Single Photon Management for Quantum Computers

In principle, quantum computers can perform calculations that are impossible or impractical using conventional computers by taking advantage of the peculiar rules of quantum mechanics. To do this, they need to operate on things that can be manipulated into specific quantum states. Photons are among the leading contenders.

The new NIST papers address one of the many challenges to a practical quantum computer: the need for a device that produces photons in ready quantities, but only one at a time, and only when the computer's processor is ready to receive them. Just as garbled data will confuse a standard computer, an information-bearing photon that enters a quantum processor together with other particles -- or when the processor is not expecting it -- can ruin a calculation.

The single-photon source has been elusive for nearly two decades, in part because no method of producing these particles individually is ideal."It's a bit like playing a game of whack-a-mole, where solving one problem creates others," says Alan Migdall of NIST's Optical Technology Division."The best you can do is keep all the issues under control somewhat. You can never get rid of them."

The team's first paper addresses the need to be certain that a photon is indeed coming when the processor is expecting it, and that none show up unexpected. Many kinds of single-photon sources create a pair of photons and send one of them to a detector, which tips off the processor to the fact that the second, information-bearing photon is on its way. But since detectors are not completely accurate, sometimes they miss the"herald" photon -- and its twin zips into the processor, gumming up the works.

The team effort, in collaboration with researchers from the Italian metrology laboratory L'Istituto Nazionale di Ricerca Metrologica (INRIM), handled the issue by building a simple gate into the source. When a herald photon reaches the detector, the gate opens, allowing the second photon past."You get a photon when you expect one, and you don't get one when you don't," Migdall says."It was an obvious solution; others proposed it long ago, we were just the first ones to build it. It makes the single photon source better."

In a second paper, the NIST team describes a photon source to address two other requirements. Quantum computers will need many such sources working in parallel, so sources must be able to be built in large numbers and operate reliably; and so that the computer can tell the photons apart, the sources must create multiple individual photons, but all at different wavelengths. The team outlines a way to create just such a source out of silicon, which has been well-understood by the electronics industry for decades as the material from which standard computer chips are built.

"Ordinarily a particular material can produce only pairs in a specific pair of wavelengths, but our design allows production of photons at a number of regular and distinct wavelengths simultaneously, all from one source," Migdall says."Because the design is compatible with microfabrication techniques, this accomplishment is the first step in the process of creating sources that are part of integrated circuits, not just prototype computers that work in the hothouse of the lab."

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Better Than the Human Eye: Tiny Camera With Adjustable Zoom Could Aid Endoscopic Imaging, Robotics, Night Vision

The"eyeball camera" has a 3.5x optical zoom, takes sharp images, is inexpensive to make and is only the size of a nickel. (A higher zoom is possible with the technology.)

While the camera won't be appearing at Best Buy any time soon, the tunable camera -- once optimized -- should be useful in many applications, including night-vision surveillance, robotic vision, endoscopic imaging and consumer electronics.

"We were inspired by the human eye, but we wanted to go beyond the human eye," said Yonggang Huang, Joseph Cummings Professor of Civil and Environmental Engineering and Mechanical Engineering at Northwestern's McCormick School of Engineering and Applied Science."Our goal was to develop something simple that can zoom and capture good images, and we've achieved that."

The tiny camera combines the best of both the human eye and an expensive single-lens reflex (SLR) camera with a zoom lens. It has the simple lens of the human eye, allowing the device to be small, and the zoom capability of the SLR camera without the bulk and weight of a complex lens. The key is that both the simple lens and photodetectors are on flexible substrates, and a hydraulic system can change the shape of the substrates appropriately, enabling a variable zoom.

The research is being published the week of Jan. 17 by theProceedings of the National Academy of Sciences(PNAS).

Huang, co-corresponding author of the PNAS paper, led the theory and design work at Northwestern. His colleague John Rogers, the Lee J. Flory Founder Chair in Engineering and professor of materials science and engineering at the University of Illinois, led the design, experimental and fabrication work. Rogers is a co-corresponding author of the paper.

Earlier eyeball camera designs are incompatible with variable zoom because these cameras have rigid detectors. The detector must change shape as the in-focus image changes shape with magnification. Huang and Rogers and their team use an array of interconnected and flexible silicon photodetectors on a thin, elastic membrane, which can easily change shape. This flexibility opens up the field of possible uses for such a system. (The array builds on their work in stretchable electronics.)

The camera system also has an integrated lens constructed by putting a thin, elastic membrane on a water chamber, with a clear glass window underneath.

