Daily Archives: June 1, 2017

May 31, 2017 by Steve Tally Purdue University and Microsoft Corp. have signed a five-year agreement to develop a useable quantum computer. Purdue is one of four international universities in the collaboration.Michael Manfra, Purdue University's Bill and Dee O'Brien Chair Professor of Physics and Astronomy, professor of materials engineering and professor of electrical and computer engineering, will lead the effort at Purdue to build a robust and scalable quantum computer by producing what scientists call a "topological qubit." Credit: Purdue University photo/Rebecca Wilcox

"If this project is successful it will cause a revolution in computing."

That's the forecast of Michael Manfra, Purdue University's Bill and Dee O'Brien Chair Professor of Physics and Astronomy, Professor of Materials Engineering and Professor of Electrical and Computer Engineering, on a new long-term enhanced collaboration between Purdue and Microsoft Corp. to build a robust and scalable quantum computer by producing what scientists call a "topological qubit."

Purdue President Mitch Daniels noted that Purdue was home to the first computer science department in the United States, and says this partnership and Manfra's work places the university at the forefront of quantum computing.

"Someday quantum computing will move from the laboratory to actual daily use, and when it does, it will signal another explosion of computing power like that brought about by the silicon chip," Daniels says. "It's thrilling to imagine Purdue at the center of this next leap forward."

In the computers that we currently use every day, information is encoded in an either/or binary system of bits, what are commonly thought of as 1s and 0s. These computers are based on silicon transistors, which, like a light switch, can only be in either an on or off position.

With quantum computers, information is encoded in qubits, which are quantum units of information. With a qubit, however, this physical state isn't just 0 or 1, but can also be a linear combination of 0 and 1. Because of a strange phenomenon of quantum mechanics called "superposition," a qubit can be in both states at the same time.

This characteristic is essential to quantum computation's potential power, allowing for solutions to problems that are intractable using classical architectures.

Advocates of quantum computing believe this never-before-seen technology will create a new global "quantum economy."

The team assembled by Microsoft will work on a type of quantum computer that is expected to be especially robust against interference from its surroundings, a situation known in quantum computing as "decoherence." The "scalable topological quantum computer" is theoretically more stable and less error-prone.

"One of the challenges in quantum computing is that the qubits interact with their environment and lose their quantum information before computations can be completed," Manfra says. "Topological quantum computing utilizes qubits that store information "non-locally" and the outside noise sources have less effect on the qubit, so we expect it to be more robust."

Manfra says that the most exciting challenge associated with building a topological quantum computer is that the Microsoft team must simultaneously solve problems of materials science, condensed matter physics, electrical engineering and computer architecture.

"This is why Microsoft has assembled such a diverse set of talented people to tackle this large-scale problem," Manfra says. "No one person or group can be expert in all aspects."

Purdue and Microsoft entered into an agreement in April 2016 that extends their collaboration on quantum computing research, effectively establishing "Station Q Purdue," one of the "Station Q" experimental research sites that work closely with two "Station Q" theory sites.

The new, multi-year agreement extends that collaboration, and includes Microsoft employees being embedded in Manfra's research team at Purdue.

Manfra's group at Station Q Purdue will collaborate with Redmond, Washington-based Microsoft team members, as well as a global experimental group established by Microsoft including experimental groups at the Niels Bohr Institute at the University of Copenhagen in Denmark, TU Delft in the Netherlands, and the University of Sydney, Australia. They are also coupled to the theorists at Microsoft Station Q in Santa Barbara. All groups are working together to solve quantum computing's biggest challenges.

"What's exciting is that we're doing the science and engineering hand-in-hand, at the same time," Manfra says. "We are lucky to be part of this truly amazing global team."

Mathematician and Fields Medal recipient Michael Freedman leads Microsoft's Station Q in Santa Barbara working on quantum computing.

"There is another computing planet out there, and we, collectively, are going to land on it. It really is like the old days of physical exploration and much more interesting than locking oneself in a bottle and traveling through space. We will find an amazing unseen world once we have general purpose programmable quantum computers," Freedman says. "Michael Manfra and Purdue University will be a key collaborator on this journey. I'm not interested in factoring numbers, but solving chemistry and materials science problems, and most ambitiously machine intelligence. Curiously, we need great materials science and transport physics Mike Manfra's work to build the systems we will use to do quantum computing and, thus, to usher in the next era of materials science."

