arXiv
(422.153 recursos)
This is one of the most extensive subject based repositories in the world in the field of physics, mathematics, astronomy, computer sciences and quantitative biology. This is the principal site with almost 20 mirror versions around the globe. The site is supported by an extensive collection of information and background documentation. An RSS feed is available for anyone interested in keeping up-to-date with newly added materials.
Mostrando recursos 1 - 20 de 25.177
1.
Quantum Markov Channels for Qubits - Daffer, Sonja; Wodkiewicz, Krzysztof; McIver, John K.
We examine stochastic maps in the context of quantum optics. Making use of
the master equation, the damping basis, and the Bloch picture we calculate a
non-unital, completely positive, trace-preserving map with unequal damping
eigenvalues. This results in what we call the squeezed vacuum channel. A
geometrical picture of the effect of stochastic noise on the set of pure state
qubit density operators is provided. Finally, we study the capacity of the
squeezed vacuum channel to transmit quantum information and to distribute EPR
states.
2.
Phase states for a three-level atom interacting with quantum fields - Klimov, A. B.; Sanchez-Soto, L. L.; Delgado, J.; Yustas, E. C.
We introduce phase operators associated with the algebra su(3), which is the
appropriate tool to describe three-level systems. The rather unusual properties
of this phase are caused by the small dimension of the system and are explored
in detail. When a three-level atom interacts with a quantum field in a cavity,
a polynomial deformation of this algebra emerges in a natural way. We also
introduce a polar decomposition of the atom-field relative amplitudes that
leads to a Hermitian relative-phase operator, whose eigenstates correctly
describe the corresponding phase properties. We claim that this is the natural
variable to deal with quantum interference effects in atom-field interactions.
We find the probability...
3.
Nonlocality of Two-Mode Squeezing with Internal Noise - Daffer, Sonja; Wodkiewicz, Krzysztof; McIver, John K.
We examine the quantum states produced through parametric amplification with
internal quantum noise. The internal diffusion arises by coupling both modes of
light to a reservoir for the duration of the interaction time. The Wigner
function for the diffused two-mode squeezed state is calculated. The
nonlocality, separability, and purity of these quantum states of light are
discussed. In addition, we conclude by studying the nonlocality of two other
continuous variable states: the Werner state and the phase-diffused state for
two light modes.
4.
Exceeding classical capacity limit in quantum optical channel - Fujiwara, Mikio; Takeoka, Masahiro; Mizuno, Jun; Sasaki, Masahide
The amount of information transmissible through a communications channel is
determined by the noise characteristics of the channel and by the quantities of
available transmission resources. In classical information theory, the amount
of transmissible information can be increased twice at most when the
transmission resource (e.g. the code length, the bandwidth, the signal power)
is doubled for fixed noise characteristics. In quantum information theory,
however, the amount of information transmitted can increase even more than
twice. We present a proof-of-principle demonstration of this super-additivity
of classical capacity of a quantum channel by using the ternary symmetric
states of a single photon, and by event selection from a weak coherent...
5.
Inequivalent classes of closed three-level systems - Klimov, Andre B.; de Guise, Hubert; Sanchez-Soto, Luis L.
We show here that the $\Lambda$ and V configurations of three-level atomic
systems, while they have recently been shown to be equivalent for many
important physical quantities when driven with classical fields [M. B. Plenio,
Phys. Rev. A \textbf{62}, 015802 (2000)], are no longer equivalent when coupled
via a quantum field. We analyze the physical origin of such behavior and show
how the equivalence between these two configurations emerges in the
semiclassical limit.
6.
Quantum algorithms for phase space tomography - Paz, Juan Pablo; Roncaglia, Augusto J.; Saraceno, Marcos
We present efficient circuits that can be used for the phase space tomography
of quantum states. The circuits evaluate individual values or selected averages
of the Wigner, Kirkwood and Husimi distributions. These quantum gate arrays can
be programmed by initializing appropriate computational states. The Husimi
circuit relies on a subroutine that is also interesting in its own right: the
efficient preparation of a coherent state, which is the ground state of the
Harper Hamiltonian.
7.
