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G. Granger,
D. Taubert,
C. E. Young,
L. Gaudreau, A. Kam,
S. A. Studenikin,
P. Zawadzki,
D. Harbusch,
D. Schuh,
W. Wegscheider,
Z. R. Wasilewski,
A. A. Clerk,
S. Ludwig,
A. S. Sachrajda
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M Busl,
G Granger,
L Gaudreau,
R Sánchez, A Kam,
M Pioro-Ladrière,
S A Studenikin,
P Zawadzki,
Z R Wasilewski,
A S Sachrajda,
G Platero
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ABSTRACT: Spin qubits based on interacting spins in double quantum dots have been demonstrated successfully. Readout of the qubit state involves a conversion of spin to charge information, which is 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. At present, more complex spin qubit circuits including triple quantum dots are being developed. Here we show, both experimentally and theoretically, that in a linear triple quantum dot circuit the spin blockade becomes bipolar with current strongly suppressed in both bias directions and also that a new quantum coherent mechanism becomes relevant. In 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 for future complex nanospintronic circuits.
Nature Nanotechnology 02/2013; · 27.27 Impact Factor
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ABSTRACT: We report on the observation and study of new effects, the quantum transport diamonds and Zero Current Anomaly (ZCA), in the non-linear differential resistance of high-mobility InxGa1-xAs/InP and GaAs/AlxGa1-xAs structures in quantizing magnetic fields. Regular Shubnikov-de Haas (SdH) oscillations are observed in differential resistance rxx=dVxx/dI at zero dc current. For non-zero values of dc current, SdH oscillations become strongly distorted and eventually invert phase, which cannot be explained by simple electron heating models. The diamond-shaped features are observed in grey-scale plots of rxx as a function of magnetic field and dc current. At higher quantizing fields when spin levels are well resolved, we observe spin-gap diamonds at odd filling factors in both InxGa1‑xAs/InP and GaAs/AlxGa1-xAs structures. Another interesting phenomenon, the ZCA effect, manifests itself as a sharp dip at zero dc current. To our knowledge the observed transport diamonds and ZCA are new phenomena which require theoretical attention and further experimental investigations.
The Quantum Hall Effect at 30 Years, University of Minisota; 02/2013
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G. Granger,
D. Taubert,
C. E. Young,
L. Gaudreau, A. Kam,
S. A. Studenikin,
P. Zawadzki,
D. Harbusch,
D. Schuh,
W. Wegscheider,
Z. R. Wasilewski,
A. A. Clerk,
S. Ludwig,
A. S. Sachrajda
[show abstract]
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ABSTRACT: 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.
01/2013;
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ABSTRACT: We employ an intermediate excited charge state of a lateral quantum dot
device to increase the charge detection contrast during the qubit state readout
procedure, allowing us to increase the visibility of coherent qubit
oscillations. This approach amplifies the coherent oscillation magnitude but
has no effect on the detector noise resulting in an increase in the signal to
noise ratio. In this letter we apply this scheme to demonstrate a significant
enhancement of the fringe contrast of coherent Landau-Zener-Stuckleberg
oscillations between singlet S and triplet T+ two-spin states.
06/2012;
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ABSTRACT: Qubits based on the singlet (S) and the triplet (T0, T+) states in double
quantum dots have been demonstrated in separate experiments. It has been
recently proposed theoretically that under certain conditions a quantum
interference could occur from the interplay between these two qubit species.
Here we report experiments and modeling which confirm these theoretical
predictions and identify the conditions under which this interference occurs.
Density matrix calculations show that the interference pattern manifests
primarily via the occupation of the common singlet state. The S/T0 qubit is
found to have a much longer coherence time as compared to the S/T+ qubit.
01/2012;
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ABSTRACT: We report on the observation and study of quantum transport diamonds and Zero Current Anomaly (ZCA) in the non‐linear differential resistance rxx = dVxx/dI of high‐mobility InxGa1−xAs/InP structures in quantizing magnetic fields. The diamond‐shaped features are observed in the grey‐scale plots of rxx as a function of magnetic field and dc current. Spin diamonds are revealed at higher magnetic fields when spin levels at odd filling factors are well resolved. Unexpectedly, a narrow dip is observed in differential resistance vs. current at Idc = 0 in quantizing magnetic fields, which we refer to as the ZCA effect.
AIP Conference Proceedings. 12/2011; 1399(1):607-608.
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Nature Physics 11/2011; 8(1):54-58. · 18.97 Impact Factor
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ABSTRACT: Spin qubits involving individual spins in single quantum dots or coupled
spins in double quantum dots have emerged as potential building blocks for
quantum information processing applications. It has been suggested that triple
quantum dots may provide additional tools and functionalities. These include
the encoding of information to either obtain protection from decoherence or to
permit all-electrical operation, efficient spin busing across a quantum
circuit, and to enable quantum error correction utilizing the three-spin
Greenberger-Horn-Zeilinger quantum state. Towards these goals we demonstrate
for the first time coherent manipulation between two interacting three-spin
states. We employ the Landau-Zener-St\"uckelberg approach for creating and
manipulating coherent superpositions of quantum states. We confirm that we are
able to maintain coherence when decreasing the exchange coupling of one spin
with another while simultaneously increasing its coupling with the third. Such
control of pairwise exchange is a requirement of most spin qubit architectures
but has not been previously demonstrated.
