Nano-diamond qubits and photonic crystals

A quantum computer operates with quantum bits (qubits) as units of information. Obeying the laws ofquantum mechanics, such a computer would be capable of addressing several of the most difficult computational tasks unsolvable with present technology. In the past few decades, scientists learned to perform room-sized experiments to optically control and read out a small number of qubits.

Now, researchers in Germany have successfully fabricated a rudimentary quantum computing hybrid system using electronic excitations in nano-diamonds as qubits and opticalnanostructures, so-called photoniccrystalswith tailored optical properties. This architecture may allow integration of multi-qubit systems on a single micrometer-sized chip for future quantum computers.

"Our results suggest a strategy for scaling up quantum information to large-scale systems, which has yet to be done,"says Janik Wolters, researcher, at Humboldt Universität in Berlin."We regard our experiment as a milestone on the long road toward on-chip integrated quantum information processing systems, bringing the dream of a quantum computer closer to reality."

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Anti-mirror optical illusion could increase LED luminosity and laser power

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Huanyang Chen, a researcher inmetamaterialsandtransformationoptics from Soochow University in Jiangsu, China, and his colleagues have discovered and modeled the anti-mirror effect. The ability to make multiple objects look like one using“overlapped illusion optics” has not been found in nature before now.

To demonstrate the basic idea, the researchers explained that two identical cylindrical perfect electric conductors (PECs) are placed on opposite sides of a perfect lens made of a negative refractive index material. When viewed from the far field (beyond a certain distance) on either side of the lens, the two PECs look like one. Further, when the scientists replaced one of the circular PECs with an illusion device with an elliptic cylindrical PEC, both PECs look like only one circular cylindrical PEC.

Further elaborating on this effect, the researchers showed that illusion devices with elliptic cylindrical PECs can be used in place of both real cylindrical PECs. Once again, the two illusion devices look like one PEC. As the scientists explained, this effect occurs because the two illusion devices are close enough together so that their virtual illusion spaces overlap. Inside this shared region, both illusion devices form a single PEC image.

“The two PECs on both sides of the perfect lens follow the image-forming principle so that each of them is overlapped with the virtual image of another,” Chen explained toPhysOrg.com.“The anti-mirror effect stems from the evanescent wave amplification of the perfect lens.”

The anti-mirror effect could have applications in both solid-state lighting, such as LEDs, and in coherent light sources, such as lasers. Currently, one of the biggest challenges in LED development is achieving a high enough luminosity for general lighting purposes. One method of increasing LED illuminance is to package many LEDs inside a single bulb; however, the problem is that the lamp's spatial illumination is not uniform. Using the new overlapped illusion optics, the researchers show that the images from multiple LEDs in different places can be overlapped to make the bulb look like a single-LED source with high, uniform illumination.

The proposed overlapped illusion optics method could also increase the power and preserve the spatial uniformity of lasers. Usually, spatial uniformity degrades when two coherent sources are aligned due to interference. Using the same configuration as the LEDs, multiple coherent sources can be operated at the same frequency and phase to double the light amplitude and quadruple the total power of the coherent system. These improvements cannot be achieved using traditional beam-combining techniques.

“Our current work is just a conceptual model,” Chen said.“We have recently realized the first illusionopticaldevice– an“invisible gateway”– by using a transmission-line medium.” See http://arxiv.org/abs/1005.3425 .

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Light on silicon better than copper?

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As good as the metal has been in zipping information from one circuit to another onsiliconinside computers and otherelectronic devices, optical signals can carry much more, according to Duke University electrical engineers. So the engineers have designed and demonstrated microscopically small lasers integrated with thin film-light guides on silicon that could replace the copper in a host of electronic products.

The structures on silicon not only contain tiny light-emitting lasers, but connect these lasers to channels that accurately guide the light to its target, typically another nearby chip or component. This new approach could help engineers who, in their drive to create tinier and faster computers and devices, are studying light as the basis for the next generation information carrier.

The engineers believe they have solved some of the unanswered riddles facing scientists trying to create and control light at such a miniscule scale.

