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Zhurnal Srednevolzhskogo Matematicheskogo Obshchestva

Simulation of switchers for CNOT-gates based on optical waveguide interaction with coupled mode theory

Alexander A. Lytaev, I. Yu. Popov

Published Dec 27, 2021• 0 citations
Publisher

Abstract

The paper is devoted to simulation of interactions in the system of two symmetrical slab optical waveguides, that guide exactly two guided modes with the aim to use the directional coupler as a switcher for CNOT gate in the waveguide model of quantum-like computations. The coupling mode theory is used to solve the system of Maxwell equations. The asymptotic analysis is applied to simplify the system of differential equations, so an approximate analytic solution can be found. The solution obtained is used for the quick directional coupler parameters adjusting algorithm, so the power exchange in the system occurs as that of correctly working CNOT-gate switcher. Moreover, the finite difference method is used to solve the stricter system of equations, that additionally takes into account the process of power exchange between different order guided modes, so the computational error of the device can be estimated. It was obtained, that the possible size of the device may not exceed 1 mm in the largest dimension, while the computational error does not exceed 3%.

DOI
10.15507/2079-6900.23.202104.433-443

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The paper is devoted to simulation of interactions in the system of two symmetrical slab optical waveguides, that guide exactly two guided modes with the aim to use the directional coupler as a switcher for CNOT gate in the waveguide model of quantum-like computations. The coupling mode theory is used to solve the system of Maxwell equations. The asymptotic analysis is applied to simplify the system of differential equations, so an approximate analytic solution can be found. The solution obtained is used for the quick directional coupler parameters adjusting algorithm, so the power exchange in the system occurs as that of correctly working CNOT-gate switcher. Moreover, the finite difference method is used to solve the stricter system of equations, that additionally takes into account the process of power exchange between different order guided modes, so the computational error of the device can be estimated. It was obtained, that the possible size of the device may not exceed 1 mm in the largest dimension, while the computational error does not exceed 3%.
Abstract The system of two coupled optical dual-mode waveguides is considered. The coupling of the system is studied to find a circuit for building a control switch for two qubit gates. The classical coupled mode theory is applied and the exact expressions for coupling coefficients are derived. The parameters of the system for performing the desired operations are numerically computed and analysed. The system describing the influence of intermodal interactions is solved numerically. The distortions are analysed.
Recently, quantum computing has gained notoriety as a computational method that has the possibility to greatly increase the speed of calculations compared with traditional, binary computing. To this end, simulations of a quantum controlled-not gate implemented by a quantum waveguide are examined. The waveguide is composed of two quantum wires and a tuned coupling region and the simulations consist of a solution to Schrödinger’s equation via the Usuki method of mode matching. The use of Gaussian wave packets in previous work on this topic is questioned and various methods of tuning the coupling region in order to guide the quantum wave are investigated. Simulations are shown for a tuned waveguide in which the wave can be switched by the application of an external magnetic field.
Quantum logic operations are the elemental operations for quantum computing and have been implemented in various time-evolved atomic systems. Here, we present a scheme to simulatively implement the typical quantum NOT- and Hadamard gates operations, alternatively with the coupled three-waveguide structure (rather than the usual atomic levels); two modes in sinuous waveguides encoding the two logic states of a qubit and the mode in the center straight waveguide serve as auxiliary the coupler. Based on the Lewis-Riesenfeld invariant theory, we show that the desired mode conversions for implementing the NOT gate and mode split to realize the Hadamard gate are fast achieved by engineering the coupling parameters between the optical waveguides. The implementations of the logic gates demonstrated here relay on the structure parameters of the waveguides, rather than the durations of time evolutions with the usual driven atomic systems, and thus should be feasible robustly with the current integrated optical technology.
A quantum mechanical analysis of the guided light in integrated photonics waveguides is presented. The analysis is made starting from one-dimensional (1D) guided vector modes by taking into account the modal orthonormalization property on a cross section of an optical waveguide, the vector structure of the guided optical modes and the reversal-time symmetry in order to quantize the 1D vector modes and to derive the quantum momentum operator and the Heisenberg equations. The results provide a quantum-consistent formulation of the linear and nonlinear quantum light propagations as a function of forward and backward creation and annihilation operators in integrated photonics. As an illustration, an application to an integrated nonlinear directional coupler is given, that is, both the nonlinear momentum and the Heisenberg equations of the nonlinear coupler are derived.
Subject of Research. The paper presents a quantum search algorithm model, suitable for integration into a linear optical chip. Error impact caused by two-qubit operator implementation and directional coupler manufacture imperfection on the algorithm output is studied. Method. Analytical calculation of the algorithm scheme was performed to assess error impact caused by two-qubit operator optical implementation. Numerical simulation of the algorithm was executed for taking into account distortions caused by directional coupler imperfections. The simulation was completed using Qutip library on Python programming language. Main Results. Two well-known implementations of the algorithm scheme main component, a two-qubit CZ gate, are compared in order to select the most optimal chip architecture. It was shown that one of two-qubit gate implementations introduces an error critical for the algorithm work. Another implementation based on projection measurements does not introduce an error, but has a lower efficiency. We have performed simulation of the proposed scheme, taking into account the imperfections of its components in the framework of unitary dynamics. We have shown that the algorithm error probability does not exceed 0.011. Two-qubit Grover’s algorithm оptical implementation with regard to directional coupler imperfections has a low error rate, but it is limited by the low two-qubit operator efficiency. Practical Relevance. The study carried out can be useful for the physical implementation of the algorithm. Creation of an integrated optical scheme that implements Grover’s algorithm will make it possible to build a quantum router for the optimal route search in quantum networks with complex topology.
The analog between an optical waveguide coupler and the quantum-mechanical double-well problem is established. Well-known results of the quantum-mechanical system are applied to the waveguide system. This way of thinking of the waveguide system provides a quick understanding of the coupling mechanism and provides new interpretations of results obtained from Maxwell’s equations. Just as ray opticals give insight into certain types of optics problems, we believe that the tunneling interpretation will give new insight into the field of guided light waves.