2,564 research outputs found
Sharp Contradiction for Local-Hidden-State Model in Quantum Steering
In quantum theory, no-go theorems are important as they rule out the
existence of a particular physical model under consideration. For instance, the
Greenberger-Horne-Zeilinger (GHZ) theorem serves as a no-go theorem for the
nonexistence of local hidden variable models by presenting a full contradiction
for the multipartite GHZ states. However, the elegant GHZ argument for Bell's
nonlocality does not go through for bipartite Einstein-Podolsky-Rosen (EPR)
state. Recent study on quantum nonlocality has shown that the more precise
description of EPR's original scenario is "steering", i.e., the nonexistence of
local hidden state models. Here, we present a simple GHZ-like contradiction for
any bipartite pure entangled state, thus proving a no-go theorem for the
nonexistence of local hidden state models in the EPR paradox. This also
indicates that the very simple steering paradox presented here is indeed the
closest form to the original spirit of the EPR paradox.Comment: 9 pages. Revised version for Scientific Report
Bipartite Entanglement in Continuous-Variable Cluster States
We present a study of the entanglement properties of Gaussian cluster states,
proposed as a universal resource for continuous-variable quantum computing. A
central aim is to compare mathematically-idealized cluster states defined using
quadrature eigenstates, which have infinite squeezing and cannot exist in
nature, with Gaussian approximations which are experimentally accessible.
Adopting widely-used definitions, we first review the key concepts, by
analysing a process of teleportation along a continuous-variable quantum wire
in the language of matrix product states. Next we consider the bipartite
entanglement properties of the wire, providing analytic results. We proceed to
grid cluster states, which are universal for the qubit case. To extend our
analysis of the bipartite entanglement, we adopt the entropic-entanglement
width, a specialized entanglement measure introduced recently by Van den Nest M
et al., Phys. Rev. Lett. 97 150504 (2006), adapting their definition to the
continuous-variable context. Finally we add the effects of photonic loss,
extending our arguments to mixed states. Cumulatively our results point to key
differences in the properties of idealized and Gaussian cluster states. Even
modest loss rates are found to strongly limit the amount of entanglement. We
discuss the implications for the potential of continuous-variable analogues of
measurement-based quantum computation.Comment: 22 page
Testing Leggett's Inequality Using Aharonov-Casher Effect
Bell's inequality is established based on local realism. The violation of
Bell's inequality by quantum mechanics implies either locality or realism or
both are untenable. Leggett's inequality is derived based on nonlocal realism.
The violation of Leggett's inequality implies that quantum mechanics is neither
local realistic nor nonlocal realistic. The incompatibility of nonlocal realism
and quantum mechanics has been urrently confirmed by photon experiments. In our
work, we propose to test Leggett's inequality using the Aharonov-Casher effect.
In our scheme, four entangled particles emitted from two sources manifest a
two-qubit-typed correlation that may result in the violation of the Leggett
inequality, while satisfying the no-signaling condition for spacelike
separation. Our scheme is tolerant to some local inaccuracies due to the
topological nature of the Aharonov-Casher phase. The experimental
implementation of our scheme can be possibly realized by a calcium atomic
polarization interferometer experiment.Comment: 7 pages, 2 figures. Accepted by Scientific Report
Parallel Architectures for Planetary Exploration Requirements (PAPER)
The Parallel Architectures for Planetary Exploration Requirements (PAPER) project is essentially research oriented towards technology insertion issues for NASA's unmanned planetary probes. It was initiated to complement and augment the long-term efforts for space exploration with particular reference to NASA/LaRC's (NASA Langley Research Center) research needs for planetary exploration missions of the mid and late 1990s. The requirements for space missions as given in the somewhat dated Advanced Information Processing Systems (AIPS) requirements document are contrasted with the new requirements from JPL/Caltech involving sensor data capture and scene analysis. It is shown that more stringent requirements have arisen as a result of technological advancements. Two possible architectures, the AIPS Proof of Concept (POC) configuration and the MAX Fault-tolerant dataflow multiprocessor, were evaluated. The main observation was that the AIPS design is biased towards fault tolerance and may not be an ideal architecture for planetary and deep space probes due to high cost and complexity. The MAX concepts appears to be a promising candidate, except that more detailed information is required. The feasibility for adding neural computation capability to this architecture needs to be studied. Key impact issues for architectural design of computing systems meant for planetary missions were also identified
Quantum entanglement
All our former experience with application of quantum theory seems to say:
{\it what is predicted by quantum formalism must occur in laboratory}. But the
essence of quantum formalism - entanglement, recognized by Einstein, Podolsky,
Rosen and Schr\"odinger - waited over 70 years to enter to laboratories as a
new resource as real as energy.
