CAPTION This is an eectron wave in a phosphorus atom, distorted by a local electric field. CREDIT Dr. Arne Laucht

Breakthrough by Australian-led team should make the construction of large-scale quantum computers more affordable

A UNSW-led research team has encoded quantum information in silicon using simple electrical pulses for the first time, bringing the construction of affordable large-scale quantum computers one step closer to reality. 

Lead researcher, UNSW Associate Professor Andrea Morello from the School of Electrical Engineering and Telecommunications, said his team had successfully realised a new control method for future quantum computers. 

The findings were published today in the open-access journal Science Advances.

Unlike conventional computers that store data on transistors and hard drives, quantum computers encode data in the quantum states of microscopic objects called qubits. 

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

The team has already improved the control of these qubits to an accuracy of above 99% and established the world record for how long quantum information can be stored in the solid state, as published in Nature Nanotechnology in 2014.

It has now demonstrated a key step that had remained elusive since 1998. 

"We demonstrated that a highly coherent qubit, like the spin of a single phosphorus atom in isotopically enriched silicon, can be controlled using electric fields, instead of using pulses of oscillating magnetic fields," explained UNSW's Dr Arne Laucht, post-doctoral researcher and lead author of the study. 

Associate Professor Morello said the method works by distorting the shape of the electron cloud attached to the atom, using a very localized electric field. 

"This distortion at the atomic level has the effect of modifying the frequency at which the electron responds.

"Therefore, we can selectively choose which qubit to operate. It's a bit like selecting which radio station we tune to, by turning a simple knob. Here, the 'knob' is the voltage applied to a small electrode placed above the atom."

The findings suggest that it would be possible to locally control individual qubits with electric fields in a large-scale quantum computer using only inexpensive voltage generators, rather than the expensive high-frequency microwave sources. 

Moreover, this specific type of quantum bit can be manufactured using a similar technology to that employed for the production of everyday computers, drastically reducing the time and cost of development. 

The device used in this experiment was fabricated at the NSW node of the Australian National Fabrication Facility, in collaboration with the group led by UNSW Scientia Professor Andrew Dzurak.

Key to the success of this electrical control method is the placement of the qubits 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," Associate Professor Morello said. 

The purified silicon was provided through collaboration with Professor Kohei Itoh from Keio University in Japan.

Artist’s concept of software system components dynamically adapting to resource changes within an operational IT ecosystem.

Program aims to generate applications capable of adapting to change, without extensive reprogramming  

As modern software systems continue inexorably to increase in complexity and capability, users have become accustomed to periodic cycles of updating and upgrading to avoid obsolescence—if at some cost in terms of frustration. In the case of the U.S. military, having access to well-functioning software systems and underlying content is critical to national security, but updates are no less problematic than among civilian users and often demand considerable time and expense. That is why today DARPA announced it will launch an ambitious four-year research project to investigate the fundamental computational and algorithmic requirements necessary for software systems and data to remain robust and functional in excess of 100 years.

The Building Resource Adaptive Software Systems, or BRASS, program seeks to realize foundational advances in the design and implementation of long-lived software systems that can dynamically adapt to changes in the resources they depend upon and environments in which they operate. Such advances will necessitate the development of new linguistic abstractions, formal methods, and resource-aware program analyses to discover and specify program transformations, as well as systems designed to monitor changes in the surrounding digital ecosystem. The program is expected to lead to significant improvements in software resilience, reliability and maintainability.

“Technology inevitably evolves, but very often corresponding changes in libraries, data formats, protocols, input characteristics and models of components in a software ecosystem undermine the behavior of applications,” said Suresh Jagannathan, DARPA program manager. “The inability to seamlessly adapt to new operating conditions undermines productivity, hampers the development of cyber-secure infrastructure and raises the long-term risk that access to important digital content will be lost as the software that generates and interprets content becomes outdated.” 