"Getting light onto silicon and controlling it is the first step toward chip scaleoptical systems,"said Sabarni Palit, who this summer received her Ph.D. while working in the laboratory of Nan Marie Jokerst, J.A. Jones Distinguished Professor of Electrical and Computer Engineering at Duke's Pratt School of Engineering.

The results of team's experiments, which were supported by the Army Research Office, were published online in the journalOptics Letters.

"The challenge has been creating light on such a small scale on silicon, and ensuring that it is received by the next component without losing most of the light,"Palit said.

"We came up with a way of creating a thin film integrated structure on silicon that not only contains alight sourcethat can be kept cool, but can also accurately guide the wave onto its next connection,"she said."This integration of components is essential for any such chip-scale, light-based system."

The Duke team developed a method of taking the thick substrate off of alaser, and bonding this thin film laser to silicon. The lasers are about one one-hundreth of the thickness of a human hair. These lasers are connected to other structures by laying down a microscopic layer of polymer that covers one end of the laser and goes off in a channel to other components. Each layer of the laser and light channel is given its specific characteristics, or functions, through nano- and micro-fabrication processes and by selectively removing portions of the substrate with chemicals.

"In the process of producing light, lasers produce heat, which can cause the laser to degrade,"Sabarni said."We found that including a very thin band of metals between the laser and the silicon substrate dissipated the heat, keeping the laser functional."

For Jokerst, the ability to reliably facilitate individual chips or components that"talk"to each other using light is the next big challenge in the continuing process of packing more processing power into smaller and smaller chip-scale packages.

"To use light in chip-scale systems is exciting,"she said."But the amount of power needed to run these systems has to be very small to make them portable, and they should be inexpensive to produce. There are applications for this in consumer electronics, medical diagnostics and environmental sensing."

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'Space-time cloak' to conceal events revealed in new study

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Previously, a team led by Professor Sir John Pendry at Imperial College London showed that metamaterials could be used to make an optical invisibility cloak. Now, a team led by Professor Martin McCall has mathematically extended the idea of a cloak that conceals objects to one that conceals events.

"Light normally slows down as it enters a material, but it is theoretically possible to manipulate the light rays so that some parts speed up and others slow down,"says McCall, from the Department of Physics at Imperial College London. When light is 'opened up' in this way, rather than being curved in space, the leading half of the light speeds up and arrives before an event, whilst the trailing half is made to lag behind and arrives too late. The result is that for a brief period the event is not illuminated, and escapes detection. Once the concealed passage has been used, the cloak can then be 'closed' seamlessly.

Such a space-time cloak would open up a temporary corridor through which energy, information and matter could be manipulated or transported undetected."If you had someone moving along the corridor, it would appear to a distant observer as if they had relocated instantaneously, creating the illusion of a Star-Trek transporter,"says McCall."So, theoretically, this person might be able to do something and you wouldn't notice!"

While using the spacetime cloak to make people move undetected is still science fiction, there are many serious applications for the new research, which was funded by the Engineering and Physical Sciences Research Council (EPSRC) and the Leverhulme Trust. Co-author Dr Paul Kinsler developed a proof of concept design using customised optical fibres, which would enable researchers to use the event cloak in signal processing and computing. A given data channel could for example be interrupted to perform a priority calculation on a parallel channel during the cloak operation. Afterwards, it would appear to external parts of the circuit as though the original channel had processed information continuously, so as to achieve 'interrupt-without-interrupt'.

Alberto Favaro, who also worked on the project, explains:"Imagine computer data moving down a channel to be like a highway full of cars. You want to have a pedestrian crossing without interrupting the traffic, so you slow down the cars that haven't reached the crossing, while the cars that are at or beyond the crossing get sped up, which creates a gap in the middle for the pedestrian to cross. Meanwhile an observer down the road would only see a steady stream of traffic."One issue that cropped up during their calculations was to speed up the transmitted data without violating the laws of relativity. Favaro solved this by devising a clever material whose properties varied in both space and time, allowing the cloak to be formed.

"We're sure that there are many other possibilities opened up by our introduction of the concept of the spacetime cloak,' says McCall,"but as it's still theoretical at this stage we still need to work out the concrete details for our proposed applications."

