Nano News: 09/07/15

Carbon nanotubes open new path toward quantum information technologies

In optical communication, critical information ranging from a credit card number to national security data is transmitted in streams of laser pulses. However, the information transmitted in this manner can be stolen by splitting out a few photons (the quantum of light) of the laser pulse. This type of eavesdropping could be prevented by encoding bits of information on quantum mechanical states (e.g. polarization state) of single photons. The ability to generate single photons on demand holds the key to realization of such a communication scheme. By demonstrating that incorporation of pristine single-walled carbon nanotubes into a silicon dioxide (SiO2) matrix could lead to creation of solitary oxygen dopant state capable of fluctuation-free, room-temperature single photon emission, Los Alamos researchers revealed a new path toward on-demand single photon generation. published their findings ("Room-temperature single-photon generation from solitary dopants of carbon nanotubes").

A solitary oxygen dopant (red sphere) covalently attached to the sidewall of the carbon nanotube (gray) can generate single photons (red) at room temperature when excited by laser pulses (green). Significance of the research Photons emitted from lasers are distributed randomly in time. Therefore, “simultaneous” emission of two or more photons is possible. True single photon generation requires an isolated quantum mechanical two-level system that can emit only one photon in one excitation-emission cycle. Technological requirements of materials for quantum communication include the ability to generate single photons in the 1,300 – 1,500 nanometer (nm) telecommunication wavelength range at room temperature and compatibility with silicon microfabrication technology to enable electrical stimulation and integration of other electronic and photonic network components. Earlier studies revealed that carbon nanotubes present technical challenges for use in quantum communications: 1) the materials were capable of single photon emission only at cryogenic temperature, and 2) their inefficient emission had strong fluctuations and degradation. The Laboratory’s new research has demonstrated that incorporation of pristine carbon nanotubes into a silicon dioxide (SiO2) matrix could lead to incorporation of solitary oxygen dopant states capable of fluctuation-free, room-temperature single photon emission in the 1100 - 1300 nm wavelength range. The oxygen-doped nanotubes can be encapsulated in a SiO2 layer deposited on a silicon wafer. This presents an opportunity to apply well-established micro-electronic fabrication technologies for the development of electrically driven single photon sources and integration of these sources into quantum photonic devices and networks. Beyond implementation of quantum communication technologies, nanotube-based single photon sources could enable transformative quantum technologies including ultra-sensitive absorption measurements, sub-diffraction imaging, and linear quantum computing. The material has potential for photonic, plasmonic, optoelectronic, and quantum information science applications. Research achievements By using a state-of-the-art photon detector, the team measured the temporal distribution of two successive photon emission events and demonstrated single photon emission. In addition, the team investigated the effects of temperature on photoluminescence emission efficiencies, fluctuations, and decay dynamics of the dopant states in the single-walled carbon nanotube. The researchers determined the conditions most suitable for the observation of single-photon emission. In principle, the emission could be tuned to 1500 nm via doping of smaller band-gap single-walled carbon nanotubes. This is a distinct advantage compared with some other materials, in which single photon emission is possible for only a few discrete wavelengths shorter than 1 µm.

Scientists witness and measure displacement of atoms

Using state of the art technology, researchers at the Monash Centre for Electron Microscopy (MCEM) have developed new methods which allow tiny displacements of atoms to be witnessed and measured. The research, published in the latest edition of prestigious journal ("Direct mapping of Li-enabled octahedral tilt ordering and associated strain in nanostructured perovskites"), provides immediate insights into lithium-ion battery performance and far-reaching implications for the design of new materials for energy generation and storage, next gen computing, green technologies, and other areas. “The Monash research is like doctors being able to “see” behaviours of individual viruses rather than just imagining them by observing the symptoms. We can study atomic behaviour with picometre precision. To put this in perspective, a hydrogen atom has an estimated diameter of ~50 picometres,” lead research author Dr. Ye Zhu said. By manipulating these atomic displacements, researchers have the potential to create ‘wonder materials’ for use in applications such as high performance computers, ultra-efficient solar cells and environmentally friendly sensors. “Atoms are the building blocks of nature. If the position of these building blocks is varied, even slightly, the impact on the function of a material can be profound,” said corresponding author, Professor Joanne Etheridge, Director of MCEM. “This new method, combined with MCEM’s powerful electron microscopes, has unveiled exquisitely subtle variations in the arrangement of atoms that drive the important properties of this material.” Researchers believe that the imaging method should be equally applicable to a variety of material systems and will become a popular and powerful tool in providing real-space structure information. “This paper reveals the extraordinary power of modern electron microscopy to directly map the fine details of complex crystal structure, in this case that of a remarkable self-assembled nanostructure with a compositionally tuneable nano-scale periodicity,” co-author Professor Ray Withers of ANU said. The MCEM is a leading research centre in electron microscopy that combines a state-of-the-art technology with international specialist expertise in the development of methods to determine atomic structures.

Semiconductor quantum dots as ideal single-photon source

With the help of a semiconductor quantum dot, physicists at the University of Basel have developed a new type of light source that emits single photons. For the first time, the researchers have managed to create a stream of identical photons. They have reported their findings in the scientific journal ("Transform-limited single photons from a single quantum dot") together with colleagues from the University of Bochum.

Semiconductor quantum dot emitting a stream of identical photons. (Image: University of Basel) A single-photon source never emits two or more photons at the same time. Single photons are important in the field of quantum information technology where, for example, they are used in quantum computers. Alongside the brightness and robustness of the light source, the indistinguishability of the photons is especially crucial. In particular, this means that all photons must be the same color. Creating such a source of identical single photons has proven very difficult in the past. However, quantum dots made of semiconductor materials are offering new hope. A quantum dot is a collection of a few hundred thousand atoms that can form itself into a semiconductor under certain conditions. Single electrons can be captured in these quantum dots and locked into a very small area. An individual photon is emitted when an engineered quantum state collapses. Noise in the semiconductor A team of scientists led by Dr. Andreas Kuhlmann and Prof. Richard J. Warburton from the University of Basel have already shown in past publications that the indistinguishability of the photons is reduced by the fluctuating nuclear spin of the quantum dot atoms. For the first time ever, the scientists have managed to control the nuclear spin to such an extent that even photons sent out at very large intervals are the same color. Quantum cryptography and quantum communication are two potential areas of application for single-photon sources. These technologies could make it possible to perform calculations that are far beyond the capabilities of today's computers.