4,586 research outputs found
Testing Bell's Inequality with Cosmic Photons: Closing the Setting-Independence Loophole
We propose a practical scheme to use photons from causally disconnected
cosmic sources to set the detectors in an experimental test of Bell's
inequality. In current experiments, with settings determined by quantum random
number generators, only a small amount of correlation between detector settings
and local hidden variables, established less than a millisecond before each
experiment, would suffice to mimic the predictions of quantum mechanics. By
setting the detectors using pairs of quasars or patches of the cosmic microwave
background, observed violations of Bell's inequality would require any such
coordination to have existed for billions of years --- an improvement of 20
orders of magnitude.Comment: 5 pages, 4 figures. Minor edits to closely match journal version to
be published in Physical Review Letter
Astronomical random numbers for quantum foundations experiments
Photons from distant astronomical sources can be used as a classical source
of randomness to improve fundamental tests of quantum nonlocality,
wave-particle duality, and local realism through Bell's inequality and
delayed-choice quantum eraser tests inspired by Wheeler's cosmic-scale
Mach-Zehnder interferometer gedankenexperiment. Such sources of random numbers
may also be useful for information-theoretic applications such as key
distribution for quantum cryptography. Building on the design of an
"astronomical random-number generator" developed for the recent "cosmic Bell"
experiment [Handsteiner et al., Phys. Rev. Lett. 118, 060401 (2017)], in this
paper we report on the design and characterization of a device that, with
20-nanosecond latency, outputs a bit based on whether the wavelength of an
incoming photon is greater than or less than 700 nm. Using the one-meter
telescope at the Jet Propulsion Laboratory (JPL) Table Mountain Observatory, we
generated random bits from astronomical photons in both color channels from 50
stars of varying color and magnitude, and from 12 quasars with redshifts up to
. With stars, we achieved bit rates of Hz /
m, limited by saturation for our single-photon detectors, and with quasars
of magnitudes between 12.9 and 16, we achieved rates between and Hz /m. For bright quasars, the resulting bitstreams exhibit
sufficiently low amounts of statistical predictability as quantified by the
mutual information. In addition, a sufficiently high fraction of bits generated
are of true astronomical origin in order to address both the locality and
freedom-of-choice loopholes when used to set the measurement settings in a test
of the Bell-CHSH inequality.Comment: 17 pages, 12 figures. References added and minor edits to match
published versio
Simple Quantum Systems in Spacetimes with Closed Timelike Curves
Three simple examples illustrate properties of path integral amplitudes in
fixed background spacetimes with closed timelike curves: non-relativistic
potential scattering in the Born approximation is non-unitary, but both an
example with hard spheres and the exact solution of a totally discrete model
are unitary.Comment: 15 pages, CALT-68-180
Path Integrals, Density Matrices, and Information Flow with Closed Timelike Curves
Two formulations of quantum mechanics, inequivalent in the presence of closed
timelike curves, are studied in the context of a soluable system. It
illustrates how quantum field nonlinearities lead to a breakdown of unitarity,
causality, and superposition using a path integral. Deutsch's density matrix
approach is causal but typically destroys coherence. For each of these
formulations I demonstrate that there are yet further alternatives in
prescribing the handling of information flow (inequivalent to previous
analyses) that have implications for any system in which unitarity or coherence
are not preserved.Comment: 25 pages, phyzzx, CALT-68-188
Erratum
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43171/1/10988_2004_Article_BF00578452.pd
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