Quantum privacy and quantum coherence

We derive a simple relation between a quantum channel's capacity to convey coherent (quantum) information and its usefulness for quantum cryptography.Comment: 6 pages RevTex; two short comments added 7 October 199

Information transmission through a noisy quantum channel

Noisy quantum channels may be used in many information-carrying applications. We show that different applications may result in different channel capacities. Upper bounds on several of these capacities are proved. These bounds are based on the coherent information, which plays a role in quantum information theory analogous to that played by the mutual information in classical information theory. Many new properties of the coherent information and entanglement fidelity are proved. Two nonclassical features of the coherent information are demonstrated: the failure of subadditivity, and the failure of the pipelining inequality. Both properties arise as a consequence of quantum entanglement, and give quantum information new features not found in classical information theory. The problem of a noisy quantum channel with a classical observer measuring the environment is introduced, and bounds on the corresponding channel capacity proved. These bounds are always greater than for the unobserved channel. We conclude with a summary of open problems

Structure of the σ\sigma-meson and diamagnetism of the nucleon

The structure of the $\sigma$ meson and the diamagnetism of the nucleon are shown to be topics which are closely related to each other. Arguments are found that the $\sigma$ meson couples to two photons via its non-strange $q\bar{q}$ structure component. This ansatz leads to a quantitative explanation of the $t$-channel component of the difference of electromagnetic polarizabilities, (\alpha-\beta)^t$, containing the diamagnetism of the nucleon. The prediction is$(\alpha-\beta)^t_{p,n}=(5\alpha_e g_{\pi MM})/(6\pi^2 m^2_\sigma f_\pi)=15.3$in units of$10^{-4}{\rm fm}^3$to be compared with the experimental value$(\alpha-\beta)^t_p=15.1\pm 1.3$for the proton and$(\alpha-\beta)^t_n=14.8\pm 2.7$for the neutron. The equivalent approach to exploit the$\pi\pi$structure component of the$\sigma$meson via the BEFT sum rule leads to$(\alpha-\beta)^t_{p,n}=14\pm 2$, what also is in agreement with the experimental results.Comment: Contribution made by Martin Schumacher to the International Workshop on the Physics of Excited Baryons, 12 - 15 Oct. 2005, Tallahasse, Florida US

Relativistic corrections to the electromagnetic polarizabilities of compound systems

The low-energy amplitude of Compton scattering on the bound state of two charged particles of arbitrary masses, charges and spins is calculated. A case in which the bound state exists due to electromagnetic interaction (QED) is considered. The term, proportional to $\omega^2$, is obtained taking into account the first relativistic correction. It is shown that the complete result for this correction differs essentially from the commonly used term $\Delta\alpha$, proportional to the r.m.s. charge radius of the system. We propose that the same situation can take place in the more complicated case of hadrons.Comment: 19 pages, LaTe

Information-theoretic approach to quantum error correction and reversible measurement

Quantum operations provide a general description of the state changes allowed by quantum mechanics. The reversal of quantum operations is important for quantum error-correcting codes, teleportation, and reversing quantum measurements. We derive information-theoretic conditions and equivalent algebraic conditions that are necessary and sufficient for a general quantum operation to be reversible. We analyze the thermodynamic cost of error correction and show that error correction can be regarded as a kind of ``Maxwell demon,'' for which there is an entropy cost associated with information obtained from measurements performed during error correction. A prescription for thermodynamically efficient error correction is given.Comment: 31 pages, REVTEX, one figure in LaTeX, submitted to Proceedings of the ITP Conference on Quantum Coherence and Decoherenc

Quantum data processing and error correction

This paper investigates properties of noisy quantum information channels. We define a new quantity called {\em coherent information} which measures the amount of quantum information conveyed in the noisy channel. This quantity can never be increased by quantum information processing, and it yields a simple necessary and sufficient condition for the existence of perfect quantum error correction.Comment: LaTeX, 20 page

“A respite thing” – A qualitative study of a creative arts leisure programme for family caregivers of people with dementia

This study explored the meanings of participating in a five-week creative arts leisure programme designed for family caregivers of people with dementia, using interpretative phenomenological analysis. Eight carers attended and four who met the eligibility criteria agreed to be interviewed. Participants experienced the arts group as providing a sense of freedom and respite, strengthening identity through promoting achievement, offering social support through a collective focus on art- and craft-making, and increasing resilience for coping with caring. Some found the five-week programme too short. Benefits were linked to the security of knowing that loved ones with dementia were close by, being well cared for. Further research is needed into the longer-term benefits of creative arts groups for promoting carer well-being