164,643 research outputs found
Unifying quantum heat transfer and superradiant signature in a nonequilibrium collective-qubit system: a polaron-transformed Redfield approach
We investigate full counting statistics of quantum heat transfer in a
collective-qubit system, constructed by multi-qubits interacting with two
thermal baths. The nonequilibrium polaron-transformed Redfield approach
embedded with an auxiliary counting field is applied to obtain the steady state
heat current and fluctuations, which enables us to study the impact of the
qubit-bath interaction in a wide regime. The heat current, current noise and
skewness are all found to clearly unify the limiting results in the weak and
strong couplings, respectively. Moreover, the superradiant heat transfer is
clarified as a system-size-dependent effect, and large number of qubits
dramatically suppresses the nonequilibrium superradiant signature.Comment: 12pages, 3figs, accepted by Chin. Phys.
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
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
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
Steady state current fluctuations and dynamical control in a nonequilibrium single-site Bose-Hubbard system
We investigate nonequilibrium energy transfer in a single-site Bose-Hubbard
model coupled to two thermal baths. By including a quantum kinetic equation
combined with full counting statistics, we investigate the steady state energy
flux and noise power. The influence of the nonlinear Bose-Hubbard interaction
on the transfer behaviors is analyzed, and the nonmonotonic features are
clearly exhibited. Particularly, in the strong on-site repulsion limit, the
results become identical with the nonequilibrium spin-boson model. We also
extend the quantum kinetic equation to study the geometric-phase-induced energy
pump. An interesting reversal behavior is unraveled by enhancing the
Bose-Hubbard repulsion strength.Comment: 12 pages,6 figure
PR Product: A Substitute for Inner Product in Neural Networks
In this paper, we analyze the inner product of weight vector w and data
vector x in neural networks from the perspective of vector orthogonal
decomposition and prove that the direction gradient of w decreases with the
angle between them close to 0 or {\pi}. We propose the Projection and Rejection
Product (PR Product) to make the direction gradient of w independent of the
angle and consistently larger than the one in standard inner product while
keeping the forward propagation identical. As a reliable substitute for
standard inner product, the PR Product can be applied into many existing deep
learning modules, so we develop the PR Product version of fully connected
layer, convolutional layer and LSTM layer. In static image classification, the
experiments on CIFAR10 and CIFAR100 datasets demonstrate that the PR Product
can robustly enhance the ability of various state-of-the-art classification
networks. On the task of image captioning, even without any bells and whistles,
our PR Product version of captioning model can compete or outperform the
state-of-the-art models on MS COCO dataset. Code has been made available
at:https://github.com/wzn0828/PR_Product.Comment: ICCV2019 ora
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
Novel X band Compact Waveguide Dual Circular Polarizer
A novel type of dual circular polarizer is developed to convert the TE10 mode
into two different polarization TE11 modes in a circular waveguide. This design
consists two major parts: a TE10 to TE10/TE20 converter and an overmoded
TE10/TE20 to circular TE11 modes converter
Approximation of Invariant Measure for Damped Stochastic Nonlinear Schr\"{o}dinger Equation via an Ergodic Numerical Scheme
In order to inherit numerically the ergodicity of the damped stochastic
nonlinear Schr\"odinger equation with additive noise, we propose a fully
discrete scheme, whose spatial direction is based on spectral Galerkin method
and temporal direction is based on a modification of the implicit Euler scheme.
We not only prove the unique ergodicity of the numerical solutions of both
spatial semi-discretization and full discretization, but also present error
estimations on invariant measures, which gives order in spatial direction
and order in temporal direction
New SLED 3 system for Multi-mega Watt RF compressor
At SLAC, we have designed and installed an X-band radio-frequency transverse
deflector system at the LCLS for measurement of the time-resolved lasing
effects on the electron beam and extraction of the temporal profile of the
pulses in routine operations.
We have designed an X-Band SLED system capable to augment the available
klystron power and to double the kick
- …