Abstract
We report on our recent progress in applying semiconductor quantum dots for spin-based quantum computation, as proposed by Loss and DiVincenzo (1998 Phys. Rev. A 57 120). For the purpose of single-electron spin resonance, we study different types of single quantum dot devices that are designed for the generation of a local ac magnetic field in the vicinity of the dot. We observe photon-assisted tunnelling as well as pumping due to the ac voltage induced by the ac current driven through a wire in the vicinity of the dot, but no evidence for ESR so far. Analogue concepts for a double quantum dot and the hydrogen molecule are discussed in detail. Our experimental results in laterally coupled vertical double quantum dot device show that the Heitler–London model forms a good approximation of the two-electron wavefunction. The exchange coupling constant J is estimated. The relevance of this system for two-qubit gates, in particular the SWAP operation, is discussed. Density functional calculations reveal the importance of the gate electrode geometry in lateral quantum dots for the tunability of J in realistic two-qubit gates.
Highlights
DEUTSCHE PHYSIKALISCHE GESELLSCHAFT particular the SWAP operation, is discussed
The charging energies of the two dots, as determined by nonlinear transport measurements, are about 1 meV. Using this to establish the gate-dot capacitances [24], the anti-crossing ranges from 0.4 meV down to 0.1 meV. This variation of the gap size reflects the difference in the strength of tunnel coupling, which is greater for the larger angular momenta of participating orbital states in the two dots [24]
In this paper we described our progress applying semiconductor quantum dots for spin-based quantum computing
Summary
We discuss our efforts aimed at realizing single-electron spin qubits in semiconductor quantum dots, in particular single-electron spin rotation by means of ESR. Using the numerical values above, we find fB ≈ 30 GHz. A microwave magnetic field, Bac, in a plane perpendicular to B0 and in resonance with the precession rate, causes coherent oscillations between the states |↑ and |↓ (ESR). The typical time for performing half a spin rotation (or π-pulse manipulation) is ∼10 ns. This value is much smaller than the expected single-electron spin decoherence time, T2 (>1 μs) [5]. A measurement of the (non-averaged) value of T2 would require single-shot read-out [8] or spin-echo pulse sequences to suppress dephasing [5]. We discuss our microwave results in subsection 2.3, in particular the influence of the (undesired) ac electric field at the site of the dot
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