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A quantum walk with both a continuous-time limit and a continuous-spacetime limit

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Abstract

Nowadays, quantum simulation schemes come in two flavours. Either they are continuous-time discrete-space models (a.k.a Hamiltonian-based), pertaining to non-relativistic quantum mechanics. Or they are discrete-spacetime models (a.k.a quantum walks or quantum cellular automata based) enjoying a relativistic continuous-spacetime limit. We provide a first example of a quantum simulation scheme that unifies both approaches. The proposed scheme supports both a continuous-time discrete-space limit, leading to lattice fermions, and a continuous-spacetime limit, leading to the Dirac equation. The transition between the two can be thought of as a general relativistic change of coordinates, pushed to an extreme. As an emergent by-product of this procedure, we obtain a Hamiltonian for lattice fermions in curved spacetime with synchronous coordinates.

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Data Availability Statement

No data sets were generated or analysed during the current study.

Notes

  1. Notice that \(H_L\) commutes with \(\sigma _x\), thus preserving the chiral symmetry w.r.t the components of the spinor \(\Psi = (\psi ^+,\psi ^-)^\intercal \). This will no longer be true when we introduce the mass, which notoriously breaks chirality.

  2. In order to recover a massless Hamiltonian commuting with \(\sigma _y\), we can choose the operator

    $$\begin{aligned} U'=\begin{pmatrix} 1 &{}\quad 0 &{}\quad 0 &{}\quad 0\\ 0 &{}\quad e^{-i \zeta }\sin \theta &{}\quad -\cos \theta &{}\quad 0\\ 0 &{}\quad \cos \theta &{}\quad -e^{i \zeta }\sin \theta &{}\quad 0\\ 0 &{}\quad 0 &{}\quad 0 &{}\quad -1 \end{pmatrix} \end{aligned}$$

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Acknowledgements

The authors would like to thank Pablo Arnault and Cédric Bény for motivating discussions.

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GDM and PA contributed equally to the main results of the manuscript.

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Correspondence to Giuseppe Di Molfetta.

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Di Molfetta, G., Arrighi, P. A quantum walk with both a continuous-time limit and a continuous-spacetime limit. Quantum Inf Process 19, 47 (2020). https://doi.org/10.1007/s11128-019-2549-2

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