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Deutsche Physikalische Gessellschaft IOP Institute of Physics

Entanglement in coined quantum walks on regular graphs

Ivens Carneiro1, Meng Loo1, Xibai Xu1, Mathieu Girerd1,2, Viv Kendon1,3 and Peter L Knight1

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Quantum walks, both discrete (coined) and continuous time, form the basis of several recent quantum algorithms. Here we use numerical simulations to study the properties of discrete, coined quantum walks. We investigate the variation in the entanglement between the coin and the position of the particle by calculating the entropy of the reduced density matrix of the coin. We consider both dynamical evolution and asymptotic limits for coins of dimensions from two to eight on regular graphs. For low coin dimensions, quantum walks which spread faster (as measured by the mean square deviation of their distribution from uniform) also exhibit faster convergence towards the asymptotic value of the entanglement between the coin and particle's position. For high-dimensional coins, the DFT coin operator is more efficient at spreading than the Grover coin. We study the entanglement of the coin on regular finite graphs such as cycles, and also show that on complete bipartite graphs, a quantum walk with a Grover coin is always periodic with period four. We generalize the 'glued trees' graph used by Childs et al (2003 Proc. STOC, pp 59–68) to higher branching rate (fan out) and verify that the scaling with branching rate and with tree depth is polynomial.


PACS

03.67.Lx Quantum computation architectures and implementations

05.40.Fb Random walks and Levy flights

05.50.+q Lattice theory and statistics (Ising, Potts, etc.)

02.60.Cb Numerical simulation; solution of equations

03.67.Mn Entanglement measures, witnesses, and other characterizations

02.10.Ox Combinatorics; graph theory

Subjects

Mathematical physics

Computational physics

Statistical physics and nonlinear systems

Quantum information and quantum mechanics

Dates

Issue 1 (July 2005)

Received 22 April 2005

Published 12 July 2005



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