Initially both detector and lens are flat. Beneath both the membranes of the detector and the simple lens are chambers filled with water. By extracting water from the detector's chamber, the detector surface becomes a concave hemisphere. (Injecting water back returns the detector to a flat surface.) Injecting water into the chamber of the lens makes the thin membrane become a convex hemisphere.

To achieve an in-focus and magnified image, the researchers actuate the hydraulics to change the curvatures of the lens and detector in a coordinated manner. The shape of the detector must match the varying curvature of the image surface to accommodate continuously adjustable zoom, and this is easily done with this new hemispherical eye camera.

In addition to Huang and Rogers, other authors of the paper are Chaofeng Lu and Ming Li, from Northwestern; Inhwa Jung, Jianliang Xiao, Viktor Malyarchuk and Jongseung Yoon, from the University of Illinois; and Zhuangjian Liu, from the Institute of High Performance Computing, Singapore.

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Coiled Nanowires May Hold Key to Stretchable Electronics

"In order to create stretchable electronics, you need to put electronics on a stretchable substrate, but electronic materials themselves tend to be rigid and fragile," says Dr. Yong Zhu, one of the researchers who created the new nanowire coils and an assistant professor of mechanical and aerospace engineering at NC State."Our idea was to create electronic materials that can be tailored into coils to improve their stretchability without harming the electric functionality of the materials."

Other researchers have experimented with"buckling" electronic materials into wavy shapes, which can stretch much like the bellows of an accordion. However, Zhu says, the maximum strains for wavy structures occur at localized positions -- the peaks and valleys -- on the waves. As soon as the failure strain is reached at one of the localized positions, the entire structure fails.

"An ideal shape to accommodate large deformation would lead to a uniform strain distribution along the entire length of the structure -- a coil spring is one such ideal shape," Zhu says."As a result, the wavy materials cannot come close to the coils' degree of stretchability." Zhu notes that the coil shape is energetically favorable only for one-dimensional structures, such as wires.

Zhu's team put a rubber substrate under strain and used very specific levels of ultraviolet radiation and ozone to change its mechanical properties, and then placed silicon nanowires on top of the substrate. The nanowires formed coils upon release of the strain. Other researchers have been able to create coils using freestanding nanowires, but have so far been unable to directly integrate those coils on a stretchable substrate.

While the new coils' mechanical properties allow them to be stretched an additional 104 percent beyond their original length, their electric performance cannot hold reliably to such a large range, possibly due to factors like contact resistance change or electrode failure, Zhu says."We are working to improve the reliability of the electrical performance when the coils are stretched to the limit of their mechanical stretchability, which is likely well beyond 100 percent, according to our analysis."

A paper describing the research was published online Dec. 28 byACS Nano. The paper is co-authored by Zhu, NC State Ph.D. student Feng Xu and Wei Lu, an assistant professor at the University of Michigan. The research was funded by the National Science Foundation.

NC State's Department of Mechanical and Aerospace Engineering is part of the university's College of Engineering.

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Light Can Control Electrical Properties of Graphene

This year's Nobel Prize for Physics was awarded for research into graphene, recognising its potential for many applications in modern life, from high-speed electronics to touchscreen technology. The UK's National Physical Laboratory, along with a team of international scientists, have further developed our understanding of graphene by showing that when this remarkable material is combined with particular polymers, its electrical properties can be precisely controlled by light and exploited in a new generation of optoelectronic devices. The polymers keep memory of light and therefore the graphene device retains its modified properties until the memory is erased by heating.

Light-modified graphene chips have already been used at NPL in ultra-precision experiments to measure the quantum of the electrical resistance.

In the future, similar polymers could be used to effectively 'translate' information from their surroundings and influence how graphene behaves. This effect could be exploited to develop robust reliable sensors for smoke, poisonous gases, or any targeted molecule.

Graphene is an extraordinary two-dimensional material made of a single atomic layer of carbon atoms. It is the thinnest material known to man, and yet is one of the strongest ever tested.

Graphene does not have volume, only surface -- its entire structure is exposed to its environment, and responds to any molecule that touches it. This makes it in principle a very exciting material for super-sensors capable of detecting single molecules of toxic gases. Polymers can make graphene respond to specific molecules and ignore all others at the same time, which also protects it from contamination.

The research team included scientists from the National Physical Laboratory (UK), Chalmers University of Technology (Sweden), University of Copenhagen (Denmark), University of California Berkeley (USA), Linköping University (Sweden) and Lancaster University (UK).

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