No abstract

Single-electron circuits of the future, consisting of a network of quantum dots, will require a mechanism to transport electrons from one functional part of the circuit to another. For example, in a quantum computer decoherence and circuit complexity can be reduced by separating quantum bit (qubit) manipulation from measurement and by providing a means of transporting electrons between the corresponding parts of the circuit. Highly controlled tunnelling between neighbouring dots has been demonstrated, and our ability to manipulate electrons in single- and double-dot systems is improving rapidly. For distances greater than a few hundred nanometres, neither free propagation nor tunnelling is viable while maintaining confinement of single electrons. Here we show how a single electron may be captured in a surface acoustic wave minimum and transferred from one quantum dot to a second, unoccupied, dot along a long, empty channel. The transfer direction may be reversed and the same electron moved back and forth more than sixty times-a cumulative distance of 0.25 mm-without error. Such on-chip transfer extends communication between quantum dots to a range that may allow the integration of discrete quantum information processing components and devices.

In a one-dimensional (1D) system of interacting electrons, excitations of spin and charge travel at different speeds, according to the theory of a Tomonaga-Luttinger Liquid (TLL) at low energies. However, the clear observation of this spin-charge separation is an ongoing challenge experimentally. We have fabricated an electrostatically-gated 1D system in which we observe spin-charge separation and also the predicted power-law suppression of tunnelling into the 1D system. The spin-charge separation persists even beyond the low-energy regime where the TLL approximation should hold. TLL effects should therefore also be important in similar, but shorter, electrostatically gated wires, where interaction effects are being studied extensively worldwide.The effects of interactions are almost impossible to calculate in a general many-particle system, though they cannot be ignored. However, for a one-dimensional (1D) system, Luttinger, building on an approximation scheme of Tomonaga, constructed a soluble 1D model with infinite linear dispersion and a restricted set of interactions. The solution has 1 arXiv:1002.2782v1 [cond-mat.str-el]

We report a detailed experimental study of the quantized acoustoelectric current induced by a surface acoustic wave in a one-dimensional channel defined in a GaAs-Al x Ga 1Ϫx As heterostructure by a split gate. The current measured as a function of the gate voltage demonstrates quantized plateaus in units of Iϭe f where e is the electron charge and f is the surface acoustic wave frequency, the effect first observed by Shilton et al. The quantization is due to trapping of electrons in the moving potential wells induced by the surface acoustic wave, with the number of electrons in each well controlled by electron-electron repulsion. The experimental results demonstrate that acoustic charge transport in a one-dimensional channel may be a viable means of producing a standard of electrical current.

The spin field effect transistor envisioned by Datta and Das[1] opens a gateway to spin information processing [2,3]. Although the coherent manipulation of electron spins in semiconductors is now possible [4][5][6][7], the realization of a functional spin field effect transistor for information processing has yet to be achieved, owing to several fundamental challenges such as the low spin-injection efficiency due to resistance mismatch [9], spin relaxation, and the spread of spin precession angles. Alternative spin transistor designs have therefore been proposed [10,11], but these differ from the field effect transistor concept and require the use of optical or magnetic elements, which pose difficulties for the incorporation into integrated circuits. Here, we present an all-electric and all-semiconductor spin field effect transistor, in which these obstacles are overcome by employing two quantum point contacts as spin injectors and detectors. Distinct engineering architectures of spin-orbit coupling are exploited for the quantum point contacts and the central semiconductor channel to achieve complete control of the electron spins-spin injection, manipulation, and detection-in a purely electrical manner. Such a device is compatible with large-scale integration and hold promise for future spintronic devices for information processing.Spin-orbit (SO) coupling-the interaction between a particle's spin and its motion-can be appreciated in the framework of an effective magnetic field B SO , which acts on charged particles when they move in an electric field E and is described by, where is Planck's constant divided by 2π, c is the speed of light, k is the particle's wavevector, and m is its mass. In semiconductor heterostructures, the electric field which gives rise to B SO can be created by breaking the structural inversion symmetry in the material, namely, the Rashba SO coupling [12,13]. Moreover, this electric field can easily be varied using metallic gates [14,15], thus controlling B SO . Such an effective magnetic field creates a link between the magnetic moment of the particle (spin) and the electric field acting upon it, offering a route for fast and coherent electrical control of spin states. While the SO coupling has been utilized for spin manipulation, approaches to spin injection and detection still rely on ferromagnetic and/or optical components, and the demonstration of an all-electric spin transistor device has remained elusive.Figure 1 illustrates our proposed spin field effect transistor (FET) and its operating principle. An InGaAs heterostructure (see Methods Summary), one of the strong contenders to replace Si in future generations of largescale integrated circuits (see International Technology Roadmap for Semiconductors; http://public.itrs.net), is used to provide a two-dimensional electron gas (2DEG) channel for ballistic electron transport under a metallic middle gate and between two gate-defined quantum point contacts (QPCs). The QPCs are narrow and short onedimensional (1D) constrictions, usually...