Purdue's role in the project will be to grow and study ultra-pure semiconductors and hybrid systems of semiconductors and superconductors that may form the physical platform upon which a quantum computer is built. Manfra's group has expertise in a technique called molecular beam epitaxy, and this technique will be used to build low-dimensional electron systems that form the basis for quantum bits, or qubits.

The work at Purdue will be done in the Birck Nanotechnology Center in the university's Discovery Park, as well as in the Department of Physics and Astronomy. The Birck facility houses the multi-chamber molecular beam epitaxy system, in which three fabrication chambers are connected under ultra-high vacuum. It also contains clean-room fabrication and necessary materials characterization tools. Laboratories for low-temperature measurement of the materials electronic properties will be conducted in the Department of Physics and Astronomy.

Suresh Garimella, executive vice president for research and partnerships, and Purdue's Goodson Distinguished Professor of Mechanical Engineering, says the tools and laboratories found in Discovery Park have enabled Purdue to become a world leader in several areas.

"Combining these world-leading facilities with our incredibly talented and knowledgeable faculty, such as Professor Manfra, has placed Purdue at the forefront of research and development of nanotechnology, nanoelectronics, next-generation silicon transistor-based electronics, and quantum computing. To have Purdue contribute to the construction of the world's first quantum computer is be a dream come true for us," he says.

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Research collaborative pursues advanced quantum computing - Phys.Org

Still waiting patiently for quantum computing to bring about the next revolution in digital processing power? We might now be a little closer, with a discovery that could help us build quantum computers at mass scale.

Scientists have refined a technique using diamond defects to store information, adding silicon to make the readouts more accurate and suitable for use in the quantum computers of the future.

To understand how the new process works, you need to go back to the basics of the quantum computing vision: small particles kept in a state of superposition, where they can represent both 1, 0, and a combination of the two at the same time.

These quantum bits, or qubits, can process calculations on a much grander scale than the bits in today's computer chips, which are stuck representing either 1 or 0 at any one time.

Getting particles in a state of superposition long enough for us to actually make use of them has proved to be a real challenge for scientists, but one potential solution is through the use of diamond as a base material.

The idea is to use tiny atomic defects inside diamonds to store qubits, and then pass around data at high speeds using light optical circuits rather than electrical circuits.

Diamond-defect qubits rely on a missing carbon atom inside the diamond lattice which is then replaced by an atom of some other element, like nitrogen. The free electrons created by this defect have a magnetic orientation that can be used as a qubit.

So far so good, but our best efforts so far haven't been accurate enough to be useful, because of the broad spectrum of frequencies in the light emitted and that's where the new research comes in.

Scientists added silicon to the qubit creation process, which emits a much narrower band of light, and supplies the precision that quantum computing requires.

At the moment, these silicon qubits don't keep their superposition as well, but the researchers are hopeful this can be overcome by reducing their temperature to a fraction of a degree above absolute zero.

"The dream scenario in quantum information processing is to make an optical circuit to shuttle photonic qubits and then position a quantum memory wherever you need it," says one of the team, Dirk Englund from MIT. "We're almost there with this. These emitters are almost perfect."

In fact, the researchers produced defects within 50 nanometres of their ideal locations on average, which is about one thousandth the size of a human hair.

Being able to etch defects with this kind of precision means the process of building optical circuits for quantum computers then becomes more straightforward and feasible.

If the team can improve on the promising results so far, diamonds could be the answer to our quantum computing needs: they also naturally emit light in a way that means qubits can be read without having to alter their states.

You still won't be powering up a quantum laptop anytime soon, but we're seeing real progress in the study of the materials and techniques that might one day bring this next-generation processing power to the masses.

The research has been published in Nature Communications.

