF, working at the IFW in Dresden. Martyna Grydlik and Moritz Brehm contributed equally to the work by designing and fabricating the samples, carrying out the experiments, the statistical analysis and writing the manuscript. Gregor Langer fabricated part of the substrate templates and contributed to their design.

NREL and Partners Demonstrate Quantum Dots that Assemble Themselves


Surprising breakthrough could bolster quantum photonics, solar cell efficiency

February 8, 2013

QDOTS imagesCAKXSY1K 8Scientists from the U.S. Department of Energy’s National Renewable Energy Laboratory and other labs have demonstrated a process whereby quantum dots can self-assemble at optimal locations in nanowires, a breakthrough that could improve solar cells, quantum computing, and lighting devices.

 

A paper on the new technology, “Self-assembled Quantum Dots in a Nanowire System for Quantum Photonics,” appears in the current issue of the scientific journal Nature Materials.

Quantum dots are tiny crystals of semiconductor a few billionths of a meter in diameter.  At that size they exhibit beneficial behaviors of quantum physics such as forming electron-hole pairs and harvesting excess energy.

The scientists demonstrated how quantum dots can self-assemble at the apex of the gallium arsenide/aluminum gallium arsenide core/shell nanowire interface. Crucially, the quantum dots, besides being highly stable, can be positioned precisely relative to the nanowire’s center. That precision, combined with the materials’ ability to provide quantum confinement for both the electrons and the holes, makes the approach a potential game-changer.

Electrons and holes typically locate in the lowest energy position within the confines of high-energy materials in the nanostructures. But in the new demonstration, the electron and hole, overlapping in a near-ideal way, are confined in the quantum dot itself at high energy rather than located at the lowest energy states. In this case, that’s the gallium-arsenide core. It’s like hitting the bulls-eye rather than the periphery.

The quantum dots, as a result, are very bright, spectrally narrow and highly anti-bunched, displaying excellent optical properties even when they are located just a few nanometers from the surface – a feature that even surprised the scientists.

“Some Swiss scientists announced that they had achieved this, but scientists at the conference had a hard time believing it,” said NREL senior scientist Jun-Wei Luo, one of the co-authors of the study. Luo got to work constructing a quantum-dot-in-nanowire system using NREL’s supercomputer and was able to demonstrate that despite the fact that the overall band edges are formed by the gallium Arsenide core, the thin aluminum-rich barriers provide quantum confinement both for the electrons and the holes inside the aluminum-poor quantum dot. That explains the origin of the highly unusual optical transitions.

Several practical applications are possible. The fact that stable quantum dots can be placed very close to the surface of the nanometers raises a huge potential for their use in detecting local electric and magnetic fields. The quantum dots also could be used to charge converters for better light-harvesting, as in the case of photovoltaic cells.

The team of scientists working on the project came from universities and laboratories in Sweden, Switzerland, Spain, and the United States.

NREL is the U.S. Department of Energy’s primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for DOE by the Alliance for Sustainable Energy, LLC.

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Visit NREL online at www.nrel.gov

Sharp Develops Solar Cell With World’s Highest Conversion Efficiency Of 37.7%


QDOTS imagesCAKXSY1K 8December 5, 2012

121205_01Sets a New Record with Triple-Junction Compound Solar Cell

 

Sharp Corporation has achieved the world’s highest solar cell conversion efficiency*1 of 37.7%*2 using a triple-junction compound solar cell in which three photo-absorption layers are stacked together.

Sharp achieved this latest breakthrough as a result of a research and development initiative promoted by Japan’s New Energy and Industrial Technology Development Organization (NEDO) *3 on the theme of “R&D on Innovative Solar Cells.” Measurement of the value of 37.7%, which sets a new record for the world’s highest conversion efficiency, was confirmed at the National Institute of Advanced Industrial Science and Technology (AIST).

Compound solar cells utilize photo-absorption layers made from compounds consisting of two or more elements, such as indium and gallium. The basic structure of this latest triple-junction compound solar cell uses proprietary Sharp technology that enables efficient stacking of the three photo-absorption layers, with InGaAs (indium gallium arsenide) as the bottom layer.

To achieve this latest increase in conversion efficiency, Sharp capitalized on the ability of the new cell to efficiently absorb light from different wavelengths in sunlight and convert it into electricity. Sharp also increased the active area*4 for converting light into electricity through optimal processing of the cell edges. These improvements led to higher maximum output levels for the solar cell and enabled Sharp to achieve a solar cell conversion efficiency of 37.7%—the highest in the world.

Sharp’s aim for the future is to apply this latest development success to concentrator photovoltaic power systems that use lenses to collect and convert sunlight into electricity. The company also foresees numerous other practical applications for the cells, such as on space satellites and vehicles.

