3,924 research outputs found
Robustness of the BB84 quantum key distribution protocol against general coherent attacks
It is demonstrated that for the entanglement-based version of the
Bennett-Brassard (BB84) quantum key distribution protocol, Alice and Bob share
provable entanglement if and only if the estimated qubit error rate is below
25% or above 75%. In view of the intimate relation between entanglement and
security, this result sheds also new light on the unconditional security of the
BB84 protocol in its original prepare-and-measure form. In particular, it
indicates that for small qubit error rates 25% is the ultimate upper security
bound for any prepare-and-measure BB84-type QKD protocol. On the contrary, for
qubit error rates between 25% and 75% we demonstrate that the correlations
shared between Alice and Bob can always be explained by separable states and
thus, no secret key can be distilled in this regime.Comment: New improved version. A minor mistake has been eliminate
Multiple Ionization under Strong XUV to X-ray Radiation
We review the main aspects of multiple photoionization processes in atoms
exposed to intense, short wavelength radiation. The main focus is the
theoretical framework for the description of such processes as well as the
conditions under which direct multiphoton multiple ionization processes can
dominate over the sequential ones. We discuss in detail the mechanisms
available in different wavelength ranges from the infrared to the hard X-rays.
The effect of field fluctuations, present at this stage in all SASE
free-electron-laser (FEL) facilities, as well as the effect of the interaction
volume integration, are also discussed
Route to Direct Multiphoton Multiple Ionization
We address the concept of direct multiphoton multiple ionization in atoms
exposed to intense, short wavelength radiation and explore the conditions under
which such processes dominate over the sequential. Their contribution is shown
to be quite robust, even under intensity fluctuations and interaction volume
integration, and reasonable agreement with experimental data is also found.Comment: Close to the version to be published in Phys. Rev. A. Additional
supplementary material can be found ther
Perfect transfer of multiple excitations in quantum networks
We present a general formalism to the problem of perfect state-transfer
(PST), where the state involves multiple excitations of the quantum network. A
key feature of our formalism is that it allows for inclusion of nontrivial
interactions between the excitations. Hence, it is perfectly suited to
addressing the problem of PST in the context of various types of physical
realizations. The general formalism is also flexible enough to account for
situations where multiple excitations are "focused" onto the same site.Comment: close to the version published in Phys. Rev. A. In version 2, a typo
has been corrected in Sec. III
Effects of relative phase and interactions on atom-laser outcoupling from a double-well Bose-Einstein condensate: Markovian and non-Markovian dynamics
We investigate aspects of the dynamics of a continuous atom-laser scheme
based on the merging of independently formed atomic condensates. Our
theoretical analysis covers the Markovian as well as the non-Markovian
operational regimes, and is based on a semiclassical (mean-field) two-mode
model. The role of the relative phase between the two condensates and the
effect of interatomic interactions on the evolution of the trapped populations
and the distribution of outcoupled atoms are discussed.Comment: to appear in J. Phys.
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