In this short note, we present a proof of a relatively expected result in a convex-optimization problem. In fact, we will show that a matrix at which the maximum of the SWAP fidelity is achieved has determinant zero.
Convex optimization problems arise naturally in quantum information theory, often in terms of minimizing a convex function over a convex subset of the space of hermitian matrices. In most cases, finding exact solutions to these problems is usually impossible. As inspired by earlier investigations into the relative entropy of entanglement (REE) (Miranowicz and Ishizaka 2008 Phys. Rev. A 78 032310 ), we introduce a general method to solve the converse problem rather than find explicit solutions. That is, given a matrix in a convex set, we determine a family of convex functions that are minimized at this point. This method allows us find explicit formulae for the REE and the Rains bound, two well-known upper bounds on the distillable entanglement, and yields interesting information about these quantities, such as the fact that they coincide in the case where at least one subsystem of a multipartite state is a qubit.
This thesis explores algorithmic applications and limitations of convex relaxation hierarchies for approximating some discrete and continuous optimization problems. - We show a dichotomy of approximability of constraint satisfaction problems (CSPs) by linear programming (LP) relaxations: for every CSP, the approximation obtained by a basic LP relaxation, is no weaker than the approximation obtained using relaxations given by super-constant levels of the Sherali-Adams hierarchy on instances of size n. - For the problem of approximating the absolute maximum of an n-variate degree-d homogeneous polynomial f with real coefficients over the unit sphere, we analyze the optimum value of the level-t sum-of-squares (SoS) SDP relaxation of the problem. Our results offer a trade-off between the approximation ratio and running time, which can take advantage of additional structure in the polynomial, such as non-negativity or sparsity of the coefficients. - We study the problem of approximating the p→q-norm of a matrix A, and prove the first NP-hardness result for approximating norms in the hypercontractive case 1<p<q<∞. We also prove almost tight algorithmic results for the case when p≥q (with 2∈[q,p]) where constant factor approximations for the matrix norms are possible. A common theme for these results is their connection to geometry. For the discrete optimization problem of CSP, geometry appears as a crucial tool for our lower bound proof. For the problem of polynomial optimization, we show that SDPs capture and extend earlier algorithms based on diameter estimation for convex bodies. For the matrix (operator) norm problem, the definition itself is geometric in nature and embedding theorems play a crucial role in our proofs.
In this technical note, we study the problem of parameter estimation for quantum channels, which are (completely positive) maps acting on quantum states. We focus on an important subclass of channels called Pauli channels which are characterized by a certain set of vectors (directions) and a number of scalar parameters. For the case where the directions are known, a special parametrization turns the parameter estimation problem into a convex optimization problem. For the case of unknown directions we give a simple algorithm to estimate the directions for qubit Pauli channels. These results assume that the identification experiment configuration is given, namely, a set of quantum states are given on which the channel acts, and a set of positive measurement (operator)s are fixed from which information is gathered. In the second part of the technical note we consider the problem of determining the experiment design, namely, determining states and measurements for optimal parameter identification of the channels. We formalize this problem as a maximization problem for Fisher information and, assuming known channel directions, prove that this problem is convex. We also prove that the optimal states to be used in experiments are pure and the optimal measurements are extremal. For qubit Pauli channels we prove that both the optimal pure input states and projective measurements should be directed towards the channel directions. We illustrate the results of the technical note with numerical examples and simulations.