Excerpt from:

Here's how we can achieve mass-produced quantum computers ... - ScienceAlert

In BriefResearchers have found a way to make the creation of qubitssimpler and more precise. The team hopes that this new techniquecould, one day, allow for the mass production of quantum computers. Commercial Quantum Computing Quantum computing is, if you are not already familiar, simply put, a type of computation that uses qubits to encode data instead of the traditional bit (1s and 0s). In short, itallows for the superposition of states, which is where data can be in more than one state at a given time.

So, while traditional computing is limited to information belonging to only one or another state, quantum computing widens those limitations. As a result,more information can be encoded into a much smaller type of bit, allowing for much larger computing capacity. And, while it is still in relatively early development, many believe that quantum computing will be the basis of future technologies, advancing our computational speed beyond what we can currently imagine.

It was extremely exciting then whenresearchers from MIT, Harvard University, and Sandia National Laboratories unveileda simpler way of using atomic-scale defects in diamond materials to build quantum computers in a way that could possibly allow them to be mass produced.

For this process, defects are they key. They are precisely and perfectly placed to function as qubits and hold information. Previous processes weredifficult, complex, and not precise enough. This new methodcreates targeted defects in a much simpler manner. Experimentally, defects created were, on average, at or under 50 nanometers of the ideal locations.

The significance of this cannot be overstated. The dream scenario in quantum information processing is to make an optical circuit to shuttle photonic qubits and then position a quantum memory wherever you need it, says Dirk Englund, an associate professor of electrical engineering and computer science, in an interview with MIT. Were almost there with this. These emitters are almost perfect.

Image Credit: carmule / Pixabay

While the reality of quantum computers, let alone mass produced quantum computers, is still a bit of a ways off, this research is promising. One of the main remaining hurdles is how these computers will read the qubits. But these diamond defects aim to solve that problem because they naturally emit light, and since the light particles emitted can retain superposition, they could help to transmit information.

The research goes on to detail how the completion of these diamond materials better allowed for the amplification of the qubit information. By the end, the researchers found that the light emitted was approximately 80-90 percent as bright as possible.

If this work eventuallyleads to the full creation of a quantum computer, life as we know it would change irrevocably. From completely upendingmodern encryption methods to allowing us to solve previously unsolvable problems, our technology and infrastructure would never be the same. Moreover, the limitations that currently exist in how we store and transmit information would shatter, opening new opportunities foras yetunimaginable exploration.

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MIT Just Unveiled A Technique to Mass Produce Quantum Computers - Futurism

Not even the greatest geniuses in the world could explain quantum computing.

In the early 1930s Einstein, in fact, called quantum mechanics the basis for quantum computing spooky action at a distance.

Then theres a famous phrase from the late Nobel Laureate in physics, Richard Feynman: If you think you understand quantum mechanics, then you dont understand quantum mechanics.

That may be so, but the mystery behind quantum has not stopped D-Wave Systems Inc. from making its mark in the field. In the 1980s it was thought maybe quantum mechanics could be used to build a computer. So people starting coming up with ideas on how to build one, says Bo Ewald, president of D-Wave in Burnaby, B.C.

Two of those people were UBC PhD physics grads Eric Ladizinsky and Geordie Rose, who had happened to take an entrepreneur course before founding D-Wave in 1999. Since there werent a lot of businesses in the field, they created and collected patents around quantum, Ewald says.

What we have with D-Wave is the mother of all ships: that is the hardware capability to unlock the future of AI

While most who were exploring the concept were looking in the direction of what is called the universal gate model, D-Wave decided to work on a different architecture, called annealing. The two do not necessarily compete, but perform different functions.

In quantum annealing, algorithms quickly search over a space to find a minimum (or solution). The technology is best suited for speeding research, modelling or traffic optimization for example.

Universal gate quantum computing can put basic quantum circuit operations together to create any sequence to run increasingly complex algorithms. (Theres a third model, called topological quantum computing, but it could be decades before it can be commercialized.)

When D-Wave sold its first commercial product to Lockheed Martin about six years ago, it marked the first commercial sale of a quantum computer, Ewald says. Google was the second to partner with D-Wave for a system that is also being run by NASA Ames Research Center. Each gets half of the machine, Ewald says. They believed quantum computing had an important future in machine learning.

