Progress in quantum computing: Researchers control rate of photon emission from luminescent imperfections in diamond

Engineers and physicists at Harvard have managed to capture light in tiny diamond pillars embedded in silver, releasing a stream of single photons at a controllable rate.

The advance represents a milestone on the road to quantum networks in which information can be encoded in spins of electrons and carried through a network via light, one photon at a time.

The finding was published in Nature Photonics, appearing online Oct. 9.

"We can make the emission of photons faster, which will allow us to do more processing per second -- for example, more computations -- in the future quantum network," explains principal investigator Marko Lončar, Associate Professor of Electrical Engineering at the Harvard School of Engineering and Applied Sciences (SEAS).

The device Lončar's research team has built consists of parallel rows of tiny, nanofabricated diamond posts, embedded in a layer of silver, that can each act as a single photon source.

By removing the silver wrapping from their nanostructures, the team was also able to achieve a slower release of photons, which is of interest for probing the dynamics of the quantum system.

The breakthrough takes advantage of imperfections in the diamond's crystal lattice, where carbon atoms are replaced by other elements. To the naked eye, these imperfections can appear as discolorations in the diamond, turning it yellow in the case of nitrogen. Occasionally, there is also a vacancy (missing carbon atom) next to the nitrogen atom.

Each nitrogen-vacancy imperfection can serve as a nearly perfect quantum emitter, capable of emitting red photons one by one, even at room temperature. The technology is a promising candidate for realization of scalable, on-chip quantum networks.

"The color centers in diamond are very interesting as qubits for quantum information processing, where they can be used as memory to store information," says Lončar. "More importantly, they can be interrogated -- they can be written into and read out -- with light."

Lončar's team fabricates diamond posts that contain negatively charged nitrogen vacancy centers, which can absorb light and hold its energy for a given amount of time, finally releasing it in the form of photons.

"The rate at which photons are emitted can be controlled by carefully nano-engineering the center's surrounding," says co-author Irfan Bulu, a research associate in the Lončar group. Attaining fine control of that release, however, has been difficult.

"One of the main challenges has been the efficiency with which you can write information into the spin of these color centers, as well as the efficiency with which you can collect photons emitted from the color centers," explains co-author Jennifer Choy, a graduate student in Lončar's lab at SEAS. "The other challenge has been the rate -- how quickly you can perform these processes."

Previous work from Lončar's group solved the collection efficiency problem by using diamond nanowires to channel and direct the flow of photons. The new research manipulates the radius of diamond pillars and adds the silver coating. The diamond-silver construction acts as an optical nanoresonator, creating a strong electromagnetic field around the emitter and offering a new level of control over the rate of emission.

Moreover, the device functions at room temperature -- an essential requirement for practical computing applications -- and the nanostructured chips are fully scalable.

"We've designed everything in parallel in a massive system, which allows us to make thousands or millions of devices with more or less the same properties, and we use conventional microfabrication and nanofabrication techniques, unlike what has been done in this field before," says Birgit Hausmann, a graduate student in Lončar's lab at SEAS and one of the co-authors.

In addition to Lončar, Choy, Hausmann, and Bulu, co-authors included Tom Babinec, a graduate student at SEAS; Mughees Khan, a staff scientist at the Wyss Institute for Biologically Inspired Engineering at Harvard; Patrick Maletinsky, a fellow of the Department of Physics at Harvard; and Amir Jacoby, Professor of Physics in the Harvard Faculty of Arts and Sciences.

The work was supported by grants and fellowships from the U.S. Department of Defense, the Defense Advanced Research Projects Agency (DARPA) QuEST program, the National Science Foundation (NSF), the King Abdullah University of Science and Technology (KAUST), the Sloan Foundation, and the NSF-supported Nanoscale Science and Engineering Center (NSEC) at Harvard. Fabrication took place at the NSF-supported Center for Nanoscale Systems (CNS) at Harvard.

The above story is reprinted (with editorial adaptations) from materials provided by Harvard University.

Journal Reference:

Jennifer T. Choy, Birgit J. M. Hausmann, Thomas M. Babinec, Irfan Bulu, Mughees Khan, Patrick Maletinsky, Amir Yacoby, Marko Lončar. Enhanced single-photon emission from a diamond–silver aperture. Nature Photonics, 2011; DOI: 10.1038/nphoton.2011.249

Molecular depth profiling modeled using buckyballs and low-energy argon

 A team of scientists led by a Penn State University chemist has demonstrated the strengths and weaknesses of an alternative method of molecular depth profiling -- a technique used to analyze the surface of ultra-thin materials such as human tissue, nanoparticles, and other substances.

