We report experimental results on gated Y-branch switches made from InAs ballistic electron waveguides. We demonstrate that gating modifies the electron wave functions as well as their interference pattern, resulting in anticorrelated oscillatory transconductances. Our data provide evidence of steering the electron wave function in a multichannel transistor structure. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1867554͔ Quantum effects in nanostructures provide insights into fundamental issues that cannot be addressed in atomic physics settings and offer perspectives for future applications in computing. 1 In the regime where quantum effects dominate, electron transport exhibits different properties. For example, when the device size becomes less than the elastic mean-free path, electrons can traverse through the conductor ballistically, leading to conductance quantization. In addition, phase coherent transport plays an important role in nanometer-scale devices. Among devices exploiting these quantum effects, the Y-channel transistor is attractive on its own right. The original proposal of the Y-channel transistor, or Y-branch switch ͑YBS͒ ͑Ref. 2͒ came from an electron wave analogy to the fiber optic coupler. The semiconductor version of YBS has a narrow electron waveguide patterned into a "Y" configuration with one source and two drain terminals. A lateral electric field perpendicular to the direction of electric current in the source waveguide steers the injected electron wave into either of the two outputs. YBS offers several advantages as a fast switch as evidenced by THz ͑Refs. 3 and 4͒ operation of quantum point contacts ͑QPC͒. Most interestingly, in the case of single mode occupation, the switching can be accomplished by a voltage of the order of ប͑e t ͒, where t is the transit time of electrons. 5 With a proper design, the switching voltage for a YBS can become smaller than the thermal voltage, k B T / e, as opposed to 40-80 times of k B T / e needed for the current transistors. Here, k B is the Boltzmann constant and T is the absolute temperature. Switching at low voltages would make such devices less noisy and consume less power, though at the expense of speed. 6

The authors demonstrate a silicon-based single-electron transistor ͑SET͒ in the few-electron regime. Our structure is similar to a metal-oxide-semiconductor field-effect transistor. The substrate, however, is undoped and could be isotope enriched so that any nonuniformity and spin decoherence due to impurity and nuclear spins can be minimized. A bilayer-gated configuration provides flexibility in manipulating single electrons. The stability chart measured at 4.2 K shows diamondlike domains with a charging energy of 18 meV, indicating a quantum dot of 20 nm in diameter. The benefits of using this enhancement-mode SET in silicon and its potential application for scalable quantum computing are discussed.

We propose and demonstrate experimentally a novel design of single-electron quantum dots. The structure consists of a narrow band gap quantum well that can undergo a transition from the hole accumulation regime to the electron inversion regime in a single-top-gate transistor configuration. We have observed large size quantization and Coulomb charging energies over 10meV. This quantum dot design can be especially important for scalable quantum computing.

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Manin, Feynman, and Deutsch have viewed quantum computing as a kind of universal physical simulation procedure. Much of the writing about quantum logic circuits and quantum Turing machines has shown how these machines can simulate an arbitrary unitary transformation on a finite number of qubits. The problem of universality has been addressed most famously in a paper by Deutsch, and later by Bernstein and Vazirani as well as Kitaev and Solovay. The quantum logic circuit model, developed by Feynman and Deutsch, has been more prominent in the research literature than Deutsch's quantum Turing machines. Quantum Turing machines form a class closely related to deterministic and probabilistic Turing machines and one might hope to find a universal machine in this class. A universal machine is the basis of a notion of programmability. The extent to which universality has in fact been established by the pioneers in the field is examined and this key notion in theoretical computer science is scrutinised in quantum computing by distinguishing various connotations and concomitant results and problems.