Effective damping in the Raman cooling of trapped ions - Klimov, A. B.; Romero, J. L.; Delgado, J.; Sanchez-Soto, L. L.
We present a method of treating the interaction of a single three-level ion
with two laser beams. The idea is to apply a unitary transformation such that
the exact transformed Hamiltonian has one of the three levels decoupled for all
values of the detunings. When one takes into account damping, the evolution of
the system is governed by a master equation usually obtained via adiabatic
approximation under the assumption of far-detuned lasers. To go around the
drawbacks of this technique, we use the same unitary transformation to get an
effective master equation.
8.
Quantum phases of a qutrit - Klimov, A. B.; Sanchez-Soto, L. L.; de Guise, H.; Bjork, G.
We consider various approaches to treat the phases of a qutrit. Although it
is possible to represent qutrits in a convenient geometrical manner by
resorting to a generalization of the Poincare sphere, we argue that the
appropriate way of dealing with this problem is through phase operators
associated with the algebra su(3). The rather unusual properties of these
phases are caused by the small dimension of the system and are explored in
detail. We also examine the positive operator-valued measures that can describe
the qutrit phase properties.
9.
Quantum Mechanics Unscrambled - Delhotel, Jean-Michel
Ab initio derivations of the elementary formalism of quantum theory are
reviewed and discussed. The theory basically functions as a predictive scheme,
which is seen to indirectly emerge in the process of setting up a
principle-based alternative to classical mechanics.
10.
Bound entanglement provides convertibility of pure entangled states - Ishizaka, Satoshi
I show that two distant parties can transform pure entangled states to
arbitrary pure states by stochastic local operations and classical
communication (SLOCC) at the single copy level, if they share bound entangled
states. This is the effect of bound entanglement since this entanglement
processing is impossible by SLOCC alone. Similar effect of bound entanglement
occurs in three qubits where two incomparable entangled states of GHZ and W can
be inter-converted. In general multipartite settings composed by $N$ distant
parties, all $N$-partite pure entangled states are inter-convertible by SLOCC
with the assistance of bound entangled states with positive partial transpose.
11.
Relativistic and Radiative Corrections to the Mollow Spectrum - Evers, Joerg; Jentschura, Ulrich D.; Keitel, Christoph H.
The incoherent, inelastic part of the resonance fluorescence spectrum of a
laser-driven atom is known as the Mollow spectrum [B. R. Mollow, Phys. Rev.
188, 1969 (1969)]. Starting from this level of description, we discuss
theoretical foundations of high-precision spectroscopy using the resonance
fluorescence light of strongly laser-driven atoms. Specifically, we evaluate
the leading relativistic and radiative corrections to the Mollow spectrum, up
to the relative orders of (Z alpha)^2 and alpha(Z alpha)^2, respectively, and
Bloch-Siegert shifts as well as stimulated radiative corrections involving
off-resonant virtual states. Complete results are provided for the hydrogen
1S-2P_{1/2} and 1S-2P_{3/2} transitions; these include all relevant correction
terms up to the specified order of...
12.
Stochastic Schrodinger equations as limit of discrete filtering - Gough, John; Sobolev, Andrei
We consider an open model possessing a Markovian quantum stochastic limit and
derive the limit stochastic Schrodinger equations for the wave function
conditioned on indirect observations using only the von Neumann projection
postulate. We show that the diffusion (Gaussian) situation is universal as a
result of the central limit theorem with the quantum jump (Poissonian)
situation being an exceptional case. It is shown that, starting from the
correponding limiting open systems dynamics, the theory of quantum filtering
leads to the same equations, therefore establishing consistency of the quantum
stochastic approach for limiting Markovian models.
13.
Inequalities for quantum channels assisted by limited resources - Giovannetti, Vittorio
The information capacities and ``distillability'' of a quantum channel are
studied in the presence of auxiliary resources. These include prior
entanglement shared between the sender and receiver and free classical bits of
forward and backward communication. Inequalities and trade-off curves are
derived. In particular an alternative proof is given that in the absence of
feedback and shared entanglement, forward classical communication does not
increase the quantum capacity of a channel.
14.