Structure of Triple-Junction Compound Solar Cell

  • InGaP: Indium Gallium Phosphide
  • GaAs: Gallium Arsenide
  • InGaAs: Indium Gallium Arsenide
  • Tunnel junction: Semiconductor junction where electricity flows as if through metal

*1 As of December 5, 2012, for non-concentrator solar cells at the research level (based on a survey by Sharp). *2 Conversion efficiency confirmed by the National Institute of Advanced Industrial Science and Technology (AIST; one of several organizations around the world that officially certifies energy conversion efficiency measurements in solar cells) in September 2012. (Cell surface: approx. 1 cm2) *3 NEDO is one of Japan’s largest public management organizations for promoting research and development as well as for disseminating industrial, energy, and environmental technologies. *4 The ratio of the effective light-reception area to the total surface area of the cell.

                    

History of Sharp Compound Solar Cell Development

1967 – Development begins of solar cells for space applications using single-crystal silicon 1976 – Launch of operational Japanese satellite, “Ume,” equipped with Sharp solar cells for space applications (single-crystal silicon solar cell) 2000 – Research and development begin on triple-junction compound solar cells to further improve efficiency, reduce weight, and increase durability of solar cells for space applications 2001 – Participation in research and development on NEDO’s photovoltaic power generation themes 2002 Triple-junction compound solar cell gains certification from the Japan Aerospace Exploration Agency (JAXA) 2003 – Conversion efficiency of 31.5% achieved (at the research level) for a triple-junction compound solar cell 2004 – Launch of small scientific satellite, “Reimei,” equipped with Sharp triple-junction compound solar cells 2007 – Conversion efficiency of 40.0% achieved (at the research level) for a triple-junction compound solar cell (concentrator type, at 1,100 times concentrated sunlight) 2009 – Launch of Greenhouse gases Observing SATellite (GOSAT), “Ibuki”, equipped with Sharp triple-junction compound solar cells 2009 – Conversion efficiency of 35.8% achieved (at the research level) for a triple-junction compound solar cell*5 2011 – Conversion efficiency of 36.9% achieved (at the research level) for a triple-junction compound solar cell*5 2012 – Conversion efficiency of 43.5% achieved (at the research level) for a concentrator triple-junction compound solar cell*5(concentrator type, at 360 times concentrated sunlight) Conversion efficiency of 37.7% achieved (at the research level) for a triple-junction compound solar cell*5

*5 Based on research and development efforts that are part of NEDO’s “R&D on Innovative Solar Cells” project.

SOURCE: Sharp Corporation

Graphene Replaces Traditional Silicon Substrates in Future Devices


Researchers at the Norwegian University of Science and Technology (NTNU) have patented and are commercializing a method by which gallium arsenide (GaAs) nanowires are grown on graphene.

The method, which was described and published in the journal Nano Letters (“Vertically Aligned GaAs Nanowires on Graphite and Few-Layer Graphene: Generic Model and Epitaxial Growth”), employs Molecular Beam Epitaxy (MBE) to grow the GaAs nanowires layer by layer. A video describing the process can be seen below.

 

“We do not see this as a new product,” says Professor Helge Weman, a professor at NTNU’s Department of Electronics and Telecommunications in the press release. “This is a template for a new production method for semiconductor devices. We expect solar cells and light emitting diodes to be first in line when future applications are planned.”

Whether it is a method or a product, Weman and his colleagues have launched a new company called Crayonano that will be commercializing the hybrid material that the researchers developed.

The researchers contend that replacing traditional semiconductor materials as a substrate will reduce material costs. The silicon materials are fairly expensive and are usually over 500µm thick for 100mm wafers. As the video explains, using graphene reduces the substrate thickness to the width of one atom. Obviously reduction in material is really only a side benefit to the use of graphene. The real advantage is that the electrode is transparent and flexible, thus its targeting for solar cells and LEDs.

Interestingly Weman sees his team’s work as a compliment to the work of companies like IBM that have used graphene “to make integrated circuits on 200-mm wafers coated with a continuous layer of the atom-thick material.”

Weman notes: “Companies like IBM and Samsung are driving this development in the search for a replacement for silicon in electronics as well as for new applications, such as flexible touch screens for mobile phones. Well, they need not wait any more. Our invention fits perfectly with the production machinery they already have. We make it easy for them to upgrade consumer electronics to a level where design has no limits.”

As magnanimous as Weman’s invitation sounds, one can’t help but think it comes from concern. The prospect of a five-year-development period before a product gets to market might be somewhat worrying for a group of scientists who just launched a new startup. A nice licensing agreement from one of the big electronics companies must look appealing right about now.