Cited by 60Quantum Computing Algorithms and ArchitectureQuantum Mechanics and ApplicationsNoncommutative and Quantum Gravity Theories

Discrete-time quantum walks: Continuous limit and symmetries

Open access
di Molfetta, G., Debbasch, F.·2012·Journal of Mathematical Physics

Abstract

The continuous limit of one-dimensional discrete-time quantum walks with time-and space-dependent coefficients is investigated. A given quantum walk does not generally admit a continuous limit but some families (1-jets) of quantum walks do. All families (1-jets) admitting a continuous limit are identified. The continuous limit is described by a Dirac-like equation or, alternately, a couple of Klein-Gordon equations. Variational principles leading to these equations are also discussed, together with local invariance properties.

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Written from the paper’s full text.

TL;DR

The paper identifies which 1-jets of one-dimensional discrete-time quantum walks with time- and space-dependent SU(2) coins admit a formal continuous limit. When it exists, the limit is described by a Dirac-like equation or, alternately, coupled Klein-Gordon equations, with variational and local symmetry properties.

Key findings

  • Generically a given discrete-time quantum walk does not admit a continuous limit; only certain families (1-jets) do, and only a few 1-jets admit one. The Hadamard walk cannot have a formal continuous limit.
  • The richest scaling is α = β = δ = 1, where all contributions balance. In this scaling the continuous limit is a first-order Dirac-like equation DΨ = 0; each component also obeys an uncoupled Klein-Gordon equation at points where θ does not vanish, but these contain time-irreversibility terms.
  • The Dirac-like dynamics derives from a Lagrangian density. It is not invariant under SU(2), but it belongs to a family of operators D(B) that exhibits a local invariance under the transformation given in the paper; for j = 3 this coincides with a standard gauge transformation along σ₃.
  • Autonomous variational principles for the individual Klein-Gordon equations exist only in special cases where the irreversible terms vanish; otherwise there is only a single variational principle delivering the coupled Dirac-like equations for Ψ = (ψ−, ψ+).

Methods

  • Theoretical analysis of one-dimensional discrete-time quantum walks on N × Z, driven by time- and space-dependent SU(2) coins parameterized by Euler angles θ, ξ, ζ, acting on a two-component wave function.
  • Continuous limit investigated by introducing time step Δt, space step Δx, scales τ, λ, infinitesimal ε, and δ; Taylor expanding the discrete equations and identifying 1-jets whose angles tend to 0 or π as ε → 0.
  • Main analysis focuses on the scaling α = β = δ = 1, derives equations in null coordinates u− and u+, and studies variational principles and local symmetries of the resulting operators.

Limitations

  • The paper states that only the α = β = δ = 1 scaling is studied in detail; other scalings should be studied separately, numerical simulations are needed, and extensions to higher-dimensional space-times, higher-dimensional coins, space-time geometry, and decoherent/relativistic stochastic processes are left for future work.
  • The paper states that autonomous variational principles for the separate Klein-Gordon equations exist only when dissipative/irreversible terms vanish; otherwise the two components cannot be decoupled without losing symmetry and simple variational principles.
  • Evident from the provided text: many displayed equations are omitted or not decoded, so explicit derivations and coefficients cannot be verified from this copy.

Model-written summary. Check it against the paper before relying on it.

Integrity signals

Registry

Crossref · 10.1063/1.4764876

Cites

45 of 45 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Claims the paper makes

0 of 28 cited sources checked · 0 supported · 0 flagged

  • Findingreasoning
    § B. Second order form of the equations · p. 10

    The mass M- appearing in (36) must be real for the Hamilton equations obeyed by ψ- and ψ-† to be equivalent.

    “Note also that the mass M -appearing in (36) must be real for the Hamilton equations obeyed by ψ -and ψ -† to be equivalent, as they should.”
    Quote in the paperSubstantial claim
  • Findingreasoning
    § A. First order form of the equations · p. 8

    By (29) and (17), the total probability preservation property is preserved under a transformation (26) if and only if ∂-α = ∂+α, i.e. ∂x α = 0.