In the new study, the researchers used computer simulations and modeling to show the effectiveness and limitations of the alternative method, which is being used by a research group in Taiwan. The new computer-simulation findings may help future researchers to choose when to use the new method of analyzing how and where particular molecules are distributed throughout the surface layers of ultra-thin materials.

The research will be published in the Journal of Physical Chemistry Letters.

Team leader Barbara Garrison, the Shapiro Professor of Chemistry and the head of the Department of Chemistry at Penn State University, explained that bombarding a material with buckyballs -- hollow molecules composed of 60 carbon atoms that are formed into a spherical shape resembling a soccer ball - is an effective means of molecular depth profiling. The name, "buckyball," is an homage to an early twentieth-century American engineer, Buckminster Fuller, whose design of a geodesic dome very closely resembles the soccer-ball-shaped 60-carbon molecule.

"Researchers figured out a few years ago that buckyballs could be used to profile molecular-scale depths very effectively," Garrison explained. "Buckyballs are much bigger and chunkier than the spacing between the molecules at the surface of the material being studied, so when the buckyballs hit the surface, they tend to break it up in a way that allows us to peer inside the solid and to actually see which molecules are arranged where. We can see, for example, that one layer is composed of one kind of molecule and the next layer is composed of another kind of molecule, similar to the way a meteor creates a crater that exposes sub-surface layers of rock."

Garrison and her colleagues decided to use computer modeling to test the effectiveness of an alternative approach that another research group had been using. The other group had used not only large, high-energy buckyballs to bombard a surface, but also another smaller, low-energy chemical element -- argon -- in the process. "In our computer simulations, we modeled the bombardment of surfaces first with high-energy buckyballs and then later, with low-energy argon atoms," Garrison said.

Garrison's group found that, with buckyball bombardment alone at grazing angles, the end result is a very rough surface with many troughs and ridges in one direction. "In many instances, this approach works out well for depth profiling. However, in other instances, using buckyballs alone makes for a bumpy surface on which to perform molecular depth profiling because the molecules can be distributed unevenly throughout the peaks and valleys," Garrison explained. "In these instances, when low-energy argon bombardment is added to the process, the result is a much more even, smoother surface, which, in turn, makes for a better area on which to do analyses of molecular arrangement. In these cases, researchers can get a clearer picture of the many layers of molecules and exactly which molecules make up each layer."

However, Garrison's team also concluded that the argon must be low enough in energy in order to avoid further damage of the molecules that are being profiled. "According to our simulations, the bottom line is that the buckyball conditions that the other research group used are not the best for depth profiling; thus, co-bombardment with low-energy argon assisted the process," Garrison said. "That is, the co-bombardment method works only in some very specific instances. We do not think low-energy argon will help in instances where the buckyballs are at sufficiently high energies." Garrison added that previous researchers had tried using smaller, simpler atomic projectiles at high, rather than low energies, but these projectiles tended to simply penetrate deeply into the surface, without giving scientists a clear view into the arrangement and identity of the molecules beneath.

Garrison said that molecular depth profiling is a crucial aspect of many chemical experiments and its applications are far-reaching. For example, molecular depth profiling is one way to get around the challenges of working with something so small and intricate as a biological cell. A cell is composed of thin layers of distinct materials, but it is difficult to slice into something so tiny to analyze the composition of those super-fine layers. In addition, molecular depth profiling can be used to analyze other kinds of human tissue, such as brain tissue -- a process that could help researchers to understand neurological disease and injury. In the future, molecular depth profiling also could be used to study nanoparticles -- extremely small objects with dimensions of between 1 and 10 nanometers, visible only with an electron microscope. Because nanoparticles already are being used experimentally as drug-delivery systems, a detailed analysis of their properties using molecular depth profiling could help researchers to test the effectiveness of the drug-delivery systems.

In addition to Garrison, other members of the research team include Zachary J. Schiffer, a high-school student at the State College Area High School near the Penn State University Park campus, Paul E. Kennedy of Penn State's Department of Chemistry, and Zbigniew Postawa of the Smoluchowski Institute of Physics at Jagiellonian University in Poland.

Funding for this research was provided by the National Science Foundation and the Polish Ministry of Science and Higher Education.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by Penn State.

Journal Reference:

Zachary J. Schiffer, Paul E. Kennedy, Zbigniew Postawa, Barbara J. Garrison. Molecular Dynamics Simulations Elucidate the Synergy of C60and Low-Energy Ar Cobombardment for Molecular Depth Profiling. The Journal of Physical Chemistry Letters, 2011; 2635 DOI: 10.1021/jz201219x

Light can detect pre-cancerous colon cells

After demonstrating that light accurately detected pre-cancerous cells in the lining of the esophagus, Duke University bioengineers turned their technology to the colon and have achieved similar results in a series of preliminary experiments.

