It is often assumed that it is not possible to alter the properties of magnetic materials once they have been prepared and put into use. For example, although magnetic materials are used in information technology to store trillions of bits (in the form of magnetization directions established by applying external magnetic fields), the properties of the magnetic medium itself remain unchanged on magnetization reversal. The ability to externally control the properties of magnetic materials would be highly desirable from fundamental and technological viewpoints, particularly in view of recent developments in magnetoelectronics and spintronics. In semiconductors, the conductivity can be varied by applying an electric field, but the electrical manipulation of magnetism has proved elusive. Here we demonstrate electric-field control of ferromagnetism in a thin-film semiconducting alloy, using an insulating-gate field-effect transistor structure. By applying electric fields, we are able to vary isothermally and reversibly the transition temperature of hole-induced ferromagnetism.

Long-distance quantum teleportation and quantum repeater technologies require entanglement between a single matter quantum bit (qubit) and a telecommunications (telecom)-wavelength photonic qubit. Electron spins in III-V semiconductor quantum dots are among the matter qubits that allow for the fastest spin manipulation and photon emission, but entanglement between a single quantum-dot spin qubit and a flying (propagating) photonic qubit has yet to be demonstrated. Moreover, many quantum dots emit single photons at visible to near-infrared wavelengths, where silica fibre losses are so high that long-distance quantum communication protocols become difficult to implement. Here we demonstrate entanglement between an InAs quantum-dot electron spin qubit and a photonic qubit, by frequency downconversion of a spontaneously emitted photon from a singly charged quantum dot to a wavelength of 1,560 nanometres. The use of sub-10-picosecond pulses at a wavelength of 2.2 micrometres in the frequency downconversion process provides the necessary quantum erasure to eliminate which-path information in the photon energy. Together with previously demonstrated indistinguishable single-photon emission at high repetition rates, the present technique advances the III-V semiconductor quantum-dot spin system as a promising platform for long-distance quantum communication.

A solid-state implementation of a quantum computer composed entirely of silicon is proposed. Qubits are 29 Si nuclear spins arranged as chains in a 28 Si (spin-0) matrix with Larmor frequencies separated by a large magnetic field gradient. No impurity dopants or electrical contacts are needed. Initialization is accomplished by optical pumping, algorithmic cooling, and pseudo-pure state techniques. Magnetic resonance force microscopy is used for readout. This proposal takes advantage of many of the successful aspects of solution NMR quantum computation, including ensemble measurement, RF control, and long decoherence times, but it allows for more qubits and improved initialization.PACS numbers: 03.67. Lx, 81.16.Rf, 76.60.Pc, 07.79.Pk The primary difficulty in the construction of quantum computers is the need to isolate the qubits from the environment to prevent decoherence, while still allowing initialization, control, and measurement. To date, the most successful experimental realizations of multiqubit, many-gate quantum computers have used roomtemperature, liquid NMR with "pseudo-pure" states [1]. These computers are able to maintain isolation from the control and measurement circuitry by employing weak measurement on a large ensemble of ∼10 18 uncoupled, identical molecules. Although such a large, highly mixed ensemble may bring the existence of entanglement into question [2], the arbitrary unitary evolution afforded by the RF-controlled quantum gates assures that these computers behave non-classically [3]. Their principal limitation results from their small initial nuclear polarization. The size of the effective sub-ensemble of nuclei contributing to the pseudo-pure state, and hence the effective Signal-to-Noise Ratio (SNR), decreases exponentially with each added qubit, leaving this method unlikely to exceed the 10-qubit level without substantial modification [4].The proposals of Kane [5] and others to use single nuclear spins in a low-temperature solid solve the scalability problem of solution NMR, but they introduce the problem of single-nuclear-spin measurement. It remains an experimental challenge to fabricate a structure in which individual nuclei are sufficiently coupled to an electronic system for single-spin measurement, but also sufficiently isolated for long coherence times.In this Letter, we propose a different solid-state NMR implementation of quantum computation which introduces electron-mediated cooling, but maintains the weak ensemble measurement that has made solution NMR quantum computers so successful. The device is made entirely of silicon, with no electrical gates or impurities. As will be discussed below, the qubits are spin-1/2 nuclei that are located in relatively isolated atomic chains, as shown in Fig. 1. The nuclei within each chain are distinguished by a large magnetic field gradient created with a nearby microfabricated ferromagnet [6]. Each nucleus has about 10 5 ensemble copies in a plane orthogonal to its chain. This structure is embedded in a thin bridge whose oscillations...