Most recently D-Wave has been working with Volkswagen to study traffic congestion in Beijing. They wanted to see if quantum computing would have applicability to their business, where there are lots of optimization problems. Another recent coup is a deal with the Los Alamos National Laboratory.

Theres no question that any quantum computing investment is a long-term prospect, but that has not hindered their funding efforts. To date, the company has acquired more than 10 rounds of funding from the likes of PSP, Goldman Sachs, Bezos Expeditions, DFJ, In-Q-Tel, BDC Capital, GrowthWorks, Harris & Harris Group, International Investment and Underwriting, and Kensington Partners Ltd.

What we have with D-Wave is the mother of all ships: that is the hardware capability to unlock the future of AI, says Jrme Nycz, executive vice-president, BDC Capital. We believe D-Waves quantum capabilities have put Canada on the map.

Now, Ewing says, the key for the company moving forward is getting more smart people working on apps and on software tools in the areas of AI, machine earning and deep learning.

To that end, D-Wave recently not only open-sourced its Qbsolv software tool, it launched an initiative with Creative Destruction Lab at the University of Torontos Rotman School of Management to create a new track focused on quantum machine learning. The intensive one-year program will go through an introductory boot camp led by Dr. Peter Wittek, author of Quantum Machine Learning: What Quantum Computing means to Data Mining, with instruction and technical support from D-Wave experts, and access to a D-Wave technology.

While it is still early days in terms of deployment for quantum computing, Ewald believes D-Waves early start gives them a leg up if and when quantum hits the mainstream. So far customers tend to be government and/or research related. Google is the notable exception. But once apps come along that are applicable for other industries, it will all make sense.

The early start has given D-Wave the experience to be able to adopt other architectures as they evolve. It may be a decade before a universal gate model machine becomes a marketable product. If that turns out to be true, we will have a 10-year lead in getting actual machines into the field and having customers working on and developing apps.

Ewald is the first to admit that as an early entrant, D-Wave faces criticism around its architecture. There are a lot of spears and things that we tend to get in the chest. But we see them coming and can deal with it. If we can survive all that, we will have a better view of the market, real customers and relationships with accelerators like Creative Destruction Lab. At the end of day we will have the ability to adapt when we need to.

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D-Wave partners with U of T to move quantum computing along - Financial Post

Due to the changes needed to algorithms and computational thinking, Telstra chief scientist Hugh Bradlow believes the first commercial users of quantum computers will need some help adjusting -- and the Australian incumbent telco will be there to offer that help at a price.

"I can assure you they are not going to walk in on day one and know how to use these things," Bradlow said on Wednesday evening.

"We want to be able to offer it as-a-service to them ... they will need a lot of hand holding, and they are not going to run the equipment themselves, it's complicated."

Telstra and the Commonwealth Bank of Australia (CBA) are two of the companies backing the work of a team at the University of New South Wales (UNSW) that is looking to develop quantum computing in silicon.

At the end of 2015, both companies contributed AU$10 million over five years to UNSW.

Despite racing against far greater funded rivals, head of UNSW's quantum effort professor Michelle Simmons said she is happy with the funding the Centre of Excellence for Quantum Computation and Communication Technology has received.

"At the moment, you have to prove you have the best hardware of anything out there to know whether you are going to go further or not," Simmons said. "I guess one of the things we've been very much driven by is milestone-based research.

"Can we actually develop the qubits, qubit by qubit, and prove that they are better than other qubits that out there? And so if you have lots of money in the beginning, and you are not doing that systematic thorough approach, it's actually not that helpful to you. You have to do it, proving it along the way."

Simmons said her team is currently looking at producing a 10-qubit system by the end of the decade, and, if successful, will be looking to move up to 100 qubits.

In October last year, the UNSW team announced that they had created a new qubit that remains in a stable superposition for 10 times longer than previously achieved.

A year earlier, the team built the first 2-qubit logic gate in silicon.

"The prototype chip we want to make within five years is a pretty shrinkable manufacturing process, and it will be able to perform a variety of calculations; we hope it will be able to potentially solve the problem that currently can't be solved on an existing computer," Andrew Dzurak, scientia professor at the university, said at the time.

