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arxiv: 2605.07968 · v1 · submitted 2026-05-08 · 💻 cs.FL

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Measure Many Quantum Finite Automata on Infinite Words

A Baskar, Abhisek Midya

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Pith reviewed 2026-05-11 02:41 UTC · model grok-4.3

classification 💻 cs.FL
keywords quantum automataBüchi acceptanceinfinite wordsmeasure-many quantum finite automataclosure propertiesdecidabilityquantum computationformal languages
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The pith

Measure-many quantum Büchi automata accept infinite words precisely when they equal the limit of a measure-many quantum finite automaton language.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper defines measure-many quantum Büchi automata (MMQBA) that process infinite words through unitary evolution interrupted by repeated projective measurements. It proves that the languages these automata accept are exactly the limits of languages accepted by ordinary measure-many quantum finite automata under the same cutpoint. This characterization supports proofs that MMQBA are closed under union but not under intersection or complement, that emptiness is semi-decidable, and that universality, inclusion, equivalence, and membership are undecidable. A reader would care because the result supplies a concrete way to translate questions about ongoing quantum processes into questions about finite-word quantum automata whose limits can be analyzed directly.

Core claim

MMQBA languages are precisely of the form lim(L(M, p)) for MMQFA M and cutpoint p, with a decomposition of the non-halting subspace into components that remain non-halting or eventually halt, which justifies the limit construction and enables the closure and decidability results.

What carries the argument

The three-part Büchi acceptance condition requiring that the run visits accepting states infinitely often, that the limiting cumulative acceptance probability is at least p, and that the limiting cumulative rejection probability is strictly less than p.

If this is right

  • Any language accepted by an MMQBA equals the limit of the language accepted by some MMQFA at the same cutpoint.
  • MMQBA are closed under union.
  • The emptiness problem for MMQBA is semi-decidable.
  • Universality, inclusion, equivalence, and membership problems for MMQBA are undecidable.
  • MMQBA are not closed under intersection or complementation.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The limit characterization transfers any property of MMQFA languages that is preserved under limits to the corresponding MMQBA languages.
  • The decomposition of the non-halting subspace isolates the quantum states whose long-term behavior determines acceptance, separating them from states that reach halting measurements.
  • Semi-decidability of emptiness yields a procedure that can confirm when an MMQBA accepts no infinite word, though it may run forever when the language is nonempty.

Load-bearing premise

The limiting cumulative acceptance and rejection probabilities must exist for every infinite run that satisfies the infinite-visit requirement.

What would settle it

An explicit infinite word and MMQBA such that the word meets the three-part acceptance condition yet the accepted language cannot be written as lim(L(M, p)) for any MMQFA M would falsify the characterization.

read the original abstract

We define a quantum computational model over infinite words, called Measure-Many Quantum B\"uchi Automata (MMQBA), which extends Measure-many Quantum Finite automata (MMQFA) to the infinite word setting with B\"uchi acceptance condition. In MMQBA, the quantum state evolves through unitary transformations followed by repeated projective measurements. An infinite word is accearaq2ppted with respect to a cutpoint p is in (0, 1] if (i) the run visits accepting states infinitely often, (ii) the limiting cumulative acceptance probability is at least p, and (iii) the limiting cumulative rejection lprobability is strictly less than p. We formalize the semantics of MMQBA, establish a language-theoretic characterization showing that MMQBA languages are precisely of the form lim(L(M, p)) for MMQFA M , and develop a decomposition of the non-halting subspace. We prove that MMQBA is closed under union but not under intersection or complementation. On the algorithmic side, we show that the emptiness problem is semi-decidable, while universality, inclusion, equivalence, and membership remain undecidable.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

3 major / 2 minor

Summary. The paper introduces Measure-Many Quantum Büchi Automata (MMQBA) as an extension of Measure-Many Quantum Finite Automata (MMQFA) to infinite words, using a Büchi-style acceptance condition that requires visiting accepting states infinitely often along with the existence of limiting cumulative acceptance probability ≥ p and rejection probability < p. It claims a language-theoretic characterization equating MMQBA languages to lim(L(M, p)) for MMQFA M, develops a decomposition of the non-halting subspace, proves closure under union (but not intersection or complement), and establishes that emptiness is semi-decidable while universality, inclusion, equivalence, and membership are undecidable.

