515 research outputs found
Coherent Transport Through a Quadruple Point in a Few Electron Triple Dot
A few electron double electrostatic lateral quantum dot can be transformed
into a few electron triple quantum dot by applying a different combination of
gate voltages. Quadruple points have been achieved at which all three dots are
simultaneously on resonance. At these special points in the stability diagram
four occupation configurations are possible. Both charge detection and
transport experiments have been performed on this device. In this short paper
we present data and confirm that transport is coherent by observing a Pi phase
shift in magneto-conductance oscillations as one passes through the quadruple
point.Comment: To be published in ICPS Conf. Proceedings 200
A Tuneable Few Electron Triple Quantum Dot
In this paper we report on a tuneable few electron lateral triple quantum dot
design. The quantum dot potentials are arranged in series. The device is aimed
at studies of triple quantum dot properties where knowing the exact number of
electrons is important as well as quantum information applications involving
electron spin qubits. We demonstrate tuning strategies for achieving required
resonant conditions such as quadruple points where all three quantum dots are
on resonance. We find that in such a device resonant conditions at specific
configurations are accompanied by novel charge transfer behaviour.Comment: 11 pages, 4 figure
Topological Hunds rules and the electronic properties of a triple lateral quantum dot molecule
We analyze theoretically and experimentally the electronic structure and
charging diagram of three coupled lateral quantum dots filled with electrons.
Using the Hubbard model and real-space exact diagonalization techniques we show
that the electronic properties of this artificial molecule can be understood
using a set of topological Hunds rules. These rules relate the multi-electron
energy levels to spin and the inter-dot tunneling , and control charging
energies. We map out the charging diagram for up to N=6 electrons and predict a
spin-polarized phase for two holes. The theoretical charging diagram is
compared with the measured charging diagram of the gated triple-dot device.Comment: 31 pages, 7 figures, accepted to March 15, 2007 issue of Phys. Rev.
B, vol. 7
Single-molecule study for a graphene-based nano-position sensor
In this study we lay the groundwork for a graphene-based fundamental ruler at
the nanoscale. It relies on the efficient energy-transfer mechanism between
single quantum emitters and low-doped graphene monolayers. Our experiments,
conducted with dibenzoterrylene (DBT) molecules, allow going beyond ensemble
analysis due to the emitter photo-stability and brightness. A quantitative
characterization of the fluorescence decay-rate modification is presented and
compared to a simple model, showing agreement with the dependence, a
genuine manifestation of a dipole interacting with a 2D material. With DBT
molecules, we can estimate a potential uncertainty in position measurements as
low as 5nm in the range below 30nm
Theory of electronic transport through a triple quantum dot in the presence of magnetic field
Theory of electronic transport through a triangular triple quantum dot
subject to a perpendicular magnetic field is developed using a tight binding
model. We show that magnetic field allows to engineer degeneracies in the
triple quantum dot energy spectrum. The degeneracies lead to zero electronic
transmission and sharp dips in the current whenever a pair of degenerate states
lies between the chemical potential of the two leads. These dips can occur with
a periodicity of one flux quantum if only two levels contribute to the current
or with half flux quantum if the three levels of the triple dot contribute. The
effect of strong bias voltage and different lead-to-dot connections on
Aharonov-Bohm oscillations in the conductance is also discussed
Landau-Zener-Stuckelberg-Majorana interferometry of a single hole
We perform Landau-Zener-Stuckelberg-Majorana (LZSM) spectroscopy on a system
with strong spin-orbit interaction (SOI), realized as a single hole confined in
a gated double quantum dot. In analogy to the electron systems, at magnetic
field B=0 and high modulation frequencies we observe the photon-assisted
tunneling (PAT) between dots, which smoothly evolves into the typical LZSM
funnel-shaped interference pattern as the frequency is decreased. In contrast
to electrons, the SOI enables an additional, efficient spin-flipping interdot
tunneling channel, introducing a distinct interference pattern at finite B.
Magneto-transport spectra at low-frequency LZSM driving show the two channels
to be equally coherent. High-frequency LZSM driving reveals complex
photon-assisted tunneling pathways, both spin-conserving and spin-flipping,
which form closed loops at critical magnetic fields. In one such loop an
arbitrary hole spin state is inverted, opening the way toward its
all-electrical manipulation.Comment: 6 pages, 4 figures, and supplementary materia
Bipolar spin blockade and coherent state superpositions in a triple quantum dot
Spin qubits based on interacting spins in double quantum dots have been
successfully demonstrated. Readout of the qubit state involves a conversion of
spin to charge information, universally achieved by taking advantage of a spin
blockade phenomenon resulting from Pauli's exclusion principle. The archetypal
spin blockade transport signature in double quantum dots takes the form of a
rectified current. Currently more complex spin qubit circuits including triple
quantum dots are being developed. Here we show both experimentally and
theoretically (a) that in a linear triple quantum dot circuit, the spin
blockade becomes bipolar with current strongly suppressed in both bias
directions and (b) that a new quantum coherent mechanism becomes relevant.
Within this mechanism charge is transferred non-intuitively via coherent states
from one end of the linear triple dot circuit to the other without involving
the centre site. Our results have implications in future complex
nano-spintronic circuits.Comment: 21 pages, 7 figure
Quantum interference and phonon-mediated back-action in lateral quantum dot circuits
Spin qubits have been successfully realized in electrostatically defined,
lateral few-electron quantum dot circuits. Qubit readout typically involves
spin to charge information conversion, followed by a charge measurement made
using a nearby biased quantum point contact. It is critical to understand the
back-action disturbances resulting from such a measurement approach. Previous
studies have indicated that quantum point contact detectors emit phonons which
are then absorbed by nearby qubits. We report here the observation of a
pronounced back-action effect in multiple dot circuits where the absorption of
detector-generated phonons is strongly modified by a quantum interference
effect, and show that the phenomenon is well described by a theory
incorporating both the quantum point contact and coherent phonon absorption.
Our combined experimental and theoretical results suggest strategies to
suppress back-action during the qubit readout procedure.Comment: 25 pages, 8 figure
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