Entropy Bound for the Classical Capacity of a Quantum Channel Assisted by Classical Feedback

We prove that the classical capacity of an arbitrary quantum channel assisted by a free classical feedback channel is bounded from above by the maximum average output entropy of the quantum channel. As a consequence of this bound, we conclude that a classical feedback channel does not improve the classical capacity of a quantum erasure channel, and by taking into account energy constraints, we conclude the same for a pure-loss bosonic channel. The method for establishing the aforementioned entropy bound involves identifying an information measure having two key properties: 1) it does not increase under a one-way local operations and classical communication channel from the receiver to the sender and 2) a quantum channel from sender to receiver cannot increase the information measure by more than the maximum output entropy of the channel. This information measure can be understood as the sum of two terms, with one corresponding to classical correlation and the other to entanglement.Comment: v2: 6 pages, 1 figure, final version published in conference proceeding

Lepton flavor violating μ→eγ\mu\to e\gamma and μ−e\mu-e conversion in unparticle physics

We have studied lepton flavor violation processes $\mu\to e\gamma$ and $\mu-e$ conversion in nuclei induced by unparticle. Both ${\rm Br}(\mu\to e\gamma)$ and $\mu-e$ conversion rate ${\rm CR}(\mu-e,{\rm Nuclei})$ strongly depend on the scale dimension $d_{\cal U}$ and the unparticle coupling $\lambda^{ff'}_{\rm K}$(K=V, A, S, P). Present experimental upper bounds on ${\rm Br}(\mu\to e\gamma)$, ${\rm CR}(\mu-e,{\rm Ti})$ and ${\rm CR}(\mu-e,{\rm Au})$ put stringent constraints on the parameters of unaprticle physics. The scale dimensions $d_{\cal U}$ around 2 are favored for the unparticle scale $\Lambda_{\cal U}$ of ${\cal O}(10 {\rm TeV})$ and the unparticle coupling of ${\cal O}(10^{-3})$. ${\rm CR}(\mu-e,{\rm Nuclei})$ is proportional to $\rm{Z^4_{eff}A^2/Z}$ for the pure vector and scalar couplings between unparticle and SM fermions, this peculiar atomatic number dependence can be used to distinguish unparticle from other theoretical models.Comment: 16 pages, 5 figure

Variational Derivation of Relativistic Fermion-Antifermion Wave Equations in QED

We present a variational method for deriving relativistic two-fermion wave equations in a Hamiltonian formulation of QED. A reformulation of QED is performed, in which covariant Green functions are used to solve for the electromagnetic field in terms of the fermion fields. The resulting modified Hamiltonian contains the photon propagator directly. The reformulation permits one to use a simple Fock-space variational trial state to derive relativistic fermion-antifermion wave equations from the corresponding quantum field theory. We verify that the energy eigenvalues obtained from the wave equation agree with known results for positronium.Comment: 25 pages, accepted in Journal of Mathematical Physics (2004

Three realizations of quantum affine algebra Uq(A2(2))U_q(A_2^{(2)})

In this article we establish explicit isomorphisms between three realizations of quantum twisted affine algebra $U_q(A_2^{(2)})$: the Drinfeld ("current") realization, the Chevalley realization and the so-called $RLL$ realization, investigated by Faddeev, Reshetikhin and Takhtajan.Comment: 15 page