    “By (29) and (17), this property is preserved under a transformation (26) if and only if ∂ -α = ∂ + α i.e. ∂ x α = 0.”
    Quote in the paperSubstantial claim
  • Findingreasoning
    § A. First order form of the equations · p. 8

    The Lagrangian density L_D can be brought to a form which closely resembles the Dirac Lagrangian density.

    “The Lagrangian density L D can also be brought to a form which closely resembles the Dirac Lagrangian density.”
    Quote in the paperNot a substantial claim
  • Findingnone given
    § A. First order form of the equations · p. 7

    The Lagrangian density (24) leads to the first-order equations of motion (15, 16).

    “leads to the first-order equations of motion (15, 16).”
    Quote in the paperSubstantial claim
  • Findingreasoning
    § III. EQUATIONS OF MOTION · p. 5

    The expansion of the discrete equations around ǫ = 0 leads to coupled first-order equations of motion for ψ^- and ψ^+.

    “The expansion of the discrete equations around ǫ = 0 then leads to the following equations of motion for ψ -and ψ + :”
    Quote in the paperSubstantial claim
  • May combine two claims. 17
  • The quote only partly states this claim. 2
  • Classified as probably not a substantial claim. 11
  • May combine two claims.

    Findingreasoning
    § B. Second order form of the equations · p. 9

    For the continuous quantum walks dynamics considered here, there are no autonomous variational principles which deliver separately the Klein-Gordon equations obeyed by ψ- and ψ+ when dissipative terms do not vanish, but there is always a single variational principle which delivers the coupled Dirac-like equations obeyed by Ψ = (ψ-, ψ+).

    “For the continuous quantum walks dynamics considered here, there are indeed no autonomous variational principles which deliver separately the Klein-Gordon equations obeyed by ψ -and ψ + when disspative terms do not vanish, but there is always a single variational principle which delivers the coupled Dirac-like equations obeyd by the Ψ = ( ψ -, ψ + ).”
    Quote in the paperSubstantial claim
  • The quote only partly states this claim.

    Findingreasoning
    § A. First order form of the equations · p. 8

    If total probability conservation is imposed as an identity valid for all values of ψ- and ψ+, then D(B) necessarily takes the form (29), where p is an arbitrary integer and ξ-, ξ+, ̄θ and ζ are arbitrary possibly time- and position-dependent real quantities.

    “If one imposes total probability conservation as an identity valid for all values of ψ -and ψ + , one obtains that D ( B ) necessarily takes the form:”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Findingreasoning
    § A. First order form of the equations · p. 7

    The operators D(B) do not commute with σ1 and σ2, and the invariance considered here is not an invariance of the continuous quantum walk under the action of SU(2).

    “Let us stress that the operators D ( B ) do not commute with σ 1 and σ 2 ? and that the invariance considered here is not an invariance of the continuous quantum walk under the action of SU (2).”
    Quote in the paperNot a substantial claim
  • May combine two claims.

    Findingreasoning
    § A. First order form of the equations · p. 7

    The generator σ3 commutes with D, but σ1 and σ2 do not.

    “The generator σ 3 does commute with D , but σ 1 and σ 2 do not.”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Findingreasoning
    § B. Second order form of the equations · p. 9

    The irreversibility is made manifest in (22) by the (∂+ ln ̄θ)∂-ψ- term and in (23) by the (∂- ln ̄θ)∂+ψ+ term.

    “This irreversibility is made manifest in (22) by the ( ∂ + ln ¯ θ ) ∂ -ψ -term and in (23) by the ( ∂ -ln ¯ θ ) ∂ + ψ + term.”
    Quote in the paperSubstantial claim
  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Findingnone given
    § A. First order form of the equations · p. 7

    The operator D characterizing the continuous dynamics of the quantum walks considered in this article belongs to the family D(B).

