Monthly Archives: May 2017

May 9, 2017 by Tom Abate Researchers are developing quantum computers based on light rather than electricity. At Stanford, new materials could be the key to progress in this field. Credit: iStock/Pobytov

For 60 years computers have become smaller, faster and cheaper. But engineers are approaching the limits of how small they can make silicon transistors and how quickly they can push electricity through devices to create digital ones and zeros.

That limitation is why Stanford electrical engineering Professor Jelena Vuckovic is looking to quantum computing, which is based on light rather than electricity. Quantum computers work by isolating spinning electrons inside a new type of semiconductor material. When a laser strikes the electron, it reveals which way it is spinning by emitting one or more quanta, or particles, of light. Those spin states replace the ones and zeros of traditional computing.

Vuckovic, who is one of the world's leading researchers in the field, said quantum computing is ideal for studying biological systems, doing cryptography or data mining in fact, solving any problem with many variables.

"When people talk about finding a needle in a haystack, that's where quantum computing comes in," she said.

Marina Radulaski, a postdoctoral fellow in Vuckovic's lab, said the problem-solving potential of quantum computers stems from the complexity of the laser-electron interactions at the core of the concept.

"With electronics you have zeros and ones," Radulaski said. "But when the laser hits the electron in a quantum system, it creates many possible spin states, and that greater range of possibilities forms the basis for more complex computing."

Capturing electrons

Harnessing information based on the interactions of light and electrons is easier said than done. Some of the world's leading technology companies are trying to build massive quantum computers that rely on materials super-cooled to near absolute zero, the theoretical temperature at which atoms would cease to move.

In her own studies of nearly 20 years, Vuckovic has focused on one aspect of the challenge: creating new types of quantum computer chips that would become the building blocks of future systems.

"To fully realize the promise of quantum computing we will have to develop technologies that can operate in normal environments," she said. "The materials we are exploring bring us closer toward finding tomorrow's quantum processor."

The challenge for Vuckovic's team is developing materials that can trap a single, isolated electron. Working with collaborators worldwide, they have recently tested three different approaches to the problem, one of which can operate at room temperature a critical step if quantum computing is going to become a practical tool.

In all three cases the group started with semiconductor crystals, material with a regular atomic lattice like the girders of a skyscraper. By slightly altering this lattice, they sought to create a structure in which the atomic forces exerted by the material could confine a spinning electron.

"We are trying to develop the basic working unit of a quantum chip, the equivalent of the transistor on a silicon chip," Vuckovic said.

Quantum dots

One way to create this laser-electron interaction chamber is through a structure known as a quantum dot. Physically, the quantum dot is a small amount of indium arsenide inside a crystal of gallium arsenide. The atomic properties of the two materials are known to trap a spinning electron.

In a recent paper in Nature Physics, Kevin Fischer, a graduate student in the Vuckovic lab, describes how the laser-electron processes can be exploited within such a quantum dot to control the input and output of light. By sending more laser power to the quantum dot, the researchers could force it to emit exactly two photons rather than one. They say the quantum dot has practical advantages over other leading quantum computing platforms but still requires cryogenic cooling, so it may not be useful for general-purpose computing. However, it could have applications in creating tamper-proof communications networks.

Color centers

In two other papers Vuckovic took a different approach to electron capture, by modifying a single crystal to trap light in what is called a color center.

In a recent paper published in Nano Letters, her team focused on color centers in diamond. In nature the crystalline lattice of a diamond consists of carbon atoms. Jingyuan Linda Zhang, a graduate student in Vuckovic's lab, described how a 16-member research team replaced some of those carbon atoms with silicon atoms. This one alteration created color centers that effectively trapped spinning electrons in the diamond lattice.

But like the quantum dot, most diamond color center experiments require cryogenic cooling. Though that is an improvement over other approaches that required even more elaborate cooling, Vuckovic wanted to do better.

So she worked with another global team to experiment with a third material, silicon carbide. Commonly known as carborundum, silicon carbide is a hard, transparent crystal used to make clutch plates, brake pads and bulletproof vests. Prior research had shown that silicon carbide could be modified to create color centers at room temperature. But this potential had not yet been made efficient enough to yield a quantum chip.

Vuckovic's team knocked certain silicon atoms out of the silicon carbide lattice in a way that created highly efficient color centers. They also fabricated nanowire structures around the color centers to improve the extraction of photons. Radulaski was the first author on that experiment, which is described in another NanoLetters paper. She said the net results an efficient color center, operating at room temperature, in a material familiar to industry were huge pluses.