Current applications execute on a software stack comprising many different layers of abstraction, providing various services and structures. Access to these layers is mediated through different kinds of interfaces, all typically specified as secondary documentation supplied along with the application program interface. Because this documentation is typically defined informally, it provides only a partial, incomplete understanding of the system as a whole and requires substantial manual effort and reasoning.

“Ensuring applications continue to function correctly and efficiently in the face of a changing operational environment is a formidable challenge,” said Jagannathan. “Failure to respond to these changes can result in technically inferior and potentially vulnerable systems. Equally concerning, the lack of automated upgrade mechanisms to restructure and transform applications leads to high software maintenance costs and premature obsolescence of otherwise functionally sound software.”

The premise on which BRASS operates is that an entirely new clean-slate approach to software design, composition and adaptation is required. This approach aims to enable the expression and discovery of new kinds of specifications, program analyses and formal methods that precisely capture the relationship between computations and the resources they use, and algorithmic transformations that enable applications to adapt to changes without the need for extensive programmer involvement.

According to Jagannathan, BRASS could lead to the construction of families of programs all generally preserving high-level functionality but with different implementations that are optimized for different sets of resources and expose opportunities for cost reduction.

For more information about BRASS, please refer to the Broad Agency Announcement at:http://1.usa.gov/1almvsw.

Physicists at the Universities of Bonn and Cambridge have succeeded in linking two completely different quantum systems to one another. In doing so, they have taken an important step forward on the way to a quantum supercomputer. To accomplish their feat the researchers used a method that seems to function as well in the quantum world as it does for us people: teamwork. The results have now been published in the "Physical Review Letters".

When facing big challenges, it is best to work together. In a team, the individual members can contribute their individual strengths – to the benefit of all those involved. One may be an absent-minded scientist who has brilliant ideas, but quickly forgets them. He needs the help of his conscientious colleague, who writes everything down, in order to remind the scatterbrain about it later. It's very similar in the world of quanta. There the so-called quantum dots (abbreviated: qDots) play the role of the forgetful genius. Quantum dots are unbeatably fast, when it comes to disseminating quantum information. Unfortunately, they forget the result of the calculation just as quickly – too quickly to be of any real use in a quantum supercomputer.

In contrast, charged atoms, called ions, have an excellent memory: They can store quantum information for many minutes. In the quantum world, that is an eternity. They are less well suited for fast calculations, however, because the internal processes are comparatively slow. The physicists from Bonn and Cambridge have therefore obliged both of these components, qDots and ions, to work together as a team. Experts speak of a hybrid system, because it combines two completely different quantum systems with one another.

Absent-minded qDots

qDots are considered the great hopes in the development of quantum supercomputers. In principle, they are extremely miniaturized electron storage units. qDots can be produced using the same techniques as normal supercomputer chips. To do so, it is only necessary to miniaturize the structures on the chips until they hold just one single electron (in a conventional PC it is 10 to 100 electrons).

The electron stored in a qDot can take on states that are predicted by quantum theory. However, they are very short-lived: They decay within a few picoseconds (for illustration: in one picosecond, light travels a distance of just 0.3 millimeters). This decay produces a small flash of light: a photon. Photons are wave packets that vibrate in a specific plane – the direction of polarization. The state of the qDots determines the direction of polarization of the photon. "We used the photon to excite an ion", explains Prof. Dr. Michael Köhl from the Institute of Physics at the University of Bonn. "Then we stored the direction of polarization of the photon".

Conscientious ions

To do so, the researchers connected a thin glass fiber to the qDot. They transported the photon via the fiber to the ion many meters away. The fiberoptic networks used in telecommunications operate very similarly. To make the transfer of information as efficient as possible, they had trapped the ion between two mirrors. The mirrors bounced the photon back and forth like a ping pong ball, until it was absorbed by the ion. "By shooting it with a laser beam, we were able to read out the ion that was excited in this way", explains Prof. Köhl. "In the process, we were able to measure the direction of polarization of the previously absorbed photon". In a sense then, the state of the qDot can be preserved in the ion – theoretically this can be done for many minutes.

