19,793 research outputs found
Strong Violations of Bell-type Inequalities for Path-Entangled Number States
We show that nonlocal correlation experiments on the two spatially separated
modes of a maximally path-entangled number state may be performed and lead to a
violation of a Clauser-Horne Bell inequality for any finite photon number N. We
present also an analytical expression for the two-mode Wigner function of a
maximally path-entangled number state and investigate a
Clauser-Horne-Shimony-Holt Bell inequality for such states. We test other
Bell-type inequalities. Some are violated by a constant amount for any N.Comment: 6 pages, LaTex; revised version accepted for publication in PR
Quantum Correlations in Nonlocal BosonSampling
Determination of the quantum nature of correlations between two spatially
separated systems plays a crucial role in quantum information science. Of
particular interest is the questions of if and how these correlations enable
quantum information protocols to be more powerful. Here, we report on a
distributed quantum computation protocol in which the input and output quantum
states are considered to be classically correlated in quantum informatics.
Nevertheless, we show that the correlations between the outcomes of the
measurements on the output state cannot be efficiently simulated using
classical algorithms. Crucially, at the same time, local measurement outcomes
can be efficiently simulated on classical computers. We show that the only
known classicality criterion violated by the input and output states in our
protocol is the one used in quantum optics, namely, phase-space
nonclassicality. As a result, we argue that the global phase-space
nonclassicality inherent within the output state of our protocol represents
true quantum correlations.Comment: 5 pages, 1 figure, comments are very welcome
Quantum Correlations and Global Coherence in Distributed Quantum Computing
Deviations from classical physics when distant quantum systems become
correlated are interesting both fundamentally and operationally. There exist
situations where the correlations enable collaborative tasks that are
impossible within the classical formalism. Here, we consider the efficiency of
quantum computation protocols compared to classical ones as a benchmark for
separating quantum and classical resources and argue that the computational
advantage of collaborative quantum protocols in the discrete variable domain
implies the nonclassicality of correlations. By analysing a toy model, it turns
out that this argument implies the existence of quantum correlations distinct
from entanglement and discord. We characterize such quantum correlations in
terms of the net global coherence resources inherent within quantum states and
show that entanglement and discord can be understood as special cases of our
general framework. Finally, we provide an operational interpretation of such
correlations as those allowing two distant parties to increase their respective
local quantum computational resources only using locally incoherent operations
and classical communication.Comment: Minor modifications and correction
What can quantum optics say about computational complexity theory?
Considering the problem of sampling from the output photon-counting
probability distribution of a linear-optical network for input Gaussian states,
we obtain results that are of interest from both quantum theory and the
computational complexity theory point of view. We derive a general formula for
calculating the output probabilities, and by considering input thermal states,
we show that the output probabilities are proportional to permanents of
positive-semidefinite Hermitian matrices. It is believed that approximating
permanents of complex matrices in general is a #P-hard problem. However, we
show that these permanents can be approximated with an algorithm in BPP^NP
complexity class, as there exists an efficient classical algorithm for sampling
from the output probability distribution. We further consider input
squeezed-vacuum states and discuss the complexity of sampling from the
probability distribution at the output.Comment: 5 pages, 1 figur
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