"We think we've demonstrated a practical approach to making a quantum chip," Radulaski said.

But the field is still in its early days and electron tapping is no simple feat. Even the researchers aren't sure which method or methods will win out.

"We don't know yet which approach is best, so we continue to experiment," Vuckovic said.

Explore further: Simultaneous detection of multiple spin states in a single quantum dot

More information: Marina Radulaski et al. Scalable Quantum Photonics with Single Color Centers in Silicon Carbide, Nano Letters (2017). DOI: 10.1021/acs.nanolett.6b05102

Journal reference: Nano Letters

Provided by: Stanford University

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New materials bring quantum computing closer to reality - Phys.org - Phys.Org

Quantum computers finally seem to be coming of age with promises of quantum supremacy by the end of the year. But theres a problemvery few people know how to work them.

The bold claim ofachieving "quantum supremacy"came on the back of Google unveiling a new quantum chip design. The hyperbolic phrase essentially means building a quantum device that can perform a calculation impossible for any conventional computer.

In theory, quantum computers can crush conventional ones at important tasks like factoring large numbers. Thats because unlike normal computers, whose bits can either be represented as 0 or 1, a quantum bitor qubitcan be simultaneously 0 and 1 thanks to a phenomenon known as superposition.

Demonstrating this would require thousands of qubits, though, which is well beyond current capabilities. So instead Google plans to compare the computers ability to simulate the behavior of a random arrangement of quantum circuits. They predict it should take 50 qubits to outdo the most powerful supercomputers, a goal they feel they can reach this year.

Clearly the nature of the experiment tips the balance in favor of their chip, but the result would be impressive nonetheless, and could act as a catalyst to spur commercialization of the technology.

This year should also see the first commercial universal quantum computing service go live, with IBM giving customers access to one of its quantum computers over the cloud for a fee. Canadian company D-Wave already provides cloud access to one of its machines, but its quantum computers are not universal, as they can only solve certain optimization problems.

But despite this apparent impetus, the technology has a major challenge to overcome. Programming these devices is much harder than programming conventional computers.

For a start, building algorithms for these machines requires a certain level of understanding about the quantum physics that gives qubits their special properties. While you dont need an advanced physics degree to get your head around it, it is a big departure from traditional computer programming.

Writing in ReadWrite, Dan Rowinski points out, Writing apps that can be translated into some form of qubit-relatable code may require some very different approaches, since among other things, the underlying logic for digital programs may not translate precisely (or at all) to the quantum-computing realm.

And while there are a number of quantum simulators that can run on a laptop for those who want to dip their toes in the water, real quantum computers are likely to behave quite differently. The real challenge is whether you can make your algorithm work on real hardware that has imperfections, Isaac Chuang, an MIT physicist, told Nature.

Convincing programmers to invest the time necessary to learn these skills is going to be tricky until commercial systems are delivering tangible benefits and securing customers, but thats going to be tough if theres no software to run on them.

The companies building these machines recognize this chicken and egg problem, and it is why there is an increasing drive to broaden access to these machines. Before the announcement of the commercial IBMQ service, the company had already released the free Quantum Experience service last year.

Earlier this year, D-Wave open sourced their Qbsolv and Qmasm tools to allow people to start getting to grips with programming its devices, while a pair of Google engineers built a Quantum Computing Playground for people to start investigating the basics of the technology. The company plans to provide access to its devices over the cloud just like IBM.

We dont just want to build these machines, Jerry Chow, the manager of IBMs Experimental Quantum Computing team told Wired. We want to build a framework that allows people to use them.

How easy it will be to translate the skills learned in one of these companies proprietary quantum computing ecosystems to another also remains to be seen, not least because the technology at the heart of them can be dramatically different. This could be a further stumbling block to developing a solid pool of quantum programmers.

Ultimately, the kinds of large-scale quantum computers powerful enough to be usefully put to work on real-world problems are still some years away, so theres no need to panic yet. But as the researchers behind Googles quantum effort note in an article in Nature, this scarcity of programming talent also presents an opportunity for those who move quickly.

If early quantum-computing devices can offer even a modest increase in computing speed or power, early adopters will reap the rewards, they write. Rival companies would face high entry barriers to match the same quality of services and products, because few experts can write quantum algorithms, and businesses need time to tailor new algorithms.

Image Credit: Shutterstock

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Quantum Computing Demands a Whole New Kind of Programmer - Singularity Hub