9,576 research outputs found
Unconditional optimality of Gaussian attacks against continuous-variable QKD
A fully general approach to the security analysis of continuous-variable
quantum key distribution (CV-QKD) is presented. Provided that the quantum
channel is estimated via the covariance matrix of the quadratures, Gaussian
attacks are shown to be optimal against all eavesdropping strategies, including
collective and coherent attacks. The proof is made strikingly simple by
combining a physical model of measurement, an entanglement-based description of
CV-QKD, and a recent powerful result on the extremality of Gaussian states
[Phys. Rev. Lett. 96, 080502 (2006)].Comment: 4 pages, 4 figure
Continuous-variable quantum key distribution protocols over noisy channels
A continuous-variable quantum key distribution protocol based on squeezed
states and heterodyne detection is introduced and shown to attain higher secret
key rates over a noisy line than any other one-way Gaussian protocol. This
increased resistance to channel noise can be understood as resulting from
purposely adding noise to the signal that is converted into the secret key.
This notion of noise-enhanced tolerance to noise also provides a better
physical insight into the poorly understood discrepancies between the
previously defined families of Gaussian protocols.Comment: Minor modifications to match published manuscrip
Higher-order supersymmetric quantum mechanics
We review the higher-order supersymmetric quantum mechanics (H-SUSY QM),
which involves differential intertwining operators of order greater than one.
The iterations of first-order SUSY transformations are used to derive in a
simple way the higher-order case. The second order technique is addressed
directly, and through this approach unexpected possibilities for designing
spectra are uncovered. The formalism is applied to the harmonic oscillator: the
corresponding H-SUSY partner Hamiltonians are ruled by polynomial Heisenberg
algebras which allow a straight construction of the coherent states.Comment: 42 pages, 12 eps figure
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