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We report on the intrinsic optoelectronic response of high-quality dual-gated monolayer and bilayer graphene p-n junction devices. Local laser excitation (of wavelength 850 nanometers) at the p-n interface leads to striking six-fold photovoltage patterns as a function of bottom-and top-gate voltages. These patterns, together with the measured spatial and density dependence of the photoresponse, provide strong evidence that nonlocal hot carrier transport, rather than the photovoltaic effect, dominates the intrinsic photoresponse in graphene. This regime, which features a long-lived and spatially distributed hot carrier population, may offer a path to hot carrier-assisted thermoelectric technologies for efficient solar energy harvesting.

The richness of optical and electronic properties of graphene attracts enormous interest. Graphene has high mobility and optical transparency, in addition to flexibility, robustness and environmental stability. So far, the main focus has been on fundamental physics and electronic devices. However, we believe its true potential lies in photonics and optoelectronics, where the combination of its unique optical and electronic properties can be fully exploited, even in the absence of a bandgap, and the linear dispersion of the Dirac electrons enables ultrawideband tunability. The rise of graphene in photonics and optoelectronics is shown by several recent results, ranging from solar cells and light-emitting devices to touch screens, photodetectors and ultrafast lasers. Here we review the state-of-the-art in this emerging field.

A wafer-scale graphene circuit was demonstrated in which all circuit components, including graphene field-effect transistor and inductors, were monolithically integrated on a single silicon carbide wafer. The integrated circuit operates as a broadband radio-frequency mixer at frequencies up to 10 gigahertz. These graphene circuits exhibit outstanding thermal stability with little reduction in performance (less than 1 decibel) between 300 and 400 kelvin. These results open up possibilities of achieving practical graphene technology with more complex functionality and performance.

A practical solution The best electronic and optoelectronic devices are built via semiconductor crystal growth on a single-crystal substrate. Over 100 papers have been published in recent years in Nature on alternative devices, produced instead from the solution phase. They have some advantages over conventional crystalline semiconductor devices: ease of fabrication, physical flexibility and — most important — low cost. The problem was the poor electronic performance of solution-processed devices, compared with single-crystal counterparts. But that could change now: a team from the University of Toronto reports that one such system — colloidal quantum dots of lead sulphide — can actually outperform the state-of-the-art crystalline alternative. A solution-processed electronic device that uses colloidal quantum dots of lead sulphide outperforms the state-of-the-art crystalline alternatives, with ease of fabrication, physical flexibility, large device areas and low cost among its benefits. Solution-processed electronic 1 and optoelectronic 2 , 3 , 4 , 5 devices offer low cost, large device area, physical flexibility and convenient materials integration compared to conventional epitaxially grown, lattice-matched, crystalline semiconductor devices. Although the electronic or optoelectronic performance of these solution-processed devices is typically inferior to that of those fabricated by conventional routes, this can be tolerated for some applications in view of the other benefits. Here we report the fabrication of solution-processed infrared photodetectors that are superior in their normalized detectivity ( D *, the figure of merit for detector sensitivity) to the best epitaxially grown devices operating at room temperature. We produced the devices in a single solution-processing step, overcoating a prefabricated planar electrode array with an unpatterned layer of PbS colloidal quantum dot nanocrystals. The devices showed large photoconductive gains with responsivities greater than 10 3  A W -1 . The best devices exhibited a normalized detectivity D * of 1.8 × 10 13  jones (1 jones = 1 cm Hz 1/2  W -1 ) at 1.3 µm at room temperature: today's highest performance infrared photodetectors are photovoltaic devices made from epitaxially grown InGaAs that exhibit peak D * in the 10 12  jones range at room temperature, whereas the previous record for D * from a photoconductive detector lies at 10 11  jones. The tailored selection of absorption onset energy through the quantum size effect, combined with deliberate engineering of the sequence of nanoparticle fusing and surface trap functionalization, underlie the superior performance achieved in this readily fabricated family of devices.