Significance. If the central claims hold, the work extends quantum automata theory to infinite words and supplies a concrete bridge between finite-word MMQFA languages and their infinite-word limits, together with closure and decidability results that delineate the model's expressive power. The explicit decomposition and semi-decidability proof for emptiness constitute concrete technical contributions that could support further algorithmic study of quantum ω-languages.

major comments (3)
  1. [§3] §3 (decomposition of the non-halting subspace): the argument that the three-part acceptance condition is well-defined relies on the claim that the limiting cumulative acceptance and rejection probabilities exist whenever the Büchi visit condition holds. The decomposition alone does not establish convergence of the cumulative sequences; without an additional spectral argument (e.g., that the restriction of the effective operator to the relevant subspace is diagonalizable over ℂ with eigenvalues of modulus 1 only at 1, or an explicit proof that the sequences are eventually monotonic), the limits may fail to exist for periodic or oscillatory behavior inside the subspace. This gap directly affects the characterization in §4 and the closure-under-union proof in §5.
  2. [§4] §4 (characterization theorem): the equality MMQBA languages = lim(L(M, p)) is stated to follow from the decomposition, yet the proof sketch does not address the case in which the cumulative probability sequences oscillate and the limits therefore do not exist. If such non-convergent runs are possible under the Büchi condition, the “precisely of the form” direction fails.
  3. [§5] §5 (closure under union): the construction that combines two MMQBA automata into one whose language is the union presupposes that the limiting probabilities exist for the combined automaton whenever they exist for the components. Because the existence question is left open by the decomposition, the closure claim is not yet secured.
minor comments (2)
  1. [Abstract] Abstract contains typographical errors (“accearaq2ppted”, “lprobability”) that should be corrected before publication.
  2. [§3] Notation for the limiting operator and the non-halting projection is introduced without an explicit summary table; adding such a table would improve readability of the decomposition argument.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for the careful reading and for highlighting the need to ensure the acceptance condition is rigorously well-defined. We address each major comment below. The key point is that the cumulative sequences are partial sums of non-negative terms and hence monotonic, so their limits exist in the extended reals; we will add an explicit clarification of this fact.

read point-by-point responses
  1. Referee: [§3] §3 (decomposition of the non-halting subspace): the argument that the three-part acceptance condition is well-defined relies on the claim that the limiting cumulative acceptance and rejection probabilities exist whenever the Büchi visit condition holds. The decomposition alone does not establish convergence of the cumulative sequences; without an additional spectral argument (e.g., that the restriction of the effective operator to the relevant subspace is diagonalizable over ℂ with eigenvalues of modulus 1 only at 1, or an explicit proof that the sequences are eventually monotonic), the limits may fail to exist for periodic or oscillatory behavior inside the subspace. This gap directly affects the characterization in §4 and the closure-under-union proof in §5.

    Authors: We agree that the decomposition by itself does not address convergence, but the cumulative acceptance (resp. rejection) probability up to step n is defined as the sum of the per-step acceptance (resp. rejection) probabilities from 1 to n. Each per-step probability is non-negative, so the partial-sum sequence is non-decreasing and the limit therefore always exists in [0, +∞]. We will insert a short paragraph in §3 making this monotonicity explicit and noting how the case of an infinite limit is handled under the Büchi condition. No spectral argument is required for existence. revision: partial

  2. Referee: [§4] §4 (characterization theorem): the equality MMQBA languages = lim(L(M, p)) is stated to follow from the decomposition, yet the proof sketch does not address the case in which the cumulative probability sequences oscillate and the limits therefore do not exist. If such non-convergent runs are possible under the Büchi condition, the “precisely of the form” direction fails.

    Authors: The characterization proof in §4 inherits the same convergence question. As explained in our response to the §3 comment, the cumulative sequences are monotonic partial sums and therefore converge (possibly to +∞). We will add a cross-reference to the new monotonicity paragraph and a brief case analysis for the infinite-limit situation, thereby completing the “precisely of the form” direction. revision: partial

  3. Referee: [§5] §5 (closure under union): the construction that combines two MMQBA automata into one whose language is the union presupposes that the limiting probabilities exist for the combined automaton whenever they exist for the components. Because the existence question is left open by the decomposition, the closure claim is not yet secured.