This success is an important step on the still long and rocky road to a quantum supercomputer. In the long term, researchers around the world are hoping for true marvels from this new type of supercomputer: Certain tasks, such as the factoring of large numbers, should be child's play for such a supercomputer. In contrast, conventional supercomputers find this a really tough nut to crack. However, a quantum supercomputer displays its talents only for such special tasks: For normal types of basic computations, it is pitifully slow.

The researchers fabricated the spintronics devices at the Nano fabrication laboratory at Chalmers University of Technology. From left: Saroj Prasad Dash, Venkata Kamalakar Mutta and André Dankert.

Researchers at Chalmers University of Technology have discovered that large area graphene is able to preserve electron spin over an extended period, and communicate it over greater distances than had previously been known. This has opened the door for the development of spintronics, with an aim to manufacturing faster and more energy-efficient memory and processors in computers. The findings are published in the journal Nature Communications.

“We believe that these results will attract a lot of attention in the research community and put graphene on the map for applications in spintronic components,” says Saroj Dash, who leads the research group at Chalmers University of Technology.

Spintronics is based on the quantum state of the electrons, and the technology is already being used in advanced hard drives for data storage and magnetic random accesses memory. But here the spin-based information only needs to move a few nanometers, or millionths of a millimetre. Which is lucky, because spin is a property in electrons that in most materials is extremely short-lived and fragile.

However, there are major advantages in exploiting spin as an information carrier, instead of, or in addition to electric charges. Spintronics could make processors significantly faster and less energy consuming than they are today.

Graphene is a promising candidate for extending the use of spintronics in the electronics industry. The thin carbon film is not only an excellent electrical conductor, but also theoretically has the rare ability to maintain the electrons with the spin intact.

“In future spin-based components, it is expected that the electrons must be able to travel several tens of micrometers with their spins kept aligned. Metals, such as aluminium or copper, do not have the capacity to handle this. Graphene appears to be the only possible material at the moment,” says Saroj Dash.

Today, graphene is produced commercially by a few companies using a number of different methods, all of which are in an early phase of development.

Put simply, you could say that high-quality graphene can only be obtained in very small pieces, while larger graphene is produced in a way that the quality is either too low or has other drawbacks from the perspective of the electronics industry.

But that general assumption is now being seriously questioned by the findings presented by the research group at Chalmers. They have conducted their experiments using CVD graphene, which is produced through chemical vapour deposition. The method gives the graphene a lot of wrinkles, roughness and other defects.

But it also has advantages: There are good prospects for the production of large area graphene on an industrial scale. The CVD graphene can also be easily removed from the copper foil on which it grows and is lifted onto a silicon wafer, which is the semiconductor industry's standard material.

Although the quality of the material is far from perfect, the research group can now show parameters of spin that are up to six times higher than those previously reported for CVD graphene on a similar substrate.

“Our measurements show that the spin signal is preserved in graphene channels that are up to 16 micrometers long. The duration over which the spins stay aligned has been measured to be over a nanosecond,” says Chalmers researcher Venkata Kamalakar who is the article's first author.

“This is promising because it suggests that the spin parameters can be further improved as we develop the method of manufacturing.

That researchers are focusing on how far the spin current can be communicated should not be thought of as just being about sending information in a new material or replacing metals or semiconductors with graphene. The goal instead is a completely new way of performing logical operations and storing information. A concept that, if successful, would take digital technology a step beyond the current dependence on semiconductors.

“Graphene is a good conductor and has no band gaps. But in spintronics there is no need for band gaps to switch between on and off, one and zero. This is controlled instead by the electron's up or down spin orientations,” Saroj Dash explains.

A short-term goal now is to construct a logical component that, not unlike a transistor, is made up of graphene and magnetic materials.

Whether spintronics can eventually fully replace semiconductor technology is an open question, a lot of research remains. But graphene, with its excellent spin conduction abilities, is highly likely to feature in this context.