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A highly efficient dye-sensitized solar cell (DSC) was fabricated using a nanocrystalline nitrogen-doped titania electrode. The properties of the nitrogen-doped titania powder, film, and solar cell were investigated. The substitution of oxygen sites with nitrogen atoms in the titania structure was confirmed by X-ray photoemission spectroscopy (XPS). The UV-vis spectrum of the nitrogen-doped powder and film showed a visible light absorption in the wavelength range from 400 to 535 nm. An enhancement of the incident photon-to-current conversion efficiency (IPCE) in the range of 380-520 nm and 550-750 nm was observed. An 8% overall conversion efficiency has been achieved. The results of the stability test indicated that the solar cell fabricated by the nitrogen-doped titania exhibited great stability.

We report NMR experiments using high-power, RF decoupling techniques to show that a 29 Si nuclear spin qubit in a solid silicon crystal at room temperature can preserve quantum phase for 10 9 precessional periods. The coherence times we report are longer than for any other observed solid-state qubit by more than four orders of magnitude. In high quality crystals, these times are limited by residual dipolar couplings and can be further improved by isotopic depletion. In defect-heavy samples, we provide evidence for decoherence limited by 1/f noise. These results provide insight toward proposals for solid-state nuclear-spin-based quantum memories and quantum computers based on silicon.PACS numbers: 03.67. Lx, 03.67.Pp, 76.60.Lz, 82.56.Jn Quantum information processing devices outperform their classical counterparts by preserving and exploiting the correlated phases of their constituent quantum oscillators, which are usually two-state systems called "qubits." An increasing number of theoretical proposals have shown that such devices allow secure long-distance communication and improved computational power [1]. Solid-state implementations of these devices are favored for reasons of both scalability and integration with existing hardware, although previous experiments have shown limited coherence times for solid-state qubits. The development of quantum error correcting codes [2] and fault tolerant quantum computation [3] showed that largescale quantum algorithms are still theoretically possible in the presence of decoherence. However, the coherence time must be dauntingly long: approximately 10 5 times the duration of a single quantum gate, and probably longer depending on the quantum computer architecture [1]. The question of whether a scalable implementation can surpass this coherence threshold is not only important for the technological future of quantum computation, but also for fundamental understanding of the border between microscopic quantum behavior and macroscopic classical behavior.Experimentally observed coherence times (T 2 ) for various qubit implementations are shown in Table I. The atomic systems shown -trapped ions and molecular nuclei in liquid solution -have already been employed for small quantum algorithms [6,13]; not coincidentally, they show very high values of Q, the product of the qubit frequency ω 0 /2π and πT 2 . Most solid-state qubits show smaller values of Q; as we demonstrate in this Letter, however, the long coherence times we observe in solid-state 29 Si nuclei afford them a Q and ΩT 2 as high or higher than atomic systems, indicating this system's promise for solid-state quantum computing.Many promising qubits and coherence time measurements are not mentioned in Table I either because reliable experimental data are not available, or because the existing experimental data are taken under conditions not sufficiently similar to the corresponding quantum computer architecture. For example, electron spins bound to phosphorous donors in pure, isotopically depleted silicon also show prom...

Let F be a non-Archimedean local field with the ring of integers O and the prime ideal p and let G = G ad (O/p n) be the adjoint Chevalley group. Let m f (G) denote the smallest possible dimension of a faithful representation of G. Using the Stone-von Neumann theorem, we determine a lower bound for m f (G) which is asymptotically the same as the results of Landazuri, Seitz and Zalesskii for split Chevalley groups over Fq. Our result yields a conceptual explanation of the exponents that appear in the aforementioned results