    Authors: The union construction in §5 is well-defined once limits exist for the product automaton. Because the per-step probabilities remain non-negative in the product, the cumulative sequences are again monotonic and converge. We will add a sentence in §5 invoking the monotonicity argument already supplied in the revised §3; the closure proof then goes through without further change. revision: partial

Circularity Check

0 steps flagged

No significant circularity; characterization via explicit limit construction is self-contained

full rationale

The paper defines MMQBA semantics directly via the three-part Büchi-style acceptance condition on infinite runs (visiting acceptors infinitely often plus existence of limiting cumulative probabilities), then states a language-theoretic characterization that MMQBA languages equal lim(L(M, p)) for MMQFA M. This is an explicit construction rather than a reduction of the claim to its own inputs. The non-halting subspace decomposition is introduced to support the infinite-run analysis, and closure, non-closure, and decidability results are presented as independently proven. No self-citations, fitted parameters renamed as predictions, or ansatzes smuggled via prior work appear as load-bearing steps in the derivation chain. The results remain externally falsifiable via the stated acceptance conditions and limit construction.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 1 invented entities

The central claim rests on standard quantum mechanics for state evolution plus the newly introduced acceptance condition and limit construction; no free parameters are fitted to data.

axioms (2)
  • standard math Unitary transformations and projective measurements act on finite-dimensional Hilbert spaces
    Invoked for the evolution and measurement steps in the automaton definition.
  • domain assumption Limiting cumulative probabilities exist for infinite runs
    Required for the acceptance condition to be well-defined.
invented entities (1)
  • Measure-Many Quantum Büchi Automata (MMQBA) no independent evidence
    purpose: Quantum model for languages over infinite words with repeated measurements and Büchi acceptance
    Newly defined model whose properties are the subject of the paper.

pith-pipeline@v0.9.0 · 5496 in / 1457 out tokens · 58281 ms · 2026-05-11T02:41:01.040359+00:00 · methodology

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Works this paper leans on

36 extracted references · 36 canonical work pages

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    = 1 3 |q0⟩ − √ 2 3 |q1⟩=ψ 2 wherex 2 =aa, acc 2 = 2 3 + 2 9 , rej 2 = 0, ψ ′ 1 = Pnon(ψ1) = 1√ 3 |q0⟩. –Va(ψ′

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    √ 3 |q0⟩ − √ 2 3

    = 1 3. √ 3 |q0⟩ − √ 2 3. √ 3 |q1⟩=ψ 3 wherex 3 =aaa, acc 3 = 2 3 + 2 9 + 2 27 , rej 3 = 0, ψ ′ 2 =P non(ψ2) = 1 3 |q0⟩. –Vb(ψ′

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    √ 3 |q0⟩+ 2 9

    = 1 9. √ 3 |q0⟩+ 2 9. √ 3 |q1⟩+ 2 9. √ 3 |q2⟩=ψ 4 wherex 4 =aaab, acc 4 = 2 3 + 2 9 + 2 27 + 4 81.3 , rej 4 = 4 81.3 , ψ ′ 3 =P non(ψ3) = 1 3 √ 3 |q0⟩. –Vb(ψ′

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    √ 3 |q0⟩+ 2 27

    = 1 27. √ 3 |q0⟩+ 2 27. √ 3 |q1⟩+ 2 27. √ 3 |q2⟩=ψ 5 wherex 4 =aaabb, acc 5 = 2 3 + 2 9 + 2 27 + 4 81.3 + 4 27.27.3 , rej 5 = 4 81.3 + 4 27.27.3 , ψ ′ 4 =P non(ψ4) = 1 27 √ 3 |q0⟩. –In general, at stepj≥1the cumulative acceptance probability satisfies accj =    2 3 , j= 1, 2 3 + 2 9 +· · ·+ 2 3j ,1≤j≤3, 2 3 + 2 9 + 2 27 + jX k=4 4 3·9 k...

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    Example 5.LetL=a(a+b) ω,Σ={a, b}wheren≥1

    Henceb ω is rejected for any cutpoint p > 1 2. Example 5.LetL=a(a+b) ω,Σ={a, b}wheren≥1. WedefinetheMMQBAM= (Q, Σ, δ, q 0, Qacc, Qrej)whereQ={q 0, q1, q2, q3}, Qacc = {q2}, Qrej ={q 3}, cut pointp= 0.8andq 0 is the initial state. Va =   0 0 0 1 2 3 1√ 2 1 3. √ 2 0 2 3 − 1√ 2 1 3. √ 2 0 1 3 0− 4 3. √ 2 0   , V b =   0 0 0 1 0 1√ 2 − ...

