Monthly Archives: May 2016

The different of Coherence and Control

Such is life for the scientists of the Martinis Group at UC Santa Barbara and Google, Inc., as they explore the exciting but also still somewhat counter-intuitive world of quantum computing. In a paper published in the journal Nature Physics, they and colleagues at Tulane University in New Orleans demonstrate a relatively simple yet complete platform for quantum processing, integrating the control of three superconducting qubits.

“We’re probing the edge of our capability,” said the paper’s lead author, Pedram Roushan. There have been quite a few efforts to build and study individual parts of a quantum processor, he explained, but this particular project involves putting them all together in a basic building block that can be fully controlled and potentially scaled up into a functional quantum computer.

However, before a fully practicable quantum computer — with all its potential for vast, rapid and simultaneous calculations — can be made, various and sometimes unpredictable and spontaneous circumstances arise that have to be understood as the researchers pursue greater control and sophistication of their system.

“You’re dealing with particles — qubits in this case — that are interacting with one another, and they’re interacting with external fields,” Roushan said. “This all leads to very complicated physics.”

To help solve this particular many-body problem, he explained, their fully controllable quantum processing system had to be built from a single qubit up, in order to give the researchers opportunities to more clearly understand the states, behaviors and interactions that can occur.

By engineering the pulse sequences used to manipulate the spins of the photons in their system, the researchers created an artificial magnetic field affecting their closed loop of three qubits, causing the photons to interact strongly with not only each other, but also with the pseudo-magnetic field. Not a small feat.

“Naturally most systems where there is good control are photonic systems,” said co-author Charles Neill. Unlike electrons, charge-less photons generally tend not to interact with each other nor with external magnetic fields, he explained. “In this article we show that we can get them to interact with each other very strongly, and interact with a magnetic field very strongly, which are the two things you need to do to get them to do interesting physics with photons,” Neill said.

Another advantage of this synthetic condensed-matter system is the ability to drive it into its lowest-lying energy state — called the ground state — to probe its properties.

But with more control comes the potential for more decoherence. As the researchers strove for greater programmability and ability to influence and read the qubits, the more open their system was likely to be to error and loss of information.

“The more control we have over a quantum system, the more complex algorithms we would be able to run,” said co-author Anthony Megrant. “However, every time we add a control line, we’re also introducing a new source of decoherence.” At the level of a single qubit, a tiny margin of error may be tolerated, the researchers explained, but even with a relatively small increase in the number of qubits, the potential for error multiplies exponentially.

“There are these corrections that are intrinsically quantum mechanical, and then they start to matter at the level of precision that we’re getting at,” Neill said.

To combat the potential for error while increasing their level of control, the team had to reconsider both the architecture of their circuit and the material that was being used in it. Instead of their traditionally single-level, planar layout, the researchers redesigned the circuit to allow control lines to “cross over” others via a self-supporting metallic “bridge.” The dielectric — the insulating material between the conducting control wires — was itself found to be a major source of errors.

“All deposited dielectrics that we know of are very lossy,” Megrant said, and so a more precisely fabricated and less defective substrate was brought in to minimize the likelihood of decoherence.

Software could help save the planet

unduhan-42Researchers at Lancaster University’s Data Science Institute have developed a software system that can for the first time rapidly self-assemble into the most efficient form without needing humans to tell it what to do.

The system — called REx — is being developed with vast energy-hungry data centres in mind. By being able to rapidly adjust to optimally deal with a huge multitude of tasks, servers controlled by REx would need to do less processing, therefore consuming less energy.

REx works using ‘micro-variation’ — where a large library of building blocks of software components (such as memory caches, and different forms of search and sort algorithms) can be selected and assembled automatically in response to the task at hand.

“Everything is learned by the live system, assembling the required components and continually assessing their effectiveness in the situations to which the system is subjected,” said Dr Barry Porter, lecturer at Lancaster University’s School of Computing and Communications. “Each component is sufficiently small that it is easy to create natural behavioural variation. By autonomously assembling systems from these micro-variations we then see REx create software designs that are automatically formed to deal with their task.

“As we use connected devices on a more frequent basis, and as we move into the era of the Internet of Things, the volume of data that needs to be processed and distributed is rapidly growing. This is causing a significant demand for energy through millions of servers at data centres. An automated system like REx, able to find the best performance in any conditions, could offer a way to significantly reduce this energy demand,” Dr Porter added.

In addition, as modern software systems are increasingly complex — consisting of millions of lines of code — they need to be maintained by large teams of software developers at significant cost. It is broadly acknowledged that this level of complexity and management is unsustainable. As well as saving energy in data centres, self-assembling software models could also have significant advantages by improving our ability to develop and maintain increasingly complex software systems for a wide range of domains, including operating systems and Internet infrastructure.

REx is built using three complementary layers. At the base level a novel component-based programming language called Dana enables the system to find, select and rapidly adapt the building blocks of software. A perception, assembly and learning framework (PAL) then configures and perceives the behaviour of the selected components, and an online learning process learns the best software compositions in real-time by taking advantage of statistical learning methods known as ‘linear bandit models’.

The work is presented in the paper ‘REx: A Development Platform and Online Learning Approach for Runtime Emergent Software Systems’ at the conference ‘OSDI ’16 12th USENIX Symposium on Operating Systems Design and Implementation’. The research has been partially supported by the Engineering and Physical Sciences Research Council (EPSRC), and also a PhD scholarship of Brazil.