This technology could be a non-invasive way for physicians to detect abnormal cells, or dysplasia, which have the potential of turning cancerous. These cells are in the epithelium, or lining, of various tissues, including the esophagus and colon.

Current biopsy techniques require physicians to take many random tissue samples, and for some disorders of the colon, these procedures can be disfiguring and life-changing. Instead of taking tissue samples, the new system would aim short bursts of light from the tip of an endoscope at locations suspected of having disease.

"When light is directed at these tissues, it scatters," said Adam Wax, Theodroe Kennedy associate professor of biomedical engineering at Duke's Pratt School of Engineering, who developed the device. The results of the Duke team's preliminary experiments were reported online in the Journal of Biomedical Optics. "We can collect and analyze that scattered light looking for the tell-tale signs of dysplasia. Significantly, the technique is noninvasive so no tissue is taken and no dyes or contrast agents are needed."

In particular, they are trying to spot characteristic changes within the cells of the epithelium. In the case of pre-cancerous cells, the nuclei are misshapen and larger than normal cells, and they scatter light in their own unique way.

"The important thing for clinicians is being able to detect these changes in the nuclei in cells just below the surface, which might not be detected by just looking at the lining of the colon through an endoscope alone," Wax said.

The technology that Wax and his team developed for cancer detection is known as angle-resolved low coherence interferometry (a/LCI). In this process, light is shined into a cell and sensors capture and analyze the light as it is reflected back. The technique separates the unique patterns of the nucleus from the other parts of the cell and provides representations of its changes in shape .

"This approach could be the future of diagnosing dysplasia of the colon," said Dr. Christopher Mantyh, colorectal surgeon at Duke University Medical Center and member of the research team. "The old-fashioned techniques we use haven't changed in years. This could be a real game-changer in how we detect, characterize and even treat precancerous or cancerous lesions. For some gastrointestinal biopsies, the procedure itself has inherent risks such as bleeding or perforation, so a non-invasive technique could greatly improve a patient's quality of life."

In their experiments, the Duke team used the device on samples of colon removed from 27 patients suspected have having colon cancer. The researchers then compared the results obtained from their device to the actual findings made by pathologists, and found that the overall accuracy of the device was 85 percent. Interestingly, the accuracy of the same technology was 86 percent when used during a recent clinical trial involving patients suspected of having Barrett's esophagus, a precursor to esophageal cancer.

Mantyh said he believes the new approach could be especially useful for people with inflammatory bowel disease, since they tend to have a higher incidence of dysplasia in the colon.

Since approximately 85 percent of all cancers begin within the layers of the epithelium in various parts of the body, Wax believes that the new system could also work in such cancers as those of the trachea, cervix or bladder.

Provided by Duke University (news : web)

Drug tracked in tissue

 When a new drug is developed, the manufacturer must be able to show that it reaches its intended goal in the body's tissue, and only that goal. Such studies could be made easier with a new method now established at Lund University in Sweden.

The method is a special type of mass spectrometry which can be used on drugs 'off the shelf', i.e. without any radioactive labelling which may change the behaviour of the drug. With this method, researchers György Marko-Varga and Thomas Fehniger have managed to create a molecular image of the drug in the tissue.

The tissue examined comes from biopsies from the lungs of patients with lung cancer and chronic obstructive lung disease (COPD), who have inhaled a drug to dilate the airways. The examination showed the precise spatial distribution of the drug within the tissue. The results are based on an analysis of 3 000 measurement points of 0.01 mm2 in each biopsy sample.

"When you want to register a new drug, you must be able to both explain its exact mechanisms of action and show that it is effective and safe. In order to avoid side-effects, the drug should reach only the cells for which it is intended. Our new technical platform makes it easier to show this", says György Marko-Varga.

He believes it will be possible to use the new technology to develop safer and more effective drug candidates. In the future it could also be used in clinical treatment, to help doctors select the right drug for a specific patient.

The researchers first conducted animal experiments, using drug doses 100 times higher than those now measured in patients. The group then optimised and refined the technology to achieve the sensitivity needed for measuring doses of drugs normally administered to patients.

Professors György Marko-Varga and Thomas Fehniger are both members of the Department of Measurement Technology and Industrial Electrical Engineering in Lund. Thomas Fehniger (who is the principal author of the article) is currently working at The Tallinn University of Technology, sponsored by the Estonian Science Foundation under the European Social Fund, while György Marko-Varga works part time for the University of Tokyo.

The two researchers have previously worked at the pharmaceuticals company AstraZeneca, which has also contributed to the study. The study was recently published in the journal Analytical Chemistry.

More information: A text about the research findings is available at http://pubs.acs.or … 41scene.html and the research article is available at http://pubs.acs.org/doi/abs/10.1021/ac2014349

Provided by Lund University