Non-equilibrium topological phase transitions in two-dimensional optical lattices

Recently, concepts of topological phases of matter are extended to non-equilibrium systems, especially periodically driven systems. In this paper, we construct an example which shows non-equilibrium topological phase transitions using ultracold fermions in optical lattices. We show that the Rabi oscillation has the possibility to induce non-equilibrium topological phases which are classified into time-reversal-invariant topological insulators for a two-orbital model of alkaline-earth-metal atoms. Furthermore we study the non-equilibrium topological phases using time-dependent Schrieffer-Wolff-type perturbation theory, and we obtain an analytical expression to describe the topological phase transitions from a high-frequency limit of external driving fields.Comment: 8 pages, 4 figure

RM-CVaR: Regularized Multiple β\beta-CVaR Portfolio

The problem of finding the optimal portfolio for investors is called the portfolio optimization problem. Such problem mainly concerns the expectation and variability of return (i.e., mean and variance). Although the variance would be the most fundamental risk measure to be minimized, it has several drawbacks. Conditional Value-at-Risk (CVaR) is a relatively new risk measure that addresses some of the shortcomings of well-known variance-related risk measures, and because of its computational efficiencies, it has gained popularity. CVaR is defined as the expected value of the loss that occurs beyond a certain probability level ($\beta$). However, portfolio optimization problems that use CVaR as a risk measure are formulated with a single $\beta$ and may output significantly different portfolios depending on how the $\beta$ is selected. We confirm even small changes in $\beta$ can result in huge changes in the whole portfolio structure. In order to improve this problem, we propose RM-CVaR: Regularized Multiple $\beta$-CVaR Portfolio. We perform experiments on well-known benchmarks to evaluate the proposed portfolio. Compared with various portfolios, RM-CVaR demonstrates a superior performance of having both higher risk-adjusted returns and lower maximum drawdown.Comment: accepted by the IJCAI-PRICAI 2020 Special Track AI in FinTec

Laser-induced phase transitions of topological Kondo insulators

In this paper, we theoretically investigate how laser fields change the nature of topological Kondo insulators(TKIs). By employing a prototypical model of TKIs, we treat the effect of the laser fields with Floquet theory, which gives effective description under high frequency laser fields. We derive the effective model of TKIs under the laser irradiation and discuss its topological properties. We demonstrate a possible realization of Floquet Chern insulators specified by various values of Chern number and reveal how the topological phase changes with increasing the laser light intensity. Furthermore, it is shown that Floquet Weyl semimetals, which have some pairs of Weyl nodes protected topologically, can emerge in the three-dimensional case. We explain how the Weyl nodes are created with varying the strength of the laser field.Comment: 8 pages, 3 figures. This paper is accepted for publication in the proceedings of ICM 2015 (Physics Procedia

Topology of Discrete Quantum Feedback Control

A general framework for analyzing topology of quantum channels of single-particle systems is developed to find a class of genuinely dynamical topological phases that can be realized by means of discrete quantum feedback control. We provide a symmetry classification of quantum channels by identifying ten symmetry classes of discrete quantum feedback control with projective measurements. We construct various types of topological feedback control by using topological Maxwell's demons that achieve robust feedback-controlled chiral or helical transport against noise and decoherence. Topological feedback control thus offers a versatile tool for creating and controlling nonequilibrium topological phases in open quantum systems that are distinct from non-Hermitian and Lindbladian systems and should provide a guiding principle for topology-based design of quantum feedback control.Comment: 38 pages, 19 figure