Qubit Channels Can Require More Than Two Inputs to Achieve Capacity
We give examples of qubit channels that require three input states in order to achieve the Holevo capacity.
quant-ph↗
arXiv subjects
Mary Beth Ruskai
Publications and source records attributed to Mary Beth Ruskai.
Pauli Exchange Errors in Quantum Computation
In many physically realistic models of quantum computation, Pauli exchange interactions cause a subset of two-qubit errors to occur as a first order effect of couplings within the computer, even in the absence of interactions with the computer's environment. We give an explicit 9-qubit code that corrects both Pauli exchange errors and all one-qubit errors.
quant-ph↗
Study of a Class of Regularizations of 1/|x| using Gaussian Integrals
This paper presents a comprehensive study of a class of functions which approximate 1/|x| for large x but which are finite at the origin. These functions arise naturally in the study of atoms in strong magnetic fields where the so-called "Landau states" give rise to Gaussian integrals. Generalizations in which e^{-x^2} is replaced by e^{-x^p} are also considered and approximate x^{1-p} for large x. The limiting behavior and monotonicity properties of these functions are discussed in terms of parameters which arise in the approximations as well as x. Several classes of inequalities, some of which provide tight bounds, are established. Some differential equations and recursion relations satisfied by these functions are given. The recursion relations give rise to two classes of polynomials, one of which is related to confluent hypergeometric functions. Finally, it is shown that the inverse of the approximating function is convex and this implies an analogue of the triangle inequality. Some comments are made about possible extensions and several open questions are raised.
math.CA↗
One Dimensional Regularizations of the Coulomb Potential with Application to Atoms in Strong Magnetic Fields
We consider one-dimensional regularizations of the Coulomb potential formed by taking a two-dimensional expectation of the Coulomb potential with respect to the Landau states. It is well-known that such functions arises naturally in the study of atoms in strong magnetic fields. For many-electron atoms consideration of the Pauli principle requires convex combinations of such potentials and interactions in which the regularizations also contain a 2^{-1/2} rescaling. We summarize the results of a comprehensive study of these functions including recursion relations, tight bounds, convexity properties, and connections with confluent hypergeometric functions. We also report briefly on their application in one-dimensional models of many-electrons atoms in strong magnetic fields.
math-ph↗
A One-Dimensional Model for Many-Electron Atoms in Extremely Strong Magnetic Fields: Maximum Negative Ionization
We consider a one-dimensional model for many-electron atoms in strong magnetic fields in which the Coulomb potential and interactions are replaced by one-dimensional regularizations associated with the lowest Landau level. For this model we show that the maximum number of electrons is bounded above by 2Z+1 + c sqrt{B}. We follow Lieb's strategy in which convexity plays a critical role. For the case of two electrons and fractional nuclear charge, we also discuss the critical value at which the nuclear charge becomes too weak to bind two electrons.
math-ph↗
Monotone Riemannian Metrics and Relative Entropy on Non-Commutative Probability Spaces
We use the relative modular operator to define a generalized relative entropy for any convex operator function g on the positive real line satisfying g(1) = 0. We show that these convex operator functions can be partitioned into convex subsets each of which defines a unique symmetrized relative entropy, a unique family (parameterized by density matrices) of continuous monotone Riemannian metrics, a unique geodesic distance on the space of density matrices, and a unique monotone operator function satisfying certain symmetry and normalization conditions. We describe these objects explicitly in several important special cases, including the familiar logarithmic relative entropy. The relative entropies, Riemannian metrics, and geodesic distances obtained by our procedure all contract under completely positive, trace-preserving maps. We then define and study the maximal contraction associated with these quantities.
math-ph↗