Quantum computing based on semiconductor nanowires
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Introduction The basic element of a quantum computer is a quantum bit, or qubit, which in analogy to a classical bit contains information. A quantum degree of freedom can be charge, spin, photon polarization, and magnetic flux. Several quantum systems are being explored as qubits, each with their specific advantages and challenges: single atoms in ion traps (see the article by Hite et al. in this issue), NV defect centers in diamond (see the article by Gordon et al. in this issue), and superconducting circuits (see the article by Oliver and Welander in this issue). Among these, semiconductor-based qubits are attractive due to their electrical tunability and ease of integration with the electronics industry. However, the search for a perfect semiconductor platform that simultaneously satisfies the requirements of fast quantum control, long coherence time, and scalability to thousands of coupled qubits continues. A prominent semiconductor system in which single and double qubit operations were demonstrated is a two-dimensional electron gas (2DEG) at the interface between GaAs and AlGaAs.1 By using metallic gates on top of the heterostructure to isolate small regions of 2DEG, quantum dots containing single electrons have been electrostatically defined. Rather than following the complementary metal oxide semiconductor route and using charge as an information carrier, electron spin is used in these quantum dot qubits for carrying information.2,3
This is because charge noise in semiconductors does not allow quantum states of charge to survive much longer than a nanosecond.4 While two-dimensional systems currently lead the race among semiconductor qubits, they are still a long way to a practical quantum computer. Among challenges going forward is the need to simultaneously carve zero-dimensional quantum dots out of a 2D sheet of electrons, and couple thousands of these dots while only being able to place control electrodes on top of a heterostructure.5 The need to increase spin coherence times may require changing the materials that host the quantum dots.6 Here, a drawback of two-dimensional systems is the limited design freedom of the material. To avoid strain and consequent incorporation of dislocations, highquality 2DEGs can only be fabricated with (nearly) latticematched materials, which is possible only for a small set of material combinations. A new solid-state platform that has recently demonstrated promise for quantum computing is semiconducting nanowires. Nanowire qubits yield the fastest electrical spin manipulation times to date for single spins in quantum dots.7 In addition, the first signatures of novel Majorana fermion quasiparticles, which are their own antiparticles and represent the building blocks of topological qubits, were obtained in nanowires.8 This progress was possible because nanowires allow for almost unlimited material design freedom in terms of chemical
Sergey M. Frolov, Department of Physics and Astronomy, University of Pittsburgh; [email protected] Sébastien R. Plissard, Technische Universiteit E
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