12,612 research outputs found
Semileptonic decays of meson to S-wave charmonium states in the perturbative QCD approach
Inspired by the recent measurement of the ratio of branching fractions
to and final states at the LHCb
detector, we study the semileptonic decays of meson to the S-wave ground
and radially excited 2S and 3S charmonium states with the perturbative QCD
approach. After evaluating the form factors for the transitions , where and denote pseudoscalar and vector S-wave charmonia,
respectively, we calculate the branching ratios for all these semileptonic
decays. The theoretical uncertainty of hadronic input parameters are reduced by
utilizing the light-cone wave function for meson. It is found that the
predicted branching ratios range from up to and could be
measured by the future LHCb experiment. Our prediction for the ratio of
branching fractions is in good
agreement with the data. For decays, the relative
contributions of the longitudinal and transverse polarization are discussed in
different momentum transfer squared regions. These predictions will be tested
on the ongoing and forthcoming experiments.Comment: 12 pages, 3 figures, 5 table
Theory of variational quantum simulation
The variational method is a versatile tool for classical simulation of a
variety of quantum systems. Great efforts have recently been devoted to its
extension to quantum computing for efficiently solving static many-body
problems and simulating real and imaginary time dynamics. In this work, we
first review the conventional variational principles, including the
Rayleigh-Ritz method for solving static problems, and the Dirac and Frenkel
variational principle, the McLachlan's variational principle, and the
time-dependent variational principle, for simulating real time dynamics. We
focus on the simulation of dynamics and discuss the connections of the three
variational principles. Previous works mainly focus on the unitary evolution of
pure states. In this work, we introduce variational quantum simulation of mixed
states under general stochastic evolution. We show how the results can be
reduced to the pure state case with a correction term that takes accounts of
global phase alignment. For variational simulation of imaginary time evolution,
we also extend it to the mixed state scenario and discuss variational Gibbs
state preparation. We further elaborate on the design of ansatz that is
compatible with post-selection measurement and the implementation of the
generalised variational algorithms with quantum circuits. Our work completes
the theory of variational quantum simulation of general real and imaginary time
evolution and it is applicable to near-term quantum hardware.Comment: 41 pages, accepted by Quantu
Enhanced quantum teleportation in the background of Schwarzschild spacetime by weak measurements
It is commonly believed that the fidelity of quantum teleportation in the
gravitational field would be degraded due to the heat up by the Hawking
radiation. In this paper, we point out that the Hawking effect could be
eliminated by the combined action of pre- and post-weak measurements, and thus
the teleportation fidelity is almost completely protected. It is intriguing to
notice that the enhancement of fidelity could not be attributed to the
improvement of entanglement, but rather to the probabilistic nature of weak
measurements. Our work extends the ability of weak measurements as a quantum
technique to battle against gravitational decoherence in relativistic quantum
information.Comment: 9 pages, 5 figures, comments are welcom
Variational quantum simulation of general processes
Variational quantum algorithms have been proposed to solve static and dynamic
problems of closed many-body quantum systems. Here we investigate variational
quantum simulation of three general types of tasks---generalised time evolution
with a non-Hermitian Hamiltonian, linear algebra problems, and open quantum
system dynamics. The algorithm for generalised time evolution provides a
unified framework for variational quantum simulation. In particular, we show
its application in solving linear systems of equations and matrix-vector
multiplications by converting these algebraic problems into generalised time
evolution. Meanwhile, assuming a tensor product structure of the matrices, we
also propose another variational approach for these two tasks by combining
variational real and imaginary time evolution. Finally, we introduce
variational quantum simulation for open system dynamics. We variationally
implement the stochastic Schr\"odinger equation, which consists of dissipative
evolution and stochastic jump processes. We numerically test the algorithm with
a six-qubit 2D transverse field Ising model under dissipation.Comment: 18 page
Two-Dimensional Transition Metal Dichalcogenides with a Hexagonal Lattice: Room Temperature Quantum Spin Hall Insulators
So far, several transition metal dichalcogenides (TMDCs) based
two-dimensional (2D) topological insulators (TIs) have been discovered, all of
them based on a tetragonal lattice. However, in 2D crystals, the hexagonal
rather than the tetragonal symmetry is the most common motif. Here, based on
first-principles calculations, we propose a new class of stable 2D TMDCs of
composition MX2 (M=Mo, W, X=S, Se, Te) with a hexagonal lattice. They are all
in the same stability range as other 2D TMDC allotropes that have been
demonstrated experimentally, and they are identified to be practical 2D TIs
with large band gaps ranging from 41 to 198 meV, making them suitable for
applications at room-temperature. Besides, in contrast to tetragonal 2D TMDs,
their hexagonal lattice will greatly facilitate the integration of theses novel
TI states van-der-Waals crystals with other hexagonal or honeycomb materials,
and thus provide a route for 2D-material-based devices for wider nanoelectronic
and spintronic applications. The nontrivial band gaps of both WSe2 and WTe2 2D
crystals are 198 meV, which are larger than that in any previously reported
TMDC-based TIs. These large band gaps entirely stem from the strong spin-orbit
coupling strength within the d orbitals of Mo/W atoms near the Fermi level. Our
findings will significantly broaden the scientific and technological impact of
both 2D TIs and TMDCs
Robust Spin Squeezing Preservation in Photonic Crystal Cavities
We show that the robust spin squeezing preservation can be achieved by
utilizing detuning modification for an ensemble of N separate two-level atoms
embedded in photonic crystal cavities (PCC). In particular, we explore the
different dynamical behaviors of spin squeezing between isotropic and
anisotropic PCC cases when the atomic frequency is inside the band gap. In both
cases, it is shown that the robust preservation of spin squeezing is completely
determined by the formation of bound states. Intriguingly, we find that unlike
the isotropic case where steady-state spin squeezing varies smoothly when the
atomic frequency moves from the inside to the outside band edge, a sudden
transition occurs for the anisotropic case. The present results may be of
direct importance for, e.g., quantum metrology in open quantum systems.Comment: 6 pages, 4 figures, accepted by Laser Physics Letter
A Note on Symplectic, Multisymplectic Scheme in Finite Element Method
We find that with uniform mesh, the numerical schemes derived from finite
element method can keep a preserved symplectic structure in one-dimensional
case and a preserved multisymplectic structure in two-dimentional case in
certain discrete version respectively. These results are in fact the intrinsic
reason that the numerical experiments indicate that such finite element
algorithms are accurate in practice.Comment: 7 pages, 3 figure
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