230 research outputs found
Rate analysis for a hybrid quantum repeater
We present a detailed rate analysis for a hybrid quantum repeater assuming
perfect memories and using optimal probabilistic entanglement generation and
deterministic swapping routines. The hybrid quantum repeater protocol is based
on atomic qubit-entanglement distribution through optical coherent-state
communication. An exact, analytical formula for the rates of entanglement
generation in quantum repeaters is derived, including a study on the impacts of
entanglement purification and multiplexing strategies. More specifically, we
consider scenarios with as little purification as possible and we show that for
sufficiently low local losses, such purifications are still more powerful than
multiplexing. In a possible experimental scenario, our hybrid system can create
near-maximally entangled (F = 0.98) pairs over a distance of 1280 km at rates
of the order of 100 Hz
Approximating Invertible Maps by Recovery Channels: Optimality and an Application to Non-Markovian Dynamics
We investigate the problem of reversing quantum dynamics, specifically via
optimal Petz recovery maps. We focus on typical decoherence channels, such as
dephasing, depolarizing and amplitude damping. We illustrate how well a
physically implementable recovery map simulates an inverse evolution. We extend
this idea to explore the use of recovery maps as an approximation of inverse
maps, and apply it in the context of non-Markovian dynamics. We show how this
strategy attenuates non-Markovian effects, such as the backflow of information.Comment: 7 pages, 8 figure
Hybrid quantum repeater with encoding
We present an encoded hybrid quantum repeater scheme using qubit-repetition
and Calderbank-Shor-Steane codes. For the case of repetition codes, we propose
an explicit implementation of the quantum error-correction protocol. Moreover,
we analyze the entangled-pair distribution rate for the hybrid quantum repeater
with encoding and we clearly identify a triple trade-off between the efficiency
of the codes, the memory decoherence time, and the local gate errors. Finally,
we show that in the presence of reasonable imperfections our system can achieve
rates of roughly 24 Hz per memory for 20 km repeater spacing, a final distance
of 1280 km, and a final fidelity of about 0.95.Comment: Published version. Eq.(10) revised, results unchange
Experimental observation of weak non-Markovianity
Non-Markovianity has recently attracted large interest due to significant
advances in its characterization and its exploitation for quantum information
processing. However, up to now, only non-Markovian regimes featuring
environment to system backflow of information (strong non-Markovianity) have
been experimentally simulated. In this work, using an all-optical setup we
simulate and observe the so-called weak non-Markovian dynamics. Through full
process tomography, we experimentally demonstrate that the dynamics of a qubit
can be non-Markovian despite an always increasing correlation between the
system and its environment which, in our case, denotes no information backflow.
We also show the transition from the weak to the strong regime by changing a
single parameter in the environmental state, leading us to a better
understanding of the fundamental features of non-Markovianity.Comment: v2: final versio
Quantum repeaters and quantum key distribution: analysis of secret key rates
We analyze various prominent quantum repeater protocols in the context of
long-distance quantum key distribution. These protocols are the original
quantum repeater proposal by Briegel, D\"ur, Cirac and Zoller, the so-called
hybrid quantum repeater using optical coherent states dispersively interacting
with atomic spin qubits, and the Duan-Lukin-Cirac-Zoller-type repeater using
atomic ensembles together with linear optics and, in its most recent extension,
heralded qubit amplifiers. For our analysis, we investigate the most important
experimental parameters of every repeater component and find their minimally
required values for obtaining a nonzero secret key. Additionally, we examine in
detail the impact of device imperfections on the final secret key rate and on
the optimal number of rounds of distillation when the entangled states are
purified right after their initial distribution.Comment: Published versio
Non-Markovianity hierarchy of Gaussian processes and quantum amplification
We investigate the dynamics of Gaussian states of continuous variable systems under Gaussianity-preserving channels. We introduce a hierarchy of such evolutions encompassing Markovian and weakly and strongly non-Markovian processes and provide simple criteria to distinguish between the classes, based on the degree of positivity of intermediate Gaussian maps. We present an intuitive classification of all one-mode Gaussian channels according to their non-Markovianity degree and show that weak non-Markovianity has an operational significance, as it leads to a temporary phase-insensitive amplification of Gaussian inputs beyond the fundamental quantum limit. Explicit examples and applications are discussed
Measurement of the t(t)over-bar production cross section in the dilepton channel in pp collisions at √s=8 TeV
The top-antitop quark (t (t) over bar) production cross section is measured in proton-proton collisions at root s = 8 TeV with the CMS experiment at the LHC, using a data sample corresponding to an integrated luminosity of 5.3 fb(-1). The measurement is performed by analysing events with a pair of electrons or muons, or one electron and one muon, and at least two jets, one of which is identified as originating from hadronisation of a bottom quark. The measured cross section is 239 +/- 2 (stat.) +/- 11 (syst.) +/- 6 (lum.) pb, for an assumed top-quark mass of 172.5 GeV, in agreement with the prediction of the standard model
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