22,667 research outputs found
Two-photon interferometry illuminates quantum measurements
The quantum measurement problem still finds no consensus. Nonlocal
interferometry provides an unprecedented experimental probe by entangling two
photons in the "measurement state" (MS). The experiments show that each photon
"measures" the other; the resulting entanglement decoheres both photons;
decoherence collapses both photons to unpredictable but definite outcomes; and
the two-photon MS continues evolving coherently. Thus, contrary to common
opinion, when a two-part system is in the MS, the outcomes actually observed at
both subsystems are definite. Although standard quantum physics postulates
definite outcomes, two-photon interferometry verifies them to be not only
consistent with, but actually a prediction of, the other principles.
Nonlocality is the key to understanding this. As a consequence of nonlocality,
the states we actually observe are the local states. These actually-observed
local states collapse, while the global MS, which can be "observed" only after
the fact by collecting coincidence data from both subsystems, continues its
unitary evolution. This conclusion implies a refined understanding of the
eigenstate principle: Following a measurement, the actually-observed local
state instantly jumps into the observed eigenstate. Various experts' objections
are rebutted.Comment: 1 figure. arXiv admin note: substantial text overlap with
arXiv:1206.518
Solution of the problem of definite outcomes of quantum measurements
Theory and experiment both demonstrate that an entangled quantum state of two
subsystems is neither a superposition of states of its subsystems nor a
superposition of composite states but rather a coherent superposition of
nonlocal correlations between incoherently mixed local states of the two
subsystems. Thus, even if one subsystem happens to be macroscopic as in the
entangled "Schrodinger's cat" state resulting from an ideal measurement, this
state is not the paradoxical macroscopic superposition it is generally presumed
to be. It is, instead, a "macroscopic correlation," a coherent quantum
correlation in which one of the two correlated sub-systems happens to be
macroscopic. This clarifies the physical meaning of entanglement: When a
superposed quantum system A is unitarily entangled with a second quantum system
B, the coherence of the original superposition of different states of A is
transferred to different correlations between states of A and B, so the
entangled state becomes a superposition of correlations rather than a
superposition of states. This transfer preserves unitary evolution while
permitting B to be macroscopic without entailing a macroscopic superposition.
This resolves the "problem of outcomes" but is not a complete resolution of the
measurement problem because the entangled state is still reversible.Comment: 21 pages, 3 figures, 1 tabl
Resolving the problem of definite outcomes of measurements
The heart of the measurement puzzle, namely the problem of definite outcomes,
remains unresolved. This paper shows that Josef Jauch's 1968 reduced density
operator approach is the solution, even though many question it: The entangled
"Measurement State" implies local mixtures of definite but indeterminate
eigenvalues even though the MS continues evolving unitarily. A second,
independent, argument based on the quantum's nonlocal entanglement with its
measuring apparatus shows that the outcomes must be definite eigenvalues
because of relativity's ban on instant signaling. Experiments with entangled
photon pairs show the MS to be a non-paradoxical superposition of correlations
between states rather than a "Schrodinger's cat" superposition of states.
Nature's measurement strategy is to shift the superposition--the
coherence--from the detected quantum to the correlations between the quantum
and its detector, allowing both subsystems to collapse locally to mixtures of
definite eigenvalues. This solution implies an innocuous revision of the
standard eigenvalue-eigenstate link. Three frequent objections to this solution
are rebutted.Comment: 16 pages, 2 figure
Avalanche photodiodes and vacuum phototriodes for the electromagnetic calorimeter of the CMS experiment at the large hadron collider
The homogeneous lead tungstate electromagnetic calorimeter for the Compact Muon Solenoid detector at the Large Hadron Collider operates in a challenging radiation environment. The central region of the calorimeter uses large-area avalanche photodiodes to detect the fast blue-violet scintillation light from the crystals. The high hadron fluence in the forward region precludes the use of these photodiodes and vacuum phototriodes are used in this region. The constructional complexity of the calorimeter, which comprises 75848 individual crystals, plus the activation of material make repair during the lifetime of the detector virtually impossible. We describe here the key features and performance of the photodetectors and the quality assurance procedures that were used to ensure that the proportion of photodetectors that fail over the lifetime of CMS will be limited to a fraction of a percent
Re-assessment of the state of Schroedinger's cat, final version
The quantum state of Schroedinger's cat is usually incorrectly described as a
superposition of "dead" and "alive," despite an argument by Rinner and Werner
that, locally, the cat should be considered to be in a mixture of
non-superposed states. Here, it is rigorously proven that the cat is not in a
superposition. This is central to the measurement problem. Nonlocal two-photon
interferometry experiments throw further light on the measurement state by
probing the effect of a variable phase factor inserted between its superposed
terms. These experiments demonstrate that both subsystems really are in locally
mixed states rather than superpositions, and they tell us what the measurement
state superposition actually superposes. They show that measurement transfers
the coherence in Schroedinger's nuclear superposition neither to the cat nor to
the nucleus, but only to the correlations between them. This explains the
collapse process--but not its subsequent irreversible dissipation--within the
context of unitary dynamics with no need for external entities such as the
environment, a human mind, other worlds, or collapse mechanisms.Comment: 11 page
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