"That particular type of problem may not be the sort of problem that is going to excite many commercial people in the first instance, but it will be an important principal."

Even though UNSW is at the frontier of quantum computing, however, Bradlow said Telstra just wants to get its hands on one.

"We are agnostic at the end of the day; we just want a quantum computer," he said. "We do hope Michelle's team wins ... we've gone and put our money on it because we think it's got the best odds, so it's not just a random bet, but we are obviously keeping across anything that is out there.

"Over the last year and a half, I've probably visited every major group in the world, and they all have very different views and by seeing multiple views you get a much better perspective.

"So it's important to keep across everything."

For its part, CBA is preparing for a quantum future by using a quantum computing simulator from QxBranch.

"The difference between the emulator of a quantum computer and the real hardware is that we run the simulator on classical computers, so we don't get the benefit of the speed up that you get from quantum, but we can simulate its behaviour and some of the broad characteristics of what the eventual hardware will do," QxBranch CEO Michael Brett told ZDNet in April.

"What we provide is the ability for people to explore and validate the applications of quantum computing so that as soon as the hardware is ready, they'll be able to apply those applications and get the benefit immediately of the unique advantages of quantum computing."

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Telstra just wants a quantum computer to offer as-a-service - ZDNet

When a company calls and says they have the best widget ever, you have to be skeptical. However, you also cant help but be curious. When they talked about how it would advance the state of the art in radar, electronic warfare, and quantum-computing test, and make an engineers workspace tidier, I was smitten.

I met up with theTektronix team, led by Product Market Manager Kip Pettigrew, and wasnt disappointed: The new AWG5200 arbitrary waveform generator is a work of art and function. Physically, its both commanding and imposing. It measures 18.13 6.05 from the front, but its 23.76 inches deepso, while itll sit nicely within a test stack and help reduce clutter, the stack had better have a deep shelf (Figs. 1 and 2).

Its whats within those dimensions, and what you have to pay to get it, though, that give the AWG5200 a certain level of gravitas. For sure, its hard to ignore a price point of $82,000, but its not surprising when you understand what youre getting in return.

1. The AWG5200 measures 18.13 6.05 and comes with a 6.5-inch touchscreen, a removable hard drive (upper right), and two, four, or eight channels (bottom right). (Source: Tektronix)

Aimed squarely at military/government and advanced research applications, the system emphasizes signal fidelity, scalability, and flexibility. It can accurately reproduce complex, real-world signals across an ever-expanding array of applications without having to physically expand a test area. Its also supported by Tektronixs SourceXpress software, which lets you create waveforms and control the AWGs remotely, and has a growing library of waveform-creation plugins.

2. The AWG5200 is designed to be compact so that it can stack easily with other equipment to reduce overall space requirements, though it is 23.76 inches deep. A synchronization feature allows it to scale up beyond eight channels by adding more AWG5200s. (Source: Tektronix)

Let the Specs Tell the Story

Digging into the specs uncovers what the AWG5200 is all about. Words like powerful, precision, and solid engineering come to mind. The system can sample at 5 Gsamples/s (10-Gsamples/s with interpolation) with 16-bit vertical resolution across two, four, or eight channels per unit. Channel-to-channel skew (typical) is <25 ps with a range of 2 ns and a resolution of 0.5 ps. The analog bandwidth is 2 GHz at 3 dB) or 4 GHz at 6 dB, and the amplitude range is 100 to 0.75 V p-p, with an accuracy of 2% of setting.

The AWG5200s multi-unit synchronization feature helps scale up beyond eight channels. Note that each channel is independent, so the classic tradeoff of sample memory for bandwidth doesnt apply here. Each channel gets 2 Gsamples of waveform memory.

The precision is embodied within its ability to generate RF signals with a spurious-free dynamic range (SFDR) of 70 dBc. Combined with a software suite and support, this is critical as new waveforms and digital-modulation techniques are explored in a time of rapid wireless evolution in military and government applications, as well as 5G and even quantum-computer test. Signal fidelity isnt something you want to worry about, and the expanding library and customizable features help kickstart and then fine-tune your research and development waveforms.

Howd They Do That?