This holistic property of compound quantum systems, which involves
nonclassical correlations between subsystems, is a potential for many quantum
processes, including ``canonical'' ones: quantum cryptography, quantum
teleportation and dense coding. However, it appeared that this new resource is
very complex and difficult to detect. Being usually fragile to environment, it
is robust against conceptual and mathematical tools, the task of which is to
decipher its rich structure.
This article reviews basic aspects of entanglement including its
characterization, detection, distillation and quantifying. In particular, the
authors discuss various manifestations of entanglement via Bell inequalities,
entropic inequalities, entanglement witnesses, quantum cryptography and point
out some interrelations. They also discuss a basic role of entanglement in
quantum communication within distant labs paradigm and stress some
peculiarities such as irreversibility of entanglement manipulations including
its extremal form - bound entanglement phenomenon. A basic role of entanglement
witnesses in detection of entanglement is emphasized.Comment: 110 pages, 3 figures, ReVTex4, Improved (slightly extended)
presentation, updated references, minor changes, submitted to Rev. Mod. Phys
Passive interferometric symmetries of multimode Gaussian pure states
As large-scale multimode Gaussian states begin to become accessible in the
laboratory, their representation and analysis become a useful topic of research
in their own right. The graphical calculus for Gaussian pure states provides
powerful tools for their representation, while this work presents a useful tool
for their analysis: passive interferometric (i.e., number-conserving)
symmetries. Here we show that these symmetries of multimode Gaussian states
simplify calculations in measurement-based quantum computing and provide
constructive tools for engineering large-scale harmonic systems with specific
physical properties, and we provide a general mathematical framework for
deriving them. Such symmetries are generated by linear combinations of
operators expressed in the Schwinger representation of U(2), called nullifiers
because the Gaussian state in question is a zero eigenstate of them. This
general framework is shown to have applications in the noise analysis of
continuous-various cluster states and is expected to have additional
applications in future work with large-scale multimode Gaussian states.Comment: v3: shorter, included additional applications, 11 pages, 7 figures.
v2: minor content revisions, additional figures and explanation, 23 pages, 18
figures. v1: 22 pages, 16 figure
Gaussian Quantum Information
The science of quantum information has arisen over the last two decades
centered on the manipulation of individual quanta of information, known as
quantum bits or qubits. Quantum computers, quantum cryptography and quantum
teleportation are among the most celebrated ideas that have emerged from this
new field. It was realized later on that using continuous-variable quantum
information carriers, instead of qubits, constitutes an extremely powerful
alternative approach to quantum information processing. This review focuses on
continuous-variable quantum information processes that rely on any combination
of Gaussian states, Gaussian operations, and Gaussian measurements.
Interestingly, such a restriction to the Gaussian realm comes with various
benefits, since on the theoretical side, simple analytical tools are available
and, on the experimental side, optical components effecting Gaussian processes
are readily available in the laboratory. Yet, Gaussian quantum information
processing opens the way to a wide variety of tasks and applications, including
quantum communication, quantum cryptography, quantum computation, quantum
teleportation, and quantum state and channel discrimination. This review
reports on the state of the art in this field, ranging from the basic
theoretical tools and landmark experimental realizations to the most recent
successful developments.Comment: 51 pages, 7 figures, submitted to Reviews of Modern Physic
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