Slow-light solitons - Leonhardt, Ulf
A new type of soliton with controllable speed is constructed generalizing the
theory of slow-light propagation to an integrable regime of nonlinear dynamics.
The scheme would allow the quantum-information transfer between optical
solitons and atomic media.
15.
From quantum circuits to adiabatic algorithms - Siu, M. Stewart
This paper explores several aspects of the adiabatic quantum computation
model. We first show a way that directly maps any arbitrary circuit in the
standard quantum computing model to an adiabatic algorithm of the same depth.
Specifically, we look for a smooth time-dependent Hamiltonian whose unique
ground state slowly changes from the initial state of the circuit to its final
state. Since this construction requires in general an n-local Hamiltonian, we
will study whether approximation is possible using previous results on ground
state entanglement and perturbation theory. Finally we will point out how the
adiabatic model can be relaxed in various ways to allow for 2-local partially
adiabatic algorithms...
16.
How much larger quantum correlations are than classical ones - Cabello, Adan
Considering as distance between two two-party correlations the minimum number
of half local results one party must toggle in order to turn one correlation
into the other, we show that the volume of the set of physically obtainable
correlations in the Einstein-Podolsky-Rosen-Bell scenario is (3 pi/8)^2 = 1.388
larger than the volume of the set of correlations obtainable in local
deterministic or probabilistic hidden-variable theories, but is only 3 pi^2/32
= 0.925 of the volume allowed by arbitrary causal (i.e., no-signaling)
theories.
17.
Multiple copy 2-state discrimination with individual measurements - Acin, A; Bagan, E.; Baig, M.; Masanes, Ll.; Munoz-Tapia, R.
We address the problem of non-orthogonal two-state discrimination when
multiple copies of the unknown state are available. We give the optimal
strategy when only fixed individual measurements are allowed and show that its
error probability saturates the collective (lower) bound asymptotically. We
also give the optimal strategy when adaptivity of individual von Neumann
measurements is allowed (which requires classical communication), and show that
the corresponding error probability is exactly equal to the collective one for
any number of copies. We show that this strategy can be regarded as Bayesian
updating.
18.
Quantum Computing with Very Noisy Devices - Knill, E.
In theory, quantum computers can efficiently simulate quantum physics, factor
large numbers and estimate integrals, thus solving otherwise intractable
computational problems. In practice, quantum computers must operate with noisy
devices called ``gates'' that tend to destroy the fragile quantum states needed
for computation. The goal of fault-tolerant quantum computing is to compute
accurately even when gates have a high probability of error each time they are
used. Here we give evidence that accurate quantum computing is possible with
error probabilities above 3% per gate, which is significantly higher than what
was previously thought possible. However, the resources required for computing
at such high error probabilities are excessive. Fortunately, they...
19.
Two-photon interference with thermal light - Scarcelli, Giuliano; Valencia, Alejandra; Shih, Yanhua
The study of entangled states has greatly improved the basic understanding
about two-photon interferometry. Two-photon interference is not the
interference of two photons but the result of superposition among
indistinguishable two-photon amplitudes. The concept of two-photon amplitude,
however, has generally been restricted to the case of entangled photons. In
this letter we report an experimental study that may extend this concept to the
general case of independent photons. The experiment also shows interesting
practical applications regarding the possibility of obtaining high resolution
interference patterns with thermal sources.
20.
Towards Efficiently Solving Quantum Traveling Salesman Problem - Goswami, Debabrata; Karnick, Harish; Jain, Prateek; Maji, Hemanta K.
We present a framework for efficiently solving Approximate Traveling Salesman
Problem (Approximate TSP) for Quantum Computing Models. Existing
representations of TSP introduce extra states which do not correspond to any
permutation. We present an efficient and intuitive encoding for TSP in quantum
computing paradigm. Using this representation and assuming a Gaussian
distribution on tour-lengths, we give an algorithm to solve Approximate TSP
(Euclidean) within BQP resource bounds. Generalizing this strategy for any
distribution, we present an oracle based Quantum Algorithm to solve Approximate
TSP. We present a realization of the oracle in the quantum counterpart of PP.
1.
