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arXiv:quant-ph/0309063 · Physical Review A

One-dimensional quantum walk with unitary noise

Daniel Shapira, Ofer Biham, Anthony J. Bracken, Michelle Hackett

Published Dec 18, 2003• 70 citations
arXivHTMLPDF

Abstract

The effect of unitary noise on the discrete one-dimensional quantum walk is studied using computer simulations. For the noiseless quantum walk, starting at the origin $(n=0)$ at time $t=0,$ the position distribution ${P}_{t}(n)$ at time t is very different from the Gaussian distribution obtained for the classical random walk. Furthermore, its standard deviation, $\ensuremath{\sigma}(t)$ scales as $\ensuremath{\sigma}(t)\ensuremath{\sim}t,$ unlike the classical random walk for which $\ensuremath{\sigma}(t)\ensuremath{\sim}\sqrt{t}.$ It is shown that when the quantum walk is exposed to unitary noise, it exhibits a crossover from quantum behavior for short times to classical-like behavior for long times. The crossover time is found to be $T\ensuremath{\sim}{\ensuremath{\alpha}}^{\ensuremath{-}2},$ where $\ensuremath{\alpha}$ is the standard deviation of the noise.

DOI
10.1103/physreva.68.062315

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Discrete-time quantum walks exhibit ballistic spreading that is highly sensitive to decoherence. We study the long-time dynamics of a one-dimensional discrete-time quantum walk under Pauli noise acting on the coin. Choosing the unitary coin such that its invariant Bloch-sphere axis is orthogonal to the axis fixed by the conditional shift, we identify the distinct effects of noise aligned with either dynamical axis or orthogonal to both. Using a Fourier-space Pauli-superoperator formalism, we derive expressions for the first two position moments and long-time asymptotics for X-, Y-, Z-, and depolarizing noise. All channels induce a crossover from ballistic to diffusive spreading, with a second moment growing linearly at long times. The approach to this regime is channel dependent: it is exponentially fast for X-, Z-, and depolarizing noise, but algebraic, with a leading t−1/2t^{-1/2}t−1/2 correction, for Y-noise. The position distributions retain distinct finite-noise signatures, including a Gaussian-like profile with a central depression for Y-noise. In the maximal-noise limit, depolarizing, Y-, and Z-noise yield the classical binomial distribution for arbitrary initial coin states, whereas X-noise does so only when the initial state has no component along the noise axis. Thus, the quantum-to-classical transition is governed by the relative alignment of the noise, the coherent walk dynamics, and the initial state.
The quantum random walk has been much studied recently, largely due to its highly nonclassical behavior. In this paper, we study one possible route to classical behavior for the discrete quantum walk on the line: the presence of decoherence in the quantum ``coin'' which drives the walk. We find exact analytical expressions for the time dependence of the first two moments of position, and show that in the long-time limit the variance grows linearly with time, unlike the unitary walk. We compare this to the results of direct numerical simulation, and see how the form of the position distribution changes from the unitary to the usual classical result as we increase the strength of the decoherence.
The behavior of one-dimensional quantum random walks is strikingly different from that of classical ones. However, when decoherence is involved, the limiting distributions take on many classical features over time. In this paper, we study the decoherence on both position and ``coin'' spaces of the particle. We propose an analytical approach to investigate these phenomena and obtain the generating functions which encode all the features of these walks. Specifically, from these generating functions, we find exact analytic expressions of several moments for the time and noise dependence of position. Moreover, the limiting position distributions of decoherent quantum random walks are shown to be Gaussian in an analytical manner. These results explicitly describe the relationship between the system and the level of decoherence.
Abstract In this article we investigate the effects of shifting position decoherence, arising from the incoherent tunneling effect in the experimental realization of the quantum walk, on the one-dimensional discrete time quantum walk. We show that in the regime of this type of noise the quantum behavior of the walker does not vanish, in contrast to the coin decoherence for which the walker undergoes a quantum-to-classical transition even for weak noise. In particular, we show that the quadratic dependence of the variance on the time and also the coin–position entanglement, i.e. two important quantum aspects of the coherent quantum walk, are preserved in the presence of tunneling decoherence. Furthermore, we present an explicit expression for the probability distribution of the decoherent one-dimensional quantum walk in terms of the corresponding coherent probabilities, and show that this type of decoherence smooths the probability distribution.
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Quantum random walks have been much studied recently,largely due to their highly non-classical behaviors.In this paper,we review the classical and quantum random walks of one-dimensional,and analyze two classical two-dimensional models and compare them.Furthermore we propose a two-dimensional quantum random walk algorithm.We compute the standard deviation,thereby proving our two-dimensional algorithm is superior to that of one-dimension.We propose a scheme,which is based on single photon with pairs of qubits,to implement it.