110 research outputs found
Spin-blockade spectroscopy of a two-level artificial molecule
Coulomb and spin blockade spectroscopy investigations have been performed on
an electrostatically defined ``artificial molecule'' connected to spin
polarized leads. The molecule is first effectively reduced to a two-level
system by placing both constituent atoms at a specific location of the level
spectrum. The spin sensitivity of the conductance enables us to identify the
electronic spin-states of the two-level molecule. We find in addition that the
magnetic field induces variations in the tunnel coupling between the two atoms.
The lateral nature of the device is evoked to explain this behavior.Comment: 4 pages, 4 figures; revised version with a minor change in Fig.2 and
additional inset in Fig.3.;accepted by PR
Time Resolved Control of Electron Tunnelling Times and Single-shot Spin Readout in a Quantum Dot
We are pursuing a capability to perform time resolved manipulations of single
spins in quantum dot circuits involving more than two quantum dots. In this
paper, we demonstrate full counting statistics as well as averaging techniques
we use to calibrate the tunnel barriers. We make use of this to implement the
Delft protocol for single shot single spin readout in a device designed to form
a triple quantum dot potential. We are able to tune the tunnelling times over
around three orders of magnitude. We obtain a spin relaxation time of 300
microseconds at 10T.Comment: Submitted to EP2DS 2009 Conference Proceeding
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
Charge Sensing of an Artificial H2+ Molecule
We report charge detection studies of a lateral double quantum dot with
controllable charge states and tunable tunnel coupling. Using an integrated
electrometer, we characterize the equilibrium state of a single electron
trapped in the doubled-dot (artificial H2+ molecule) by measuring the average
occupation of one dot. We present a model where the electrostatic coupling
between the molecule and the sensor is taken into account explicitly. From the
measurements, we extract the temperature of the isolated electron and the
tunnel coupling energy. It is found that this coupling can be tuned between 0
and 60 micro electron-volt in our device.Comment: 5 pages, 4 figures. Revised version with added material. To be
published in Physical Review
Coulomb and Spin blockade of two few-electrons quantum dots in series in the co-tunneling regime
We present Coulomb Blockade measurements of two few-electron quantum dots in
series which are configured such that the electrochemical potential of one of
the two dots is aligned with spin-selective leads. The charge transfer through
the system requires co-tunneling through the second dot which is in
resonance with the leads. The observed amplitude modulation of the resulting
current is found to reflect spin blockade events occurring through either of
the two dots. We also confirm that charge redistribution events occurring in
the off-resonance dot are detected indirectly via changes in the
electrochemical potential of the aligned dot.Comment: 6 pages, 5 figures, submitted to Phys. Rev.
Spin filling of a quantum dot derived from excited-state spectroscopy
We study the spin filling of a semiconductor quantum dot using excited-state
spectroscopy in a strong magnetic field. The field is oriented in the plane of
the two-dimensional electron gas in which the dot is electrostatically defined.
By combining the observation of Zeeman splitting with our knowledge of the
absolute number of electrons, we are able to determine the ground state spin
configuration for one to five electrons occupying the dot. For four electrons,
we find a ground state spin configuration with total spin S=1, in agreement
with Hund's first rule. The electron g-factor is observed to be independent of
magnetic field and electron number.Comment: 11 pages, 7 figures, submitted to New Journal of Physics, focus issue
on Solid State Quantum Informatio
High-fidelity single-shot readout for a spin qubit via an enhanced latching mechanism
The readout of semiconductor spin qubits based on spin blockade is fast but
suffers from a small charge signal. Previous work suggested large benefits from
additional charge mapping processes, however uncertainties remain about the
underlying mechanisms and achievable fidelity. In this work, we study the
single-shot fidelity and limiting mechanisms for two variations of an enhanced
latching readout. We achieve average single-shot readout fidelities > 99.3% and
> 99.86% for the conventional and enhanced readout respectively, the latter
being the highest to date for spin blockade. The signal amplitude is enhanced
to a full one-electron signal while preserving the readout speed. Furthermore,
layout constraints are relaxed because the charge sensor signal is no longer
dependent on being aligned with the conventional (2, 0) - (1, 1) charge dipole.
Silicon donor-quantum-dot qubits are used for this study, for which the dipole
insensitivity substantially relaxes donor placement requirements. One of the
readout variations also benefits from a parametric lifetime enhancement by
replacing the spin-relaxation process with a charge-metastable one. This
provides opportunities to further increase the fidelity. The relaxation
mechanisms in the different regimes are investigated. This work demonstrates a
readout that is fast, has one-electron signal and results in higher fidelity.
It further predicts that going beyond 99.9% fidelity in a few microseconds of
measurement time is within reach.Comment: Supplementary information is included with the pape
The origin of switching noise in GaAs/AlGaAs lateral gated devices
We have studied the origin of switching (telegraph) noise at low temperature
in lateral quantum structures defined electrostatically in GaAs/AlGaAs
heterostructures by surface gates. The noise was measured by monitoring the
conductance fluctuations around on the first step of a quantum point
contact at around 1.2 K. Cooling with a positive bias on the gates dramatically
reduces this noise, while an asymmetric bias exacerbates it. We propose a model
in which the noise originates from a leakage current of electrons that tunnel
through the Schottky barrier under the gate into the doped layer. The key to
reducing noise is to keep this barrier opaque under experimental conditions.
Bias cooling reduces the density of ionized donors, which builds in an
effective negative gate voltage. A smaller negative bias is therefore needed to
reach the desired operating point. This suppresses tunnelling from the gate and
hence the noise. The reduction in the density of ionized donors also
strengthens the barrier to tunneling at a given applied voltage. Support for
the model comes from our direct observation of the leakage current into a
closed quantum dot, around for this device. The current
was detected by a neighboring quantum point contact, which showed monotonic
steps in time associated with the tunneling of single electrons into the dot.
If asymmetric gate voltages are applied, our model suggests that the noise will
increase as a consequence of the more negative gate voltage applied to one of
the gates to maintain the same device conductance. We observe exactly this
behaviour in our experiments.Comment: 8 pages, 7 figure
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