06/2011;
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ABSTRACT: We study the electron transport properties of gated quantum dots formed in InGaAs/InP quantum well structures grown by chemical-beam epitaxy on prepatterned substrates. Quantum dots form directly underneath narrow gates due to potential fluctuations. We measure the Coulomb-blockade diamonds in the few-electron regime of a single dot and observe photon-assisted tunneling peaks under microwave irradiation. A singlet-triplet transition at high magnetic field and Coulomb-blockade in the quantum Hall regime are also observed.
Applied Physics Letters 03/2011; 98(13):132107-132107-3. · 3.84 Impact Factor
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ABSTRACT: This paper reports on the observation and analysis of magnetotransport
phenomena in the nonlinear differential resistance $r_{xx}=dV_{xx}/dI$ of
high-mobility InGaAs/InP and GaAs/AlGaAs Hall bar samples driven by direct
current, $\Idc$. Specifically, it is observed that Shubnikov -de Haas (SdH)
oscillations at large filling factors invert their phase at sufficiently large
values of $\Idc$.
This phase inversion is explained as being due to an electron heating effect.
In the quantum Hall effect regime the $r_{xx}$ oscillations transform into
diamond-shaped patterns with different slopes corresponding to odd and even
filling factors. The diamond-shaped features at odd filling factors can be used
as a probe to determine spin energy gaps.
A Zero Current Anomaly (ZCA) which manifests itself as a narrow dip in the
$r_{xx}(\Idc)$ characteristics at zero current, is also observed. The ZCA
effect strongly depends upon temperature, vanishing above 1 K while the
transport diamonds persist to higher temperatures. The transport diamonds and
ZCA are fully reproduced in a higher mobility GaAs/AlGaAs Hall bar structure
confirming that these phenomena reflect intrinsic properties of two-dimensional
systems.
11/2010;
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ABSTRACT: We measure a triple quantum dot in the regime where three addition lines, corresponding to the addition of an electron to each of three dots, pass through each other. In particular, we probe the interplay between transport and the three-dimensional nature of the stability diagram. We choose the regime most pertinent for spin qubit applications. We find that at low bias transport through the triple quantum dot circuit is only possible at six quadruple point locations. The results are consistent with an equivalent circuit model.
Phys. Rev. B. 08/2010; 82(7).
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ABSTRACT: We study experimentally the electron transport properties of gated quantum dots formed in InGaAs/InP and InAsP/InP quantum well structures grown by chemical-beam epitaxy. For the case of the InGaAs quantum well, quantum dots form directly underneath narrow gate electrodes due to potential fluctuations. We measure the Coulomb-blockade diamonds in the few-electron regime of a single quantum dot and observe photon-assisted tunneling peaks under microwave irradiation. A singlet-triplet transition at high magnetic field and Coulomb-blockade effects in the quantum Hall regime are also observed. For the InAsP quantum well, an incidental triple quantum dot forms also due to potential fluctuations within a single dot layout. Tunable quadruple points are observed via transport measurements. Comment: 3.2 pages, 3 figures
07/2010;
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ABSTRACT: We report an investigation of quantum Hall induced currents by simultaneous measurements of their magnetic moment and their effect on the conductance of a quantum point contact (QPC). Features in the magnetic moment and QPC resistance are correlated at Landau-level filling factors nu=1, 2 and 4, which demonstrates the common origin of the effects. Temperature and non-linear sweep rate dependences are observed to be similar for the two effects. Furthermore, features in the noise of the induced currents, caused by breakdown of the quantum Hall effect, are observed to have clear correlations between the two measurements. In contrast, there is a distinct difference in the way that the induced currents decay with time when the sweeping field halts at integer filling factor. We attribute this difference to the fact that, while both effects are sensitive to the magnitude of the induced current, the QPC resistance is also sensitive to the proximity of the current to the QPC split-gate. Although it is clearly demonstrated that induced currents affect the electrostatics of a QPC, the reverse effect, the QPC influencing the induced current, was not observed.
07/2010;
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ABSTRACT: We measure a triple quantum dot in the regime where three addition lines, corresponding to the addition of an electron to each of three dots, pass through each other. In particular, we probe the interplay between transport and the tridimensional nature of the stability diagram. We choose the regime most pertinent for spin qubit applications. We find that at low bias transport through the triple quantum dot circuit is only possible at six quadruple point locations. The results are consistent with an equivalent circuit model. Comment: 4.3 pages, 5 figures, 1 table. This second version contains expanded exprimental details and analysis sections
06/2010;
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ABSTRACT: In this paper, we report on a tunable 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 complex charge transfer behavior.
Applied Physics Letters 12/2009; · 3.84 Impact Factor
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ABSTRACT: 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 Proceedings
10/2009;
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ABSTRACT: The purpose of this work is to fabricate ribbon-like InGaAs and InAsP wires embedded in InP ridge structures and investigate their transport properties. The InP ridge structures that contain the wires are selectively grown by chemical beam epitaxy (CBE) on pre-patterned InP substrates. To optimize the growth and micro-fabrication processes for electronic transport, we explore the Ohmic contact resistance, the electron density, and the mobility as a function of the wire width using standard transport and Shubnikov–de Haas measurements. At low temperatures the ridge structures reveal reproducible mesoscopic conductance fluctuations. We also fabricate ridge structures with submicron gate electrodes that exhibit non-leaky gating and good pinch-off characteristics acceptable for device operation. Using such wrap gate electrodes, we demonstrate that the wires can be split to form quantum dots evidenced by Coulomb blockade oscillations in transport measurements.
Physica E: Low-dimensional Systems and Nanostructures. 08/2009;
