Quantum Entanglement Capacity with Classical Feedback

For any quantum discrete memoryless channel, we define a quantity called quantum entanglement capacity with classical feedback ($E_B$), and we show that this quantity lies between two other well-studied quantities. These two quantities - namely the quantum capacity assisted by two-way classical communication ($Q_2$) and the quantum capacity with classical feedback ($Q_B$) - are widely conjectured to be different: there exists quantum discrete memoryless channel for which $Q_2>Q_B$. We then present a general scheme to convert any quantum error-correcting codes into adaptive protocols for this newly-defined quantity of the quantum depolarizing channel, and illustrate with Cat (repetition) code and Shor code. We contrast the present notion with entanglement purification protocols by showing that whilst the Leung-Shor protocol can be applied directly, recurrence methods need to be supplemented with other techniques but at the same time offer a way to improve the aforementioned Cat code. For the quantum depolarizing channel, we prove a formula that gives lower bounds on the quantum capacity with classical feedback from any $E_B$ protocols. We then apply this formula to the $E_B$ protocols that we discuss to obtain new lower bounds on the quantum capacity with classical feedback of the quantum depolarizing channel

Asymptotic entanglement capacity of the Ising and anisotropic Heisenberg interactions

We compute the asymptotic entanglement capacity of the Ising interaction ZZ, the anisotropic Heisenberg interaction XX + YY, and more generally, any two-qubit Hamiltonian with canonical form K = a XX + b YY. We also describe an entanglement assisted classical communication protocol using the Hamiltonian K with rate equal to the asymptotic entanglement capacity.Comment: 5 pages, 1 figure; minor corrections, conjecture adde

New geometries for high spatial resolution hall probes

The Hall response function of symmetric and asymmetric planar Hall effect devices is investigated by scanning a magnetized tip above a sensor surface while simultaneously recording the topography and the Hall voltage. Hall sensor geometries are tailored using a Focused Ion Beam, in standard symmetric and new asymmetric geometries. With this technique we are able to reduce a single voltage probe to a narrow constriction 20 times smaller than the other device dimensions. We show that the response function is peaked above the constriction, in agreement with numerical simulations. The results suggest a new way to pattern Hall sensors for enhanced spatial resolution.Comment: 12 pages, 5 figures, submitted to Journal of Applied Physic

The autoepitope of the 74-kD mitochondrial autoantigen of primary biliary cirrhosis corresponds to the functional site of dihydrolipoamide acetyltransferase.

Autoantibodies to mitochondrial antigens are characteristic of the autoimmune liver disease primary biliary cirrhosis (PBC), but the precise antigenic determinants recognized by these antibodies have not been defined. Recently, our laboratory identified a 1,370-bp rat liver cDNA clone that coded for a polypeptide recognized specifically by sera from patients with PBC but not by sera from patients with other forms of liver disease. This recombinant protein was identified as the 74-kD M2 mitochondrial inner membrane autoantigen, now known to be dihydrolipoamide acetyltransferase. In the present study, we have identified a 603-bp fragment that codes for a polypeptide containing all of the autoreactivity of the original clone. In addition, based on hydrophobicity/hydrophilicity plots of the amino acid sequence of this polypeptide segment, several peptides were synthesized and tested for reactivity by an inhibition assay using sera from patients with PBC. One peptide, defined by the amino acids AEIETDKATIGFEVQEEGYL, absorbed serum reactivity to the protein product of the original clone. Of particular interest was the finding that this peptide contains the lipoic acid binding site KATIGF of the dihydrolipoamide acetyltransferase found in the inner mitochondrial membrane. Thus, it appears that for this autoantigen, the target of the autoantibodies corresponds to a functional site of the dihydrolipoamide acetyltransferase

Coherent control of microwave pulse storage in superconducting circuits

Coherent pulse control for quantum memory is viable in the optical domain but nascent in microwave quantum circuits. We show how to realize coherent storage and on-demand pulse retrieval entirely within a superconducting circuit by exploiting and extending existing electromagnetically induced transparency technology in superconducting quantum circuits. Our scheme employs a linear array of superconducting artificial atoms coupled to a microwave transmission line.Comment: 13 pages, 4 figures and some supplementary materia

Two-way quantum communication channels

We consider communication between two parties using a bipartite quantum operation, which constitutes the most general quantum mechanical model of two-party communication. We primarily focus on the simultaneous forward and backward communication of classical messages. For the case in which the two parties share unlimited prior entanglement, we give inner and outer bounds on the achievable rate region that generalize classical results due to Shannon. In particular, using a protocol of Bennett, Harrow, Leung, and Smolin, we give a one-shot expression in terms of the Holevo information for the entanglement-assisted one-way capacity of a two-way quantum channel. As applications, we rederive two known additivity results for one-way channel capacities: the entanglement-assisted capacity of a general one-way channel, and the unassisted capacity of an entanglement-breaking one-way channel.Comment: 21 pages, 3 figure

Characterization of silicon nanowire by use of full-vectorial finite element method.

We have carried out a rigorous H-field-based full-vectorial modal analysis and used it to characterize, more accurately, the abrupt dielectric discontinuity of a high index contrast optical waveguide. The full-vectorial H and E fields and the Poynting vector profiles are described in detail. It has been shown through this work that the mode profile of a circular silicon nanowire is not circular and also contains a strong axial field component. The single-mode operation, vector field profiles, modal hybridness, modal ellipticity, and group velocity dispersion of this silicon nanowire are also presented