Metamaterials is an expanding field of science, with a vast array of potential uses, spanning defence, security, medicine, data transfer and computing. Many ordinary household devices that work using electromagnetic fields could be made more cheaply or to work at higher speeds. Metamaterials could also be used to control other types of waves as well as light, such as sound or water waves, opening up potential applications for protecting coastal or offshore installations, or even engineering buildings to withstand earthquake waves.

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Making better biosensors with electron density waves

According to Fainman, tapping the potential of plasmonics for biomolecule detection systems has been a challenge, because localized optical field scales are usually much larger than the molecules in question. In order to make a useful optical biosensor, he says,"We need to increase the interaction cross-section by finding ways to localize optical interrogation fields ideally to the scales comparable to those of biomolecules."Since that is not currently possible, he and his team used an approach of integrating microfluidics and plasmonics on single chips, allowing fluid to ferry the molecules into the cross-section of the optical field.

Fainman expects the system to be particularly useful in studying large arrays of protein-protein interactions for identifying potential drugs that bind to specific target molecules, which may lead to earlier cancer diagnoses and faster discovery of new drugs. Unlike most current methods,optical detectiondoes not require labeling of molecules with fluorescent or radioactive entities -- labels often hinder interaction by covering up or blocking binding surfaces.

The new platform also carries the advantage of being high throughput and multiplexed, offering researchers an opportunity to examine thousands of arrayed compounds simultaneously, which, he says,"biologists and physicians get very excited about."

Fainman will present these results at Frontiers in Optics (FiO) 2010/Laser Science XXVI -- the 94th annual meeting of the Optical Society (OSA).

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World's fastest camera takes a new look at biosensing

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Faster, higher resolution cameras

Since the introduction of solid-stateoptical sensors, like those found in digital cameras, the main trend has been towards increasing the resolution (i.e. number of pixels) while miniaturising the chip.

However, the other factor is the number of frames the chip is capable of recording in a given time. Until recently, fast cameras (i.e. those capturing more than the 24 frames per second required for 'normal' video) were only used in niche markets in science and entertainment.

Ultrafast cameras

Now that higher-than-video speeds are achievable, a whole new range of previously unthinkable applications have emerged– such as: cellular / sub-cellular imaging; neural imaging; biochemical sensors; DNA / protein microarray scanning; automotive collision studies; and high-sensitivity astronomical observations.

The Megaframe Imager uses an extremely sensitive single photon avalanche diode (SPAD) device, and bespoke on-chip intelligence and has shown for the first time that it could potentially be a powerful technology in biosensing.

Reporting in the Optical Society of America's new journalBiomedical Optics Expressthe research team have demonstrated detection of viral DNA binding events using fluoresence lifetime imaging at the very low target concentrations relevant in biosensing applications with acquisition times of less than 30 seconds.

DNA microarrays are important tools for biomolecular detection. Widely used for gene expression profiling, disease screening, mutation and forensic analysis, they also hold much promise for the future development of personalised drugs and point of care testing devices.

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Physicists show that superfluid light is possible

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In their study published in a recent issue ofPhysical Review Letters, Patricio Leboeuf and Simon Moulieras from the University Paris-Sud and CNRS explain thatsuperfluidityis the ability of a fluid to move with zero dissipation or viscosity. A fluid behaves like a superfluid only under a certain critical velocity; above this critical velocity, superfluidity disappears. Most commonly demonstrated in liquid helium, superfluidity occurs when the helium is cooled and some helium atoms have reached their lowest possible energy. At this point, these atoms' quantum wave functions begin to overlap so that they form a Bose-Einstein condensate, in which all the atoms behave as one large atom, and their quantum nature is manifested on the macroscopic scale.

Previously, investigations of the superfluid motion oflighthave not revealed clear evidence of the existence of a superfluid critical velocity. Although some recent experiments have observed superfluidity related to light, these experiments did not use photons, but a composite particle, called a polariton, which is a mixture of a photon and an exciton.

