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Acta Physica Sinica

Effects of initial states on continuous-time quantum walk in the optical waveguide array

Chun-Nian Ren, Shi Peng, Liu Kai, Wendong Li, Jie Zhao, Gu Yong-Jian

Published Jan 1, 2013• 2 citations
Publisher

Abstract

Continuous-time quantum random walk is constructed when photons propagate passing the branches of the waveguide array. It is possible to make quantum simulator, based on the quantum walk in waveguides, on a commercial scale firstly, but there are still some problems such as input state, the structure and boundary of the waveguides that should be treated at present. A nearest-neighbor coupling model is used to deal with the question of coupled waveguides and an explicit analytical solution can be derived. Using the analytical solution, we analyze the effects of input state on particle number probability distribution function and the second-order coherence degree of the quantum walk process in periodic waveguides. The results show that the symmetry properties of the input state would influence the distribution of second-order coherence degree, but have little effect on the probability distribution function.

DOI
10.7498/aps.62.090301

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Continuous-time quantum random walk is constructed when photons propagate passing the branches of the waveguide array. It is possible to make quantum simulator, based on the quantum walk in waveguides, on a commercial scale firstly, but there are still some problems such as input state, the structure and boundary of the waveguides that should be treated at present. A nearest-neighbor coupling model is used to deal with the question of coupled waveguides and an explicit analytical solution can be derived. Using the analytical solution, we analyze the effects of input state on particle number probability distribution function and the second-order coherence degree of the quantum walk process in periodic waveguides. The results show that the symmetry properties of the input state would influence the distribution of second-order coherence degree, but have little effect on the probability distribution function.
The continuous-time quantum walk (CTQW) is the quantum analogue of the continuous-time classical walk and is widely used in universal quantum computations. Here, taking the advantages of the waveguide arrays, we implement large-scale CTQWs on chips. We couple the single-photon source into the middle port of the waveguide arrays and measure the emergent photon number distributions by utilizing the fiber coupling platform. Subsequently, we simulate the photon number distributions of the waveguide arrays by considering the boundary conditions. The boundary conditions are quite necessary in solving the problems of quantum mazes.
Two-dimensional continuous time quantum random walks (CTQRW) are physical processes where quantum particles simultaneously evolve in different permissible directions within discrete graphs. In order to force the quantum walkers (QWs) to evolve in such a fashion, one generally requires periodic potentials. Here, we demonstrate that two-dimensional CTQRW can be generated in free space by properly tailoring the initial wave functions. We analytically show that within a certain spatial region the arising probability distribution quantitatively resembles the probability pattern exhibited by a QW traversing a periodic lattice potential. These theoretical predictions were experimentally verified using classical laser light, appropriately shaped by a spatial light modulator. Expanding the presented results to the case of multiple walkers may open new possibilities in quantum information technology using bulk optics. (C) 2014 Optical Society of America
Random processes are ubiquitous in the natural world as well as in man-made environments. While classical random walks may behave highly complex or even chaotic, there outcome can, in principle, always be predicted from the parameters of the system and the initial conditions. In the realm of quantum mechanics, however, this is not possible as the underlying wave mechanics leads to intrinsically indeterministic outcomes. Moreover, if multiple indistinguishable particles are subjected to such a quantum random walk, their exchange symmetry causes quantum interference, thereby enriching the dynamics of the system even further. \nIn this work, quantum random walks of pairs of indistinguishable photons, the quanta of light, are investigated. Networks of coupled optical waveguides are chosen as the experimental platform of choice, offering a high degree of coherence and versatility. In these photonic lattices, light propagates along one spatial dimension, whereas the individual waveguides are connected by evanescent coupling in the transverse dimensions. \nIn particular, it is investigated how the various degrees of freedom, which are available in such photonic lattices, affect the trajectories in the quantum walks and their complexity. It is shown how the facilitation of both transverse dimensions allows for much richer quantum walks with properties unencountered in planar arrangements. But even if just a single transverse dimension is available, the coupling properties of the lattice along this dimension are a potent degree of freedom, which can be used to manipulate the quantum walk. Finally, an experimental technique is developed which enables a convenient characterisation of the expected quantum walk in an arbitrary waveguide lattice by classical light. \nThe thesis concludes with a summary of the results and an outlook onto further developments in the field.
We investigate a quasi-one-dimensional periodic array of coupled waveguides, with one extended and one bound dimension, incorporating both first- and second-order coupling. We study the evolution of optical fields in this system, and measure quantum correlations when path-entangled photon pairs are launched into them. We observe a surprisingly large and nontrivial effect of second-order coupling on these correlations---while quantum correlations are symmetric when only first-order coupling is present, the introduction of next-nearest-neighbor coupling often breaks the symmetry to reflections.
Abstract We investigate the dynamical properties of the two-boson quantum walk in systems with different degrees of coherence, and where the effect of the coherence on the two-boson quantum walk can be naturally introduced. A general analytical expression for the two-boson correlation function, for both pure states and mixed states, is given.We propose a possible two-photon quantum-walk scheme with a mixed initial state, and find that the twophoton correlation function and the average distance between two photons can be influenced by the initial photon distribution, the relative phase, or the degree of coherence. The propagation features of our numerical results can be explained by our analytical two-photon correlation function.