959 research outputs found
Semiconductor quantum dots for electron spin qubits
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
Charge-noise-free Lateral Quantum Dot Devices with Undoped Si/SiGe Wafer
We develop quantum dots in a single layered MOS structure using an undoped
Si/SiGe wafer. By applying a positive bias on the surface gates, electrons are
accumulated in the Si channel. Clear Coulomb diamond and double dot charge
stability diagrams are measured. The temporal fluctuation of the current is
traced, to which we apply the Fourier transform analysis. The power spectrum of
the noise signal is inversely proportional to the frequency, and is different
from the inversely quadratic behavior known for quantum dots made in doped
wafers. Our results indicate that the source of charge noise for the doped
wafers is related to the 2DEG dopant.Comment: Proceedings of the 12th Asia Pacific Physics Conferenc
Coherent single electron spin control in a slanting Zeeman field
We consider a single electron in a 1D quantum dot with a static slanting
Zeeman field. By combining the spin and orbital degrees of freedom of the
electron, an effective quantum two-level (qubit) system is defined. This
pseudo-spin can be coherently manipulated by the voltage applied to the gate
electrodes, without the need for an external time-dependent magnetic field or
spin-orbit coupling. Single qubit rotations and the C-NOT operation can be
realized. We estimated relaxation () and coherence () times, and
the (tunable) quality factor. This scheme implies important experimental
advantages for single electron spin control.Comment: 4 pages, 3 figure
Localization effects in the tunnel barriers of phosphorus-doped silicon quantum dots
We have observed a negative differential conductance with singular gate and
source-drain bias dependences in a phosphorus-doped silicon quantum dot. Its
origin is discussed within the framework of weak localization. By measuring the
current-voltage characteristics at different temperatures as well as simulating
the tunneling rates dependences on energy, we demonstrate that the presence of
shallow energy defects together with an enhancement of localization
satisfactory explain our observations. Effects observed in magnetic fields are
also discussed.Comment: 15 page
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