In this study, Leboeuf and Moulieras have shown that a superfluid critical velocity does exist in a nonlinear medium. They explain how superfluid light can be observed in an array of waveguides. From a dynamical point of view, light propagating through a nonlinear medium is formally equivalent to a Bose gas of interacting massive particles. Light can travel straight along the waveguides in the longitudinal direction, or it can tunnel between adjacent guides in the transverse direction. The benefit of this set-up is that it allows the scientists to engineer different characteristics of the array and control the light's flow.

The physicists were specifically interested in what happens to alight pulseas it travels through the array at different velocities in the presence of a defect. If the light is scattered by the defect, it means dissipative processes have occurred. If the light pulse moves through the defect without changing its shape (i.e., without losing collectivity), there is no dissipation and the light has superfluid motion. Through their calculations, the physicists showed that, for certain low velocities, the transverse motion of light is superfluid with zero dissipation. When the velocity increases, dissipative processes occur that destroy the collectivity of the light's oscillations, and superfluidity breaks down.

In the future, the physicists plan to further investigate additional details of superfluid light, such as how it relates to an underlying quantum theory of light and how it is connected to Bose-Einstein condensation. They predict that superfluid motion is a general property of light that exists in a variety of scenarios, and is not limited to the waveguide array proposed here. Superfluid light could also have applications in light transport optimization.

“One straightforward implication is related to transport in the presence of noise,” Leboeuf said.“Such a noise is expected to be present generically, since any material has imperfections and impurities. The impurities are responsible for the scattering of light. In the superfluid regime, we expect a light pulse to be able to propagate through a noisy medium without being affected or scattered (perfect transmission).”

Leboeuf and Moulieras plan to perform their proposed experiment and are discussing the opportunity with experimental groups at the Laboratoire de Photonique et de Nanostructures (LPN) at Marcoussis, France. However, the scientists said that superfluid light is not likely to have any strange effect analogous to a superfluid flowing up a container.

“The most basic 'strange' quantum effect that light shows related to superfluidity is, as shown in our article, dissipationless motion,” Moulieras said.“Another, though more indirect or spectacular, effect is related to quantized vortices, which were observed in laser patterns propagating through nonlinear media. Concerning other possibilities, such as fluid motion up the walls of a container, they are related, for atoms, to the forces between these atoms and a substrate, and the balance between capillary, gravity and viscous forces. We do not see a straightforward application of these concepts to photons, and therefore do not expect them for light.”

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This little light of mine: Changing the color of single photons emitted by quantum dots

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Two important resources forquantum information processingare the transmission of data encoded in thequantum stateof a photon and its storage in long-lived internal states of systems like trapped atoms, ions or solid-state ensembles. Ideally, one envisions devices that are good at both generating and storing photons. However, this is challenging in practice because while typical quantum memories are suited to absorbing and storing near-visible photons, transmission is best accomplished at near-infrared wavelengths where information loss in telecommunications optical fibers is low.

To satisfy these two conflicting requirements, the NIST team combined a fiber-coupled single photon source with a frequency up-conversion single photon detector. Both developed at NIST, the frequency up-conversion detector uses a strong pump laser and a special non-linear crystal to convert longwavelength(low frequency) photons into short wavelength (high frequency) photons with high efficiency and sensitivity (See http://www.physorg.com/news170516085.html).

According to Matthew Rakher and Kartik Srinivasan, two authors of the paper, previous up-conversion experiments looked at the color conversion of highly attenuated laser beams that contained less than one photon on average. However, these light sources still exhibited"classical"photon statistics exactly like that of an unattenuated laser, meaning that the photons are organized in such as way that at most times there are no photons while at other times there are more than one. Secure quantum communications relies upon the use of single photons.

"The quantum dot can act as a true single photon source,"says Srinivasan."Each time we excite the dot, it subsequently releases that energy as a single photon. In the past, we had little control over the wavelength of that photon, but now we can generate a single photon of one color on demand, transmit it over long distances with fiber optics, and convert it to another color."

Converting the photon's wavelength also makes it easier to detect, say co-authors Lijun Ma and Xiao Tang. While commercially available single photon detectors in the near-infrared suffer noise problems, detectors in the near-visible are a comparatively mature and high-performance technology. The paper describes how the wavelength conversion of thephotonsimproved their detection sensitivity by a factor of 25 with respect to what was achieved prior to conversion.

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