Achieving higher or improved specifications is almost always a labor of love: The test companys engineers constant urge to make things better combines with customer feedback and an analysis of where to focus energy and development to have the most impact. However, at a fundamental level, the AWG5200s advances go back to the digital-to-analog converter (DAC) technology at the heart of the system.

Advances in DAC technologies, particularly with respect to signal processing and functional integration, allow them to directly generate detailed and complex RF and electronic-warfare (EW) signals. This is an area worth digging into in more detail, so Christopher Skach and Sahandi Noorizadeh developed a feature specially for Electronic Design on DAC technology advances and how its changing signal generation for test. Its worth a look.

Rapidly Evolving Applications

Pettigrew also provided a quick run through of the newer and more interesting applications, as well as the key market trends that the system is solving for. In general electronic test, go wide technologies like MIMO need test systems that can scale as they need multiple, independent, wide-bandwidth RF streams (Fig. 3).

3. Rapid expansion in the use of techniques such as MIMO requires more advanced and flexible waveform generators to generate multiple high-fidelity, RF signals with complex modulation schemes. (Source: Tektronix)

This translates over to mil/gov, too, where systems must be tested for their ability to detect and respond to adaptive threats. The signals of interest are able to be generated on two channels, while the others can be used to generate expected noise, Wi-Fi interferers, and other MIMO channels.

However, just being able to reproduce the signals isnt enough: The AWG must be capable of enabling stress and margin testing, as well as verification and characterization.1

On the research front, it turns out that quantum computing needs advanced AWGs, too, said Pettigrew, as they lack the fidelity, latency, and scalability. In quantum computers, the qubits are often controlled using precision-pulsed microwave signals, each requiring multiple independent RF channels. This is only going to get more interesting and challenging as companies like IBM and Google, along with many independent physicists and engineers, work to scale up quantum-computing technology and applications.

For all three of these applications, cost remains a factor. So, instead of developing multiple custom solutions, the AWG5200 may be a good commercial off-the-shelf (COTS) option.

References:

1. How New DAC Technologies are Changing Signal Generation for Test

Continued here:

Tektronix AWG Pulls Test into Era of Quantum Computing - Electronic Design

Purdue researchers are partnering with Microsoft and scientists at three other universities around the globe to determine whether theyve found a way to create a stable form of whats known as quantum computing.

A new five-year agreement aims to build a type of system that could perform computations that are currently impossible in a short timespan, even for supercomputers.

Purdue physics and astronomy professor Michael Manfra is heading up the West Lafayette team, which will work with Microsoft scientists and university colleagues in Australia, the Netherlands and Denmark to construct, manipulate and strengthen tiny building blocks of information called topological qubits."

The real win that topological quantum computing suggests is that if you devise your system in which you store your information cleverly enough, that you can make the qubit insensitive basically deaf to the noise thats all around it in the environment, Manfra says.

He says that deafness is important because of whats held quantum computing back the ease with which its disturbed.

It can interact with photons; electromagnetic fields. It can interact with vibrations of the lattice. And those interactions, what they can do is cause a decoherence of that qubit basically cause it to lose the stored information.

Manfra says its an open question whether quantum computing will ever overtake the current zeroes-and-ones system of information storing, but he says hes interested in either proving or disproving the concept.

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Purdue, Microsoft Partner On Quantum Computing Research | WBAA - WBAA

Quantum computers are experimental devices that offer large speedups on some computational problems. One promising approach to building them involves harnessing nanometer-scale atomic defects in diamond materials.

But practical, diamond-based quantum computing devices will require the ability to position those defects at precise locations in complex diamond structures, where the defects can function as qubits, the basic units of information in quantum computing. In todays of Nature Communications, a team of researchers from MIT, Harvard University, and Sandia National Laboratories reports a new technique for creating targeted defects, which is simpler and more precise than its predecessors.

In experiments, the defects produced by the technique were, on average, within 50 nanometers of their ideal locations.

The dream scenario in quantum information processing is to make an optical circuit to shuttle photonic qubits and then position a quantum memory wherever you need it, says Dirk Englund, an associate professor of electrical engineering and computer science who led the MIT team. Were almost there with this. These emitters are almost perfect.