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    For everyψ∈S 1 and every infinite input wordx= #σ 1σ2 · · · ∈Γ ω, the run ofMremains entirely withinS 1, and the cumulative halting probability is zero such that ∀k≥1, ψ ′ k ∈S 1, ∞X k=1 ∥Phalt(ψk)∥2 = 0

  28. [28]

    Letψ∈S non be the initial state and define the run as follows: ψ0 :=ψ, ψ 1 :=V σ1 ψ0, ψ ′ 1 :=P non(ψ1)

    For everyψ∈S 2,lim k→∞ ∥ψ′ k∥= 0 Proof.LetV σ be the unitary transition operator for input symbolσ∈Σ, and define the projected transition operator asV′ σ :=P nonVσ. Letψ∈S non be the initial state and define the run as follows: ψ0 :=ψ, ψ 1 :=V σ1 ψ0, ψ ′ 1 :=P non(ψ1). For everyk≥2, define recursively: ψk :=V σk ψ′ k−1, ψ ′ k :=P non(ψk). The state space ...

  29. [29]

    Cumulative acceptance too small.For allj > k, we may haveaccj < p

    Sincep ′ > 1 2, this violates the MMQBA rejection bound, and therefore x /∈L(M′, p′). Cumulative acceptance too small.For allj > k, we may haveaccj < p. Then limj→∞ accj ≤p, and sincep > 1 2 butlim acc j does not reachp′, the MMQBA acceptance cutpoint requirement fails. Hencex /∈L(M′, p′). Measure Many Quantum Finite Automata on Infinite Words 39 Büchi co...

  30. [30]

    limj→∞ accj ≥p ′, 3.lim j→∞ rejj < p ′

    infinitely many indicesjwith∥P acc(ψj)∥2 >0(Büchi condition), 2. limj→∞ accj ≥p ′, 3.lim j→∞ rejj < p ′. We construct the following words to analyze the run ofM-x1 =aab k1 , x2 = aabk1 aabk2 , x3 =aab k1 aabk2 aabk3, which can be extended to an infinite sequence x=aab k1 aabk2 aabk3 · · · ∈Σ ω. Let the initial state ofMbe represented as ψ0. The state evol...

  31. [31]

    IfP(x)never halts, then along the unique run onaω there are only finitely many (indeed, zero) projections intoQacc (Büchi fails), and the cumulative acceptance and rejection masses satisfylimj→∞ accj = 0andlim j→∞ rejj = 0< p

  32. [32]

    IfP(x)halts in exactlyksteps, then at stepkthe run projects into an accepting halting subspace with nonzero amplitude, and from every subse- quent step it leaks a fixed, nonzero amount intoQacc andnomass intoQ rej. Consequently: (a) Büchi holds because accepting projections occur at allj≥k; (b)rej j is monotone and bounded by0< p, hencelim j→∞ rejj < p; (...

  33. [33]

    Therefore all three conditions of Definition 3.1 hold, and we conclude thataω ∈ L(MP,x, p)

    Although some rejecting projections may occur, they are bounded so thatlim j→∞ rejj < p. Therefore all three conditions of Definition 3.1 hold, and we conclude thataω ∈ L(MP,x, p). 46 Abhisek Midya and A Baskar Combining the two directions, we haveaω ∈L(M P,x, p)⇐ ⇒P(x)halts. Therefore, if the assumed algorithmAfor membership existed, we could decide the ...

  34. [34]

    Then the following statements are equivalent:

  35. [35]

    The languageL(M, p)is nonempty

  36. [36]

    Proof.(1)⇒(2) :AssumeL(M, p)̸=∅

    There exists a wordw=uv ω ∈Σ ω, whereu∈Σ ∗ andv∈Σ +, such that the run ofMonw Measure Many Quantum Finite Automata on Infinite Words 47 –visits final states infinitely often, –satisfieslim j→∞ accj ≥p, and –satisfieslim j→∞ rejj < p. Proof.(1)⇒(2) :AssumeL(M, p)̸=∅. Then there exists an infinite input word w=σ 1σ2σ3 · · · ∈Σ ω such that the run ofMonwsati...