    “It is obvious that the operator D characterizing the continuous dynamics of the quantum walks considered in this article belongs to this family.”
    Quote in the paperNot a substantial claim
  • May combine two claims. The quote only partly states this claim.

    Findingnone given
    § A. Summary · p. 11

    The Lagrangian density and hence the dynamics is not invariant under the global or local action of SU(2).

    “Neither the dynamics of the discrete-time walk nor the dynamics of its continuous limit are invariant under SU (2) , but the continuous dynamics nevertheless belongs to a family of Dirac-like equations which exhibit an elegant symmetry.”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Findingreasoning
    § A. First order form of the equations · p. 7

    The family of operators D(B) displays local invariance with respect to transformation (26), and the operator fixing the continuous quantum walk dynamics is one member of this locally invariant family.

    “The family of operators D ( B ) thus displays local invariance with respect to the transformation (26) and the operator fixing the continuous quantum walk dynamics is one member of this locally invariant family.”
    Quote in the paperNot a substantial claim
  • May combine two claims.

    Findingreasoning
    § B. Second order form of the equations · p. 10

    The only non-possibly-vanishing components of the electromagnetic tensor F are F-+ = F+-; these components vanish if the conditions from identifying (36) with (22) are satisfied.

    “The only non possibly vanishing components of the electromagnetic tensor F ( i.e. the curvature of the connection A ) are F -+ = F + -. A straightforward calculation shows that these components actually vanish if the above conditions are satisfied.”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Findingreasoning
    § B. Second order form of the equations · p. 11

    In the other situations, each Klein-Gordon equation contains terms which induce time-irreversibility and the equations do not follow from autonomous variational principles.

    “In the other situations, each Klein-Gordon equation contains terms which induce time-irreversibility and the equations do not follow from autonomous variational principles.”
    Quote in the paperSubstantial claim
  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Findingnone given
    § B. Discussion · p. 12

    The scaling studied in this article is not the only one which allows for a continuous limit of the discrete walk.

    “the scaling studied in this article is not the only one which allows for a continuous limit of the discrete walk.”
    Quote in the paperNot a substantial claim
  • May combine two claims.

    Findingnone given
    § A. Summary · p. 11

    The continuous limit is described by a simple Dirac-like equation which derives from a variational principle.

    “The continuous limit is described by a simple Dirac-like equation which derives from a variational principle.”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Interpretationreasoning
    § B. Second order form of the equations · p. 10

    The conditions for ψ+ make the electromagnetic tensor F+ vanish; thus A+ does not represent a real, physical electromagnetic field.

    “As above, these conditions makes the electromagnetic tensor F + vanish. Thus, A + does not represent a real, physical electromagnetic field.”
    Quote in the paperSubstantial claim
  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Interpretationreasoning
    § III. EQUATIONS OF MOTION · p. 6

    The natural initial conditions for (22) are the values of ψ^- and ∂_t ψ^- on the sub-manifold t = 0.

    “The natural initial conditions for (22) are the values of ψ -and ∂ t ψ -on the sub-manifold t = 0.”
    Quote in the paperNot a substantial claim
  • May combine two claims.

    Interpretationreasoning
    § B. Scaling for the continuous limit · p. 4

    The two functions θ and ξ must depend on ǫ and tend to 0 or π as ǫ tends to zero.

    “This trivial remark implies that the two functions θ and ξ must actually depend on ǫ and tend to 0 or π as ǫ tends to zero.”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Interpretationreasoning
    § B. Second order form of the equations · p. 10

    A mass with imaginary part would generate an imaginary part in the frequency (energy) or wave-vector (impulse) of the particle, and this would signal irreversibility.

    “The physical reason for this restriction on the mass M -is that a mass with imaginary part would generate an imaginary part in the frequency (energy) or wave-vector (impulse) of the particle, and this would signal irreversibility.”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Interpretationnone given
    § I. INTRODUCTION · p. 2

    When it exists, the continuous limit can be described by a 2D Dirac-like equation or, alternately, two independent Klein-Gordon equations.

