633 research outputs found
Experimental realization of Wheeler's delayed-choice GedankenExperiment
The quantum "mystery which cannot go away" (in Feynman's words) of
wave-particle duality is illustrated in a striking way by Wheeler's
delayed-choice GedankenExperiment. In this experiment, the configuration of a
two-path interferometer is chosen after a single-photon pulse has entered it :
either the interferometer is \textit{closed} (\textit{i.e.} the two paths are
recombined) and the interference is observed, or the interferometer remains
\textit{open} and the path followed by the photon is measured. We report an
almost ideal realization of that GedankenExperiment, where the light pulses are
true single photons, allowing unambiguous which-way measurements, and the
interferometer, which has two spatially separated paths, produces high
visibility interference. The choice between measuring either the 'open' or
'closed' configuration is made by a quantum random number generator, and is
space-like separated -- in the relativistic sense -- from the entering of the
photon into the interferometer. Measurements in the closed configuration show
interference with a visibility of 94%, while measurements in the open
configuration allow us to determine the followed path with an error probability
lower than 1%
Wheeler's delayed-choice thought experiment: Experimental realization and theoretical analysis
Wheeler has strikingly illustrated the wave-particle duality by the
delayed-choice thought experiment, in which the configuration of a 2-path
interferometer is chosen after a single-photon light-pulsed has entered it. We
present a quantitative theoretical analysis of an experimental realization of
Wheeler's proposal
Prediction and Measurement of the local extinction coefficient in sprays for 3D simulation/experiment data comparison
AbstractIn the recent years, large progresses in laser imaging techniques have allowed to extract spatially resolved 2D and 3D quantitative spray information even in optically dense situations. The main breakthrough of these techniques is the possibility of suppressing unwanted effects from multiple light scattering using Structured Illumination. Thanks to this new feature, effects due to light extinction can also be corrected allowing the measurement of the local extinction coefficient. These quantitative information which is available even in challenging conditions, where Phase Doppler does not work anymore, can be used for data comparison between experiment and simulation. The local extinction coefficient is particularly valuable for the description of the droplet field, defined as the “spray region”, as it contains information related to both droplets size and concentration. In this article we detail, then, the procedure enabling the modelers to obtain numerically this local extinction coefficient over the full 3D spray system. Following this procedure, results can now be adequately compared between simulation and experiment. The proposed comparison approach can better guide model adjustments in situation where the initial droplet size distribution is unknown or approximated and presents a step towards future validations of spray simulations, especially those based on Lagrangian Particle Tracking. The approach is exemplified here for the case of a Diesel-type spray. The results reveal at which specific spray locations discrepancies occur, and highlight the sensitivity of the initial droplet size distribution on the resulting extinction coefficient
Quantum Cryptography Approaching the Classical Limit
We consider the security of continuous-variable quantum cryptography as we
approach the classical-limit, i.e., when the unknown preparation noise at the
sender's station becomes significantly noisy or thermal (even by as much as
10,000 times the variance of the vacuum mode). We show that, provided the
channel transmission losses do not exceed 50%, the security of quantum
cryptography is not dependent on the channel transmission, and is therefore,
incredibly robust against significant amounts of excess preparation noise. We
extend these results to consider for the first time quantum cryptography at
wavelengths considerably longer than optical and find that regions of security
still exist all the way down to the microwave.Comment: Letter (4 pages) followed by appendix (4 pages). Updated from
published version with some minor correction
Continuous-Variable Quantum Key Distribution using Thermal States
We consider the security of continuous-variable quantum key distribution
using thermal (or noisy) Gaussian resource states. Specifically, we analyze
this against collective Gaussian attacks using direct and reverse
reconciliation where both protocols use either homodyne or heterodyne
detection. We show that in the case of direct reconciliation with heterodyne
detection, an improved robustness to channel noise is achieved when large
amounts of preparation noise is added, as compared to the case when no
preparation noise is added. We also consider the theoretical limit of infinite
preparation noise and show a secure key can still be achieved in this limit
provided the channel noise is less than the preparation noise. Finally, we
consider the security of quantum key distribution at various electromagnetic
wavelengths and derive an upper bound related to an entanglement-breaking
eavesdropping attack and discuss the feasibility of microwave quantum key
distribution.Comment: 12 pages, 11 figures. Updated from published version with some minor
correction
Linear optical logical Bell state measurements with optimal loss-tolerance threshold
Quantum threshold theorems impose hard limits on the hardware capabilities to
process quantum information. We derive tight and fundamental upper bounds to
loss-tolerance thresholds in different linear-optical quantum information
processing settings through an adversarial framework, taking into account the
intrinsically probabilistic nature of linear optical Bell measurements. For
logical Bell state measurements - ubiquitous operations in photonic quantum
information - we demonstrate analytically that linear optics can achieve the
fundamental loss threshold imposed by the no-cloning theorem even though,
following the work of Lee et al., (Phys. Rev. A 100, 052303 (2019)), the
constraint was widely assumed to be stricter. We spotlight the assumptions of
the latter publication and find their bound holds for a logical Bell
measurement built from adaptive physical linear-optical Bell measurements. We
also give an explicit even stricter bound for non-adaptive Bell measurements.Comment: 17pages, 14 figure
Continuous variable quantum cryptography using coherent states
We propose several methods for quantum key distribution (QKD) based upon the
generation and transmission of random distributions of coherent or squeezed
states, and we show that they are are secure against individual eavesdropping
attacks. These protocols require that the transmission of the optical line
between Alice and Bob is larger than 50 %, but they do not rely on
"non-classical" features such as squeezing. Their security is a direct
consequence of the no-cloning theorem, that limits the signal to noise ratio of
possible quantum measurements on the transmission line. Our approach can also
be used for evaluating various QKD protocols using light with gaussian
statistics.Comment: 5 pages, 1 figure. In v2 minor rewriting for clarity, references
adde
Experimental Demonstration of Post-Selection based Continuous Variable Quantum Key Distribution in the Presence of Gaussian Noise
In realistic continuous variable quantum key distribution protocols, an
eavesdropper may exploit the additional Gaussian noise generated during
transmission to mask her presence. We present a theoretical framework for a
post-selection based protocol which explicitly takes into account excess
Gaussian noise. We derive a quantitative expression of the secret key rates
based on the Levitin and Holevo bounds. We experimentally demonstrate that the
post-selection based scheme is still secure against both individual and
collective Gaussian attacks in the presence of this excess noise.Comment: 4 pages, 4 figure
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