20,471 research outputs found
Noise filtering of composite pulses for singlet-triplet qubits
Semiconductor quantum dot spin qubits are promising candidates for quantum
computing. In these systems, the dynamically corrected gates offer considerable
reduction of gate errors and are therefore of great interest both theoretically
and experimentally. They are, however, designed under the static-noise model
and may be considered as low-frequency filters. In this work, we perform a
comprehensive theoretical study of the response of a type of dynamically
corrected gates, namely the {\sc supcode} for singlet-triplet qubits, to
realistic noises with frequency spectra . Through
randomized benchmarking, we have found that {\sc supcode} offers improvement of
the gate fidelity for and the improvement becomes
exponentially more pronounced with the increase of the noise exponent in the
range studied. On the other hand, for small ,
{\sc supcode} will not offer any improvement. The -{\sc supcode},
specifically designed for systems where the nuclear noise is absent, is found
to offer additional error reduction than the full {\sc supcode} for charge
noises. The computed filter transfer functions of the {\sc supcode} gates are
also presented.Comment: 9 pages, 5 figure
Magic angle for barrier-controlled double quantum dots
We show that the exchange interaction of a singlet-triplet spin qubit
confined in double quantum dots, when being controlled by the barrier method,
is insensitive to a charged impurity lying along certain directions away from
the center of the double-dot system. These directions differ from the polar
axis of the double dots by the magic angle, equaling
, a value previously found in
atomic physics and nuclear magnetic resonance. This phenomenon can be
understood from an expansion of the additional Coulomb interaction created by
the impurity, but also relies on the fact that the exchange interaction solely
depends on the tunnel coupling in the barrier-control scheme. Our results
suggest that for a scaled-up qubit array, when all pairs of double dots rotate
their respective polar axes from the same reference line by the magic angle,
cross-talks between qubits can be eliminated, allowing clean single-qubit
operations. While our model is a rather simplified version of actual
experiments, our results suggest that it is possible to minimize unwanted
couplings by judiciously designing the layout of the qubits.Comment: 8 pages, 5 figure
Suppression of charge noise using barrier control of a singlet-triplet qubit
It has been recently demonstrated that a singlet-triplet spin qubit in
semiconductor double quantum dots can be controlled by changing the height of
the potential barrier between the two dots ("barrier control"), which has led
to a considerable reduction of charge noises as compared to the traditional
tilt control method. In this paper we show, through a
molecular-orbital-theoretic calculation of double quantum dots influenced by a
charged impurity, that the relative charge noise for a system under the barrier
control not only is smaller than that for the tilt control, but actually
decreases as a function of an increasing exchange interaction. This is
understood as a combined consequence of the greatly suppressed detuning noise
when the two dots are symmetrically operated, as well as an enhancement of the
inter-dot hopping energy of an electron when the barrier is lowered which in
turn reduces the relative charge noise at large exchange interaction values. We
have also studied the response of the qubit to charged impurities at different
locations, and found that the improvement of barrier control is least for
impurities equidistant from the two dots due to the small detuning noise they
cause, but is otherwise significant along other directions.Comment: 9+ pages, 7 figure
Nonparametric Clustering of Mixed Data Using Modified Chi-square Tests
We propose a non-parametric method to cluster mixed data containing both
continuous and discrete random variables. The product space of continuous and
categorical sample spaces is approximated locally by analyzing neighborhoods
with cluster patterns. Detection of cluster patterns on the product space is
determined by using a modified Chi-square test. The proposed method does not
impose a global distance function which could be difficult to specify in
practice. Results from simulation studies have shown that our proposed methods
out-performed the benchmark method, AutoClass, for various settings
Power System Transient Stability Analysis Using Truncated Taylor Expansion Systems
Small signal analysis is a special case of analytical approaches using Taylor
expansions of power system differential equations with the truncation performed
at order one. The truncated Taylor expansions (TTEs) at higher orders can lead
to better approaches for stability analysis by considering higher order
nonlinearities, e.g. normal form, modal series and nonlinear modal decoupling.
