1,675 research outputs found
Geometric Phases for Mixed States during Cyclic Evolutions
The geometric phases of cyclic evolutions for mixed states are discussed in
the framework of unitary evolution. A canonical one-form is defined whose line
integral gives the geometric phase which is gauge invariant. It reduces to the
Aharonov and Anandan phase in the pure state case. Our definition is consistent
with the phase shift in the proposed experiment [Phys. Rev. Lett. \textbf{85},
2845 (2000)] for a cyclic evolution if the unitary transformation satisfies the
parallel transport condition. A comprehensive geometric interpretation is also
given. It shows that the geometric phases for mixed states share the same
geometric sense with the pure states.Comment: 9 pages, 1 figur
Quantum confinement effects on the ordering of the lowest-lying excited states in conjugated chains
The symmetrized density matrix renormalization group approach is applied
within the extended Hubbard-Peierls model (with parameters U/t, V/t, and bond
alternation \delta) to study the ordering of the lowest one-photon
(1^{1}B^{-}_u) and two-photon (2^{1}A^{+}_g) states in one- dimensional
conjugated systems with chain lengths, N, up to N=80 sites. Three different
types of crossovers are studied, as a function of U/t, \delta, and N. The
U-crossover emphasizes the larger ionic character of the 2A_g state compared to
the lowest triplet excitation. The \delta crossover shows strong dependence on
both N and U/t. The N-crossover illustrates the more localized nature of the
2A_g excitation relative to the 1B_u excitation at intermediate correlation
strengths.Comment: Latex file; figures available upon request. Submitted to PR
Bayesian Semiparametric Multivariate Density Deconvolution
We consider the problem of multivariate density deconvolution when the
interest lies in estimating the distribution of a vector-valued random variable
but precise measurements of the variable of interest are not available,
observations being contaminated with additive measurement errors. The existing
sparse literature on the problem assumes the density of the measurement errors
to be completely known. We propose robust Bayesian semiparametric multivariate
deconvolution approaches when the measurement error density is not known but
replicated proxies are available for each unobserved value of the random
vector. Additionally, we allow the variability of the measurement errors to
depend on the associated unobserved value of the vector of interest through
unknown relationships which also automatically includes the case of
multivariate multiplicative measurement errors. Basic properties of finite
mixture models, multivariate normal kernels and exchangeable priors are
exploited in many novel ways to meet the modeling and computational challenges.
Theoretical results that show the flexibility of the proposed methods are
provided. We illustrate the efficiency of the proposed methods in recovering
the true density of interest through simulation experiments. The methodology is
applied to estimate the joint consumption pattern of different dietary
components from contaminated 24 hour recalls
Current-voltage characteristics in donor-acceptor systems: Implications of a spatially varying electric field
We have studied the transport properties of a molecular device composed of
donor and acceptor moieties between two electrodes on either side. The device
is considered to be one-dimensional with different on-site energies and the
non-equilibrium properties are calculated using Landauer's formalism. The
current-voltage characteristics is found to be asymmetric with a sharp Negative
Differential Resistance at a critical bias on one side and very small current
on the other side. The NDR arises primarily due to the bias driven electronic
structure change from one kind of insulating phase to another through a highly
delocalized conducting phase. Our model can be considered to be the simplest to
explain the experimental current-voltage characteristics observed in many
molecular devices.Comment: 5 pages, 4 figures (accepted for publication in Physical Review B
Low-Lying Electronic Excitations and Nonlinear Optic Properties of Polymers via Symmetrized Density Matrix Renormalization Group Method
A symmetrized Density Matrix Renormalization Group procedure together with
the correction vector approach is shown to be highly accurate for obtaining
dynamic linear and third order polarizabilities of one-dimensional Hubbard and
models. The model is seen to show characteristically different
third harmonic generation response in the CDW and SDW phases. This can be
rationalized from the excitation spectrum of the systems.Comment: 4 pages Latex; 3 eps figures available upon request; Proceedings of
ICSM '96, to appear in Synth. Metals, 199
Continuous variable remote state preparation
We extend exact deterministic remote state preparation (RSP) with minimal
classical communication to quantum systems of continuous variables. We show
that, in principle, it is possible to remotely prepare states of an ensemble
that is parameterized by infinitely many real numbers, i.e., by a real
function, while the classical communication cost is one real number only. We
demonstrate continuous variable RSP in three examples using (i) quadrature
measurement and phase space displacement operations, (ii) measurement of the
optical phase and unitaries shifting the same, and (iii) photon counting and
photon number shift.Comment: 7 pages, RevTeX
Signatures of Nucleon Disappearance in Large Underground Detectors
For neutrons bound inside nuclei, baryon instability can manifest itself as a
decay into undetectable particles (e.g., ), i.e.,
as a disappearance of a neutron from its nuclear state. If electric charge is
conserved, a similar disappearance is impossible for a proton. The existing
experimental lifetime limit for neutron disappearance is 4-7 orders of
magnitude lower than the lifetime limits with detectable nucleon decay products
in the final state [PDG2000]. In this paper we calculated the spectrum of
nuclear de-excitations that would result from the disappearance of a neutron or
two neutrons from C. We found that some de-excitation modes have
signatures that are advantageous for detection in the modern high-mass,
low-background, and low-threshold underground detectors, where neutron
disappearance would result in a characteristic sequence of time- and
space-correlated events. Thus, in the KamLAND detector [Kamland], a
time-correlated triple coincidence of a prompt signal, a captured neutron, and
a decay of the residual nucleus, all originating from the same
point in the detector, will be a unique signal of neutron disappearance
allowing searches for baryon instability with sensitivity 3-4 orders of
magnitude beyond the present experimental limits.Comment: 13 pages including 6 figures, revised version, to be published in
Phys.Rev.
General impossible operations in quantum information
We prove a general limitation in quantum information that unifies the
impossibility principles such as no-cloning and no-anticloning. Further, we
show that for an unknown qubit one cannot design a universal Hadamard gate for
creating equal superposition of the original and its complement state.
Surprisingly, we find that Hadamard transformations exist for an unknown qubit
chosen either from the polar or equatorial great circles. Also, we show that
for an unknown qubit one cannot design a universal unitary gate for creating
unequal superpositions of the original and its complement state. We discuss why
it is impossible to design a controlled-NOT gate for two unknown qubits and
discuss the implications of these limitations.Comment: 15 pages, no figures, Discussion about personal quantum computer
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