1,155 research outputs found
The visibility study of S-T Landau-Zener-St\"uckelberg oscillations without applied initialization
Probabilities deduced from quantum information studies are usually based on
averaging many identical experiments separated by an initialization step. Such
initialization steps become experimentally more challenging to implement as the
complexity of quantum circuits increases. To better understand the consequences
of imperfect initialization on the deduced probabilities, we study the effect
of not initializing the system between measurements. For this we utilize
Landau-Zener-St\"uckelberg oscillations in a double quantum dot circuit.
Experimental results are successfully compared to theoretical simulations.Comment: 8 pages, 5 figure
Enhanced charge detection of spin qubit readout via an intermediate state
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.Comment: 3 pages, 3 figure
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
Quantum Hall induced currents and the magnetoresistance of a quantum point contact
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
Reversible Metal-Semiconductor Transition of ssDNA-Decorated Single-Walled Carbon Nanotubes
A field effect transistor (FET) measurement of a SWNT shows a transition from
a metallic one to a p-type semiconductor after helical wrapping of DNA. Water
is found to be critical to activate this metal-semiconductor transition in the
SWNT-ssDNA hybrid. Raman spectroscopy confirms the same change in electrical
behavior. According to our ab initio calculations, a band gap can open up in a
metallic SWNT with wrapped ssDNA in the presence of water molecules due to
charge transfer.Comment: 13 pages, 6 figure
Quantum interference between three two-spin states in a double quantum dot
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.Comment: 4 pages including 5 figure
PEG Branched Polymer for Functionalization of Nanomaterials with Ultralong Blood Circulation
Nanomaterials have been actively pursued for biological and medical
applications in recent years. Here, we report the synthesis of several new
poly(ethylene glycol) grafted branched-polymers for functionalization of
various nanomaterials including carbon nanotubes, gold nanoparticles (NP) and
gold nanorods (NRs), affording high aqueous solubility and stability for these
materials. We synthesize different surfactant polymers based upon
poly-(g-glutamic acid) (gPGA) and poly(maleic anhydride-alt-1-octadecene)
(PMHC18). We use the abundant free carboxylic acid groups of gPGA for attaching
lipophilic species such as pyrene or phospholipid, which bind to nanomaterials
via robust physisorption. Additionally, the remaining carboxylic acids on gPGA
or the amine-reactive anhydrides of PMHC18 are then PEGylated, providing
extended hydrophilic groups, affording polymeric amphiphiles. We show that
single-walled carbon nanotubes (SWNTs), Au NPs and NRs functionalized by the
polymers exhibit high stability in aqueous solutions at different pHs, at
elevated temperatures and in serum. Morever, the polymer-coated SWNTs exhibit
remarkably long blood circulation (t1/2 22.1 h) upon intravenous injection into
mice, far exceeding the previous record of 5.4 h. The ultra-long blood
circulation time suggests greatly delayed clearance of nanomaterials by the
reticuloendothelial system (RES) of mice, a highly desired property for in vivo
applications of nanomaterials, including imaging and drug delivery
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
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