Caption: The researchers fabricated the spintronics devices at the Nano fabrication laboratory at Chalmers University of Technology. From left: Saroj Prasad Dash, Venkata Kamalakar Mutta and André Dankert. Photo: Oscar Mattsson

Facts/This is spin
Spin is a quantum mechanical property of elementary particles, which among other things gives rise to the phenomenon of magnetism. The spin can be directed either up or down. For the electrons in a normal electric current, the spin is randomly distributed, and the stream carries no spin signal. But with the help of magnets, electrons that are fed into a conductor can be polarised, which means they all have their spin directed up or down. You could liken the electrons to a series of small compass needles, all pointing towards north or south. 
The challenge is to maintain this state long enough and over sufficiently long distances.

Facts/Why spin works in graphene
The spin of electrons can easily be disturbed by environmental factors. Atoms and their crystal structures in the conductive material have an electric field, which is perceived as a magnetic field by the electrons rushing by. But as carbon is such a light atom with only six protons arranged in a symmetrical hexagonal structure, this magnetic interference will be very limited.
The internal spin in an atomic nucleus is also a potential source of interference. But the net spin from the nucleus is negligible, as the majority of the carbon atoms are of the C12 isotope, with as many neutrons as protons.

Facts/Three ways of producing graphene

  • The Nobel Laureates Geim and Novoselov manufactured graphene from graphite using ordinary household tape. Similar methods are used today to produce high quality graphene. But the pieces are small.
  • The Graphensic company, created by researchers at Swedish Linköping University, manufactures large area graphene that is “cultivated” from a substrate of silicon carbide.
  • At Chalmers University of Technology, large area graphene is produced using the chemical vapour deposition method (CVD). For the study in Nature Communications, the researchers have used CVD graphene purchased from the company Graphenea in Spain.


Facts about the research
The research group at Chalmers University of Technology consists of Saroj Dash, who heads the group, Venkata Kamalakar, who is the article's first author, along with André Dankert and Christiaan Groenveld. The research has been funded by Chalmers' Area of Advance Nanoscience and Nanotechnology.
The scientific article is published at:
http://www.nature.com/naturecommunications

The team demonstrated a quantum on/off switching time of about a millionth of a millionth of a second - the fastest-ever quantum switch to be achieved with silicon and over a thousand times faster than previous attempts.

"Quantum computing exploits the fact that, according to quantum mechanics atoms can exist in two states at once, being both excited and unexcited at the same time. This is known as a superposition state, and is most famously illustrated by Schrödinger's quantum cat which is simultaneously dead and alive" said Dr. Ellis Bowyer, one of the Surrey researchers who made the laser measurements, He added "This superposition of orbital states is very delicate, but we discovered that silicon provides an amazingly clean environment for the phosphorus atoms trapped inside where our quantum information is being stored. We put the atoms into a superposition state with a very short (a few trillionths of seconds) laser pulse from the FELIX laser facility, and then, we showed we can create a new superposition which depends on the exact time at which a second laser pulse arrives. We found that the superposition state even survives when electrons are flying around the trapped atom while current was flowing through the chip, and even more strangely, the current itself depends on the superposition state".

The team has recently been awarded further funding from the UK EPSRC (Engineering and Physical Sciences Research Council) to investigate how to connect many of these quantum objects to each other, creating the bigger building blocks needed for quantum computers. This next phase of research could enable the creation of fast quantum silicon chips, and other kinds of devices such as super-accurate clocks and ultra-sensitive bio-medical sensors.

"Quantum superpositions and the resulting quantum technologies are only just beginning to make an impact, but we believe that with new advances in silicon, it is only a matter of time before it becomes more part of the everyday. This work brings that time closer by showing that exotic quantum features, more usually demonstrated with unimaginably tiny things in university physics labs can also be seen using an ordinary voltmeter," said Dr Thornton Greenland of UCL. "What is exciting is that we can see these exotic quantum phenomena in that most common material, silicon, using a measurement as simple as that of the electrical resistance" Thus the time is drawing nearer when we'll be able to take advantage of make a computer that does a tremendous number of calculations simultaneously, and that provides unprecedentedly secure computing, impenetrable to hackers." 

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