Despite several years of research into graphene electronics, sufficient on/off current ratio / on // off in graphene transistors with conventional device structures has been impossible to obtain. We report on a three-terminal active device, a graphene variable-barrier \"barristor\" (GB), in which the key is an atomically sharp interface between graphene and hydrogenated silicon. Large modulation on the device current (on/off ratio of 10⁵) is achieved by adjusting the gate voltage to control the graphene-silicon Schottky barrier. The absence of Fermi-level pinning at the interface allows the barrier's height to be tuned to 0.2 electron volt by adjusting graphene's work function, which results in large shifts of diode threshold voltages. Fabricating GBs on respective 150-mm wafers and combining complementary p-and n-type GBs, we demonstrate inverter and half-adder logic circuits.

By monitoring the environment of a superconducting quantum bit in real time, the quantum bit can be maintained in a pure state and its time evolution, as described by its ‘quantum trajectory’, can be tracked. To stabilize quantum systems — measure them A quantum state, such as a particle's superposition between two energy levels, quickly reverts to a classically described state on contact with the environment. To avoid this 'decoherence', large efforts are usually made to decouple quantum devices from their surroundings. But there is another way. Kater Murch et al . show that quantum coherence can be preserved by continuous, accurate monitoring of the environmental fluctuations. They studied a qubit consisting of a superconducting device embedded in a microwave cavity with fluctuations likely to cause decoherence. The act of accurately measuring either phase or amplitude of the fluctuations was found to steer the qubit's state along random trajectories that are purely quantum in nature. This work suggests a new type of control, harnessing action at a distance through measurement, for the manipulation of quantum systems in complex environments, ranging from biological systems to quantum computers. The length of time that a quantum system can exist in a superposition state is determined by how strongly it interacts with its environment. This interaction entangles the quantum state with the inherent fluctuations of the environment. If these fluctuations are not measured, the environment can be viewed as a source of noise, causing random evolution of the quantum system from an initially pure state into a statistical mixture—a process known as decoherence. However, by accurately measuring the environment in real time, the quantum system can be maintained in a pure state and its time evolution described by a ‘quantum trajectory’ 1 , 2 determined by the measurement outcome. Here we use weak measurements to monitor a microwave cavity containing a superconducting quantum bit (qubit), and track the individual quantum trajectories 3 of the system. In this set-up, the environment is dominated by the fluctuations of a single electromagnetic mode of the cavity. Using a near-quantum-limited parametric amplifier 4 , 5 , we selectively measure either the phase or the amplitude of the cavity field, and thereby confine trajectories to either the equator or a meridian of the Bloch sphere. We perform quantum state tomography at discrete times along the trajectory to verify that we have faithfully tracked the state of the quantum system as it diffuses on the surface of the Bloch sphere. Our results demonstrate that decoherence can be mitigated by environmental monitoring, and validate the foundation of quantum feedback approaches based on Bayesian statistics 6 , 7 , 8 . Moreover, our experiments suggest a new means of implementing ‘quantum steering’ 9 —the harnessing of action at a distance to manipulate quantum states through measurement.

Although electrochemical capacitors (ECs), also known as supercapacitors or ultracapacitors, charge and discharge faster than batteries, they are still limited by low energy densities and slow rate capabilities. We used a standard LightScribe DVD optical drive to do the direct laser reduction of graphite oxide films to graphene. The produced films are mechanically robust, show high electrical conductivity (1738 Siemens per meter) and specific surface area (1520 square meters per gram), and can thus be used directly as EC electrodes without the need for binders or current collectors, as is the case for conventional ECs. Devices made with these electrodes exhibit ultrahigh energy density values in different electrolytes while maintaining the high power density and excellent cycle stability of ECs. Moreover, these ECs maintain excellent electrochemical attributes under high mechanical stress and thus hold promise for high-power, flexible electronics.

The on-demand emission of coherent and indistinguishable electrons by independent synchronized sources is a challenging task of quantum electronics, in particular regarding its application for quantum information processing. Using two independent on-demand electron sources, we triggered the emission of two single-electron wave packets at different inputs of an electronic beam splitter. Whereas classical particles would be randomly partitioned by the splitter, we observed two-particle interference resulting from quantum exchange. Both electrons, emitted in indistinguishable wave packets with synchronized arrival time on the splitter, exited in different outputs as recorded by the low-frequency current noise. The demonstration of two-electron interference provides the possibility of manipulating coherent and indistinguishable single-electron wave packets in quantum conductors.

