Younghun Kwon

Generalized quantum search Hamiltonian

There are Hamiltonians that solve the search problem of finding one of N items in O(√N) steps. These are Hamiltonians describing an oscillation between two states. In this paper we propose a generalized search Hamiltonian H g . Then the known search Hamiltonians become special cases of H g . For the generalized search Hamiltonian, we present the remarkable result that searching with 100% success is subject only to the phase factor in H g and independent of the number of states or initialization.

Revisiting Quantum Discord for Two-Qubit X States: The Error Bound to an Analytical Formula

In this article, we investigate the error bound of quantum discord, obtained from the analytic formula of Ali et al. (Phys. Rev. A 81, 042105, 2010) for the case of general X states and from the analytic formula of Fanchini et al. (Phys. Rev. A 81, 052107, 2010) for the case of symmetric X states. For this purpose we consider 3-element POVM. We find that the results of Ali et al. and Fanchini et al. may have the worst-case errors of 0.004565 and 0.0009, respectively.We study the Schrodinger operator with a potential given by the sum of the potentials for harmonic oscillator and imaginary cubic oscillator and we focus on its pseudospectral properties. A summary of known results about the operator and its spectrum is provided and the importance of examining its pseudospectrum as well is emphasized. This is achieved by employing scaling techniques and treating the operator using semiclassical methods. The existence of pseudoeigenvalues very far from the spectrum is proven, and as a consequence, the spectrum of the operator is unstable with respect to small perturbations and the operator cannot be similar to a self-adjoint operator via a bounded and boundedly invertible transformation. It is shown that its eigenfunctions form a complete set in the Hilbert space of square-integrable functions; however, they do not form a Schauder basis.

Complete analysis for three-qubit mixed-state discrimination

In this article, by treating minimum error state discrimination as a complementarity problem, we obtain the geometric optimality conditions. These can be used as the necessary and sufficient conditions to determine whether every optimal measurement operator can be nonzero. Using these conditions and an inductive approach, we demonstrate a geometric method and the intrinsic polytope for N-qubit mixed-state discrimination. When the intrinsic polytope becomes a point, a line segment, or a triangle, the guessing probability, the necessary and sufficient condition for the exact solution, and the optimal measurement are analytically obtained. We apply this result to the problem of discrimination to arbitrary three-qubit mixed states with given a priori probabilities and obtain the complete analytic solution to the guessing probability and optimal measurement.

Quantum mechanism of biological search

We wish to suggest an algorithm for biological search including DNA search. Our argument supposes that biological search be performed by quantum search. If we assume this, we can naturally answer the following long lasting puzzles such that “Why does DNA use the helix structure?” and “How can the evolution in biological system occur?”.

Speech Enhancement by Wavelet Packet Transform with Best Fitting Regression Line in Various Noise Environments

In this paper, we suggest a speech enhancement method which can be applied in various noise environments. This method uses a wavelet packet transform (WPT) and a best fitting regression line (BFRL) in order to accurately estimate parameters for the spectral subtraction method based on the time-varying gain function. It should be noted that our method does not use the statistical information of pause region detected by voice activity detector. The evaluation is performed on various environments where the noisy speech are between SNR -5 ~ 15 dB, in various noises. We compare the performance of the proposed method, with that of magnitude spectral subtraction in WPT and nonlinear magnitude spectral subtraction in WPT. We can see that the performance of the proposed method is better than that of any other methods, with regard to objective test (segmental SNR, weighted spectral slope), spectrogram analysis, and subjective one (mean opinion score). Especially, our method showed reliable result even at low SNR

An optimal discrimination of two mixed qubit states with a fixed rate of inconclusive results

In this paper we consider the optimal discrimination of two mixed qubit states for a measurement that allows a fixed rate of inconclusive results. Our strategy is to transform the problem of two qubit states into a minimum error discrimination for three qubit states by adding a specific quantum state

Asymptotic relation between Bell-inequality violations and entanglement distillability

\rho _{0}

Analysis of Optimal Sequential State Discrimination for Linearly Independent Pure Quantum States

ρ0 and a prior probability