    “when it exists, the continuous limit can be described by a 2 D Dirac -like equation or, alternately, two independent Klein-Gordon equations”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Interpretationreasoning
    § A. First order form of the equations · p. 7

    σ1 and σ2 interchange ψ- and ψ+ while these two components of Ψ play fundamentally different roles, one propagating towards the left while the other propagates towards the right.

    “This reflects the fact that σ 1 and σ 2 interchange ψ -and ψ + while these two components of Ψ play fundametally different roles, one propagating towards the left while the other propagates towards the right.”
    Quote in the paperNot a substantial claim
  • May combine two claims.

    Interpretationreasoning
    § B. Second order form of the equations · p. 10

    The potential A- can be eliminated by the zero of phase for ψ- and does not represent a real, physical electromagnetic field.

    “Thus, the potential A -can be eliminated by the zero of phase for ψ -and it does not represent a real, physical electromagnetic field.”
    Quote in the paperSubstantial claim
  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Premisenone given
    § A. First order form of the equations · p. 9

    Ψ belongs to a two-dimensional Hilbert space and is not a Dirac spinor.

    “Of course, Ψ here belongs to a two -dimensional Hilbert space, and is not a Dirac spinor.”
    Quote in the paperNot a substantial claim
  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation
    § B. Discussion · p. 11

    At least formally, discrete time quantum walks become continuous time quantum walks when the time step tends to zero.

    “It is shown in 32 that, at least formally, discrete time quantum walks become continuous time quantum walks when the time step tends to zero.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [32] 1986, Feynman and the visualization of space-time processes, Rev. Mod. Phys. source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation
    § B. Discussion · p. 12

    References 17,32 mention that a Dirac-like equation obeyed by Ψ can be formally obtained from a discrete quantum walk when the angle ξ tends to zero with the time and length steps.

    “These references mention that a Dirac-like equation obeyed by Ψ can be formally obtained from a discrete quantum walk when the angle ξ tends to zero with the time and length steps”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites [17] 1998, Quantum computation and decision trees, Phys. Rev. A source

      Not checked in the free run.

    • cites [32] 1986, Feynman and the visualization of space-time processes, Rev. Mod. Phys. source

      Not checked in the free run.

  • Classified as probably not a substantial claim.

    Premisecitation
    § B. Discussion · p. 12

    References 6,20 propose continuous limits for several discrete walks with constant coefficients, including the Hadamard walk.

    “References 6,20 propose continuous limits for several discrete walks with constant coefficients, including the Hadamard walk.”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites [6] 2004, Quantum random walks and piecewise deterministic evolutions, Phys. Rev. Lett. source

      Not checked in the free run.

    • cites [20] 2009, Science source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation
    § B. Discussion · p. 12

    The equations presented in 10 contain various trigonometric functions of θ.

    “the equations presented in 10 contain various trigonometric functions of θ .”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [10] D. Burgarth, Quantum State Transfer with Spin Chains, Ph.D. dissertation (University College, London, 2006).

      Not checked in the free run.

  • Classified as probably not a substantial claim.

    Premisenone given
    § B. Discussion · p. 12

    References 17,32 offer little detailed computations.

    “but they offer little detailed computations”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites [17] 1998, Quantum computation and decision trees, Phys. Rev. A source

      Not checked in the free run.

    • cites [32] 1986, Feynman and the visualization of space-time processes, Rev. Mod. Phys. source

      Not checked in the free run.

  • Classified as probably not a substantial claim. May combine two claims.

    Premisecitation
    § B. Discussion · p. 12

    Reference 10 presents a detailed computation and both Dirac and Klein-Gordon equations are obtained.

    “This reference does present a detailed computation and both Dirac and Klein-Gordon equations are obtained”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [10] D. Burgarth, Quantum State Transfer with Spin Chains, Ph.D. dissertation (University College, London, 2006).

      Not checked in the free run.

  • Classified as probably not a substantial claim.

    Premisenone given
    § B. Discussion · p. 12

    References 17,32 do not mention the existence of Klein-Gordon equations.

