915 research outputs found
Quantum probabilistic sampling of multipartite 60-qubit Bell inequality violations
We show that violation of genuine multipartite Bell inequalities can be
obtained with sampled, probabilistic phase space methods. These genuine Bell
violations cannot be replicated if any part of the system is described by a
local hidden variable theory. The Bell violations are simulated
probabilistically using quantum phase-space representations. We treat
mesoscopically large Greenberger-Horne-Zeilinger (GHZ) states having up to 60
qubits, using both a multipartite SU(2) Q-representation and the positive
P-representation. Surprisingly, we find that sampling with phase-space
distributions can be exponentially faster than experiment. This is due to the
classical parallelism inherent in the simulation of quantum measurements using
phase-space methods. Our probabilistic sampling method predicts a contradiction
with local realism of "Schr\"odinger-cat" states that can be realized as a GHZ
spin state, either in ion traps or with photonic qubits. We also present a
quantum simulation of the observed super-decoherence of the ion-trap "cat"
state, using a phenomenological noise model
Philosophy Enters the Optics Laboratory: Bell's Theorem and its First Experimental Tests (1965-1982)
This paper deals with the ways that the issue of completing quantum mechanics
was brought into laboratories and became a topic in mainstream quantum optics.
It focuses on the period between 1965, when Bell published what now we call
Bell's theorem, and 1982, when Aspect published the results of his experiments.
I argue that what was considered good physics after Aspect's experiments was
once considered by many a philosophical matter instead of a scientific one, and
that the path from philosophy to physics required a change in the physics
community's attitude about the status of the foundations of quantum mechanics.Comment: 57 pages, accepted by Studies in History and Philosophy of Modern
Physic
Deterministic creation of entangled atom-light Schr\"odinger-cat states
Quantum physics allows for entanglement between microscopic and macroscopic
objects, described by discrete and continuous variables, respectively. As in
Schr\"odinger's famous cat gedanken experiment, a box enclosing the objects can
keep the entanglement alive. For applications in quantum information
processing, however, it is essential to access the objects and manipulate them
with suitable quantum tools. Here we reach this goal and deterministically
generate entangled light-matter states by reflecting a coherent light pulse
with up to four photons on average from an optical cavity containing one atom.
The quantum light propagates freely and reaches a remote receiver for quantum
state tomography. We produce a plethora of quantum states and observe
negative-valued Wigner functions, a characteristic sign of non-classicality. As
a first application, we demonstrate a quantum-logic gate between an atom and a
light pulse, with the photonic qubit encoded in the phase of the light field.Comment: includes Methods and Supplementary Informatio
Is Bell's theorem relevant to quantum mechanics? On locality and non-commuting observables
Bell's theorem is a statement by which averages obtained from specific types
of statistical distributions must conform to a family of inequalities. These
models, in accordance with the EPR argument, provide for the simultaneous
existence of quantum mechanically incompatible quantities. We first recall
several contradictions arising between the assumption of a joint distribution
for incompatible observables and the probability structure of
quantum-mechanics, and conclude that Bell's theorem is not expected to be
relevant to quantum phenomena described by non-commuting observables,
irrespective of the issue of locality. Then, we try to disentangle the locality
issue from the existence of joint distributions by introducing two models
accounting for the EPR correlations but denying the existence of joint
distributions. We will see that these models do not need to resort explicitly
to non-locality: the first model relies on conservation laws for ensembles, and
the second model on an equivalence class by which different configurations lead
to the same physical predictions.Comment: Extended with new materia
Quantum Nonlocality
This book presents the current views of leading physicists on the bizarre property of quantum theory: nonlocality. Einstein viewed this theory as “spooky action at a distance” which, together with randomness, resulted in him being unable to accept quantum theory. The contributions in the book describe, in detail, the bizarre aspects of nonlocality, such as Einstein–Podolsky–Rosen steering and quantum teleportation—a phenomenon which cannot be explained in the framework of classical physics, due its foundations in quantum entanglement. The contributions describe the role of nonlocality in the rapidly developing field of quantum information. Nonlocal quantum effects in various systems, from solid-state quantum devices to organic molecules in proteins, are discussed. The most surprising papers in this book challenge the concept of the nonlocality of Nature, and look for possible modifications, extensions, and new formulations—from retrocausality to novel types of multiple-world theories. These attempts have not yet been fully successful, but they provide hope for modifying quantum theory according to Einstein’s vision
Wigner function negativity and contextuality in quantum computation on rebits
We describe a universal scheme of quantum computation by state injection on
rebits (states with real density matrices). For this scheme, we establish
contextuality and Wigner function negativity as computational resources,
extending results of [M. Howard et al., Nature 510, 351--355 (2014)] to
two-level systems. For this purpose, we define a Wigner function suited to
systems of rebits, and prove a corresponding discrete Hudson's theorem. We
introduce contextuality witnesses for rebit states, and discuss the
compatibility of our result with state-independent contextuality.Comment: 18 + 4 page
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