The new paper has 15 co-authors. Seven are from MIT, including Englund and first author Tim Schrder, who was a postdoc in Englunds lab when the work was done and is now an assistant professor at the University of Copenhagens Niels Bohr Institute. Edward Bielejec led the Sandia team, and physics professor Mikhail Lukin led the Harvard team.

Appealing defects

Quantum computers, which are still largely hypothetical, exploit the phenomenon of quantum superposition, or the counterintuitive ability of small particles to inhabit contradictory physical states at the same time. An electron, for instance, can be said to be in more than one location simultaneously, or to have both of two opposed magnetic orientations.

Where a bit in a conventional computer can represent zero or one, a qubit, or quantum bit, can represent zero, one, or both at the same time. Its the ability of strings of qubits to, in some sense, simultaneously explore multiple solutions to a problem that promises computational speedups.

Diamond-defect qubits result from the combination of vacancies, which are locations in the diamonds crystal lattice where there should be a carbon atom but there isnt one, and dopants, which are atoms of materials other than carbon that have found their way into the lattice. Together, the dopant and the vacancy create a dopant-vacancy center, which has free electrons associated with it. The electrons magnetic orientation, or spin, which can be in superposition, constitutes the qubit.

A perennial problem in the design of quantum computers is how to read information out of qubits. Diamond defects present a simple solution, because they are natural light emitters. In fact, the light particles emitted by diamond defects can preserve the superposition of the qubits, so they could move quantum information between quantum computing devices.

Silicon switch

The most-studied diamond defect is the nitrogen-vacancy center, which can maintain superposition longer than any other candidate qubit. But it emits light in a relatively broad spectrum of frequencies, which can lead to inaccuracies in the measurements on which quantum computing relies.

In their new paper, the MIT, Harvard, and Sandia researchers instead use silicon-vacancy centers, which emit light in a very narrow band of frequencies. They dont naturally maintain superposition as well, but theory suggests that cooling them down to temperatures in the millikelvin range fractions of a degree above absolute zero could solve that problem. (Nitrogen-vacancy-center qubits require cooling to a relatively balmy 4 kelvins.)

To be readable, however, the signals from light-emitting qubits have to be amplified, and it has to be possible to direct them and recombine them to perform computations. Thats why the ability to precisely locate defects is important: Its easier to etch optical circuits into a diamond and then insert the defects in the right places than to create defects at random and then try to construct optical circuits around them.

In the process described in the new paper, the MIT and Harvard researchers first planed a synthetic diamond down until it was only 200 nanometers thick. Then they etched optical cavities into the diamonds surface. These increase the brightness of the light emitted by the defects (while shortening the emission times).

Then they sent the diamond to the Sandia team, who have customized a commercial device called the Nano-Implanter to eject streams of silicon ions. The Sandia researchers fired 20 to 30 silicon ions into each of the optical cavities in the diamond and sent it back to Cambridge.

Mobile vacancies

At this point, only about 2 percent of the cavities had associated silicon-vacancy centers. But the MIT and Harvard researchers have also developed processes for blasting the diamond with beams of electrons to produce more vacancies, and then heating the diamond to about 1,000 degrees Celsius, which causes the vacancies to move around the crystal lattice so they can bond with silicon atoms.

After the researchers had subjected the diamond to these two processes, the yield had increased tenfold, to 20 percent. In principle, repetitions of the processes should increase the yield of silicon vacancy centers still further.

When the researchers analyzed the locations of the silicon-vacancy centers, they found that they were within about 50 nanometers of their optimal positions at the edge of the cavity. That translated to emitted light that was about 85 to 90 percent as bright as it could be, which is still very good.

Its an excellent result, says Jelena Vuckovic, a professor of electrical engineering at Stanford University who studies nanophotonics and quantum optics. I hope the technique can be improved beyond 50 nanometers, because 50-nanometer misalignment would degrade the strength of the light-matter interaction. But this is an important step in that direction. And 50-nanometer precision is certainly better than not controlling position at all, which is what we are normally doing in these experiments, where we start with randomly positioned emitters and then make resonators.

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Toward mass-producible quantum computers | MIT News - MIT News