    “and do not mention the existence of Klein-Gordon equations.”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites [17] 1998, Quantum computation and decision trees, Phys. Rev. A source

      Not checked in the free run.

    • cites [32] 1986, Feynman and the visualization of space-time processes, Rev. Mod. Phys. source

      Not checked in the free run.

27 of 27
  • The authors state this as an assumption.

    All functions can be chosen at least C^2 in both space and time variables for all sufficiently small values of ∆t and ∆x.

    “We now suppose, that all functions can be chosen at least C 2 in both space and time variables for all sufficiently small values of ∆ t and ∆ x .”
1 of 1

Claims other papers make about this one

Each is checked against this paper’s own text: does it say what the citing paper says it does?

  • Dirac Hamiltonian for a non-zero mass particle has been recovered from QW by taking the evolution into the continuum limit and changing to null coordinates, or by taking each step evolution operator to continuous form and introducing a rotation.

    “In order to recover the DE for a non-zero mass particle from the QW evolution operator, the evolution was taken into continuum limit and the coordinate was changed to null coordinate48. DH for a non-zero mass particle was also recovered by taking each step evolution operators to continuous form and by introducing a rotation 47.”

    From Dirac Cellular Automaton from Split-step Quantum Walk (2016) · cites this paper as [48]

    Section III (p. 5) derives the continuous-limit equations using null coordinates and calls them the Dirac form of the walk dynamics. The First order form section (pp. 8–9) identifies a mass-like term, but describes the result as Dirac-like and explicitly says the two-component wave function is not a Dirac spinor. The separate claim about a rotation is attached to citation 47, not this source. (LLM, conf=0.84)

    Experimental — accuracy not yet measured

Self-check

Percentages, sums and reported differences re-computed from the paper’s own numbers.

Each contradiction between two passages listed here was confirmed by an independent check.

  • Total-probability expression repeats the minus component

    Minor

    The later expression counts |ψ−|2 twice and omits |ψ+|2, contradicting the paper's earlier definition of total probability.

1 of 1

Cites

Flagged first, then by citations

Research works printed but not linked: 1

  1. 2003, Proceedings of the Thirty-Fifth Annual ACM Symposium

Cited by

Sorted by citations

Authors
di Molfetta, G., & Debbasch, F.
Affiliations
  • Université Paris 6 , ERGA-LERMA, UMR 8112, 3, rue Galilée, F-94200 Ivry, France
  • Laboratoire d’Etudes du Rayonnement et de la Matière en Astrophysique et Atmosphères (FR)
Venue
Journal of Mathematical Physics
Published
2012-11-08
Volume
53
Issue
12
Pages
123302
ISSN
  • 0022-2488 (ISSN-L)
  • 1089-7658
OpenAlex
W2012636081
Cited by
60
References
45
Citations by year
2026: 1 · 2025: 2 · 2024: 1 · 2023: 3 · 2022: 3 · 2021: 4 · 2020: 3 · 2019: 6 · 2018: 5 · 2017: 6 · 2016: 9 · 2015: 7 · 2014: 5 · 2013: 3 · 2012: 2
FWCI
5.31
Type
journal article
Language
en
Open access
Locations
  • Journal of Mathematical Physics (publishedVersion)
  • arXiv (Cornell University) (submittedVersion)
  • HAL (submittedVersion)
  • arXiv (Cornell University)
Topics
  • Physical Sciences › Computer Science › Artificial Intelligence › Quantum Computing Algorithms and Architecture
  • Physical Sciences › Physics and Astronomy › Atomic and Molecular Physics, and Optics › Quantum Mechanics and Applications
  • Physical Sciences › Physics and Astronomy › Statistical and Nonlinear Physics › Noncommutative and Quantum Gravity Theories
Keywords
discrete-time quantum walks, continuum limit, time-dependent coefficients, space-dependent coefficients, Dirac equation, Klein-Gordon equation, variational principle, local invariance, symmetry, one-dimensional quantum walk

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