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Limit distribution of a quantum walk driven by a CMV matrix

Takuya Machida

Published Mar 17, 2019• 0 citations
OpenAlex

Abstract

Quantum walks with CMV matrices, five-diagonal unitary matrices, started to be studied in 2003 [1]. The spectral analysis for CMV matrices told us that the quantum walks could localize in distribution depending on the Verblunsky parameters of the matrices. In this paper, we work on a quantum walk whose system is manipulated by a CMV matrix with homogeneous Vervlunsky parameters, and present long-time limit distributions. One can understand from the theory that the quantum walk does not localize and how it approximately distributes after the long-time evolution has been executed on the walk.

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Quantum walks with CMV matrices, five-diagonal unitary matrices, started to be studied in 2003 [1]. The spectral analysis for CMV matrices told us that the quantum walks could localize in distribution depending on the Verblunsky parameters of the matrices. In this paper, we work on a quantum walk whose system is manipulated by a CMV matrix with homogeneous Vervlunsky parameters, and present long-time limit distributions. One can understand from the theory that the quantum walk does not localize and how it approximately distributes after the long-time evolution has been executed on the walk.
In this paper, we work on a quantum walk whose system is manipulated by a five-diagonal unitary matrix, and present long-time limit distributions. The quantum walk launches off a location and delocalizes in distribution as its system is getting updated. The five-diagonal matrix contains a phase term and the quantum walk becomes a standard coined walk when the phase term is fixed at special values. Or, the phase term gives an effect on the quantum walk. As a result, we will see an explicit form of a long-time limit distribution for a quantum walk driven by the matrix, and thanks to the exact form, we understand how the quantum walker approximately distributes in space after the long-time evolution has been executed on the walk.
We study discrete-time quantum walks on a half line by means of spectral analysis. Cantero et al. showed that the CMV matrix, which gives a recurrence relation for the orthogonal Laurent polynomials on the unit circle, expresses the dynamics of the quantum walk. Using the CGMV method introduced by them, the name is taken from their initials, we obtain the spectral measure for the quantum walk. As a corollary, we give another proof for localization of the quantum walk on homogeneous trees shown by Chisaki et al.
We study discrete-time quantum walks on a half line by means of spectral analysis. Cantero et al. [1] showed that the CMV matrix, which gives a recurrence relation for the orthogonal Laurent polynomials on the unit circle [2], expresses the dynamics of the quantum walk. Using the CGMV method introduced by them, the name is taken from their initials, we obtain the spectral measure for the quantum walk. As a corollary, we give another proof for localization of the quantum walk on homogeneous trees shown by Chisaki et al. [3].
Long-time limit distributions are key quantities for understanding the asymptotic dynamics of quantum walks, and they are known for most forms of one-dimensional quantum walks using two-state coin systems. For two-dimensional quantum walks using a four-state coin system, however, the only known limit distribution is for a walk using a parameterized Grover coin operation and analytical complexities have been a major obstacle for obtaining long-time limit distributions for other coins. In this work however, we present two new types of long-time limit distributions for walks using different forms of coin-flip operations in a four-state coin system. This opens the road towards understanding the dynamics and asymptotic behaviour for higher state coin system from a mathematical view point.
The probability distributions of discrete-time quantum walks have been often investigated, and many interesting properties of them have been discovered. The probability that the walker can be find at a position is defined by diagonal elements of the density matrix. On the other hand, although off-diagonal parts of the density matrices have an important role to quantify quantumness, they have not received attention in quantum walks. We focus on the off-diagonal parts of the density matrices for discrete-time quantum walks on the line and derive limit theorems for them.