A qubit candidate shines brighter

In the race to design the world's first universal quantum computer, a special kind of diamond defect called a nitrogen vacancy (NV) center is playing a big role. NV centers consist of a nitrogen atom and a vacant site that together replace two adjacent carbon atoms in diamond crystal. The defects can record or store quantum information and transmit it in the form of light, but the weak signal is hard to identify, extract and transmit unless it is intensified. Now a team of researchers at Harvard, the University of California, Santa Barbara and the University of Chicago has taken a major step forward in effectively enhancing the fluorescent light emission of diamond nitrogen vacancy centers - a key step to using the atom-sized defects in future quantum computers. The technique, described in the journal ("Deterministic coupling of delta-doped nitrogen vacancy centers to a nanobeam photonic crystal cavity"), hinges on the very precise positioning of NV centers within a structure called a photonic cavity that can boost the light signal from the defect. A scanning electron microscope image of a diamond photonic cavity A scanning electron microscope image of the diamond photonic cavity shows the nanoscale holes etched through the layer containing NV centers. The scale bar indicates 200 nanometers. (Image: Evelyn Hu/Harvard) A Potential Qubit Power Couple NV centers contain an unpaired electron that can store information in a property known as spin. Researchers can "read" the spin state of the electron by observing the intensity of particular frequencies of the light that the NV center emits when illuminated by a laser. At room temperatures, this pattern of light emission couples to multiple "sideband" frequencies, making it difficult to interpret. To amplify the most important element of the signal researchers can use a structure called a photonic cavity, which consists of a pattern of nanoscale holes that serve to enhance the NV center's light emission at its main frequency. "A photonic cavity that is properly matched to the NVs can substantially augment their capabilities," said Evelyn Hu, a researcher at Harvard whose group studies the optical and electronic behavior of materials that have been carefully sculpted at the nanoscale. NV centers whose signal is enhanced by photonic cavities could act as qubits, the fundamental units of quantum information in a quantum computer. Matchmaker, Matchmaker, Make Me a Match Photonic cavities best enhance the signal of NV centers located in a "hot spot" where the cavities' resonant fields are strongest, but making sure an atom-sized defect's location matches up with this spot is extremely tricky. "Strong spatial overlap is the hardest [task] to achieve in designing and fabricating a photonic cavity for NV centers," Hu said. She compared the task to turning on a fixed small light beam in a dark room containing ultra-small transmitters that send out information once they are illuminated by the 'right' beam. If the match is right, the signal from the transmitter is returned strongly, but the challenge is that the chances of the light hitting the transmitter are very small. Hu and her colleagues ultimately aim to make sure the beam (or field of the photonic cavity) will always hit the transmitter (or NV center), so that information will always be read out. They can do this by knowing the exact position of the tiny NV centers. The team took an important first step toward this goal by controlling the depth of the diamond defects using a technique called delta doping. "Integrating a plane of spins into these structures enables us to engineering the spin-photon interaction and exploit quantum effects for future technologies," said David Awschalom, a researcher at the University of Chicago whose group grows and characterizes these systems. The technique confines the possible location of NV centers to a layer approximately 6 nanometers thick sandwiched inside a diamond membrane approximately 200 nanometers thick. The researchers then etched holes into the membrane to create the photonic cavities. Using this method the researchers were able to increase the intensity of the light emitted by the NV centers by a factor of about 30 times. The team believes they can further enhance the emission by also controlling the position of the defects in the horizontal plane and are currently working on possible ways to achieve full 3-D control. Computers, Sensors and More Nitrogen vacancy centers aren't the only candidate for qubits, but they have attracted a lot of interest because their electrons have long spin lifetimes at room temperature, meaning they can maintain quantum information for a relatively long time. And the promise of NV centers doesn't stop at ultrafast computers. NV centers can also be used in non-computing applications, for examples as molecular-scale magnetic and temperature sensors that could measure the properties within single cells.
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Study unveils new half-light half-matter quantum particles

Prospects of developing computing and communication technologies based on quantum properties of light and matter may have taken a major step forward thanks to research by City College of New York physicists led by Dr. Vinod Menon. In a pioneering study ("Strong light–matter coupling in two-dimensional atomic crystals"), Professor Menon and his team were able to discover half-light, half-matter particles in atomically thin semiconductors (thickness ∼ a millionth of a single sheet of paper) consisting of two-dimensional (2D) layer of molybdenum and sulfur atoms arranged similar to graphene. They sandwiched this 2D material in a light trapping structure to realize these composite quantum particles. "Besides being a fundamental breakthrough, this opens up the possibility of making devices which take the benefits of both light and matter," said Professor Menon. For example one can start envisioning logic gates and signal processors that take on best of light and matter. The discovery is also expected to contribute to developing practical platforms for quantum computing. Dr. Dirk Englund, a professor at MIT whose research focuses on quantum technologies based on semiconductor and optical systems, hailed the City College study. "What is so remarkable and exciting in the work by Vinod and his team is how readily this strong coupling regime could actually be achieved. They have shown convincingly that by coupling a rather standard dielectric cavity to exciton-polaritons in a monolayer of molybdenum disulphide, they could actually reach this strong coupling regime with a very large binding strength," he said. Professor Menon's research team included City College PhD students, Xiaoze Liu, Tal Galfsky and Zheng Sun, and scientists from Yale University, National Tsing Hua University (Taiwan) and Ecole Polytechnic -Montreal (Canada).
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Gummy bears under antiparticle fire

Gelatin is used in the pharmaceutical industry to encapsulate active agents. It protects against oxidation and overly quick release. Nanopores in the material have a significant influence on this, yet they are difficult to investigate. In experiments on gummy bears, researchers at Technische Universität München (TUM) have now transferred a methodology to determine the free volume of gelatin preparations ("The Free Volume in Dried and H2O-Loaded Biopolymers Studied by Positron Lifetime Measurements"). red Gummy bear Gummy bear on the experimental set-up. To prevent color influences, the researchers used red gummy bears only. Custom-tailored gelatin preparations are widely used in the pharmaceutical industry. Medications that do not taste good can be packed into gelatin capsules, making them easier to swallow. Gelatin also protects sensitive active agents from oxidation. Often the goal is to release the medication gradually. In these cases slowly dissolving gelatin is used. Nanopores in the material play a significant role in all of these applications. “The larger the free volume, the easier it is for oxygen to penetrate it and harm the medication, but also the less brittle the gelatin,” says PD Dr. Christoph Hugenschmidt, a physicist at TU München. However, characterizing the size and distribution of these free spaces in the unordered biopolymer is difficult. A methodology adapted by the Garching physicists Christoph Hugenschmidt and Hubert Ceeh provides relief. “Using positrons as highly mobile probes, the volume of the nanopores can be determined, especially also in unordered systems like netted gelatins,” says Christoph Hugenschmidt. Positrons are the antiparticles corresponding to electrons. They can be produced in the laboratory in small quantities, as in this experiment, or in large volumes at the Heinz Maier Leibnitz Research Neutron Source (FRM II) of the TU München. If a positron encounters an electron they briefly form an exotic particle, the so-called positronium. Shortly after it annihilates to a flash of light. To model gelatin capsules that slowly dissolve in the stomach, the scientists bombarded red gummy bears in various drying stages with positrons. Their measurements showed, that in dry gummy bears the positroniums survive only 1.2 nanoseconds on average while in soaked gummy bears it takes 1.9 nanoseconds before they are annihilated. From the lifetime of the positroniums the scientists can deduce the number and size of nanopores in the material.
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