A computer built entirely using transistors based on carbon nanotubes, which is capable of multitasking and emulating instructions from the MIPS instruction set, is enabled by methods that overcome inherent challenges with this new technology. Computing with carbon nanotube transistors Carbon nanotubes have long been touted as promising building blocks for computers based on carbon rather than silicon. A main motivation towards this goal is the potential for circuits using carbon nanotube transistors to achieve high energy efficiency. Various carbon nanotube electronic circuit blocks have been demonstrated previously, but Max Shulaker et al . now reach a true milestone in the fields of carbon electronics and nanoelectronics by building a simple but functional computer made entirely from carbon nanotube transistors. Composed of 178 transistors, each containing between 10 and 200 carbon nanotubes, it runs a simple operating system and is capable of multitasking: it performs four tasks (summarized as instruction fetch, data fetch, arithmetic operation and write-back) and can run two different programs concurrently. The miniaturization of electronic devices has been the principal driving force behind the semiconductor industry, and has brought about major improvements in computational power and energy efficiency. Although advances with silicon-based electronics continue to be made, alternative technologies are being explored. Digital circuits based on transistors fabricated from carbon nanotubes (CNTs) have the potential to outperform silicon by improving the energy–delay product, a metric of energy efficiency, by more than an order of magnitude. Hence, CNTs are an exciting complement to existing semiconductor technologies 1 , 2 . Owing to substantial fundamental imperfections inherent in CNTs, however, only very basic circuit blocks have been demonstrated. Here we show how these imperfections can be overcome, and demonstrate the first computer built entirely using CNT-based transistors. The CNT computer runs an operating system that is capable of multitasking: as a demonstration, we perform counting and integer-sorting simultaneously. In addition, we implement 20 different instructions from the commercial MIPS instruction set to demonstrate the generality of our CNT computer. This experimental demonstration is the most complex carbon-based electronic system yet realized. It is a considerable advance because CNTs are prominent among a variety of emerging technologies that are being considered for the next generation of highly energy-efficient electronic systems 3 , 4 .

Taking aim at silicon Silicon transistors in microelectronics are shrinking to close to the size at which quantum effects begin to have an impact on device performance. As silicon looks certain to remain the semiconductor material of choice for a while yet, such effects may be turned into an advantage by designing silicon devices that can process quantum information. One approach is to make use of electron spins generated by phosphorus dopant atoms buried in silicon, as they are known to represent well-isolated quantum bits (qubits) with long coherence times. It has not been possible to control single electrons in silicon with the precision for qubits, but now Andrea Morello and colleagues report single-shot, time-resolved readout of electron spins in silicon. This is achieved by placing the phosphorus donor atoms near a charge-sensing device called a single-electron transistor, which is fully compatible with current microelectronic technology. The demonstrated high-fidelity single-shot spin readout opens a path to the development of a new generation of quantum computing and spintronic devices in silicon. Electron spins generated by phosphorus dopant atoms buried in silicon represent well-isolated quantum bits with long coherence times, but so far the control of such single electrons has been insufficient to use them in this way. These authors report single-shot, time-resolved readout of electron spins in silicon, achieved by coupling the donor atoms to a charge-sensing device called a single-electron transistor. This opens a path to the development of a new generation of quantum computing and spintronic devices in silicon. The size of silicon transistors used in microelectronic devices is shrinking to the level at which quantum effects become important 1 . Although this presents a significant challenge for the further scaling of microprocessors, it provides the potential for radical innovations in the form of spin-based quantum computers 2 , 3 , 4 and spintronic devices 5 . An electron spin in silicon can represent a well-isolated quantum bit with long coherence times 6 because of the weak spin–orbit coupling 7 and the possibility of eliminating nuclear spins from the bulk crystal 8 . However, the control of single electrons in silicon has proved challenging, and so far the observation and manipulation of a single spin has been impossible. Here we report the demonstration of single-shot, time-resolved readout of an electron spin in silicon. This has been performed in a device consisting of implanted phosphorus donors 9 coupled to a metal-oxide-semiconductor single-electron transistor 10 , 11 —compatible with current microelectronic technology. We observed a spin lifetime of ∼6 seconds at a magnetic field of 1.5 tesla, and achieved a spin readout fidelity better than 90 per cent. High-fidelity single-shot spin readout in silicon opens the way to the development of a new generation of quantum computing and spintronic devices, built using the most important material in the semiconductor industry.