This paper presents fundamental studies on how accurate transient stability
analysis results can be obtained from the TTE systems compared to that on the
original system. The analytical investigation is conducted on
single-machine-infinite-bus power systems. Conclusions are drawn from there and
verified on two multi-machine power systems by extensive numerical simulations.Comment: This paper has been submitted to 2019 Texas Power and Energy
Conferenc
Nonlinear Modal Decoupling Based Power System Transient Stability Analysis
Nonlinear modal decoupling (NMD) was recently proposed to nonlinearly
transform a multi-oscillator system into a number of decoupled oscillators
which together behave the same as the original system in an extended
neighborhood of the equilibrium. Each oscillator has just one degree of freedom
and hence can easily be analyzed to infer the stability of the original system
associated with one electromechanical mode. As the first attempt of applying
the NMD methodology to realistic power system models, this paper proposes an
NMD-based transient stability analysis approach. For a multi-machine power
system, the approach first derives decoupled nonlinear oscillators by a
coordinates transformation, and then applies Lyapunov stability analysis to
oscillators to assess the stability of the original system. Nonlinear modal
interaction is also considered. The approach can be efficiently applied to a
large-scale power grid by conducting NMD regarding only selected modes. Case
studies on a 3-machine 9-bus system and an NPCC 48-machine 140-bus system show
the potentials of the approach in transient stability analysis for
multi-machine systems.Comment: This paper has been submitted to IEEE Transactions on Power System
Prometheus: LT Codes Meet Cooperative Transmission in Cellular Networks
Following fast growth of cellular networks, more users have drawn attention
to the contradiction between dynamic user data traffic and static data plans.
To address this important but largely unexplored issue, in this paper, we
design a new data plan sharing system named Prometheus, which is based on the
scenario that some smartphone users have surplus data traffic and are willing
to help others download data. To realize this system, we first propose a
mechanism that incorporates LT codes into UDP. It is robust to transmission
errors and encourages more concurrent transmissions and forwardings. It also
can be implemented easily with low implementation complexity. Then we design an
incentive mechanism using a Stackelberg game to choose assistant users (),
all participants will gain credits in return, which can be used to ask for
future help when they need to download something. Finally real environment
experiments are conducted and the results show that users in our Prometheus not
only can manage their surplus data plan more efficiently, but also achieve a
higher speed download rate
A Note on Entropy Relations of Black Hole Horizons
We focus on the entropy relations of black holes in three, four and higher
dimensions. These entropy relations include entropy product, "part" entropy
product and entropy sum. We also discuss their differences and similarities, in
order to make a further study on understanding the origin of black hole entropy
at the microscopic level.Comment: 17 pages, no figures, 2 tables, minor corrections, the version
appears in International Journal of Modern Physics
Entropy relations and the application of black holes with cosmological constant and Gauss-Bonnet term
Based on the entropy relations, we derive thermodynamic bound for entropy and
area of horizons of Schwarzschild-dS black hole, including the event horizon,
Cauchy horizon and negative horizon (i.e. the horizon with negative value),
which are all geometrical bound and made up of the cosmological radius.
Consider the first derivative of entropy relations together, we get the first
law of thermodynamics for all horizons. We also obtain the Smarr relation of
horizons by using the scaling discussion. For thermodynamics of all horizons,
the cosmological constant is treated as a thermodynamical variable. Especially
for thermodynamics of negative horizon, it is defined well in the side of
spacetime. The validity of this formula seems to work well for three-horizons
black holes. We also generalize the discussion to thermodynamics for event
horizon and Cauchy horizon of Gauss-Bonnet charged flat black holes, as the
Gauss-Bonnet coupling constant is also considered as thermodynamical variable.
These give further clue on the crucial role that the entropy relations of
multi-horizons play in black hole thermodynamics and understanding the entropy
at the microscopic level.Comment: 12 pages, 0 figures, references added. Accepted for publication in
Physics Letters
The Entropy Sum of (A)dS Black Holes in Four and Higher Dimensions
We present the "entropy sum" relation of (A)dS charged black holes in higher
dimensional Einstein-Maxwell gravity, gravity, Gauss-Bonnet gravity and
gauged supergravity. For their "entropy sum" with the necessary effect of the
un-physical "virtual" horizon included, we conclude the general results that
the cosmological constant dependence and Gauss-Bonnet coupling constant
dependence do hold in both the four and six dimensions, while the "entropy sum"
is always vanishing in odd dimensions. Furthermore, the "entropy sum" of all
horizons is related to the geometry of the horizons in four and six dimensions.
In these explicitly four cases, one also finds that the conserved charges
(the mass), (the charge from Maxwell field or supergravity) and the
parameter (the angular momentum) play no role in the "entropy sum"
relations.Comment: 19 pages, 0 figures, some references added, conclusions improved, an
appendix added, correct typos, published versio
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